Effect of urban development on solute transport in groundwater

Transcription

1 Effect of urban development on solute transport in groundwater Anthony Barr and Olga Barron October 29

2 Water for a Healthy Country Flagship Report series ISSN: X Australia is founding its future on science and innovation. Its national science agency, CSIRO, is a powerhouse of ideas, technologies and skills. CSIRO initiated the National Research Flagships to address Australia s major research challenges and opportunities. They apply large scale, long term, multidisciplinary science and aim for widespread adoption of solutions. The Flagship Collaboration Fund supports the best and brightest researchers to address these complex challenges through partnerships between CSIRO, universities, research agencies and industry. The Water for a Healthy Country Flagship aims to achieve a tenfold increase in the economic, social and environmental benefits from water by 225. For more information about Water for a Healthy Country Flagship or the National Research Flagship Initiative visit Citation: Barr, A. and Barron, O. 29. Effect of urban development on solute transport in groundwater. CSIRO: Water for a Healthy Country National Research Flagship Copyright and Disclaimer 29 CSIRO To the extent permitted by law, all rights are reserved and no part of this publication covered by copyright may be reproduced or copied in any form or by any means except with the written permission of CSIRO. Important Disclaimer: CSIRO advises that the information contained in this publication comprises general statements based on scientific research. The reader is advised and needs to be aware that such information may be incomplete or unable to be used in any specific situation. No reliance or actions must therefore be made on that information without seeking prior expert professional, scientific and technical advice. To the extent permitted by law, CSIRO (including its employees and consultants) excludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using this publication (in part or in whole) and any information or material contained in it. Cover Photograph: From CSIRO s ScienceImage: File: BU6454.jpg Description: Aerial view of the western suburbs of Sydney, NSW Photographer: Gregory Heath 29 CSIRO

3 CONTENTS Acknowledgments... xii Executive Summary... xiii 1. Introduction Regional modelling Topography, geology, hydrogeology and land use Climate Surface waters Regional model Flow modelling of urban development areas Urbanisation and relevant changes in the water regime in the catchment Local area modelling for solute transport Modelling and underlying assumptions Presentation of urban land cover in the model and associated forcing parameters Solute transport modelling Water and solute mass balances Validation Scenarios Flow modelling Legacy concentrations Changes in solute mass in shallow groundwater Changes in solute mass in deep superficial groundwater Solute export from the area by drains Solute export via groundwater abstraction from deep superficial aquifer Legacy solute transport with groundwater flux into and from the model domain Legacy solute transport into the domain Legacy solute transport from the domain Garden and verge recharge Changes in solute mass in shallow groundwater Changes in solute mass in deep groundwater Solute export in drains Solute export from the area with groundwater abstraction from deeper superficial aquifer Solute transport with groundwater flux from the model domain Solutes associated with roof runoff Application of model results Chloride Nitrogen and Phosphorus Effect of climate Effect of urban density Effect of groundwater abstraction Summary and conclusions References... 7 Solutes transport in an urbanising area iii

4 APPENDIX A Predictive climate sequences APPENDIX B Sensitivity analysis B.1 Conductance of subsurface drains...73 B.2 Recharge in the gardens...75 APPENDIX C Simulated solute distributions C.1 Legacy solute...79 C.2 Garden recharge solute...82 C.3 Solutes associated with roof runoff...84 C.3.1 Changes in solute mass in deep groundwater C.3.2 Solute export in drains C.3.3 Solute export from the area with groundwater abstraction from deeper superficial aquifer C.3.4 Solute transport with groundwater flux from the model domain APPENDIX D Total solute export Solutes transport in an urbanising area iv

5 LIST OF FIGURES Figure 2-1 Ground surface elevation within the regional model domain... 3 Figure 2-2 Surface geology of the catchment within the regional model domain... 4 Figure 2-3 Land cover for the Southern River catchment within the regional model domain... 5 Figure 2-4 Computational grid for the regional model... 6 Figure 2-5 Wungong Urban Water and associated MODHMS model domain... 7 Figure 2-6 Comparison of cumulative rainfall for the three climate scenarios... 8 Figure 2-7 Fraction of cumulative rainfall difference between specified climate scenarios and current climate... 8 Figure 3-1 Area of Wungong Urban Water showing extent of local and district models Figure 3-2 Schematic of the transition between low horizontal discretised WUW submodel with high discretisation of upper superficial aquifer just below the ground surface to highly horizontally discretised WUW local area model with no vertical discretisation in upper superficial aquifer. The vertical lines represent the horizontal discretisation, the lines across represent the vertical discretisation Figure 3-3 Urban land cover for (a) low, (b) medium and (c) high density developments Figure 3-4 Location of head observation points and abstraction wells within the WUW local model, The vertical lines represent the subsurface drains Figure 4-1 Monthly sub-surface drainage for scenario under current climate, medium urban density, abstraction for POS and drains as the only watertable control Figure 4-2 Monthly evapotranspiration for scenario under current climate, medium urban density, abstraction for POS and drains as the only watertable control... 2 Figure 4-3 Time series of heads at two locations in the domain: (a) at a drain and (b) up-gradient mid-way between two drains... 2 Figure 4-4 Downward flux into the lower superficial aquifer layer as a function of (a) climate, (b) the urban density, (c) groundwater abstraction rates and (d) the shallow watertable control; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) Figure 4-5 Annual cycle (June year 4 to June year 5) of downward flux into lower superficial aquifer layer as a function of groundwater abstraction rates using current climate, medium urban density and drains as the only watertable control option. Irrigation turns on at the end of September and turns off at the end of March Figure 4-6 Infiltration rates as a function of urban density for current climate, abstraction for POS and drains as only watertable control measure Figure 5-1 Mass of legacy solute in upper superficial aquifer for all scenarios as a function of time Figure 5-2 Relative mass of legacy solute in upper superficial layer in WUW as a function of (a) climate, (b) the urban density, (c) groundwater abstraction rates and (d) the shallow watertable control; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) Figure 5-3 Time till (a) 5% (b) 75% and (c) 9% of legacy solutes removed from upper superficial for all scenarios Figure 5-4 Distribution of relative legacy solute concentrations in upper superficial layer (a) at the end of summer irrigation period in the first year (Stress period 4, April); (b) at the end of winter non-irrigation period in the first Solutes transport in an urbanising area v

6 year (Stress period 9, September), (c) at the end of summer irrigation period in the final year (Stress period 112, April) and (d) at the end of winter non-irrigation period in the final year (Stress period 117, September); the simulation uses current climate, medium urban density, abstraction for POS, and drainage as the only watertable control measure Figure 5-5 Simulated fraction of original mass of legacy solute in the lower superficial layer as a function of time for all urban scenarios Figure 5-6 Relative mass of legacy solute in lower superficial layer as a function of (a) climate, (b) the urban density, (c) groundwater abstraction, and (d) the shallow watertable control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) Figure 5-7 Average relative concentration of legacy solute in drainage water during (a) first year and (b) final year for all simulations Figure 5-8 Legacy solute relative mass transfer to drainage during (a) first year and (b) final year for all simulations Figure 5-9 Relative mass flux of legacy solutes into the drains for various (a) climates, (b) urban densities, (c) groundwater abstractions rates and (d) watertable control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) Figure 5-1 Average monthly relative concentration of legacy solute into the drains for various (a) climates, (b) urban densities, (c) groundwater abstractions rates and (d) watertable control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) (gaps on the plots are related to no flow period) Figure 5-11 Relative mass of legacy solutes in abstraction for various (a) climates, (b) urban densities, (c) groundwater abstractions rates (note the different scale on the vertical axis) and (d) watertable control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) (gaps on the plots are related to a period with no irrigation) Figure 5-12 Simulated relative concentration of legacy solutes in abstraction for various (a) climates, (b) urban densities, (c) groundwater abstractions rates and (d) watertable control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) (gaps on the plots are related to a period with no irrigation)... 4 Figure 5-13 Simulated legacy solute mass entering the upper superficial aquifer for all scenarios Figure 5-14 Total mass inflow as percentage of original legacy solute mass for all scenarios Figure 5-15 Simulated relative total mass outflow from model domain of legacy solutes for all scenarios Figure 5-16 Mass outflow as percentage of original legacy solute mass for all scenarios Figure 5-17 Average concentration of outflow through the boundary in the lower superficial aquifer as a function of the abstraction rate Figure 6-1 Simulated relative mass of garden recharge solute in the upper superficial layer for all scenarios Figure 6-2 Simulated relative maximum concentration of garden recharge solute in upper superficial over whole layer for all scenarios Solutes transport in an urbanising area vi

7 Figure 6-3 Relative mass of garden recharge solute in upper superficial layer as a function of various (a) climates, (b) urban densities, (c) groundwater abstractions rates and (d) watertable control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) Figure 6-4 Simulated relative concentration of garden recharge solute in upper superficial layer (a) at the end of summer irrigation period in the final year (Stress period 112, April) and (b) at the end of winter non-irrigation period in the final year (Stress period 117, September) for current climate, medium urban density, abstraction for POS, and drainage as the only watertable control measure Figure 6-5 Simulated relative mass of garden recharge solute in the lower superficial layer for all scenarios Figure 6-6 Simulated relative concentration of garden recharge solute in lower superficial layer (a) at the end of summer irrigation period in the final year (Stress period 112, April) and (b) at the end of winter non-irrigation period in the final year (Stress period 117, September) for current climate, medium urban density, abstraction for POS, and drainage as the only watertable control measure Figure 6-7 Vertical conductance between upper superficial and aquitard layers in the local model... 5 Figure 6-8 Relative solute mass flux associated with groundwater recharge in pervious area into the drains for various (a) climate scenarios, (b) urban densities, (c) groundwater abstraction rates and (d) shallow groundwater control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) Figure 6-9 Relative concentration of drainage water related to groundwater recharge in pervious area for various (a) climate scenarios, (b) urban densities, (c) groundwater abstraction rates and (d) shallow groundwater control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) Figure 6-1 Relative mass solute flux associated with groundwater recharge in pervious area with groundwater abstraction for various (a) climate scenarios, (b) urban densities, (c) groundwater abstractions rates (note the different scale) and (d) shallow groundwater control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) Figure 6-11 Relative concentration of abstracted groundwater associated with groundwater recharge in pervious area for various (a) climate scenarios, (b) urban densities, (c) groundwater abstractions rates and (d) shallow groundwater control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) Figure 6-12 Simulated relative total mass outflow from model domain for garden recharge solutes for all scenarios Figure 6-13 Simulated relative concentration of garden recharge out through general head boundary condition in the lower superficial layer as a function of groundwater abstraction rate for current climate, medium urban density, and watertable control through drainage only Figure 7-1 Relative concentration of drainage water with roof runoff solutes for various (a) climate scenarios, (b) urban densities, (c) groundwater Solutes transport in an urbanising area vii

8 abstraction rates and (d) shallow groundwater control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) Figure 8-1 Total chloride flux to drainage based on first quartile, median and third quartile of legacy concentration (rows) for current climate, medium density, no abstraction, and drainage only as the watertable control scenarios in the first column and current climate, medium density, hot water abstraction, and drainage only as the watertable control scenarios in the second column Figure 8-2 Observed chloride concentrations in Bayswater Main Drain (BW) at Slade Street and Mill Street Main Drain at Palm Place. The fitted line to each set of observations is a 5 point moving average Figure 8-3 Locations of observation sites in currently urbanised areas used for comparison with simulated results Figure 8-4 Total nutrient flux to drainage based on first quartile, median and third quartile of all nutrient concentrations (rows) and total nitrogen (TN) and total phosphorous (TP) in columns for current climate, medium density, POS abstraction, and drainage only as the watertable control scenarios Figure 8-5 Nitrogen concentrations in (a) Bayswater and Mills Street Main Drain Figure 8-6 Total nutrient flux to drainage based on first quartile, median and third quartile of total nitrogen concentrations (rows) and drier and wetter climates in columns for medium density, POS abstraction, and drainage only as the watertable control scenarios Figure 8-7 Total nutrient flux to drainage based on first quartile, median and third quartile of total nitrogen concentrations (rows) and low and high urban densities in columns for current climate, POS abstraction, and drainage only as the watertable control scenarios Figure 8-8 Median total nitrogen concentration in flux to drainage for different nonpotable water abstraction rates for current climate, medium density, and drainage only as the watertable control scenarios Figure A-1 Annual rainfall for the three climate scenarios Figure B-1 Total modelled solute mass for original and sensitivity analysis for subsurface drainage conductance; (a) legacy solute in upper superficial layer, (b) legacy solute in lower superficial aquifer, (c ) garden recharge solute in upper superficial layer, (d) garden recharge solute in lower superficial aquifer, (e) roof runoff solute in upper superficial layer, and (f) roof runoff solute in lower superficial layer. All results are from the scenario using current climate, medium urban density, abstraction for POS and drains-only as the watertable control option Figure B-2 Mass flux to the subsurface drains for (a) legacy solutes, (b) garden recharge solutes, and (c) roof runoff solutes. r. All results are from the scenario using current climate, medium urban density, abstraction for POS and drains-only as the watertable control option Figure B-3 Annual drainage discharge as a function of drain conductance Figure B-4 Total modelled solute mass for original and 2% reduction in garden recharge; (a) legacy solute in upper superficial layer, (b) legacy solute in lower superficial aquifer, (c ) garden recharge solute in upper superficial layer, (d) garden recharge solute in lower superficial aquifer, (e) roof runoff solute in upper superficial layer, and (f) roof runoff solute in lower superficial layer. All results are from the scenario using current climate, medium urban density, abstraction for POS and drains-only as the watertable control option Figure B-5 Comparison of mass flux to the subsurface drains between model results reported in main part of report (original) and model results from sensitivity Solutes transport in an urbanising area viii

9 analysis with varying conductance and recharge (sensitivity) for (a) legacy solutes, (b) garden recharge solutes, and (c) roof runoff solutes. All results are from the scenario using current climate, medium urban density, abstraction for POS and drains-only as the watertable control option Figure C-1 Legacy solute concentration in the lower superficial layer for four times in simulation: (a) at the end of summer irrigation period in the first year (Stress period 4, April); (b) at the end of winter non-irrigation period in the first year (Stress period 9, September), (c) at the end of summer irrigation period in the final year (Stress period 112, April) and (d) at the end of winter non-irrigation period in the final year (Stress period 117, September); the simulation uses current climate, medium urban density, abstraction for POS, and drainage as the only watertable control measure Figure C-2 Legacy solute distributions in the top and bottom layers of the superficial aquifer at the end of year 4 for three different abstraction scenarios Figure C-3 Legacy solute mass flux into and out of the lower superficial aquifer for current climate, medium density urban area, abstraction for POS and drainage-only watertable control Figure C-4 Garden recharge solute concentration in the lower superficial layer for four times in simulation: (a) at the end of summer irrigation period in the first year (Stress period 4, April); (b) at the end of winter non-irrigation period in the first year (Stress period 9, September), (c) at the end of summer irrigation period in the final year (Stress period 112, April) and (d) at the end of winter non-irrigation period in the final year (Stress period 117, September); the simulation uses current climate, medium urban density, abstraction for POS, and drainage as the only watertable control measure Figure C-5 Garden recharge solute mass flux into and out of the lower superficial aquifer for current climate, medium density urban area, abstraction for POS and drainage-only watertable control Figure C-6 Simulated relative mass of roof recharge solute in the upper superficial layer for all scenarios Figure C-7 Maximum concentration of roof runoff solutes in upper superficial layer for all scenarios Figure C-8 Roof runoff solute concentration in the upper superficial layer for four times in simulation: (a) at the end of summer irrigation period in the first year (Stress period 4, April); (b) at the end of winter non-irrigation period in the first year (Stress period 9, September), (c) at the end of summer irrigation period in the final year (Stress period 112, April) and (d) at the end of winter non-irrigation period in the final year (Stress period 117, September); the simulation uses current climate, medium urban density, abstraction for POS, and drainage as the only watertable control measure Figure C-9 Simulated relative mass of roof recharge solute in the lower superficial layer for all scenarios Figure C-1 Roof runoff solute concentration in the lower superficial layer for two times in simulation: (a) at the end of summer irrigation period in the final year (Stress period 112, April) and (b) at the end of winter non-irrigation period in the final year (Stress period 117, September); the simulation uses current climate, medium urban density, abstraction for POS, and drainage as the only watertable control measure Figure C-11 Relative mass flux of roof runoff solutes into the drains for various (a) climates, (b) urban densities, (c) groundwater abstractions rates and (d) watertable control measures; the simulations were for current climate (b, Solutes transport in an urbanising area ix

10 c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) Figure C-12 Average monthly relative concentration of roof runoff solute into the drains for various (a) climates, (b) urban densities, (c) groundwater abstractions rates and (d) watertable control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) (gaps on the plots are related to no flow period) Figure C-13 Relative mass of roof runoff solutes in abstraction for various (a) climates, (b) urban densities, (c) groundwater abstractions rates (note the different scale on the vertical axis) and (d) watertable control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) (gaps on the plots are related to a period with no irrigation) Figure C-14 Simulated relative concentration of roof runoff solutes in abstraction for various (a) climates, (b) urban densities, (c) groundwater abstractions rates and (d) watertable control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) (gaps on the plots are related to a period with no irrigation) Figure C-15 Simulated relative total mass outflow from model domain for roof recharge solutes Figure C-16 Simulated relative concentration of roof recharge out through general head boundary condition in the lower superficial layer as a function of groundwater abstraction rate and current climate, medium urban density, and watertable control through drainage only Figure D-1 Time series of simulated total nutrient concentration with individual components in drainage water for wetter climate, medium urban density, POS abstraction and drains as the sole watertable control measure. The concentrations are: (a) total nitrogen (TN) legacy of 1.6 mg/l, garden recharge 1.5 mg/l and roof runoff.27 mg/l; (b) total phosphorous (TP) legacy of.5 mg/l, garden recharge.1 mg/l and roof runoff.22 mg/l; (c) TN legacy of 4.5 mg/l, garden recharge 1.5 mg/l and roof runoff.27 mg/l; (d) TP legacy of.5 mg/l, garden recharge.1 mg/l and roof runoff.22 mg/l; (e) TN legacy of 7.4 mg/l, garden recharge 1.5 mg/l and roof runoff.27 mg/l; (f) TP legacy of.58 mg/l, garden recharge.1 mg/l and roof runoff.22 mg/l Figure D-2 Time series of simulated total nutrient concentration with individual components in drainage water for drier climate, medium urban density, POS abstraction and drains as the sole watertable control measure. The concentrations are: (a) total nitrogen (TN) legacy of 1.6 mg/l, garden recharge 1.5 mg/l and roof runoff.27 mg/l; (b) total phosphorous (TP) legacy of.5 mg/l, garden recharge.1 mg/l and roof runoff.22 mg/l; (c) TN legacy of 4.5 mg/l, garden recharge 1.5 mg/l and roof runoff.27 mg/l; (d) TP legacy of.5 mg/l, garden recharge.1 mg/l and roof runoff.22 mg/l; (e) TN legacy of 7.4 mg/l, garden recharge 1.5 mg/l and roof runoff.27 mg/l; (f) TP legacy of.58 mg/l, garden recharge.1 mg/l and roof runoff.22 mg/l. The gaps in the records are where the watertable is below the inverts for the subsurface drainage Solutes transport in an urbanising area x

11 LIST OF TABLES Table 2-1 Average annual rainfall, effective rainfall and potential evaporation (BoM station 91)... 4 Table 2-2 Summary of the urbanisation effects on the water catchment regime and adopted modelling approach to simulate those effects... 9 Table 3-1 Dimensions (length) of land use components for different urban densities Table 3-2 Block widths, area and roof size, adopted for simulation Table 3-3 Concentrations used in modelling Table 3-4 Groundwater abstraction rates for various scenarios (+ indicates the inclusion of the relevant water demand type) Table 4-1 The water balance in the local model domain for a modelling scenario under current climate, medium urban density, abstraction for POS and drains as the only watertable control Table A-1 Annual rainfall [mm] for different climate regimes used in predictive scenarios Solutes transport in an urbanising area xi

12 ACKNOWLEDGMENTS We would like to thank the Western Australian Department of Water for the funding provided through the project Urbanisation Modelling and CSIRO National Flagship Research Water for a Healthy Country for the funding provided through the Swan Future program. This work is a continuation of the project Investigation of Techniques to Better Manage Western Australia s Non-Potable Water Resources and we would like to thank the contributors to that project. We would also like to thank our internal reviewers Dr Janek Greskowiak and Dr Matt Lenahan for adding to the clarity of the document and ensuring the results were fully explained. Solutes transport in an urbanising area xii

13 EXECUTIVE SUMMARY The Southern River catchment is one of the fastest developing areas of Perth. Local urban development is challenged by shallow groundwater and multiple wetlands, some of which are conservation category. It is expected that the future land use alteration may significantly affect the current hydrological cycle. As inundation and shallow watertables are incompatible with much urban infrastructure, it is expected that the urban development will include implementation of watertable control measures. A project funded by the Department of Water and CSIRO National Flagship Water for a Healthy Country aims to evaluate the likely impact of the changes in the land use on the water fluxes and the water quality within the Southern River catchment. This report examines the effects of urbanisation on solute transfer with groundwater to urban drainage. The adopted modelling is based on previously reported the Southern River catchment MODHMS model (Barr and Barron, 29), which was also applied to the Wungong Urban Water (WUW) sub-catchment: (Barron and Barr, 29). This previous work concentrated on the water levels and flows in the region under predevelopment and post development conditions. The current work extends the analysis to examine the fate of pre-existing (or legacy) solutes in the upper superficial aquifer and the addition of alternative solute sources associated with urban development. The main objectives of the undertaken research were: to define the likely fate of the pre-existing or legacy solute pool within the upper superficial aquifer after urbanisation to examine the accumulation and discharge of alternative solute sources associated with urban development such as roof runoff and irrigation throughflow The model, based on the MODFLOW package, was used to examine the effect of land and water management variables and their combinations under three climate scenarios over a 1 years period including: shallow groundwater table control measures using only subsurface drains or a combination of surface fill and subsurface drains density of urban development, including high (R35), medium (R25) and low (R17.5) urban density (The R code refers to the number of residential blocks per hectare (ha), thus R25 has 25 residential blocks per ha) effect of groundwater abstraction from the lower superficial aquifer for non-potable water use in the new development for irrigation of Public Open Space (POS) and domestic gardens and for indoor use for toilet flushing and hot water use in the laundry The results of undertaken analysis indicate that the legacy solute stored in the shallow groundwater may influence water quality in subsurface drains for up to 9 years, depending on climate variability and adopted water and land management options. Within the constraints of the undertaken modelling it was indentified that flushing rates of the legacy solute from shallow groundwater are greater for the scenarios that are associated with a greater rate of the groundwater recharge. These are wetter climate and higher urban density. Groundwater abstraction has the most significant effect on the legacy solute fluxes to drains, which has a smaller rate for scenarios under higher groundwater abstraction. An increase in the downwards flux from shallow to deep superficial aquifer as the vertical hydraulic gradient increases during abstraction. These fluxes carry the legacy solutes resulting in depletion of the solutes mass in the top layer. Subsequently the solute concentration in drains is smaller when groundwater abstraction rate increases. Solutes transport in an urbanising area xiii

14 The different watertable control options of subsurface drains only and fill and subsurface drains have a negligible long term impact on the solute balances. However within the first 1-2 years of urbanisation, the subsurface drains only option showed a greater rate of the legacy solute removal from shallow groundwater. The analysis also indicated that the effect of solutes introduced in new urban forms increases over 5-7 years before reaching equilibrium and the maximum concentrations. Two solute sources were considered: the solutes associated with groundwater recharge within irrigated/fertilised areas (named garden recharge) and with groundwater recharge from roof runoff. A constant concentration in these recharge sources was assumed. Accordingly the mass flux of solutes entering the shallow groundwater increases with the rate of the groundwater recharge and the equilibrium concentrations in drains is higher for scenarios with wetter climate and higher urban density. Deep groundwater abstraction reduces the accumulation of these solutes in shallow groundwater and leads to a lower concentration in the drainage water. The different watertable control options have a negligible impact on these solute balances. Combining the observed and previous reported ranges of the concentrations of the Chloride, Total Phosphorus and Total Nitrogen from different sources with the volumetric fluxes of each source, the evolution of the solute concentration in drainage discharge after urbanisation was evaluated. It was demonstrated that there is a transition in a prevailing source of solutes to the drainage discharge from the predominately legacy solute to the combined garden recharge and roof runoff solutes. The legacy solutes deplete rapidly within a few years and are displaced by the roof runoff and garden recharge solutes. The transition between the pre-existing and new sources can be well defined over time. The length of time to displace the legacy solutes depends on the relative concentrations in groundwater prior to urbanisation and the new urban sources. There are also seasonal changes in the solute concentrations of the drainage discharge, as the concentration increases during the summer dry period (and the volume of discharge decrease), and decreases during the winter period. This is in agreement with the observed data in the existing urban drains, such as Bayswater Main Drain or Mills Street Main Drain. The limitation of the undertaken analysis is related to limited opportunities for validation in terms of both water fluxes and solute transport. The model was a subset of the large urban area model. Limitations are also related to the adopted assumption of the future land cover within the model domain. Solutes were considered to be conservative tracers, so no reactive processes were included in solute transport analysis. As a result the model was used to explore the relative effects of water and land management options for new urban developments. The model outcomes are indicative of the maximum concentrations of the solute fluxes from the subsurface to the drainage network. Future work may include monitoring and therefore validation on these results in newly developing areas; consideration of temporal changes in concentrations of the solute sources; and an extension of the solute model to a reactive nutrient transport model, which allows simulation of nutrient attenuation, enrichment and speciation within the unsaturated, saturated and phreatic zones. Solutes transport in an urbanising area xiv

15 1. INTRODUCTION The Southern River catchment is one of the fastest developing areas of Perth. Local urban development is challenged by shallow groundwater and the requirement to protect wetlands, some of which are conservation category. Large areas of the catchment are inundated during the wet season, often as an expression of the local watertable. Conversely, at the end of the dry season only the deeply-incised lower reaches of the river contain water, which is maintained by a continuous regional groundwater discharge. This hydrological cycle means that the dynamics of local groundwater levels is dominated by the rainfall/recharge evaporation annual cycle rather then by the lateral flow. Thus the residence time of groundwater in these areas can be great which are conducive to the local build-up of solutes. It was shown that the alteration of the land use in the catchment may significantly affect the current hydrological cycle (Barron and Barr, 29). As inundation and shallow watertables are incompatible with most urban infrastructure the urban development will implement watertable control measures. This project, funded by the Western Australia Department of Water and CSIRO Flagship Water for a Healthy County, was initiated to evaluate the likely impact of the changes in the land use on the water quality within the urbanising areas. The project objectives include investigation of: 1. Effect of legacy solute stored in shallow groundwater on water quality in urban drainage in post-development conditions 2. Effect of new urban land cover and water management on water quality in urban drainage 3. Time-scale for changes in solute concentration and fluxes from the urbanising areas This report examines the urbanisation effects on the solutes transport in the environment using a groundwater model and associated solute transport models. In order to predict the variations in the catchment water regime under the new land use, a process-based, coupled surface water-groundwater model was deployed to simulate predevelopment hydrological process in the Southern River catchment. The commercially available MODHMS modelling package (Panday and Huyakorn, 24; Hydrogeologic Inc., 26) was adopted. The model validation and modelling results are described in Barr and Barron (29) and is briefly summarised in Chapter 2 of this report. The sub-models of the regional MODHMS model were further used for a district development scale in two development areas in the catchment. One application was to the Wungong Urban Waters (WUW) district, the proposed urban development in the southeast part of the Southern River catchment on the Swan Coastal Plain (Barron and Barr, 29). The outcomes are also summarised in Chapter 2, including discussion on key features of the impact of the urbanisation on water balance in the area. Solute transport was not included in this district-scale model. These models were further used as a basis for a solute transport model. This modelling was undertaken to evaluate variations in conservative solute concentrations for selected land and water management scenarios. The modelling approach adopted in this study is described in Chapter 3, including references to the input data, considerations and assumptions deployed in the simulations of the impact of the urban development on the solute transport on a local scale. The modelling results are presented in Chapters 4 to 7, with the summary and discussion of these results presented in Chapter 8. Report Title Page 1

16 2. REGIONAL MODELLING Creating a regional hydrological and hydrogeological model able to evaluate the surface water-groundwater interaction requires a range of data with extensive spatial and temporal coverage. Importantly, the modelling requires a good understanding of the regional and local conditions based upon a review of the available data. Accordingly, a brief overview of the regional conditions will be presented first, followed by a summary of the regional model setup Topography, geology, hydrogeology and land use The Southern River catchment on the Swan Costal Plain is the southernmost catchment of the Swan-Canning Estuary. The tributaries of the river (Neerigen Creek and Wungong Brook) drain approximately 4 km 2 of the hills sub-catchment. Downstream from the confluence of these streams the river is known as Southern River. The topography of the catchment and the computational domain for the regional model are shown in Figure 2-1. The catchment has topographic highs in the east, adjacent to the Darling Scarp (65 mahd), and some high dunes in the west (48 mahd). The central part of the catchment is mainly flat (~25 mahd) and contains the major drainage channel with the Southern River draining to the north and the Birrega Main Drain flowing to the south. The largest wetland within the catchment is Forrestdale Lake, which lies in a depression close to the relief saddle point between the Birrega Main Drain and Southern River. The catchment is located within two unconsolidated geological units in the superficial formation (Figure 2-2). To the east there is the Guildford Formation, which is predominantly of fluvial origin and consists of clayey-sands and clays with lenses of coarse sands particularly at the base (Davidson, 1995). There are also extensive areas of surface sands. The western part of the catchment consists of Bassendean Sands, which are predominantly medium-grained, moderately sorted, quartz sands, and commonly contain a layer of weaklylimonite-cemented sands at or about the watertable (Davidson, 1995). Lacustrine sediments occur in low-lying areas of the Bassendean Sands and are a build up of peaty deposits in inundated or seasonally inundated areas. Solutes transport in an urbanising area 2

17 Figure 2-1 Ground surface elevation within the regional model domain There are also peaty deposits associated with inter-dunal wetlands, major rivers and drainage channels and the Canning estuary. The thickness of the superficial formation varies between 1 m in the Brookdale and Canning River areas to 7 m in the vicinity of the Jandakot Mound. Hydrogeologically the Guildford Formation consists of a thin layer of seasonally saturated clayey-sand sand overlying an aquitard consisting of heavier clays which in turn overlies the regional superficial aquifer. Similarly the Bassendean Sands consist of highly-conductive seasonally saturated surface layer of sand overlying a weaklylimonite-cemented sand layer, which acts as a minor aquitard, above the regional superficial aquifer. The lacustrine sediments have the same lithology as the Bassendean Sands, with the difference being the upper layer consists of low-conductivity peaty sands. The superficial formation overlies Mesozoic sedimentary formations. The current land use in the catchment, as shown in Figure 2-3, consists of urban areas, grassland and native vegetation. The urban areas are located on the eastern margins close to the Darling scarp and in the north where the Southern River joins the Canning River. The remaining area consists of a mosaic of grassland and native vegetation, with the native vegetation centred about low-lying wetlands Climate The climate on the Swan Coastal Plain is of Mediterranean type. Up to 8% of annual rainfall is recorded during the winter months, from May until September. On an annual basis the potential evaporation is greater than rainfall, but monthly potential evaporation can be lower than the monthly rainfall during winter months. According to meteorological observation in Armadale (Bureau of Meteorology (BoM) station 91) the long term average rainfall is 891 mm (period ). Over the recent decades the average annual rainfall has reduced to 828 mm (period ) or 786 mm (period ). However, as shown in Table 2-1, the reduction in average effective rainfall (as difference between rainfall and potential evaporation on a daily basis) is even greater. Solutes transport in an urbanising area 3

18 Table 2-1 Average annual rainfall, effective rainfall and potential evaporation (BoM station 91) average over long term average Average annual rainfall (mm) Average annual effective rainfall (mm) n/a Average annual potential evaporation (mm) n/a Surface waters The hydrological condition of the catchment has greatly changed since European settlement. The network of channels in the area is of generally anthropogenic origin, with only the lower and upper reaches of the Southern River consisting of natural channels. The channels flowing from the Darling Scarp into the Southern River catchment are Neerigen Brook, Wungong Brook and the Brickworks A and B streams which lie between Neerigen and Wungong Brooks. The observed average annual outflow from Southern River for the period was 13.8 GL/annum. Figure 2-2 Surface geology of the catchment within the regional model domain Solutes transport in an urbanising area 4

19 Figure 2-3 Land cover for the Southern River catchment within the regional model domain There are many wetland types occurring in the catchment within the area designated for urban development. There is permanent baseflow occurring in the lower reaches of Southern River where the channel is deeply incised in the landscape. In the remaining areas however, the baseflow is seasonal. In the Forrestdale Main Drain tributary on the Bassendean sands (western part of the catchment), the groundwater contribution to the drainage network, is small until late in the wet season when the watertable rises close to the ground surface (and the drainage inverts) Regional model The MODHMS model (Panday and Huyakorn, 24; Hydrogeologic Inc., 26) was applied to the Southern River catchment and verified based upon the available regional data (Barr and Barron, 29). The regional model domain covers the area where superficial aquifers occur in the Southern River catchment and thus excludes the Darling Scarp. The regional model grid contained 123 columns and 163 rows in a rectangular finitedifference grid (Figure 2-4) in the horizontal plane. The dimensions of the computational cells range from 5 m in the north, south and west of the domain to m in the vicinity of the Southern River catchment where urbanisation will occur. This grid is used for both the overland flow and subsurface flows in the model. The vertical discretisation was based on the topographic information and soil profile as interpreted from available geological information and bore logs. The thickness of the vertical layers in the model corresponds to the identified structure of the hydrogeological systems, with the exception of the upper superficial, which was divided into four layers to better simulate the unsaturated zone processes (Barr and Barron, 29). The model layer thickness varied from.224 m to 59.4 m. The superficial formation is represented in the model as a 6 layered hydrogeological system. The lowest layer represents the deep superficial regional aquifer, the second lowest layer the superficial aquitard and the top four layers have been assigned for the upper superficial layer to better simulate unsaturated zone fluxes. Solutes transport in an urbanising area 5

20 Figure 2-4 Computational grid for the regional model The selective calibration and verification of major hydrological and hydrogeological processes have shown that the parameters derived for the regional model successfully reproduced the current conditions in the catchment. Details on model development, verification and application are given in Barr and Barron (29). Following its verification, the regional model was used as a base to develop a sub-model for a smaller domain. This sub-model was used to assess the changes in the water and land use within the selected area as a result of urbanisation. These encompassed the changes to the catchment drainage, introduction of hard surfaces and use of local groundwater as a source of the non-potable water for the new urban developments Flow modelling of urban development areas The urban area models of Wungong Urban Water (WUW) area and North Forrestdale were created as sub-models of the regional MODHMS model. The sub-models were used to study changes in the local water regime that may arise from the proposed urban development in the area. The main objectives of this analysis were to define: drainage outflow from the area sustainable rates of groundwater abstraction to supply water for non-potable use in the new development WUW covers an area of 15 km 2, which comprises 2% of the Wungong Brook catchment upstream from the outflow form WUW (Armadale Rd) (Figure 2-5). In WUW there are many streams and drains which flow from the hills to the Wungong Brook including Neerigen Brook and Brickwork drains. Based on the relevant catchment areas Neerigen Brook contributes about 6% of total inflow to the WUW area. Solutes transport in an urbanising area 6

21 Figure 2-5 Wungong Urban Water and associated MODHMS model domain The WUW sub-model domain covers an area of 51 km 2 including the WUW area. The buffer area around the WUW area in the sub-model was used to reduce the influence of the submodel boundary conditions on the development area. Results from the regional model were adopted to construct boundary conditions for the sub-model where the boundaries of the sub-model are within the regional model. Whilst no additional verifications were undertaken for these sub-models, the results from the sub-regional model were in agreement with the results from a regional model when the same stresses were applied to the model. This confirmed that the model has been set up correctly. Both models were used to examine the changes in watertable levels, channel flows and water balance due to different land and water management variables including: shallow groundwater table control measures (two scenarios) density of urban development (three scenarios) effect of groundwater abstraction for non-potable water use in the new development (five scenarios) This analysis was undertaken for three rainfall scenarios: current, drier (as 1% less rainfall on average over 1 years simulation period) and wetter conditions (as 1% more rainfall on average over 1 years simulation period), using the sequences in Appendix A and illustrated in Figure 2-6. The adopted approach allowed a greater annual rainfall for individual years in simulation for a drier climate scenario than in others (Figure 2-7). Solutes transport in an urbanising area 7

22 Cumulative rainfall (mm) 1 Drier 8 Current Wetter Fraction difference Drier climate Wetter climate Year Figure 2-6 Comparison of cumulative rainfall for the three climate scenarios Figure 2-7 Fraction of cumulative rainfall difference between specified climate scenarios and current climate 2.6. Urbanisation and relevant changes in the water regime in the catchment The key aspects of the land and water use changes under urbanisation that may, in the considered case, have an effect on the local water regime include introduction of hard (impervious) surfaces (roofs, roads, and pavements) redirection of roof runoff to subsurface provision for shallow groundwater control measures removal or establishment of deep-rooted vegetation irrigation in Public Open Space (POS) and domestic gardens potentially greater groundwater abstraction than for predevelopment conditions Some of the above alterations are dependant on the urban density, while others are consistent for all urban developments. For the considered case the following options and relevant parameters were adopted: 1. 15% of the urban area was designated for Public Open Space (POS). 2. The requirement to control shallow groundwater levels and water logging requires a subsurface drainage network to be established. A common practice is to maintain the water table at 1.2 m below the surface and thus below house footings. This may be achieved by watertable reductions (drainage), by raising the ground surface with imported 1.2 m fill or a combination of both. 3. Three types of the residential development density were considered: high (R35), medium (R25) and low (R17.5) (The R code refers to the number of residential blocks per hectare, thus R25 has 25 residential blocks per ha). 4. Proportion of the various land use types was, in general, defined as 15% POS, 65% residential area, 1% trees, including native conservation areas, and 1% impervious surfaces outside residential area including roads and commercial areas. 5. Water demand for non-potable water supply sourced from local groundwater may be potentially used for irrigation of POS and domestic gardens. Additionally it may be considered for indoor use for toilet flushing and hot water use in laundry as suggested in the WUW District Water Management Strategy (JDA et al., 28). 6. Apart from the irrigation needs for POS, the non-potable water demands are largely controlled by urban density as it influences the domestic garden size and population. Solutes transport in an urbanising area 8

23 In order to simulate the effect of land use alteration on the water balance, the modifications to the original (i.e. regional) model were undertaken as shown in Table 2-2. Table 2-2 Summary of the urbanisation effects on the water catchment regime and adopted modelling approach to simulate those effects Assumptions Roof runoff is infiltrated Modelling approach Equivalent of rainfall over roof area in each cell is injected into the subsurface Rainfall to the cell is reduced by the amount of rainfall over the roof areas. Evaporation from roof does not occur Sub-surface evaporation from non-roof hard surfaces does not occur The land surface is levelled for urban development, which limits local pooling of water and decreases the resistance to overland flow Shallow groundwater control Potential evaporative flux from the cell is reduced by the fraction of roofs in the cell. Evaporation coefficients for the subsurface are adjusted by the fraction of non-roof hard surfaces in the cell The rill height, the obstruction height and the Manning Coefficient were reduced according to an area-weighted fraction which was specified as constant over all WUW urban cells Subsurface drains were added in each WUW urban cell to prevent the watertable rising above 1.2m below the surface. In addition, when applicable, fill was added to raise the land surface 1.2 m above the average annual maximum groundwater level, as calculated in the regional model using the current climate, in each cell The modelling results demonstrated that in predevelopment condition the water balance in the area is dominated by rainfall and evaporative losses. The volumetric runoff from the WUW area in predevelopment conditions is extremely low (<4% annual rainfall), occurring within 2-3 winter months. The WUW area contribution to Wungong Brook is low, composing 4-7% annual brook discharge. It was demonstrated that when urban development occurs, the magnitude of the urbanisation impact on the catchment fluxes are strongly influenced by the urban density, the extent of local groundwater resources use for non-potable water supply and climate variability. The changes in fluxes are linked with the effect of urbanisation on sub-surface evaporative losses. The latter is a dominant water balance component in the predevelopment conditions, but is reduced from 73% of infiltration to 13% of infiltration after urbanisation takes place. Up to 9% of predevelopment evapotranspiration losses were losses from the shallow groundwater table, which was most significant during the period when the groundwater table was the shallowest (August to October). The shallow watertable control measures introduced with urbanisation remove a significant part of the rainfall, thus reducing the influence of evapotranspiration. The urbanisation also leads to an increase in infiltration rates mainly due to roof runoff being directed to the subsurface. This alongside with a reduction in evaporative losses causes a significant increase in net groundwater recharge. This effect is most significant for high density urban development. Consequently, the increase in net recharge triggers a greater drainage discharge, but also it results in an increase in available groundwater resources for local non-potable water supply. High rates of groundwater abstraction allow reducing drainage discharge from the urbanised area, because it leads to groundwater table drawdown during summer months. This Solutes transport in an urbanising area 9

24 generates an additional storage capacity in the local aquifer for increased recharge during wet seasons. The two simulated shallow watertable control measures, namely subsurface drains only and a combination of subsurface drains and fill, had similar effects on the drainage discharge. The sustainable yield within the WUW under the selected bore field design (64 bores) varies across the area. The abstraction rate in an individual bore is greatest (>6 L/s) in the areas where the superficial aquifer is characterised by a higher hydraulic transmissivity in the west and north-west. Conversely the abstraction rate is smallest (< 2 L/s) in the south and southeast, in areas of lower hydraulic transmissivity. The sustainable yield increases with wetter climate and higher urban densities. Solutes transport in an urbanising area 1

25 3. LOCAL AREA MODELLING FOR SOLUTE TRANSPORT The local area model for the solute transport has been developed as a subset of the WUW MODHMS models. The model domain size of the local area model was equal to the size of 1 by 1 model cells of the WUW sub-model as shown in Figure 3-1 and is approximately 84 x 84 m. The spatial discretisation of the local area model is discussed in Section 3.2. The boundary conditions used in the local area model are extracted from the district scale models, including both the lateral boundaries and the infiltration rates as estimated from the MODHMS WUW model. Figure 3-1 Area of Wungong Urban Water showing extent of local and district models 3.1. Modelling and underlying assumptions The modelling packages adopted for simulation of the solute transport in the saturated part of the aquifer were MODFLOW (Harbaugh et al., 2) and associated utilities MT3DMS (Zheng and Wang, 1999; Zheng, 26) and ZONEBUDGET (Harbaugh, 199). As the watertable is controlled by sub-surface drains within urban areas, surface and groundwater interaction was simplified and the use of coupled surface/groundwater MODHMS model was not critical. On the other hand using MODFLOW package was favourable due to the reduced time required for simulation of individual scenarios. MODFLOW is also used for the groundwater component in MODHMS and thus the underlying parameterisation of the saturated zone model remains the same in the local model. The six layers used in MODHMS model were reduced to three for the MODFLOW model. The 4 upper layers, used to simulate the unsaturated zone in the MODHMS model, are combined into a single layer for the MODFLOW model. As the unsaturated zone is not modelled in MODFLOW, this subdivision was no longer needed. The watertable in the model is generally within the lower layer of the upper superficial aquifer in the sub-model and in the Solutes transport in an urbanising area 11

26 upper superficial layer in the WUW local area model. Since the parameters for all layers in the upper superficial aquifer in WUW district model were the same, no changes in parameterisation were needed for this single layer (see Figure 3-2). The parameters for the other two layers in the local model were also derived from the district model, including the top and bottom elevations of the cell as well as the hydraulic conductivity and storage parameters. The parameters were assigned on the basis that if the centre of the local model cell lay within the district model cell, then the local cell acquired the same properties as the district cell. As the model was totally enclosed by the district model, boundary conditions were needed to maintain the connection with the aquifer outside the modelled area. A general head boundary (GHB) was chosen to represent the external influences of the regional models. This boundary condition consisted of the head at the closest external cell centre outside the local model. The conductance term for the boundary condition was calculated using the saturated thickness, local horizontal hydraulic conductivity, local model cell width and distance between the centre of the local cell and the external regional cell. Some variation to the groundwater fluxes were expected as a result of the changes in the model cell size, as discussed below. Upper superficial aquifer Upper superficial aquifer Aquitard Aquitard Lower superficial aquifer Lower superficial aquifer Figure 3-2 Schematic of the transition between low horizontal discretised WUW sub-model with high discretisation of upper superficial aquifer just below the ground surface (left) to highly horizontally discretised WUW local area model with no vertical discretisation in upper superficial aquifer (right). The vertical lines represent the horizontal discretisation, the horizontal lines represent the vertical discretisation Presentation of urban land cover in the model and associated forcing parameters The urban land cover includes the land cover types in Table 3-1. These consist of a road and associated verges and two residential blocks with back-to-back houses. The block width was assumed to vary proportional to adopted urban density (Table 3-2). However the block depth was assumed to remain 7 m for all considered urban densities. The repeating pattern of land cover consists of a road with associated verges and a residential block. It was also assumed that the front of the block (closest to the road) would be paved, whilst the rear of the block would be used as a garden. The simulated urban land cover is shown in Figure 3-3 for low, medium and high density developments respectively. They show progressive reduction in the garden area and increase in the roof area. Roads were assumed to run in a north-south direction only, running across the main groundwater flow direction, thus ensuring effective control of the watertable as the subsurface drains are perpendicular to the groundwater flow. Solutes transport in an urbanising area 12

27 Table 3-1 Dimensions (length) of land use components for different urban densities Land cover Lengths (m) External forcing Low Medium High Recharge ET* Road verge Yes Yes Verge and sub-surface drain Yes Yes Road No No Road verge Yes Yes Block1: Hard surface No No Block1: House (roof) Roof No Block1: Garden Yes Yes Block2: Garden Yes Yes Block2: House (roof) Roof No Block2: Hard surface * evapotranspiration Table 3-2 Block widths, area and roof size, adopted for simulation Urban Density Block width (m) Block size (m 2 ) Roof Area (m 2 ) Low Medium High Solutes transport in an urbanising area 13

28 Figure 3-3 Urban land cover for (a) low, (b) medium and (c) high density developments. The recharge rates to the local model were extracted from the district model. The recharge into a cell was calculated based on the land use and the following assumptions: no recharge occurred under non-roof hard surfaces such as roads or paved surfaces. Any rainfall on this area is assumed to runoff to an adjacent verge or garden and become part of the recharge in those cells redirected recharge (injection) under roofs was equal to daily rainfall minus the first 1 mm over the roof area recharge to the verge and garden areas was calculated from the infiltration at the surface of the regional models. The calculation adjusted the rate of recharge so that the recharge within the local model was the same as the larger district model in the same area the recharge included irrigation. The irrigation was applied over the period between October and April inclusive, based on the average effective evaporation for the period of 4 mm/week. Due to the variations in domestic garden areas the total irrigation quantities varied for different urban densities The evaporation rates applied to an individual cell was dependent on the land cover: for roofs and other hard surfaces, it was assumed that the evaporation was zero for garden and verge areas, the average monthly potential evaporation rate was used with an extinction depth of 2 m Solutes transport in an urbanising area 14

29 The recharge and rainfall have been calculated and supplied respectively as daily quantities. However the input and output for other fluxes were on monthly basis, as monthly stress periods were adopted. The sub-surface drains in the model are modelled using the MODFLOW drains package. In this package they are assumed to exist in the whole area of the cell. The degree of connectedness is defined by a conductance which is used to calculate the flux into the cell. The flux is calculated as Q K A H B where Q is the drainage flux, K is the conductance, A is the cell area, H is the watertable level in the cell and B is the base of the drain. If the head is at or below the base of the drain, then no water is removed from the cell. There is no flux from the drain into the aquifer. The water removed from the cell is removed from the model. Apart from the presence of the drains, these cells are treated the same as the road verge cells. The depth of the drains in the local model is set to 1.5 m below the ground surface. This is a greater depth than the 1.3 m used in the district modelling as the depth in the district model considered the presence of the watertable mound between the drains. Drains were also associated with designated cells, while in the district model drains were included in all urban model cells, constraining the water level to a maximum level of 1.3 m below the ground surface Solute transport modelling The modelling was undertaken to evaluate the trends in solute transport in the new urban development, initially based on the flow analysis. The modelling of the solute transport was simulated using the MT3DMS package (Zheng and Wang, 1999; Zheng, 26), which is operated employing the hybrid method of characteristics (HMOC). Though the initial project motivation was to investigate the nutrient transfer in the pre- and post-development condition, it was agreed that as a first step it is important to identify the patterns in conservative solute transport. Nutrient transfer from sources, such as soil, to surface water is usually associated with transformation between nutrient forms and can be associated with attenuation or enrichment processes. Modelling of such processes required more specific investigation, and can be undertaken as a follow-up step to the analysis presented in this report. The following solute categories were used for analyses: transport of a legacy solute associated with shallow groundwater under new urban land cover transport of a solute associated with irrigated (and fertilised) area within new urban development transport of a solute associated with roof runoff, which is recharged to the groundwater, within new urban development In an initial stage each of the above cases were considered separately. For the legacy solute models, the initial concentration in layers 2 (aquitard) and 3 (regional superficial aquifer) was zero, and a nominal unit concentration (taken equal to 1. mg/l) was used for the upper superficial layer representing the legacy solutes. For the garden and roof recharge solute models, the initial concentration distribution in all modelled layers was zero. It is assumed that solutes can be associated with recharge fluxes in both cases: road verges/gardens and the roof runoff. These fluxes were also assigned with a constant nominal unit concentration (taken equal to 1. mg/l). Such approach enables post-processing to superimpose the concentrations from different sources to calculate the total concentration of a solute. Solutes transport in an urbanising area 15

30 The boundary conditions used for the solute transport model are that by default all fluxes into the model have zero concentration. The exceptions to this in addition to the recharge to the gardens associated with the garden recharge solutes and the recharge from the roof associated with the roof recharge solutes, was the general head inflow boundaries in the upper superficial layer for the legacy solutes which have a nominal concentration of 1. mg/l. It was assumed that no solutes were entrained in the evaporation. Additionally observed water quality data was used to demonstrate the applicability of the undertaken method to the evaluation of the solute transfer in urban catchments. The statistical values of a solute concentration (presented in terms of chloride ion) and total Phosphorus and Nitrogen concentrations for Southern River catchment are given in Table 3.2 and Table 3.3 of Barron et al. (29) as well as published data. These values were used to investigate a combined effect of legacy solutes and new solute sources on water quality in the modelled area. Table 3-3 Concentrations used in modelling Solute Legacy a Roof runoff b Aquifer recharge c 1 st Qu Median 3 rd Qu 1 st Qu Median 3 rd Qu 1 st Qu Median 3 rd Qu Chloride (mg/l) Total Nitrogen (mg/l) Total Phosphorous (mg/l) a As measured in shallow groundwater (<1m from surface) in the Southern River catchment b The roof runoff concentrations use the range for rainfall for TN and TP, source for nutrient concentrations was Barron et al. (29) c Wendling et al. (28) Water and solute mass balances The groundwater fluxes in the model were post-processed using the USGS package ZoneBudget (Harbaugh, 199). For this purpose, each model layer was specified as a zone and the boundary and inter-zone fluxes were calculated for each stress period. The budget for the solutes was created by combining the solute concentrations in the source cells or in the solute transport boundary conditions with the fluid fluxes. The mass of solutes in the different zones was found by multiplying the concentration in the cell by the saturated thickness and the porosity of the cell. Model outputs are presented for the following: relative solute mass in the upper and low layers of the superficial aquifer and their temporal and spatial changes relative solute concentrations in the upper and low layers of the superficial aquifer and their temporal and spatial changes relative mass fluxes from the upper layer of the superficial aquifer with sub-surface drainage over the simulation period (1 years) relative mass fluxes from the lower layer of the superficial aquifer with groundwater abstraction over the simulation period (1 years) relative concentrations in sub-surface drainage over the simulation period (1 years) relative concentrations in groundwater abstracted over the simulation period (1 years) Solutes transport in an urbanising area 16

31 Term relative is used to indicate that the nominal concentrations were used in the analysis Validation Since the modelling activity was exploratory, the model validation was limited. The drain fluxes were compared with the baseflow discharge of existing urban drains. The variation in the major fluxes for alternative parameterisations, including variations in conductance and recharge, are discussed in Appendix B. As there are currently no available observations of solute concentrations within the new developments considered, there is no data to directly compare the solute concentration results of the modelling. However there is monitoring of the solute concentrations in drains and groundwater within existing urban areas in the Perth metropolitan area and this data may be compared to the results of this model Scenarios The scenarios for the modelling are based on the scenarios used in the modelling of the WUW (Barron and Barr, 29). The variation include different climates, urban densities, groundwater abstraction rates for non-potable water demands, and type of watertable controls in the modelling The urban area is represented by a number of scenarios. The major scenarios include a north-south orientation of features that are perpendicular to the general direction of groundwater flow in the area. Alternative scenarios that are used for comparison with the major scenarios are an east-west orientation of major features, and a gridded road network overlain on a north-south orientation. Overall 9 scenarios were considered with the following model inputs: three rainfall scenarios (d=dry, c=current, w=wet) three urban density scenarios (l=low, m=medium, h=high) five groundwater abstraction scenarios (n=none, P=Public Open Space (POS) only; g=p+garden; t=g+toilet; h=t+hot water) (each scenario builds on the abstraction required for the previous scenario) one non-potable water source (s=superficial) two water table control scenarios (d=drains only, f=fill and drains) The single letter codes for these model inputs are bundled together to give the names of each scenario. Thus dlnsd refers to dry climate with low urban density, no groundwater abstraction from the superficial aquifer and drains as the only watertable control measure. The abstraction in the model for the different scenarios depends on the abstraction scenario and the urban density. Table 3-4 shows the abstraction rate for each bore. Figure 3-4 shows the location of the three groundwater abstraction bores used in this modelling. Table 3-4 Groundwater abstraction rates for various scenarios (+ indicates the inclusion of the relevant water demand type) Non-potable water demands Abstraction rate (ML/year/bore) POS Domestic gardens Toilet flush Hot water for laundry High density Medium density Low density Solutes transport in an urbanising area 17

32 Figure 3-4 Location of head observation points and abstraction wells within the WUW local model. The vertical lines represent the subsurface drains The following chapters describe the results of undertaken analysis. Chapter 4 outlines the flow modelling results, Chapter 5 examines the legacy solute transport under urbanisation scenarios, Chapter 6 and 7 discuss the solute transfer associated with the recharge within pervious area and roof runoff respectively, Chapter 8 provides the outcome of the solute transport when, in contrast to the nominal concentrations of 1 mg/l, measured solute concentrations given in Table 3-3 are adopted. Solutes transport in an urbanising area 18

33 4. FLOW MODELLING The water balance for a single scenario in the local model domain is shown in Table 4-1. This scenario, which is based on the current climate, medium urban density, groundwater abstraction for POS and watertable control by drainage only, has an annual groundwater discharge to the drains equivalent to 127 mm/m 2 (or 19% infiltration). The greatest fluxes to the subsurface drains occur during July-September (Figure 4-1). Summer baseflow also occurs at an average flow rate 15 m 3 /day for October - April. As baseflow in Perth urban drains composes more than 45-5% of annual drain discharge (Barron et al, 29), the simulated flux in the subsurface drains appear to be in agreement with the reported range of total annual runoff of mm/m 2 /annum (Swan River Trust, 29). Annual evaporative losses from the model compose 23% infiltration (Table 4-1, Figure 4-2). Higher evapotranspiration occurs in the spring when the watertable is closer to the surface. The variation in groundwater table at the drains and at a distance from the drains is illustrated by Figure 4-3. The figure shows the watertable level in the local model at two locations: the first location being on a sub-surface drainage line in the local model and the second point being in the middle between two drainage lines. The first part of figure shows the water level being constrained by the drainage invert. The second part of Figure 4-3 shows greater watertable variation as there are no longer any local constraints such as a drain in the immediate vicinity of the cell. Table 4-1 The water balance in the local model domain for a modelling scenario under current climate, medium urban density, abstraction for POS and drains as the only watertable control Type of Flux Local (ML/annum/km 2 ) % of infiltration Infiltration Evapotranspiration Drainage Abstraction 24 General head boundary inflow (all layers) 85 - General head boundary outflow (all layers) 31 - Vertical flux into deep superficial layer 42-2 Q (m3/day) Year Figure 4-1 Simulated monthly sub-surface drainage for current climate, medium urban density, abstraction for POS and drains as the only watertable control Solutes transport in an urbanising area 19

34 1 Q (m3/day) Period Figure 4-2 Simulate monthly evapotranspiration for current climate, medium urban density, abstraction for POS and drains as the only watertable control Head (mahd) Head (mahd) Year Year a) at a drain b) up-gradient mid-way between two drains Figure 4-3 Time series of piezometric head at two locations in the domain: (a) at a drain and (b) up-gradient mid-way between two drains Comparisons of the monthly vertical fluxes downwards into the deep superficial aquifer are shown in Figure 4-4 for various scenarios. The vertical fluxes all show a distinct seasonal pattern, with a maximum downwards flux occurring each year in winter or spring, and a minimum occurring late in the summer. The rainfall does have some effect on the fluxes, with the wetter climate scenario fluxes generally being higher, and the very high rainfall year in the current climate sequence associated with the greatest downwards flux observed (Figure 4-4a). The urban housing density also has an effect on the peak fluxes, with the higher density being associated with the greater downward flux during the winter/spring period. However the flux in the remainder of the year seems to be independent of the urban density (Figure 4-4b). The greatest influence on the downward flux is the abstraction rate in the lower superficial aquifer as shown in Figure 4-4c. As the rate increases the vertical flux also increases. There is a substantial increase in the vertical flux as the abstraction for irrigation starts in October, and although the vertical flux declines during the irrigation season, there is a considerable drop in the downwards flux at the end of the irrigation season. Detail of an annual cycle is shown in Figure 4-5 showing the response to the start and end of the pumping for irrigation for the different abstraction scenarios. The watertable control options seem to have little effect on the downwards flux (Figure 4-4d). Solutes transport in an urbanising area 2

35 Flux (ML/km 2 /month) Drier Current Wetter 5 1 Year (a) Climate Flux (ML/km 2 /month) Low Medium High 5 1 Year (b) Urban density Flux (ML/km 2 /month) 12 None POS 1 Garden Toilet Hot Year (c) Abstraction Flux (ML/km 2 /month) Fill and drains Drains only 5 1 Year (d) Watertable control Figure 4-4 Downward flux into the lower superficial aquifer layer as a function of (a) climate, (b) the urban density, (c) groundwater abstraction rates and (d) the shallow watertable control. The simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c). Flux (ML/km2/month) None Garden Hot POS Toilet J J A S O N D J F M A M J Year Figure 4-5 Annual cycle (June year 4 to June year 5) of downward flux into lower superficial aquifer layer as a function of groundwater abstraction rates using current climate, medium urban density and drains as the only watertable control option. Irrigation begins at the end of September and stops at the end of March. The different sources of recharge with the total recharge are shown in Figure 4-6 as a function of urban density for current climate, abstraction for POS and drainage as the Solutes transport in an urbanising area 21

36 watertable control option. This shows that within the model at low urban densities the recharge in gardens is the dominant infiltration source, whereas for high urban densities the recharge from the roof runoff is dominant. Infiltration (ML/km 2 /year) Total Roof Garden Urban density (R code) Figure 4-6 Infiltration rates as a function of urban density for current climate, abstraction for POS and drains as only watertable control measure. Solutes transport in an urbanising area 22

37 5. LEGACY CONCENTRATIONS The legacy solutes are the solutes that are in the aquifer prior to the urbanisation process. These may have accumulated under an area covered by native vegetation, or be a product of the previous anthropogenic land uses such as night soil disposal sites, piggeries, orchards or pastoral lands Changes in solute mass in shallow groundwater The mass of the legacy solute in all scenarios in the upper superficial layer is shown in Figure 5-1. Prior to land use alteration, the initial legacy solute mass in the upper superficial aquifer layer was approximately 8 kg/km 2 for the nominal initial solute concentration of 1 mg/l. The mass of the legacy solute in the upper superficial reduces by half after years depending on the development scenario. The faster reduction rate is related to higher urban density, higher pumping rates and the scenarios when the groundwater level control is by drainage. The flushing of the legacy solutes is also faster during wetter climate scenarios. Changes in the solute mass in the shallow groundwater for selected scenarios are shown in Figure 5-2. All the POS scenarios, irrespective of urban density, have the same abstraction rate so all such scenarios can be directly compared. The seasonal changes in the residual mass for the majority of scenarios occurs when the abstraction ceases or reduces for the winter and restarts or increases again for the summer when irrigation takes place. As the different climate regimes, with the 1% less rainfall (drier) and 1% more rainfall (wetter), are averages over the period of the simulation, the variations between the legacy solute relative mass under different climates in Figure 5-2a is due to both the temporal variations in rainfall through the simulation as well as the absolute difference in rainfall. The abstraction scenario has a major impact on the distribution of solute within the layers. Higher abstraction scenarios induce higher rates of downwards leakage into the lower superficial regional aquifer and thus deplete the legacy solute faster from the upper superficial aquifer (Figure 5-2c). Urban density and shallow groundwater control measures have a smaller effect on the rate of the legacy solute flushing from shallow groundwater (upper layer of the superficial aquifer) (Figure 5-2b, d) than the abstraction and climate variations. At the end of the 1 year simulation for all scenarios less than 1% of the original legacy solute remains in the shallow groundwater (upper model layer). Figure 5-3 shows, for each scenario, the time when 5%, 75% and 9% of the legacy solute mass has been removed from the upper superficial layer. The wetter climate scenarios show a shorter time to remove 5% of the legacy solutes than the drier and current climate scenarios. The time for the 5% reduction in mass for the current climate scenario is slightly longer than that for the drier climate scenario. This arises because of the aforementioned differences in the distribution of rainfall through the simulation. The time required to remove 9% of the legacy solute mass within the upper layer is less than 1 years for all scenarios. This indicates that the flushing of legacy solutes will occur over a short period of time. Solutes transport in an urbanising area 23

38 Relative Mass (kg/km 2 ) Figure 5-1 Mass of legacy solute in upper superficial aquifer for all scenarios as a function of time Relative mass (kg/km 2 ) Drier Current Wetter 5 1 (a) Climate Relative mass (kg/km 2 ) Low Medium High 5 1 (b) Urban density Relative mass (kg/km 2 ) None POS Garden Toilet Hot 5 1 (c) Abstraction Relative mass (kg/km 2 ) Fill and drains Drains only 5 1 (d) Watertable control Figure 5-2 Relative mass of legacy solute in upper superficial layer in WUW as a function of (a) climate, (b) the urban density, (c) groundwater abstraction rates and (d) the shallow watertable control; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c). Solutes transport in an urbanising area 24

39 dlnsd dlnsf dlpsd dlpsf dmnsd dmnsf dmpsd dmpsf dmgsd dmgsf dmtsd dmtsf dhnsd dhnsf dhpsd dhpsf dhgsd dhgsf dhtsd dhtsf dhhsd dhhsf clnsd clnsf clpsd clpsf cmnsd cmnsf cmpsd cmpsf cmgsd cmgsf cmtsd cmtsf cmhsd cmhsf chnsd chnsf chpsd chpsf chgsd chgsf chtsd chtsf chhsd chhsf wlnsd wlnsf wlpsd wlpsf wlgsd wlgsf wmnsd wmnsf wmpsd wmpsf wmgsd wmgsf wmtsd wmtsf wmhsd wmhsf whnsd whnsf whpsd whpsf whgsd whgsf whtsd whtsf whhsd whhsf (a) Time required for removal of 5% of legacy solute from upper superficial layer Solutes transport in an urbanising area 25

40 dlnsd dlnsf dlpsd dlpsf dmnsd dmnsf dmpsd dmpsf dmgsd dmgsf dmtsd dmtsf dhnsd dhnsf dhpsd dhpsf dhgsd dhgsf dhtsd dhtsf dhhsd dhhsf clnsd clnsf clpsd clpsf cmnsd cmnsf cmpsd cmpsf cmgsd cmgsf cmtsd cmtsf cmhsd cmhsf chnsd chnsf chpsd chpsf chgsd chgsf chtsd chtsf chhsd chhsf wlnsd wlnsf wlpsd wlpsf wlgsd wlgsf wmnsd wmnsf wmpsd wmpsf wmgsd wmgsf wmtsd wmtsf wmhsd wmhsf whnsd whnsf whpsd whpsf whgsd whgsf whtsd whtsf whhsd whhsf (b) Time required for removal of 75% of legacy solute from upper superficial aquifer Solutes transport in an urbanising area 26

41 dlnsd dlnsf dlpsd dlpsf dmnsd dmnsf dmpsd dmpsf dmgsd dmgsf dmtsd dmtsf dhnsd dhnsf dhpsd dhpsf dhgsd dhgsf dhtsd dhtsf dhhsd dhhsf clnsd clnsf clpsd clpsf cmnsd cmnsf cmpsd cmpsf cmgsd cmgsf cmtsd cmtsf cmhsd cmhsf chnsd chnsf chpsd chpsf chgsd chgsf chtsd chtsf chhsd chhsf wlnsd wlnsf wlpsd wlpsf wlgsd wlgsf wmnsd wmnsf wmpsd wmpsf wmgsd wmgsf wmtsd wmtsf wmhsd wmhsf whnsd whnsf whpsd whpsf whgsd whgsf whtsd whtsf whhsd whhsf (c ) Time required for removal of 9% of legacy solute from upper superficial layer Figure 5-3 Time required for removal of (a) 5% (b) 75% and (c) 9% of legacy solutes from upper superficial for all scenarios The distribution of the legacy solute concentration in the upper superficial aquifer for the current climate, medium urban density, POS irrigation abstraction and watertable control by drains is shown in Figure 5-4. This figure includes the legacy solute distribution at four times: at the end of the irrigation period (April) and the end of the winter wet (prior to commencement of irrigation abstraction) (September) in the first year of simulation, and for the same times in the final year of simulation. Comparing the legacy solute distributions with the locations of the different urban land cover in Figure 3-3, the initial reduction in solute concentration during the initial summer period occurs in the garden areas. By the end of the winter, the concentration in the upper superficial aquifer has substantially reduced due to the dilution of the solute mass by the additional recharge associated with winter rainfall. The legacy solute concentration in the vicinity of the gardens and roof recharge areas has the greatest dilution and commensurate reduction in legacy solute concentration. However there are still local areas with high legacy solute concentration around the verge, road and subsurface drain areas. Solutes transport in an urbanising area 27

42 In the final year the majority of the modelled region is characterised by low solute concentration. There are high legacy solute concentrations on the eastern and northern boundaries associated with boundary conditions. The higher solute concentration areas internal to the model, although still relatively low concentration, are associated with areas of sub-surface drains, roads and verges. Figure 5-4 Distribution of relative legacy solute concentrations in upper superficial layer (a) at the end of summer irrigation period in the first year (Stress period 4, April); (b) at the end of winter non-irrigation period in the first year (Stress period 9, September), (c) at the end of summer irrigation period in the final year (Stress period 112, April) and (d) at the end of winter non-irrigation period in the final year (Stress period 117, September); the simulation uses current climate, medium urban density, abstraction for POS, and drainage as the only watertable control measure Changes in solute mass in deep superficial groundwater As indicated above the groundwater abstraction from the deeper superficial aquifer has the most significant effect on solute transfer from shallow groundwater (layer 1). This section illustrates how abstraction influences the solute mass balance in the deeper aquifer (layer 3). In Figure 5-5 the fraction of the legacy solute mass, which initially resided in shallow superficial layer only, that is present in the deep superficial layer is shown. The figure Solutes transport in an urbanising area 28

43 presents the results of all scenarios. The mass of legacy solute rises faster for the scenarios with high abstraction rates and reaches an earlier, higher maximum of over 4% of the initial legacy solute mass in years 3 to 6. The decrease after the maximum is due to lower concentration fluxes from the upper superficial aquifer and removal of solute via abstraction or lateral flow from the lower superficial aquifer. The increase of legacy solute mass in the deep superficial aquifer is slower for the scenarios with low abstraction rates or without abstraction, and within the simulation period, the decline is also slower. There are some differences between the climates, with the wetter climate scenarios showing an initial greater solute increase and drier climate scenarios taking longer to reach a maximum. Both higher urban densities and the use of the drainage only watertable control option result in a reduced mass of legacy solute reaching the lower superficial aquifer..6 Fraction original mass Figure 5-5 Simulated fraction of original mass of legacy solute in the lower superficial layer as a function of time for all urban scenarios The changes in solute mass in the deep groundwater for selected scenarios are shown in Figure 5-6. The greatest influence on the mass of solute in the deep superficial is the abstraction rate. The higher abstraction rates increase the downwards mass flux, as discussed in Chapter 4. This leads to a greater rate of solute mass increase in the lower superficial aquifer during first 4-5 years. But the high abstraction rates also lead to faster depletion of solute mass in the upper superficial aquifer as well as removal of the solute mass from the lower superficial aquifer with abstraction. As a result the mass in the lower superficial aquifer declines faster for the higher abstraction rates during last 5-6 years of simulation and the total solute mass in the layer in a final year of simulation falls below that associated with the lower abstraction rates. The wetter climate and the higher urban densities increase the groundwater recharge into the upper superficial aquifer, which increases the head and downwards flux to the lower superficial aquifer and thus initially increases the solute mass within the lower superficial aquifer. This solute mass gets depleted earlier through abstraction and lateral flow and eventually there is less solute mass in the deep superficial for the wetter and/or higher urban densities climates scenarios than for the drier and/or lower urban densities climates scenarios. The greater legacy solute mass in the lower superficial aquifer for fill and drains compared to the drains only watertable control option occurs because of the different invert levels in the drains for the two scenarios. The drain invert levels for the fill-and-drain scenario are higher than the invert levels for the drains only scenarios. This means that there is greater storage available in the fill-and-drain scenario below the invert level of the drains. The higher invert levels also mean that the there is less likelihood of the initial legacy solute being intercepted by the drains in the fill-and-drains scenario, eventually leading to a greater mass of legacy solute in the bottom layer of the superficial aquifer. Solutes transport in an urbanising area 29

44 Relative mass (kg/km2) Drier Current Wetter 5 1 (a) Climate Relative mass (kg/km 2 ) Low Medium High 5 1 (b) Urban density Relative mass (kg/km 2 ) None POS Garden Toilet Hot 5 1 (c) Abstraction Relative mass (kg/km 2 ) Fill and drains Drains only 5 1 (d) Watertable control Figure 5-6 Relative mass of legacy solute in lower superficial layer as a function of (a) climate, (b) the urban density, (c) groundwater abstraction, and (d) the shallow watertable control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) Solute export from the area by drains In addition to downward flux, the legacy solute may be also transferred to the sub-surface drains which are used to control the shallow groundwater table. The major factor affecting the drainage mass flux is the rate of abstraction. The abstraction associated with maximum groundwater abstraction (POS and garden irrigation, toilet and hot water) is 37 ML/annum/km 2 reducing the mass flux to the drains to an average of 2.1 kg/km 2 compared to 35.8 kg/km 2 for no abstraction under scenario with current climate, medium urban density and drainage the only watertable control. The urban density also affects the mass flux with a greater removal rate under scenario with a higher urban density. The climate and the watertable control measure have only minor effects with the mass flux increasing slightly with an increase in average rainfall, and the drainage only as a watertable control measure. As the mass of legacy solute decreases in the upper superficial with time, the amount of mass discharged with the drainage and therefore drainage water concentration also decrease with time. Figure 5-7 shows the average relative drainage water concentration during the first and final years of the simulation and Figure 5-8 shows the relative legacy mass in the drainage for the same periods. For a scenario with a wetter climate, high urban density and without any groundwater abstraction, the relative mass discharge in the first year with the drainage is about 18 kg/km 2 or around 22% of the original mass. However after 1 Solutes transport in an urbanising area 3

45 years, the maximum discharge is likely to be 1 kg/km 2 or less. Figure 5-7a shows that in the first year of urbanisation, the concentration of legacy solutes is in the range of 55-75% of the pre-urbanisation concentration. Solutes transport in an urbanising area 31

46 dlnsd dlnsf dlpsd dlpsf dmnsd dmnsf dmpsd dmpsf dmgsd dmgsf dmtsd dmtsf dhnsd dhnsf dhpsd dhpsf dhgsd dhgsf dhtsd dhtsf dhhsd dhhsf Concentration (mg/l) Concentration (mg/l) clnsd clnsf clpsd clpsf cmnsd cmnsf cmpsd cmpsf cmgsd cmgsf cmtsd cmtsf cmhsd cmhsf chnsd chnsf chpsd chpsf chgsd chgsf chtsd chtsf chhsd chhsf Concentration (mg/l) wlnsd wlnsf wlpsd wlpsf wlgsd wlgsf wmnsd wmnsf wmpsd wmpsf wmgsd wmgsf wmtsd wmtsf wmhsd wmhsf whnsd whnsf whpsd whpsf whgsd whgsf whtsd whtsf whhsd whhsf (a) Year 1 Solutes transport in an urbanising area 32

47 dlnsd dlnsf dlpsd dlpsf dmnsd dmnsf dmpsd dmpsf Concentration (mg/l) dmgsd Concentration (mg/l) dmgsf dmtsd dmtsf dhnsd dhnsf dhpsd dhpsf dhgsd dhgsf dhtsd dhtsf dhhsd dhhsf clnsd clnsf clpsd clpsf cmnsd cmnsf cmpsd cmpsf cmgsd cmgsf cmtsd cmtsf cmhsd cmhsf chnsd chnsf chpsd chpsf chgsd chgsf chtsd chtsf chhsd chhsf Concentration (mg/l) wlnsd wlnsf wlpsd wlpsf wlgsd wlgsf wmnsd wmnsf wmpsd wmpsf wmgsd wmgsf wmtsd wmtsf wmhsd wmhsf whnsd whnsf whpsd whpsf whgsd whgsf whtsd whtsf whhsd whhsf (b) Year 1 Figure 5-7 Average relative concentration of legacy solute in drainage water during (a) first year and (b) final year for all simulations Solutes transport in an urbanising area 33

48 dlnsd dlnsf dlpsd dlpsf dmnsd dmnsf dmpsd dmpsf dmgsd dmgsf dmtsd dmtsf dhnsd dhnsf dhpsd dhpsf dhgsd dhgsf dhtsd dhtsf dhhsd dhhsf Relative mass (kg/km 2 ) Relative mass (kg/km 2 ) clnsd clnsf clpsd clpsf cmnsd cmnsf cmpsd cmpsf cmgsd cmgsf cmtsd cmtsf cmhsd cmhsf chnsd chnsf chpsd chpsf chgsd chgsf chtsd chtsf chhsd chhsf Relative mass (kg/km 2 ) wlnsd wlnsf wlpsd wlpsf wlgsd wlgsf wmnsd wmnsf wmpsd wmpsf wmgsd wmgsf wmtsd wmtsf wmhsd wmhsf whnsd whnsf whpsd whpsf whgsd whgsf whtsd whtsf whhsd whhsf (a) Year 1 Solutes transport in an urbanising area 34

49 dlnsd dlnsf dlpsd dlpsf dmnsd dmnsf dmpsd dmpsf dmgsd dmgsf Relative mass dmtsd dmtsf dhnsd dhnsf dhpsd dhpsf dhgsd dhgsf dhtsd Relative mass dhtsf dhhsd dhhsf clnsd Relative mass clnsf clpsd clpsf cmnsd cmnsf cmpsd cmpsf cmgsd cmgsf cmtsd cmtsf cmhsd cmhsf chnsd chnsf chpsd chpsf chgsd chgsf chtsd chtsf chhsd chhsf wlnsd wlnsf wlpsd wlpsf wlgsd wlgsf wmnsd wmnsf wmpsd wmpsf wmgsd wmgsf wmtsd wmtsf wmhsd wmhsf whnsd whnsf whpsd whpsf whgsd whgsf whtsd whtsf whhsd whhsf (b) Year 1 Figure 5-8 Legacy solute relative mass transfer to drainage during (a) first year and (b) final year for all simulations Figure 5-9 shows the legacy relative solute mass flux, based on the initial and inflow concentrations of 1 mg/l, and Figure 5-1 shows the relative concentration of the legacy solute into the drains over time for various scenarios. The gaps in the records are due to no drainage from the aquifer in that scenario during that month. As shown in Figure 5-9a and Figure 5-1a there is a strong dependence on the climate for the mass and concentration of the legacy solutes in the drainage. The high rainfall in the first year of the wet climate scenario mobilises the legacy solute and exporting up to 23% of the total legacy mass. In subsequent years, although the mass removal is still high relative to the other climate scenarios, the concentration of legacy solute in the drainage is less than the other scenarios. The wet year 4 in the current climate scenario also contributes considerable mass to the drainage. The urban density has an effect on both the mass flux and the concentration. With increasing density, less water is transpired and more water enters the drains. This leads to a greater Solutes transport in an urbanising area 35

50 rate of solute flushing from the groundwater and higher mass fluxes with the drainage (Figure 5-9b and Figure 5-1b). In Figure 5-9c and Figure 5-1c the abstraction regime has a large influence on both the mass and the concentration of legacy solute in the drainage. Increased abstraction lowers the watertable, particularly in summer, and reduces the period the drains are active. There is not much difference in the concentrations for the various abstraction rates in the first few years of the simulation, but in years 3-6 the concentration in the drainage reduces if greater abstraction rates are applied. The different watertable control measures show approximately the same mass flux in drainage over all scenarios except the first couple of years (Figure 5-9d and Figure 5-1d). Initially the mass flux is less in the fill and drain scenario with, the drain invert higher where the fill is used and thus time required till the drains become active in the fill-and-drain scenario is greater. However in later years the concentration in the fill and drains scenario during the summer is greater than the concentration in the drains only scenario. The volume of flow in these instances is small and this difference does not register on the annual mass flux. Relative mass flux (g/day) (a) Climate Drier Current Wetter 5 1 Relative mass flux (g/day) Low Medium High 5 1 (b) Urban density Relative mass flux (g/day) None POS Garden Toilet Hot 5 1 (c) Abstraction Relative mass flux (g/day) Fill and drains Drains only 5 1 (d) Watertable control Figure 5-9 Relative mass flux of legacy solutes into the drains for various (a) climates, (b) urban densities, (c) groundwater abstractions rates and (d) watertable control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c). Solutes transport in an urbanising area 36

51 Relative concentration Drier Current Wetter Relative concentration Low Medium High. 5 1 (a) Climate. 5 1 (b) Urban density Relative concentration None POS Garden Toilet Hot Relative concentration Fill and drains Drains only. 5 1 (c) Abstraction. 5 1 (d) Watertable control Figure 5-1 Average monthly relative concentration of legacy solute into the drains for various (a) climates, (b) urban densities, (c) groundwater abstractions rates and (d) watertable control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) (gaps on the plots are related to no flow period). Solutes transport in an urbanising area 37

52 5.4. Solute export via groundwater abstraction from deep superficial aquifer As mentioned previously, the abstraction rate has an effect on the legacy solute transport in and from the upper superficial aquifer. Figure 5-11 shows the monthly relative mass flux of legacy solute abstracted for different scenarios and Figure 5-12 shows the average monthly relative concentration of the legacy solute in the abstracted water for the same scenarios. As the abstraction rates are specified in the model depending on urban density and abstraction scenario, there is little difference between the mass flux and concentration plots for similar abstraction rates. For instance all the POS groundwater abstraction scenarios use the same abstraction rate regardless of urban density. The gaps between the concentrations in Figure 5-12 indicate intervals over which no abstraction was taking place. The mass flux and concentration of the legacy solutes in the abstracted water show only a small variation under the different climate regimes. The wetter climate results in a slightly lower mass flux and concentration in the final years of the simulation and the drier climate leads to a slightly higher mass flux and concentration. This is due to the increased dilution of the legacy solute mass with the increased rainfall in the wetter climate, and reduced dilution in the drier climate. Similarly the variations in urban density show slightly less mass flux and concentration for the high urban density and slightly greater mass flux and concentration for the low urban density scenarios. This is a function of the greater overall recharge in the high urban density scenario which dilutes the residual legacy solutes in the upper layer of the superficial aquifer. The mass flux of legacy solute for different abstraction scenarios increases with the abstraction rate. However the concentration of the abstracted water shows no clear trends. The effects of the different watertable control measures are small; however the mass discharge is slightly less for the drainage only scenarios. This is shown in Figure 5-12d, where the concentration is lower for the drains only scenario. Solutes transport in an urbanising area 38

53 Relative mass flux (g/day) Drier Current Wetter 5 1 (a) Climate Relative mass flux (g/day) Low Medium High 5 1 (b) Urban density Relative mass flux (g/day) None POS Garden Toilet Hot 5 1 (c) Abstraction Relative mass flux (g/day) Fill and drains Drains only 5 1 (d) Watertable control Figure 5-11 Relative mass of legacy solutes in abstraction for various (a) climates, (b) urban densities, (c) groundwater abstractions rates (note the different scale on the vertical axis) and (d) watertable control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) (gaps on the plots are related to a period with no irrigation). Solutes transport in an urbanising area 39

54 Relative concentration Drier Current Wetter 5 1 (a) Climate Relative concentration Low Medium High 5 1 (b) Urban density Relative concentration None POS Garden Toilet Hot 5 1 (c) Abstraction Relative concentration Fill and drains Drains only 5 1 (d) Watertable control Figure 5-12 Simulated relative concentration of legacy solutes in abstraction for various (a) climates, (b) urban densities, (c) groundwater abstractions rates and (d) watertable control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) (gaps on the plots are related to a period with no irrigation) Legacy solute transport with groundwater flux into and from the model domain In addition to solute transport from shallow groundwater to drains and downward to the deep superficial aquifer within the model domain, solute may also be transferred to and from the surrounding area. The effect of this component on the mass balance is discussed below Legacy solute transport into the domain The only boundary conditions which have solute concentrations associated with it (apart from the recharge) are the general head boundary conditions in the upper superficial for the legacy solutes. Figure 5-13 shows the relative average annual legacy solute mass entering the domain expressed as a mass per unit area of the domain. The quantity of solute entering the domain increases with an increase in the groundwater abstraction rates, higher urban densities, and for wetter climates. The influx is also greater when the combination of fill and drainage is used compared to the use of drainage only to control the watertable levels. Figure 5-14 shows the amount of mass associated with the boundary condition as a percentage of the original legacy solute mass in the model domain. This is conservative quantity of legacy solute to enter the domain in the upper superficial as it is the maximum possible concentration of inflow and may only be observed where the urban area abuts undeveloped land. Solutes transport in an urbanising area 4

55 It is noted that inflows in the other model layers may also transport solute into the domain, but this has been ignored in the current analysis. Mass inflow (kg/km 2 /annum) dlnsd dlnsf dlpsd dlpsf dmnsd dmnsf dmpsd dmpsf dmgsd dmgsf dmtsd dmtsf dhnsd dhnsf dhpsd dhpsf dhgsd dhgsf dhtsd dhtsf dhhsd dhhsf Mass inflow (kg/km 2 /annum) clnsd clnsf clpsd clpsf cmnsd cmnsf cmpsd cmpsf cmgsd cmgsf cmtsd cmtsf cmhsd cmhsf chnsd chnsf chpsd chpsf chgsd chgsf chtsd chtsf chhsd chhsf Mass inflow (kg/km 2 /annum) wlnsd wlnsf wlpsd wlpsf wlgsd wlgsf wmnsd wmnsf wmpsd wmpsf wmgsd wmgsf wmtsd wmtsf wmhsd wmhsf whnsd whnsf whpsd whpsf whgsd whgsf whtsd whtsf whhsd whhsf Figure 5-13 Simulated legacy solute mass entering the upper superficial aquifer for all scenarios Solutes transport in an urbanising area 41

56 Mass inflow (% original mass) 16% 12% 8% 4% % dlnsd dlnsf dlpsd dlpsf dmnsd dmnsf dmpsd dmpsf dmgsd dmgsf dmtsd dmtsf dhnsd dhnsf dhpsd dhpsf dhgsd dhgsf dhtsd dhtsf dhhsd dhhsf Mass inflow (% original mass) 16% 12% 8% 4% % clnsd clnsf clpsd clpsf cmnsd cmnsf cmpsd cmpsf cmgsd cmgsf cmtsd cmtsf cmhsd cmhsf chnsd chnsf chpsd chpsf chgsd chgsf chtsd chtsf chhsd chhsf Mass inflow (% original mass) 16% 12% 8% 4% % wlnsd wlnsf wlpsd wlpsf wlgsd wlgsf wmnsd wmnsf wmpsd wmpsf wmgsd wmgsf wmtsd wmtsf wmhsd wmhsf whnsd whnsf whpsd whpsf whgsd whgsf whtsd whtsf whhsd whhsf Figure 5-14 Total mass inflow as percentage of original legacy solute mass for all scenarios Legacy solute transport from the domain The total relative mass outflow of legacy solutes over the 1 year period of the simulation through the lateral sides of the domain is shown in Figure This shows that the dominant component of the lateral flux is in the lower superficial. The total lateral mass flux and the mass flux in the lower superficial increases with abstraction but for the upper superficial layer it decreases with increasing abstraction. The mass outflow is plotted as a fraction of the original mass of legacy solutes in Figure This shows that a large proportion of the solute mass is lost through lateral flow. Figure 5-17 shows the average concentration of the lateral outflow as a function of time for the lower layer of the superficial aquifer for different abstraction rates. These show that the concentration of legacy solutes in the outflow from the upper superficial aquifer declines rapidly with time, whereas the concentration of legacy solutes in the outflow from the lower superficial aquifer increases in time to between 1 and 14% of the original concentration, and after 1 years averages at about 8% of the original legacy concentration. Solutes transport in an urbanising area 42

57 Mass outflow (kg/km 2 ) Upper superficial Aquitard Lower superficial dlnsd dlnsf dlpsd dlpsf dmnsd dmnsf dmpsd dmpsf dmgsd dmgsf dmtsd dmtsf dhnsd dhnsf dhpsd dhpsf dhgsd dhgsf dhtsd dhtsf dhhsd dhhsf Mass outflow (kg/km 2 ) Upper superficial Aquitard Lower superficial clnsd clnsf clpsd clpsf cmnsd cmnsf cmpsd cmpsf cmgsd cmgsf cmtsd cmtsf cmhsd cmhsf chnsd chnsf chpsd chpsf chgsd chgsf chtsd chtsf chhsd chhsf Mass outflow (kg/km 2 ) Upper superficial Aquitard Lower superficial wlnsd wlnsf wlpsd wlpsf wlgsd wlgsf wmnsd wmnsf wmpsd wmpsf wmgsd wmgsf wmtsd wmtsf wmhsd wmhsf whnsd whnsf whpsd whpsf whgsd whgsf whtsd whtsf whhsd whhsf Figure 5-15 Simulated relative total mass outflow from model domain of legacy solutes for all scenarios Solutes transport in an urbanising area 43

58 Mass outflow (% original mass) 5% 4% 3% 2% 1% % dlnsd dlnsf dlpsd dlpsf dmnsd dmnsf dmpsd dmpsf dmgsd dmgsf dmtsd dmtsf dhnsd dhnsf dhpsd dhpsf dhgsd dhgsf dhtsd dhtsf dhhsd dhhsf Mass outflow (% original mass) 5% 4% 3% 2% 1% % clnsd clnsf clpsd clpsf cmnsd cmnsf cmpsd cmpsf cmgsd cmgsf cmtsd cmtsf cmhsd cmhsf chnsd chnsf chpsd chpsf chgsd chgsf chtsd chtsf chhsd chhsf Mass outflow (% original mass) 5% 4% 3% 2% 1% % wlnsd wlnsf wlpsd wlpsf wlgsd wlgsf wmnsd wmnsf wmpsd wmpsf wmgsd wmgsf wmtsd wmtsf wmhsd wmhsf whnsd whnsf whpsd whpsf whgsd whgsf whtsd whtsf whhsd whhsf Figure 5-16 Mass outflow as percentage of original legacy solute mass for all scenarios Relative conc. (mg/l) None POS Garden Toilet Hot water Figure 5-17 Average concentration of outflow through the boundary in the lower superficial aquifer as a function of the abstraction rate. Solutes transport in an urbanising area 44

59 6. GARDEN AND VERGE RECHARGE For this series of simulation it was assumed that concentration in all fluxes is equal to zero, except for recharge from the pervious areas (gardens and road verges). As in a previous case the nominal concentration was 1 mg/l Changes in solute mass in shallow groundwater The mass of solute associated with garden and verge recharge, henceforth called garden recharge, in the upper superficial aquifer increases with time (Figure 6-1), although there is a distinct seasonal cycle with a maximum level occurring in winter and a minima at the end of summer. The increase during winter is due to the addition of the solute with the rainfall/recharge. The decrease in summer is due to the downward flux of water and solute. Although not all the scenarios have reached an equilibrium solute concentration, the majority of scenarios are close to equilibrium. The mass of solute in the upper part of the superficial aquifer starts reaching a steady value after 5 years of urbanisation, with the exact time depending on the scenario. The higher mass accumulation in the upper superficial occurs in the simulations with low urban density and no abstraction. In contrast to the legacy solutes, the mass of garden recharge solute increases in the upper superficial layer with smaller or no abstraction. The climate shows an initial increase in the garden recharge mass with wetter climates although in the last 5 years of the simulation, the differences between the different climate scenarios become less pronounced. Urban density has a large affect on the accumulation of mass, with the lower the urban density, the higher the percentage of gardens in the area and the greater the mass in the upper superficial aquifer, whereas the method of watertable control has very little effect. Figure 6-2 shows the maximum relative concentration of the garden recharge in the upper superficial aquifer. The maximum recharge occurs for no or low abstraction rates (POS). The maximum is greater than 1.o due to the concentration of the solute through evaporation. Figure 6-3 shows the evolution of the relative mass of garden recharge for variations in the climate, urban density, abstraction rate and watertable control measures. These show that the urban density and the abstraction rate have the greatest influence on the addition of mass to the upper superficial. The lower the abstraction rate, the more mass remains within the upper superficial, and the higher the urban density, the smaller the garden area and thus the smaller the amount of garden recharge solute in the aquifer. The climate has a small effect on the increase of the garden recharge solute mass in the upper superficial. There is a slight increase in the mass with the wetter climate as the quantity of mass entering the model is proportional to the rainfall. There are only minor differences between the watertable control measures, with the fill-and-drains scenario having slightly more mass of garden recharge solute in the later years of the simulation. This is due to the slightly greater storage available in this scenario due to the higher invert levels on the drains. Solutes transport in an urbanising area 45

60 Relative Mass (kg/km2) Figure 6-1 Simulated relative mass of garden recharge solute in the upper superficial layer for all scenarios Maximum relative concentration dlnsd dlnsf dlpsd dlpsf dmnsd dmnsf dmpsd dmpsf dmgsd dmgsf dmtsd dmtsf dhnsd dhnsf dhpsd dhpsf dhgsd dhgsf dhtsd dhtsf dhhsd dhhsf Maximum relative concentration clnsd clnsf clpsd clpsf cmnsd cmnsf cmpsd cmpsf cmgsd cmgsf cmtsd cmtsf cmhsd cmhsf chnsd chnsf chpsd chpsf chgsd chgsf chtsd chtsf chhsd chhsf Maximum relative concentration wlnsd wlnsf wlpsd wlpsf wlgsd wlgsf wmnsd wmnsf wmpsd wmpsf wmgsd wmgsf wmtsd wmtsf wmhsd wmhsf whnsd whnsf whpsd whpsf whgsd whgsf whtsd whtsf whhsd whhsf Figure 6-2 Simulated relative maximum concentration of garden recharge solute in upper superficial over whole layer for all scenarios Solutes transport in an urbanising area 46

61 Relative mass (kg/km 2 ) Drier Current Wetter 5 1 (a) Climate Relative mass (kg/km 2 ) Low Medium High 5 1 (b) Urban density Relative mass (kg/km 2 ) None POS Garden Toilet 1 Hot 5 1 (c) Abstraction Relative mass (kg/km 2 ) Fill and drains Drains only 5 1 (d) Watertable control Figure 6-3 Relative mass of garden recharge solute in upper superficial layer as a function of various (a) climates, (b) urban densities, (c) groundwater abstractions rates and (d) watertable control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c). The garden recharge solute distribution in the upper superficial layer is shown in Figure 6-4 for the scenario using current climate, medium urban density, POS abstraction and watertable control using drainage only. This shows that large areas of the gardens located between houses have relative concentrations in excess of 1 mg/l. This is because the addition of recharge and removal of the water through evapotranspiration tends to increase the concentrations in these areas. In the area of the verge, the proximity of the subsurface drain provides an outlet for the groundwater and associated solute so that the build up of concentration is not as evident. However this area receives some lateral flow from the garden areas and concentration may occur in areas where the watertable remains below the invert for the subsurface drains for the majority of the year. Solutes transport in an urbanising area 47

62 Figure 6-4 Simulated relative concentration of garden recharge solute in upper superficial layer (a) at the end of summer irrigation period in the final year (Stress period 112, April) and (b) at the end of winter non-irrigation period in the final year (Stress period 117, September) for current climate, medium urban density, abstraction for POS, and drainage as the only watertable control measure Changes in solute mass in deep groundwater In the lower superficial layer, the mass of solute is still steadily increasing at the end of the 1 year simulation period (Figure 6-5). There is again a small seasonal signal with the mass increasing during the summer irrigation season and slightly rising or steady during the winter. The climate has a significant effect on the accumulation of mass in the lower superficial aquifer, with wetter climates increasing the mass of garden recharge solute. Lower urban density, as in the upper superficial aquifer, has an increased mass due to the greater garden areas, whereas greater abstraction in the lower superficial results in an increase in the solute mass in contrast to what is happening in the upper superficial aquifer. This is because the pumping is increasing the hydraulic gradient between the upper and lower parts of the superficial aquifer thus increasing the solute mass flux from the upper superficial aquifer to the lower superficial aquifer. There are no significant differences between the two watertable control options used in the modelling. Relative Mass (kg/km2) Figure 6-5 Simulated relative mass of garden recharge solute in the lower superficial layer for all scenarios Solutes transport in an urbanising area 48

63 The distribution of the solute associated with garden recharge in the lower superficial is shown in Figure 6-6 at the end of the irrigation period in the final year of the simulation. It shows a large area of lower concentration in the northern central part of the domain. This is associated with the lower vertical conductivity in the northern part of the domain as shown in Figure 6-7. Figure 6-6 Simulated relative concentration of garden recharge solute in lower superficial layer (a) at the end of summer irrigation period in the final year (Stress period 112, April) and (b) at the end of winter non-irrigation period in the final year (Stress period 117, September) for current climate, medium urban density, abstraction for POS, and drainage as the only watertable control measure. Solutes transport in an urbanising area 49

64 Figure 6-7 Vertical conductance between upper superficial and aquitard layers in the local model 6.3. Solute export in drains The simulated relative mass flux of garden recharge solutes to the drains are shown in Figure 6-8, for variations in climate, urban density, abstraction rates and watertable control measures. The concentrations for the same scenarios are shown in Figure 6-9. Once again the wetter the climate the greater the mass discharge in the drainage outflows, although the concentration of the drainage waters are relatively similar. The mass flux of the garden recharge solutes is greater for higher density, but the additional quantity of water generated through roof runoff reduces the concentration in shallow groundwater. Since abstraction increases the downward flux, less mass is exported in the drains, and the lower the predicted concentration of the drainage waters. Solutes transport in an urbanising area 5

65 Relative mass flux (g/day) (a) Climate Drier Current Wetter 5 1 Relative mass flux (g/day) Low Medium High 5 1 (b) Urban density Relative mass flux (g/day) None POS Garden Toilet Hot 5 1 (c) Abstraction Relative mass flux (g/day) Fill and drains Drains only 5 1 (d) Watertable control Figure 6-8 Relative solute mass flux associated with groundwater recharge in pervious area into the drains for various (a) climate scenarios, (b) urban densities, (c) groundwater abstraction rates and (d) shallow groundwater control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) Solutes transport in an urbanising area 51

66 Relative concentration Drier Current Wetter 5 1 (a) Climate Relative concentration Low Medium High 5 1 (b) Urban density Relative concentration None POS Garden Toilet Hot 5 1 (c) Abstraction Relative concentration Fill and drains Drains only 5 1 (d) Watertable control Figure 6-9 Relative concentration of drainage water related to groundwater recharge in pervious area for various (a) climate scenarios, (b) urban densities, (c) groundwater abstraction rates and (d) shallow groundwater control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) 6.4. Solute export from the area with groundwater abstraction from deeper superficial aquifer The groundwater abstraction has a substantial effect on the solute distribution in both the upper and lower parts of the superficial aquifer. However as the garden recharge solute in the lower superficial aquifer has not reached a steady-state conditions, both the mass flux and concentration of the solute in the abstracted water are still increasing at the end of the simulation period. Figure 6-1 shows the monthly relative mass flux of garden recharge solutes associated with the different scenarios. The results show an increase in the solute mass flux under wetter climates, lower urban densities, and higher abstraction rates. There is very little difference between the two scenarios of the watertable control options. Figure 6-11 shows the monthly average concentrations of garden recharge solutes in the abstracted water. As the quantity of abstraction remains constant, then the relationships between the concentrations of the various scenarios are the same as those for the mass flux in the abstraction. Solutes transport in an urbanising area 52

67 Relative mass flux (g/day) Drier Current Wetter 5 1 (a) Climate Relative mass flux (g/day) Low Medium High 5 1 (b) Urban density Relative mass flux (g/day) None POS Garden Toilet Hot 5 1 (c) Abstraction Relative mass flux (g/day) Fill and drains Drains only 5 1 (d) Watertable control Figure 6-1 Relative mass solute flux associated with groundwater recharge in pervious area with groundwater abstraction for various (a) climate scenarios, (b) urban densities, (c) groundwater abstractions rates (note the different scale) and (d) shallow groundwater control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) Solutes transport in an urbanising area 53

68 Relative concentration Drier Current Wetter 5 1 (a) Climate Relative concentration Low Medium High 5 1 (b) Urban density Relative concentration None POS Garden Toilet Hot 5 1 (c) Abstraction Relative concentration Fill and drains Drains only 5 1 (d) Watertable control Figure 6-11 Relative concentration of abstracted groundwater associated with groundwater recharge in pervious area for various (a) climate scenarios, (b) urban densities, (c) groundwater abstractions rates and (d) shallow groundwater control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) 6.5. Solute transport with groundwater flux from the model domain For the garden recharge solutes, the recharge in the gardens and verge areas of the domain is a single source of solute. Lateral flow into the model domain is assumed to have no solute associated with it. However solute may flow out of the domain through the general head boundary conditions. Figure 6-12 shows the total mass outflow over the simulation through the general head boundary conditions. The total relative mass flux through the boundary is much greater than the relative flux for the legacy solutes in Figure Figure 6-13 shows the average outflow concentration for the lower superficial layer. This is approaching 5% of the incident recharge concentration of the garden recharge and is still rising. Solutes transport in an urbanising area 54

69 Mass outflow (kg/km 2 ) Upper superficial Aquitard Lower superficial dlnsd dlnsf dlpsd dlpsf dmnsd dmnsf dmpsd dmpsf dmgsd dmgsf dmtsd dmtsf dhnsd dhnsf dhpsd dhpsf dhgsd dhgsf dhtsd dhtsf dhhsd dhhsf Mass outflow (kg/km 2 ) Upper superficial Aquitard Lower superficial clnsd clnsf clpsd clpsf cmnsd cmnsf cmpsd cmpsf cmgsd cmgsf cmtsd cmtsf cmhsd cmhsf chnsd chnsf chpsd chpsf chgsd chgsf chtsd chtsf chhsd chhsf Mass outflow (kg/km 2 ) Upper superficial Aquitard Lower superficial wlnsd wlnsf wlpsd wlpsf wlgsd wlgsf wmnsd wmnsf wmpsd wmpsf wmgsd wmgsf wmtsd wmtsf wmhsd wmhsf whnsd whnsf whpsd whpsf whgsd whgsf whtsd whtsf whhsd whhsf Figure 6-12 Simulated relative total mass outflow from model domain for garden recharge solutes for all scenarios Solutes transport in an urbanising area 55

70 Relative concentration None POS Garden Toilet Hot water Figure 6-13 Simulated relative concentration of garden recharge out through general head boundary condition in the lower superficial layer as a function of groundwater abstraction rate for current climate, medium urban density, and watertable control through drainage only Solutes transport in an urbanising area 56

71 7. SOLUTES ASSOCIATED WITH ROOF RUNOFF In a similar investigation to the trends in solute transport entering the shallow aquifer with infiltration within the pervious areas (gardens and road verges), an additional series of analysis was undertaken to investigate the solute transport associated with roof runoff, diverted to the shallow groundwater. As in a previous case the nominal concentration assigned to the roof runoff was 1 mg/l, while concentrations in all other fluxes in this case were equal to zero. The overall results of this series simulation were similar to the outcomes of the analysis related to solute transport associated with pervious areas. The spatial and temporal trends in mass accumulation in shallow and deep layer of the superficial aquifer, relative concentrations in drainage and abstracted water were comparable in magnitude being 15-2% less for the roof runoff case, as shown in Figure 7-1. The results for solutes associated with roof runoff are described in greater details in Appendix C.3. The mass of solute associated with roof recharge, in the upper superficial aquifer increases with time, which also results in an increase in drainage water concentration (Figure 7-1). The concentration of roof runoff solutes in the drains decrease during winter due to the mixing with garden/verge recharge (which is mg/l), and then increase in the summer is due to the concentration with evapotranspiration. The concentration of the roof runoff solute in the upper superficial aquifer starts reaching an equilibrium condition for some of the simulated scenarios after 5 years of urbanisation. The higher concentrations in the upper superficial occur in the simulations with higher urban density and no abstraction. The major difference between the roof runoff solutes and the garden recharge solutes is that the effect of urban density reverses the concentration trend. That is for a higher urban density the quantity of roof runoff increases (see Figure 4-6) and the resulting concentration of the roof runoff solutes increases (Figure 7-1b), whereas for the garden recharge solute, as the urban density increase the concentration of the garden recharge solutes decrease (Figure 6-9b). Solutes transport in an urbanising area 57

72 Relative concentration Drier Current Wetter Relative concentration Low Medium High. 5 1 (a) Climate. 5 1 (b) Urban density Relative concentration None POS Garden Toilet Hot 5 1 (c) Abstraction Relative concentration Fill and drains Drains only 5 1 (d) Watertable control Figure 7-1 Relative concentration of drainage water with roof runoff solutes for various (a) climate scenarios, (b) urban densities, (c) groundwater abstraction rates and (d) shallow groundwater control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c). Solutes transport in an urbanising area 58

73 8. APPLICATION OF MODEL RESULTS The model results to this time have assumed an initial unit concentration of 1. mg/l for the legacy solutes and input concentrations of 1 mg/l for the other solutes. Some observed field concentrations of different solutes are listed in Table Chloride Chloride is a conservative ion and as such its concentration in the urban environment is likely to be better correlated with the outcomes of the current analyses. Applying the quartile concentration values for chloride concentrations from Table 3-3 to the various inputs, the chloride concentration in drainage discharge from the urban development was simulated. Figure 8-1 shows the results for the scenario using current climate, medium urban density, watertable control by drainage only, and the two extreme groundwater abstraction regimes: no-abstraction and the maximum abstraction for non-potable usage. It shows that the legacy chloride contribution to the total chloride pool reduces quickly in the first few years and subsequently the chloride associated with the garden recharge dominates the total chloride pool. Comparing the two abstraction scenarios shows that abstraction reduces the concentration of the chloride in the drainage as well as the period of the year the drainage is active. Thus an increase in abstraction is likely to reduce both volume and concentration of solutes in the drain discharge. The gaps in the plots indicate where there is no drainage occurring in the simulation as the watertable has fallen below the drainage inverts in the model. As shown in Figure 8-2 the modelled results for median observed concentrations appears to be in agreement with the chloride concentration in Mills Street Main Drain, which is inferred from electrical conductivity (EC) measurements at the outflow from the catchment. Chloride concentration appears to be greater in Bayswater Main Drain, where regional groundwater discharge occurs. The locations of these observation points in currently urbanised areas is shown in Figure 8-3. The similarity extends to both total chloride concentrations and its seasonal variation. Solutes transport in an urbanising area 59

74 12 1 Legacy Gardens Roof (a) 1st Quartile, no abstraction Concentration (mg/l) Concentration (mg/l) Legacy Gardens Roof (b) 1st Quartile, hot abstraction Legacy Gardens Roof (c) Median, no abstraction Concentration (mg/l) Concentration (mg/l) Legacy Gardens Roof (d) Median, hot abstraction Concentration (mg/l) Legacy Gardens Roof (e) 3rd Quartile, no abstraction Concentration (mg/l) Legacy Gardens Roof (f) 3rd Quartile, hot abstraction Figure 8-1 Total chloride flux to drainage based on first quartile, median and third quartile of legacy concentration (rows) for current climate, medium density, no abstraction, and drainage only as the watertable control scenarios in the first column and current climate, medium density, hot water abstraction, and drainage only as the watertable control scenarios in the second column. Solutes transport in an urbanising area 6

75 Chloride (mg/l) 5 BW Mill St 5 per. Mov. Avg. (BW) 5 per. Mov. Avg. (Mill St) Year Figure 8-2 Observed chloride concentrations in Bayswater Main Drain (BW) at Slade Street and Mill Street Main Drain at Palm Place. The fitted line to each set of observations is a 5 point moving average. Figure 8-3 Locations of observation sites in currently urbanised areas used for comparison with simulated results. Solutes transport in an urbanising area 61

76 8.2. Nitrogen and Phosphorus Similarly applying the quartile concentration values for total nitrogen (TN) and total phosphorous (TP) from Table 3-3, the relative concentration in the drainage discharge from the urban development is given in Figure 8-4 for scenario under POS abstraction, current climate, medium urban density, and watertable control by drainage only. These show that the concentration in the drainage discharge may vary by an order of magnitude depending on the actual concentrations of the various solutes within an urban development. The maximum (third quartile) concentrations for nutrients used in this analysis are related to leachate from turf, so can be considered as extreme concentrations and is unlikely to be realistic for land use in urban areas. The simulated nutrients concentrations associated with the minimum (first quartile) and median input concentrations appear to be lower than those observed in Perth urban drains (Figure 8-5). Considering that attenuation processes was not included in solute transfer simulation, this might indicate that nutrient concentrations in leachate form soil and in roof runoff are greater than suggested in Table 3-3. The results for the median value input concentrations indicate the equilibrium values for total nitrogen concentration of mg/l and total phosphorous concentration of.12 mg/l. at the end of the simulation. The modelling results suggest a seasonal variation in solute concentrations. This outcome was compared with the observed TN concentrations (Figure 8-5), and the seasonality was particularly evident for Nitrate and Ammonia, which are predominately delivered to the drains with groundwater discharge (Barron et al., 29). The total nitrogen and total phosphorous quartile concentrations in Table 3-3 have approximately the same ratios between the different solute sources and produce similarly shaped time series of drainage discharge concentration. Therefore in the remaining figures in this chapter, total nitrogen is used to represent both nutrient sources Effect of climate Figure 8-6 contrasts the concentration of total nitrogen in the drainage discharge for drier and wetter climates using POS abstraction, medium urban density, and watertable control by drainage only. These can be compared to total nitrogen concentration in the current climate in the first column in Figure 8-4. The results show that there are some seasonal differences of up to 2% in the drainage discharge concentrations. However the average concentrations are similar in both climates and the dominant contributor is the garden recharge. It also shows that the concentrations in the drainage discharge decrease under a wetter climate scenario and that the magnitude of seasonal variations in concentration through the year also decrease Effect of urban density The solute concentrations for low and high urban densities in discharge are shown in Figure 8-7 for the total nitrogen under current climate, POS abstraction, and watertable control by drainage only. These can be compared to the medium density in the first column in Figure 8-4. The results show substantial differences between the scenarios, with the concentration in lower urban density scenarios about 25% greater than under the current climate scenarios, and the concentration under the higher urban density scenarios about 25% less than under the current climate Effect of groundwater abstraction The total nitrogen median concentrations in the drainage discharge for various abstraction rates are shown in Figure 8-8. All results are for current climate, medium urban density and watertable control by drainage only. This shows the period the drains are active decreases with increasing abstraction and the concentration of TN in the discharge halves from the no abstraction scenario to the maximum groundwater abstraction scenario. Solutes transport in an urbanising area 62

77 5 Legacy Gardens Roof (a) TN, 1st Quartile Concentration (mg/l) Concentration (mg/l).5 Legacy Gardens Roof (b) TP, 1st Quartile 5 Legacy Gardens Roof (c) TN, median Concentration (mg/l) Concentration (mg/l).5 Legacy Gardens Roof (d) TP, median 25 Legacy Gardens Roof (e) TN, 3rd Quartile Concentration (mg/l) Concentration (mg/l) 2. Legacy Gardens Roof (f) TP, 3rd Quartile Figure 8-4 Total nutrient flux to drainage based on first quartile, median and third quartile of all nutrient concentrations (rows) and total nitrogen (TN) and total phosphorous (TP) in columns for current climate, medium density, POS abstraction, and drainage only as the watertable control scenarios Solutes transport in an urbanising area 63

78 Figure 8-5 Nitrogen concentrations in (a) Bayswater and Mills Street Main Drain Solutes transport in an urbanising area 64

79 5 Legacy Gardens Roof (a) Drier climate, 1st Quartile Concentration (mg/l) Concentration (mg/l) 5 Legacy Gardens Roof (b) Wetter climate, 1st Quartile 5 Legacy Gardens Roof (c) Drier climate, median Concentration (mg/l) Concentration (mg/l) 5 Legacy Gardens Roof (d) Wetter climate, median 25 Legacy Gardens Roof (e) Drier climate, 3rd Quartile Concentration (mg/l) Concentration (mg/l) 25 Legacy Gardens Roof (f) Wetter climate, 3rd Quartile Figure 8-6 Total nutrient flux to drainage based on first quartile, median and third quartile of total nitrogen concentrations (rows) and drier and wetter climates in columns for medium density, POS abstraction, and drainage only as the watertable control scenarios Solutes transport in an urbanising area 65

80 5 Legacy Gardens Roof (a) Low urban density, 1st Quartile Concentration (mg/l) Concentration (mg/l) 5 Legacy Gardens Roof (b) High urban density, 1st Quartile 5 Legacy Gardens Roof (c) Low urban density, median Concentration (mg/l) Concentration (mg/l) 5 Legacy Gardens Roof (d) High urban density, median 3 25 Legacy Gardens Roof (e) Low urban density, 3rd Quartile Concentration (mg/l) Concentration (mg/l) 3 25 Legacy Gardens Roof (f) High urban density, 3rd Quartile Figure 8-7 Total nutrient flux to drainage based on first quartile, median and third quartile of total nitrogen concentrations (rows) and low and high urban densities in columns for current climate, POS abstraction, and drainage only as the watertable control scenarios Solutes transport in an urbanising area 66

81 5 Legacy Gardens Roof (a) No abstraction Concentration (mg/l) Concentration (mg/l) 5 Legacy Gardens Roof (b) Garden abstraction 5 Legacy Gardens Roof (c) Internal toilet abstraction Concentration (mg/l) Concentration (mg/l) 5 Legacy Gardens Roof (d) Internal hot water abstraction Figure 8-8 Median total nitrogen concentration in flux to drainage for different non-potable water abstraction rates for current climate, medium density, and drainage only as the watertable control scenarios Overall it appears that the modelling results reasonably represent the seasonal variation in and long-term water quality in urban drainage, similar to those observed in Perth urban drains controlling shallow groundwater table. Solutes transport in an urbanising area 67

82 9. SUMMARY AND CONCLUSIONS The modelling in this report uses a small local model that is a subset of a larger district model. The smaller scale model was required as to include the finer details of the urban area in the district model would have created a very large model which would take excessive time to execute and consume considerable computational resources. Such a model would not be conducive to investigating multiple scenarios within the time frame of the project. Instead a small local sub-model was created from the district model and used to investigate multiple scenarios. The usage of the local model was validated against the fluxes into the same area from the district model. It was found that although some of the fluxes varied between the two scales of models, this was due to the changes in land use discretisation and it is expected that such changes would also occur if the district model was subject to a similar level of discretisation. The main objectives of this analysis were to define: Variation in the nutrient concentration in the drainage outflow from the urbanised area; Rate of depletion of legacy nutrients from the urban sub-surface Rate of accretion of new sources of nutrients to the urban sub-surface. The local scale model was used to examine the effect of land and water management variables and their combinations including: shallow groundwater table control measures using either subsurface drains only or combination of surface fill and subsurface drains; density of urban development, including high (R35), medium (R25) and low (R17.5) urban density; effect of groundwater abstraction for non-potable water use in the new development for irrigation of POS and domestic gardens and for indoor use for toilet flushing and hot water use in laundry. The modelling found that the urbanisation had a large immediate effect on the pool of legacy solutes, reducing the mass in the upper layer of the superficial aquifer by 5% within 2 years regardless of the urban density, abstraction from the lower superficial aquifer or the climate, with 75% removed within 4 years and 9% removed within 9 years. The major pathways for the removal of the legacy solute pool were subsurface drainage, lateral flow and flux to the lower superficial aquifer. The subsurface drainage had the greatest effect for wetter climates, higher urban densities, lower abstraction from the deep superficial aquifer and when drains were the only watertable control measure. The subsurface drains removed between 1 and 5% of the legacy solute mass, with the higher rates of removal associated with the wetter climates and no abstraction from the lower superficial aquifer. The flux to the lower superficial aquifer was enhanced by an increase in the hydraulic gradient to the deeper layer created by a combination of additional recharge associated with urban development and the abstraction in the lower superficial aquifer. The greatest downwards flux of the legacy solutes occurred with the highest abstraction rates in the deep superficial. Although considerable solute was abstracted from the lower superficial aquifer, and the associated water was used for irrigation, it was assumed that no changes to the solute concentration associated with the garden recharge occurred within the simulation period. The solutes introduced by the urbanisation process were separated into two sources: the recharge to the watertable beneath vegetated areas and the direct infiltration of roof runoff into the subsurface. The behaviour of both these sources was mostly similar. One difference Solutes transport in an urbanising area 68

83 arises because, in addition to recharge from rainfall, the garden recharge also has a component associated with the irrigation during the period October-April of every year. The mass of these solutes in the upper superficial aquifer increases with urbanisation. Some of the scenarios reach a steady mass of solutes within the upper superficial aquifer in the first five years of urbanisation. The majority of land and water management variables have a single impact on the solute concentrations. However due to the balance between garden and verge areas and roof area on the other, there is considerable difference in the responses of the individual new urban sources to the urban density. For high urban density, the percentage of roof area in the model increases at the expense of the garden and verge areas. This changes the balance between these two components of recharge and thus fraction of roof solutes increases with urban density, whilst the fraction of garden recharge decreases with increasing urban density. The current model does not consider any reduction of concentration of solute with time through such processes as denitrification, adsorption or geochemical reactions. For nutrients it is likely that some of the nutrients associated with the recharge will undergo changes that may reduce the total nutrient load in the groundwater. There may also be changes in the solute load in the various sources through the year. It is likely that rainfall after a long dry period would have higher concentrations in roof runoff, than rainfall during a prolonged wet spell. This could be the subject for further work. Solutes transport in an urbanising area 69

84 REFERENCES Barr, A. and Barron, O., (29). Application of a coupled surface water-groundwater model to evaluate environmental conditions in a shallow groundwater catchment. CSIRO: Water for a Healthy Country National Research Flagship. Barron, O. and Barr, A., (29). Effect of urban development on the water balance in Southern River catchment. CSIRO: Water for a Healthy Country National Research Flagship. Barron, O., Donn, M.J. and Pollock D., (29). Determining the effectiveness of best management practices to reduce nutrient flows in urban drains managed by the water corporation. CSIRO: Water for a Healthy Country National Research Flagship. Davidson, W.A., (1995). Hydrogeology and groundwater resources of the Perth Region, Western Australia. Western Australia Geological Survey, Bulletin 142, 257pp. Harbaugh, A.W. (199). A Computer Program for Calculating Subregional Water Budgets Using Results from the U.S. Geological Survey Modular Three-dimensional Finite-difference Ground-water Flow Model. U.S. Geological Survey, Open-File Report 9-392, 27pp. Harbaugh, A.W., Banta, E.R., Hill, M.C., and McDonald, M.G. (2). MODFLOW-2, the U.S. Geological Survey modular ground-water model -- User guide to modularization concepts and the Ground-Water Flow Process: U.S. Geological Survey Open-File Report - 92, 121 p. HydroGeoLogic, Inc. (26). MODHMS: A comprehensive MODFLOW-based hydrologic modeling system, Version 3.. HydroGeoLogic Incorporated, Herndon, VA. JDA, CSIRO, and GHD. (28). Armadale Redevelopment Authority: Wungong Urban Water Master Plan District Water Management Strategy. Volume 1: Main Report. Perth, WA. Panday, S., and Huyakorn, P.S., (24). A fully coupled physically-based spatially distributed model for evaluating surface/subsurface flow, Advances in Water Resources,.27, Swan River Trust (29). Reducing nutrients in the Swan and Canning Rivers, Swan- Canning Water Quality Improvement Plan Draft for Public Comment, February. Wendling, L. et al., (28). Best Management Practices: Mineral-Based By-Product Materials for the Removal of DOC and Nutrients from Surface Waters of the Swan Coastal Plain. CSIRO: Water for a Healthy Country National Research Flagship. Zheng, C. (26). MT3DMS v5.2 Supplemental User's Guide, Technical Report to the U.S. Army Engineer Research and Development Center, Department of Geological Sciences, University of Alabama, 24 p. Zheng, C., and Wang, P.P. (1999). MT3DMS, A modular three-dimensional multi-species transport model for simulation of advection, dispersion and chemical reactions of contaminants in groundwater systems; documentation and user s guide, U.S. Army Engineer Research and Development Center Contract Report SERDP-99-1, Vicksburg, MS, 22 p. Solutes transport in an urbanising area 7

85 APPENDIX A PREDICTIVE CLIMATE SEQUENCES The different climate sequences used in the models are to examine the natural variability in the catchment. Three different climate scenarios are simulated: a drier climate which is labelled dry in the subsequent analysis, the current climate labelled current and a wetter climate labelled wet. The climate scenarios used in this modelling are based on 1% changes in the average rainfall over the 1 year simulation period. The three model inputs based on the climate are the rainfall, potential evaporation and runoff from the Darling Scarp. There are currently 1 years of contiguous data for all three parameters ( ), with the limitation being the runoff data from the Darling Scarp (as these are not simulated in the current model). The 1% reduction in rainfall is of the order of the change adopted in the Gnangara Sustainability Strategy (GSS) (27) for climate change in the Gnangara area 65 km to the north of the Southern River catchment on the Swan Costal Plain. The GSS defines an average climate scenario based on the years The period used for climate in the base simulation ( ) has 2% less rainfall than the average. The period used for wet scenario ( ) has an increase of 21% in the average rainfall. This is much greater than the 1% increase used in this modelling, but with only one year (2) in the contiguous data set having rainfall in excess of this average, there would be a high repetition of that year in the wet climate scenario and it is unlikely that the antecedent catchment condition would be captured in the modelling. Therefore a more modest wet climate increase of 1% is adopted. In order to fit the existing climate to the increase/decrease in rainfall in the altered climates, the existing annual rainfall data is combined to produce the required average annual rainfall for the simulation. As there are usually multiple repeats of annual data in the constructed records, the years are distributed randomly with the sequences used in the modelling shown in Table A-1 and Figure A-1. Table A-1 Annual rainfall [mm] for different climate regimes used in predictive scenarios Year Current Dry Wet Average Report Title Page 71

86 12 1 Drier Current Wetter Rainfall (mm) Year Figure A-1 Annual rainfall for the three climate scenarios Solutes transport in an urbanising area 72

87 APPENDIX B SENSITIVITY ANALYSIS This appendix examines the sensitivity of the parameters used in the local area flow model on the flows and solute concentrations and fluxes. B.1 Conductance of subsurface drains The conductance (C) used for the drainage was calculated using the horizontal hydraulic conductivity in the cell (K), and assumed the cross-sectional area of the drain (A) was 1 square metre and that the extent of influence (D) was also 1 m. KA C D This gave values of the conductance in the range of.1-1. m 2 /day for the model domain. The results from the flow modelling using this value were reported in Chapter 4. The influence of the conductance on these results was investigated by increasing the value of the conductance by one order of magnitude. This increased the drainage flow from an average of 127 ML/year/km 2 over the local model area to 33 ML/year/km 2. This had a commensurate effect on the solute mass and outflow. Figure B-1 shows the influence of the higher conductance values on the quantity of mass of all solute sources in both the upper and lower layers of the model. These results show less mass present in each of the layers under each scenario. This is because of the increased flow to the drains increases the mass flux to the drains (Figure B-2) and reduces the residual mass in the aquifer layers. Figure B-3 shows the effect of three different multipliers of the calculated conductance on the average annual drainage discharge. This shows that there is a large increase in the drainage. The higher drainage discharge also reduces the times for the flushing of the legacy solutes. The 5, 75 and 9% reductions in mass occur to.9, 2.1 and 3.6 years respectively from 1.2, 2.8 and 4.8 years, i.e. by 25%. Solutes transport in an urbanising area 73

88 Mass (kg/km 2 ) x Conductance Conductance Mass (kg/km 2 ) x Conductance Conductance (a) Year (b) Year Mass (kg/km 2 ) (c) x Conductance Conductance Year Mass (kg/km 2 ) (d) x Conductance Conductance Year Mass (kg/km 2 ) x Conductance Conductance Mass (kg/km 2 ) x Conductance Conductance (e) Year (f) Year Figure B-1 Total modelled solute mass for original and sensitivity analysis for subsurface drainage conductance; (a) legacy solute in upper superficial layer, (b) legacy solute in lower superficial aquifer, (c ) garden recharge solute in upper superficial layer, (d) garden recharge solute in lower superficial aquifer, (e) roof runoff solute in upper superficial layer, and (f) roof runoff solute in lower superficial layer. All results are from the scenario using current climate, medium urban density, abstraction for POS and drains-only as the watertable control option. Solutes transport in an urbanising area 74

89 Relative mass flux (g/day) (a) x Conductance Conductance Year Relative mass flux (g/day) (b) x Conductance Conductance Year Relative mass flux (g/day) (c) x Conductance Conductance Year Figure B-2 Mass flux to the subsurface drains for (a) legacy solutes, (b) garden recharge solutes, and (c) roof runoff solutes. r. All results are from the scenario using current climate, medium urban density, abstraction for POS and drains-only as the watertable control option. 4 Drain discharge (ML/yr/km 2 ) Conductance multiplier Figure B-3 Annual drainage discharge as a function of drain conductance B.2 Recharge in the gardens Solutes transport in an urbanising area 75

90 The recharge in the gardens and verges was based on the infiltration through the surface. As not all this water reaches the watertable, the sensitivity of the model to this rate of recharge was examined by reducing the quantity of recharge by 2%. It is noted that a similar sensitivity could be interpreted from the different climate scenarios but in this case the recharge will not have different inter-annual variations. The changes in the flow in the model are such that the total recharge (including roof runoff) has reduced from 654 ML/year/km 2 to 585 ML/year/km 2, a decrease of about 11%. The drainage discharge also decreases from 127 ML/year/km 2 to 13 ML/year/km 2, a decrease of 19%. The influence on the concentrations of the various solutes is shown in Figure B-4. This shows that there is a major difference in the concentration of the garden recharge solute as would be expected with the mass in the lower superficial down by 1% in the upper superficial layer and by 2% in the lower superficial aquifer. There are also changes in the mass of the other two solutes, with the legacy solute increasing by 23% in the upper superficial aquifer and 1% in the lower superficial aquifer, and the roof runoff solutes increasing by 1% in the upper superficial aquifer and by only 1% in the lower superficial aquifer. These increases in the mass of the non-garden-recharge solutes arise for a number of reasons. The first reason is that the reduction in garden recharge increases the relative concentration of the other solutes in the aquifer. Thus the mass of the other solutes in the lower superficial aquifer increase as the concentration in the downward flux is greater. However this increase is counterbalanced to some extent by the reduction in downward flux due to the lower heads in the upper superficial aquifer as a result of the lower recharge. A second reason for the increase of the non-garden-recharge solute mass is the reduction in the volume of drainage and thus the mass entrained in the discharge. Figure B-5 shows the mass flux to the subsurface drains for the three types of solutes. The total reduction in mass discharge for the non-garden recharge solutes is 8%, whilst there is a 27% drop in the mass discharge of the garden recharge solutes. Solutes transport in an urbanising area 76

91 Mass (kg/km 2 ) % recharge Original Mass (kg/km 2 ) % recharge Original (a) Year (b) Year Mass (kg/km 2 ) (c) % recharge Original Year Mass (kg/km 2 ) (d) % recharge Original Year Mass (kg/km 2 ) % recharge Original Mass (kg/km 2 ) % recharge Original (e) Year (f) Year Figure B-4 Total modelled solute mass for original and 2% reduction in garden recharge; (a) legacy solute in upper superficial layer, (b) legacy solute in lower superficial aquifer, (c ) garden recharge solute in upper superficial layer, (d) garden recharge solute in lower superficial aquifer, (e) roof runoff solute in upper superficial layer, and (f) roof runoff solute in lower superficial layer. All results are from the scenario using current climate, medium urban density, abstraction for POS and drains-only as the watertable control option. Solutes transport in an urbanising area 77

92 Relative mass flux (g/day) (a) % Recharge Original Year Relative mass flux (g/day) (b) % Recharge Original Year Relative mass flux (g/day) (c) % Recharge Original Year Figure B-5 Comparison of mass flux to the subsurface drains between model results reported in main part of report (original) and model results from sensitivity analysis with varying conductance and recharge (sensitivity) for (a) legacy solutes, (b) garden recharge solutes, and (c) roof runoff solutes. All results are from the scenario using current climate, medium urban density, abstraction for POS and drains-only as the watertable control option. Solutes transport in an urbanising area 78

93 APPENDIX C SIMULATED SOLUTE DISTRIBUTIONS This appendix contains results from the modelling that are not included in the main text. C.1 Legacy solute The distribution of legacy solute concentrations in the upper superficial layer was shown in Figure 5-4 for four time periods (end of April in year 1; end of September in year1; end of April in year 1; and end of September in year 1) for the scenario using current climate, medium urban density, abstraction for POS and drains as the only watertable control option. Figure C-1 shows the solute distribution for the same scenario in the lower superficial layer. This shows that the concentration in the lower superficial aquifer is low, with the majority less than 2% of the initial legacy concentration in the upper superficial aquifer. Figure 5-5 shows that between 2 and 35% of the original solute mass is in the lower superficial aquifer. The difference between the concentrations observed in Figure C-1 and the mass in Figure 5-5 is that the saturated thickness in the lower superficial aquifer is greater than the saturated thickness in the upper superficial aquifer such that an equivalent mass will have a smaller concentration in the lower superficial aquifer. Solutes transport in an urbanising area 79

94 Figure C-1 Legacy solute concentration in the lower superficial layer for four times in simulation: (a) at the end of summer irrigation period in the first year (Stress period 4, April); (b) at the end of winter non-irrigation period in the first year (Stress period 9, September), (c) at the end of summer irrigation period in the final year (Stress period 112, April) and (d) at the end of winter non-irrigation period in the final year (Stress period 117, September); the simulation uses current climate, medium urban density, abstraction for POS, and drainage as the only watertable control measure. The variations in the solute concentration for different scenarios were examined in Figure 5-6. This showed that the greatest variations in the solute mass in the lower superficial aquifer occurred for the different abstraction rates. Figure C-2 shows the legacy solute concentrations in the upper and lower superficial aquifers for three different abstraction scenarios. These solute distributions are at the end of year 4 of the simulation, occurring at the time that the mass in the lower superficial aquifer is at a maximum for the hot water abstraction scenario. These show that as the abstraction rate increases, the distribution of legacy mass in the upper superficial aquifer becomes sparser and sparser. Conversely in the lower superficial aquifer, as the abstraction rate rises, the distribution of legacy mass increases and becomes more connected. Solutes transport in an urbanising area 8

95 Figure C-2 Legacy solute distributions in the top and bottom layers of the superficial aquifer at the end of year 4 for three different abstraction scenarios The legacy solute mass flux to the deep layers is shown in Figure C-3. It shows a long-term trend of declining mass flux as the mass of legacy solutes in the upper superficial aquifer Solutes transport in an urbanising area 81

96 gradually declines. The annual pattern of mass fluxes shows a minimum mass flux occurring at about May each year, and maxima occurring throughout the period June-September, with multiple maxima occurring during some winters. There is some return flow from the deeper layers at the end of summer. This generally occurs in May and occasionally at other times during the winter when the abstraction for irrigation is not active. Mass flux (g/day) Downwards Upwards Year Figure C-3 Legacy solute mass flux into and out of the lower superficial aquifer for current climate, medium density urban area, abstraction for POS and drainage-only watertable control C.2 Garden recharge solute The fate of the garden recharge solute within the superficial aquifer was discussed in Chapter 6. The results in this appendix are some additional information that is not included in the main report. Figure C-4 shows the distribution of garden recharge solute within the lower superficial aquifer. It compliments the solute distribution for the upper superficial layer in Figure 6-4. It shows that although extensive areas of high concentration occur in the upper superficial layer, in the deep superficial layer the majority of the aquifer has a concentration less than 5% of the recharge concentration. Solutes transport in an urbanising area 82

97 Figure C-4 Garden recharge solute concentration in the lower superficial layer for four times in simulation: (a) at the end of summer irrigation period in the first year (Stress period 4, April); (b) at the end of winter non-irrigation period in the first year (Stress period 9, September), (c) at the end of summer irrigation period in the final year (Stress period 112, April) and (d) at the end of winter non-irrigation period in the final year (Stress period 117, September); the simulation uses current climate, medium urban density, abstraction for POS, and drainage as the only watertable control measure. The garden recharge solute mass flux to the deep layers is shown in Figure C-5. It shows a long-term trend of increasing mass flux as the mass of garden recharge solutes in the upper superficial aquifer increases. Similarly to the legacy solute mass flux, the annual pattern of mass fluxes shows a minimum mass flux occurring at about May each year, and maxima occurring throughout the period June-September, with multiple maxima occurring during some winters. Again there is some return flow from the deeper layers at the end of summer. Solutes transport in an urbanising area 83

98 Mass flux (g/day) Downwards Upwards Year Figure C-5 Garden recharge solute mass flux into and out of the lower superficial aquifer for current climate, medium density urban area, abstraction for POS and drainage-only watertable control C.3 Solutes associated with roof runoff The fluxes of the roof recharge solutes was briefly covered in Chapter 7 of the main body of the report. It exhibited much the same behaviour as the garden recharge solutes, with the major difference in that the concentration of the solutes increases with urban density for this solute, whereas for the garden recharge solute, it decreases with increasing urban density. This section of the Appendix completes the analysis of the roof runoff solutes in the urban domain. The mass of solute associated with roof recharge, in the upper superficial aquifer increases with time (Figure C-6), although there is a distinct seasonal cycle with a maximum level occurring in winter and a minima at the end of summer. The increase during winter is due to the addition of the solute with the rainfall/recharge, and the decline in the summer is due to the downward flux of water and solute exceeding the quantity of solute in the summer roof runoff. The figure shows that some scenarios, particularly those with low abstraction rates, reach an equilibrium in the upper superficial layer after 5 years. However some of the scenarios with higher abstraction rates have not yet reached equilibrium after the 1 years of the simulation. Figure C-7 shows the maximum relative concentration of the roof recharge in the upper superficial. The maximum concentration occurs for no or low abstraction rates (POS). This is because in these scenarios, the smaller hydraulic gradients to the lower superficial aquifer mean that the downwards flux is smaller and the residence times of the solutes in the upper superficial layers are higher and thus the solutes may become more concentrated. Solutes transport in an urbanising area 84

99 Relative Mass (kg/km2) Figure C-6 Simulated relative mass of roof recharge solute in the upper superficial layer for all scenarios Maximum relative concentration dlnsd dlnsf dlpsd dlpsf dmnsd dmnsf dmpsd dmpsf dmgsd dmgsf dmtsd dmtsf dhnsd dhnsf dhpsd dhpsf dhgsd dhgsf dhtsd dhtsf dhhsd dhhsf Maximum relative concentration clnsd clnsf clpsd clpsf cmnsd cmnsf cmpsd cmpsf cmgsd cmgsf cmtsd cmtsf cmhsd cmhsf chnsd chnsf chpsd chpsf chgsd chgsf chtsd chtsf chhsd chhsf Maximum relative concentration wlnsd wlnsf wlpsd wlpsf wlgsd wlgsf wmnsd wmnsf wmpsd wmpsf wmgsd wmgsf wmtsd wmtsf wmhsd wmhsf whnsd whnsf whpsd whpsf whgsd whgsf whtsd whtsf whhsd whhsf Figure C-7 Maximum concentration of roof runoff solutes in upper superficial layer for all scenarios Solutes transport in an urbanising area 85

100 The roof recharge solute distributions in the upper superficial layer at various times during the simulation are shown in Figure C-8 for the scenario using current climate, medium urban density, POS abstraction and watertable control using drainage only. This shows that high concentrations occur in areas which correspond to places where the watertables are below the inverts of the drainage for substantial periods of the year. There are also higher concentrations along the lines of gardens and verges, indicating that evapotranspiration increases the concentrations in these areas. In the area of the verge, the close location of the subsurface drain provides an outlet for the groundwater and associated solute so that the build up of concentration is not as evident. In the vicinity of the abstraction wells in the northeast, southeast and central-west of the domain, there are areas of lower concentration as the induced additional downward flux of water in these locations keeps the watertable relatively fresh. Figure C-8 Roof runoff solute concentration in the upper superficial layer for four times in simulation: (a) at the end of summer irrigation period in the first year (Stress period 4, April); (b) at the end of winter non-irrigation period in the first year (Stress period 9, September), (c) at the end of summer irrigation period in the final year (Stress period 112, April) and (d) at the end of winter non-irrigation period in the final year (Stress period 117, September); the simulation uses current climate, medium urban density, abstraction for POS, and drainage as the only watertable control measure. Solutes transport in an urbanising area 86

101 C.3.1 Changes in solute mass in deep groundwater In the lower superficial layer, the mass of solute is still steadily increasing at the end of the 1 year simulation period (Figure C-9). There is again a small seasonal signal with the mass increasing during the summer irrigation season and slightly rising or steady during the winter. Relative Mass (kg/km2) Figure C-9 Simulated relative mass of roof recharge solute in the lower superficial layer for all scenarios The distribution of the solute associated with roof recharge in the lower superficial is shown in Figure C-1 at the end of the irrigation period in the final year of the simulation. It again shows a large area of lower concentration in the northern central part of the domain. This may be associated with a lower vertical conductivity in the northern part of the domain as shown in Figure 6-7. The higher concentrations associated with the downward flux induced by the abstraction can be observed in the northwest and southwest of the domain. Figure C-1 Roof runoff solute concentration in the lower superficial layer for two times in simulation: (a) at the end of summer irrigation period in the final year (Stress period 112, April) and (b) at the end of winter non-irrigation period in the final year (Stress period 117, September); the simulation uses current climate, medium urban density, abstraction for POS, and drainage as the only watertable control measure. Solutes transport in an urbanising area 87

102 C.3.2 Solute export in drains The simulated relative mass flux of roof recharge solutes to the drains are shown in Figure C-11 for variations in climate, urban density, abstraction rates and watertable control measures. The concentrations for the same scenarios are shown in Figure C-12 respectively. Once again the wetter the climate the greater the mass in the drainage outflows, although the concentration of the drainage waters are relatively similar. The mass of the roof recharge solutes is greater for higher density, but the additional quantity of water generated through garden recharge dilutes the concentration such that the concentration associated with the high density is the lower than the other two simulated urban densities. The effect of the abstraction scenarios is as usual, with the higher the abstraction rate, the smaller the mass and the lower the concentration of the drainage waters. Relative mass flux (g/day) (a) Climate Drier Current Wetter 5 1 Relative mass flux (g/day) Low Medium High 5 1 (b) Urban density Relative mass flux (g/day) None POS Garden Toilet Hot (c) Abstraction Relative mass flux (g/day) Fill and drains Drains only 5 1 (d) Watertable control Figure C-11 Relative mass flux of roof runoff solutes into the drains for various (a) climates, (b) urban densities, (c) groundwater abstractions rates and (d) watertable control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c). Solutes transport in an urbanising area 88

103 Relative concentration Drier Current Wetter Relative concentration Low Medium High. 5 1 (a) Climate. 5 1 (b) Urban density Relative concentration None POS Garden Toilet Hot 5 1 (c) Abstraction Relative concentration Fill and drains Drains only 5 1 (d) Watertable control Figure C-12 Average monthly relative concentration of roof runoff solute into the drains for various (a) climates, (b) urban densities, (c) groundwater abstractions rates and (d) watertable control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) (gaps on the plots are related to no flow period). C.3.3 Solute export from the area with groundwater abstraction from deeper superficial aquifer The groundwater abstraction has a substantial effect on the solute distribution in both the upper and lower parts of the superficial aquifer. However as the roof runoff solute in the lower superficial has not yet reached a steady-state, both the mass and concentration of the solute in the abstraction is still increasing at the end of the 1 year simulation. Figure C-13 shows the monthly relative mass of garden recharge solutes associated with the abstraction for different scenarios. The results show an increase in the mass abstracted with wetter climates and higher urban densities. There is very little difference between the two watertable control options and the different abstraction rates. Figure C-14 shows the monthly average concentrations of garden recharge solutes in the abstracted water. For scenarios using the same abstraction rates (i.e. all POS abstraction simulations), the relationship between the concentrations of the various scenarios are the same as those for the mass flux in the abstraction. Solutes transport in an urbanising area 89

104 Relative mass flux (g/day) Drier Current Wetter 5 1 (a) Climate Relative mass flux (g/day) Low Medium High 5 1 (b) Urban density Relative mass flux (g/day) None POS Garden Toilet Hot 5 1 (c) Abstraction Relative mass flux (g/day) Fill and drains Drains only 5 1 (d) Watertable control Figure C-13 Relative mass of roof runoff solutes in abstraction for various (a) climates, (b) urban densities, (c) groundwater abstractions rates (note the different scale on the vertical axis) and (d) watertable control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) (gaps on the plots are related to a period with no irrigation). Solutes transport in an urbanising area 9

105 Relative concentration Drier Current Wetter Relative concentration Low Medium High. 5 1 (a) Climate. 5 1 (b) Urban density Relative concentration None POS Garden Toilet Hot Relative concentration Fill and drains Drains only. 5 1 (c) Abstraction. 5 1 (d) Watertable control Figure C-14 Simulated relative concentration of roof runoff solutes in abstraction for various (a) climates, (b) urban densities, (c) groundwater abstractions rates and (d) watertable control measures; the simulations were for current climate (b, c, d) medium urban density (a, c, d), abstraction for POS (a, b, d) and drains as the only watertable control (a, b, c) (gaps on the plots are related to a period with no irrigation). C.3.4 Solute transport with groundwater flux from the model domain For the roof recharge solutes, the sole source is the recharge from the roofs of the domain. Solute may flow out of the domain through the general head boundary conditions. Figure C- 15 shows the total mass outflow over the simulation through the general head boundary conditions. The total relative mass flux through the boundary is much greater than the relative flux for the legacy solutes in Figure 5-15 though less than the flux for the garden recharge solutes in Figure Figure C-16 shows the average outflow concentration for the lower superficial layer. This is approaching 35% of the incident recharge concentration of the roof runoff and is still rising. Solutes transport in an urbanising area 91

106 Mass outflow (kg/km 2 ) Upper superficial Aquitard Lower superficial dlnsd dlnsf dlpsd dlpsf dmnsd dmnsf dmpsd dmpsf dmgsd dmgsf dmtsd dmtsf dhnsd dhnsf dhpsd dhpsf Mass outflow (kg/km 2 ) dhgsd dhgsf dhtsd dhtsf dhhsd dhhsf Upper superficial Aquitard Lower superficial Mass outflow (kg/km 2 ) clnsd clnsf clpsd clpsf cmnsd cmnsf cmpsd cmpsf cmgsd cmgsf cmtsd cmtsf cmhsd cmhsf chnsd chnsf chpsd chpsf chgsd chgsf chtsd chtsf chhsd chhsf Upper superficial Aquitard Lower superficial wlnsd wlnsf wlpsd wlpsf wlgsd wlgsf wmnsd wmnsf wmpsd wmpsf wmgsd wmgsf wmtsd wmtsf wmhsd wmhsf whnsd whnsf whpsd whpsf whgsd whgsf whtsd whtsf whhsd whhsf Figure C-15 Simulated relative total mass outflow from model domain for roof recharge solutes Relative concentration None POS Garden Toilet Hot water Figure C-16 Simulated relative concentration of roof recharge out through general head boundary condition in the lower superficial layer as a function of groundwater abstraction rate and current climate, medium urban density, and watertable control through drainage only. Solutes transport in an urbanising area 92

107 Solutes transport in an urbanising area 93

108 APPENDIX D TOTAL SOLUTE EXPORT An examination of the total concentration of nutrients in the subsurface drainage was performed in Chapter 8 for different abstraction rates and different legacy concentrations. Figure D-1 and Figure D-2 show the concentrations of total nitrogen and total phosphorous in the subsurface drainage water for different climates. Concentration (mg/l) (a) Legacy Gardens Roof Concentration (mg/l) (b) Legacy Gardens Roof Concentration (mg/l) (c) Legacy Gardens Roof Concentration (mg/l) (d) Legacy Gardens Roof Concentration (mg/l) (e) Legacy Gardens Roof Concentration (mg/l) (f) Legacy Gardens Roof Figure D-1 Time series of simulated total nutrient concentration with individual components in drainage water for wetter climate, medium urban density, POS abstraction and drains as the sole watertable control measure. The concentrations are: (a) total nitrogen (TN) legacy of 1.6 mg/l, garden recharge 1.5 mg/l and roof runoff.27 mg/l; (b) total phosphorous (TP) legacy of.5 mg/l, garden recharge.1 mg/l and roof runoff.22 mg/l; (c) TN legacy of 4.5 mg/l, garden recharge 1.5 mg/l and roof runoff.27 mg/l; (d) TP legacy of.5 mg/l, garden recharge.1 mg/l and roof runoff.22 mg/l; (e) TN legacy of 7.4 mg/l, garden recharge 1.5 mg/l and roof runoff.27 mg/l; (f) TP legacy of.58 mg/l, garden recharge.1 mg/l and roof runoff.22 mg/l. Solutes transport in an urbanising area 94

109 Concentration (mg/l) (a) Legacy Gardens Roof Concentration (mg/l) (b) Legacy Gardens Roof Concentration (mg/l) (c) Legacy Gardens Roof Concentration (mg/l) (d) Legacy Gardens Roof Concentration (mg/l) (e) Legacy Gardens Roof Concentration (mg/l) (f) Legacy Gardens Roof Figure D-2 Time series of simulated total nutrient concentration with individual components in drainage water for drier climate, medium urban density, POS abstraction and drains as the sole watertable control measure. The concentrations are: (a) total nitrogen (TN) legacy of 1.6 mg/l, garden recharge 1.5 mg/l and roof runoff.27 mg/l; (b) total phosphorous (TP) legacy of.5 mg/l, garden recharge.1 mg/l and roof runoff.22 mg/l; (c) TN legacy of 4.5 mg/l, garden recharge 1.5 mg/l and roof runoff.27 mg/l; (d) TP legacy of.5 mg/l, garden recharge.1 mg/l and roof runoff.22 mg/l; (e) TN legacy of 7.4 mg/l, garden recharge 1.5 mg/l and roof runoff.27 mg/l; (f) TP legacy of.58 mg/l, garden recharge.1 mg/l and roof runoff.22 mg/l. The gaps in the records are where the watertable is below the inverts for the subsurface drainage Solutes transport in an urbanising area 95

110