Water Budgets - Source Water Protection and Beyond

Transcription

1 Water Budgets - Source Water Protection and Beyond Donald Ford, P.Geo 1, Gayle Soo Chan, P.Geo 2 1 Manager, Geoenvironmental, Toronto and Region Conservation Authority, Toronto, Ontario, Canada 2 Director Groundwater Resources, Central Lake Ontario Conservation Authority, Oshawa, Ontario, Canada ABSTRACT Water budgets have been completed in Ontario using a tiered approach to focus resources on the highest stressed areas. The technical work has included data collection, as well as modelling of both surface water hydrology and groundwater flow. Despite extensive input data and use of complex numerical models by subject matter experts, uncertainties remain regarding water use, surface water flow, and groundwater flow across subwatershed boundaries. This paper summarizes these uncertainties while outlining benefits and precautions regarding use of the data and model outputs. RÉSUMÉ Des bilans hydrologiques ont été produits en Ontario à l'aide d une approche à plusieurs niveaux afin de concentrer les ressources sur les domaines sujets aux plus grandes pressions. Le travail technique comprend la collecte de données, ainsi que la modélisation de l écoulement des eaux de surface et souterraines. En dépit de nombreuses données d'entrée et de l'utilisation de modèles numériques complexes par des experts en la matière, des incertitudes demeurent en ce qui concerne l utilisation de l'eau, l écoulement des eaux de surface et le débit des eaux souterraines traversant les frontières des sous-bassins versants. Cet article résume ces incertitudes tout en identifiant les bénéfices et les précautions dont il faut tenir compte lors de l utilisation des données et résultats des modèles. 1 INTRODUCTION Conservation Authorities are unique entities within the province of Ontario. They are local environmental agencies that have jurisdictions based on watershed boundaries. Under The Clean Water Act, 2006 (CWA; MOE, 2006), Conservation Authorities (called Source Protection Areas or SPAs) are grouped into Source Protection Regions (SPRs) for the purposes of completing Assessment Reports and Source Protection Plans. The overall goal is to complete Assessment Reports and Source Protection Plans for each SPA. The Assessment Reports document the environmental conditions, vulnerable areas, and threats to drinking water for each SPA. Under the CWA, three conservation authorities - Credit Valley (CVC), Toronto and Region (TRCA) and Central Lake Ontario (CLOCA) were joined into the Credit Valley, Toronto and Region, Central Lake Ontario (CTC) Source Protection Region, as shown on Figure 1. The Ministry of the Environment (MOE) developed Technical Rules (MOE, 2009) under the CWA that require a number of technical components for the Assessment Reports, including the preparation of water budgets on a watershed basis for all Source Protection Areas in Ontario. These water budgets characterize each component of the hydrologic system on a catchment basis and describe, in a quantitative manner, the pathways that water takes through the watersheds. In accordance with the Technical Rules, water budgets are prepared in a tiered approach (Conceptual, Tiers 1 to 3) with increasingly more detail and level of discretization for an increasingly more focused area. The first tier requires the development of a conceptual understanding of the hydrology and hydrogeology of the entire SPA, while the Tier 1 level requires estimates of precipitation, runoff, evapotranspiration and recharge on a subwatershed basis. These estimates may be generated based on simple numerical models (unless more complex tools or numerical models are available). Tier 2 and Tier 3 analyses are required to use progressively more complex groundwater and surface water flow models to estimate the available water supplies. Prior to the CWA, two numerical models (USGS PRMS and USGS MODFLOW), developed as part of the York Peel Durham Toronto Groundwater initiative (Kassenaar and. Wexler, 2006) were available to Toronto and Region Conservation Authority (TRCA) and Central Lake Ontario Conservation Authority (CLOCA). These models were applied in an integrated manner for this study. The models provided a quantitative assessment of groundwater recharge, interaction between the groundwater and surface water systems, and groundwater movement across watershed boundaries. The groundwater model extended beyond the boundary of the conservation authorities and was used to estimate inflow from recharge areas external to the study areas.

2 Figure 1. The CTC (Credit Valley, Toronto and Region, Central Lake Ontario) Source Protection Region In the calculation of water budgets, the components of the water cycle must be estimated, including: water supply (Q Supply ); an allowance for ecological functions called the Water Reserve (Q Reserve ); and water use (Q Demand ). For a Tier 1 Water Budget, these parameters must be calculated on a monthly basis for surface water and both monthly and annually for groundwater. As per Technical Rules 9-34 (MOE, 2009), a watershed is deemed stressed for groundwater or surface water supplies when the Q Demand plus the Q Reserve exceeds pre-defined percentages of the Q Supply specified in the Technical Rules (ibid). In the prescribed Tiered approach, where significant or moderate stress is determined and a municipal supply is located in the watershed, more advanced studies must be completed for the subwatershed (Tier 2) or water system (Tier 3). In the TRCA jurisdiction, a Tier 2 study was required for the Little Rouge River and the Reesor/Stouffville Creek subwatersheds. Based on the outcome of that work, a Tier 3 study was recommended for the Whitchurch-Stouffville and Uxville municipal drinking water systems. As there are no municipal drinking water systems located within any of the CLOCA watersheds (all drinking water supplies come from Lake Ontario), water budget studies were not advanced beyond the Tier 1 level. These studies are required by the legislation and were completed to assess and protect municipal drinking water supplies. They, however, provide invaluable information and serve to inform many other decisions associated with the management of water resources. Water budget results are currently being used in the development of watershed plans, in the review of development proposals and water taking applications, and in the modelling of future land-use scenarios. While provincial methodologies represent accepted technical approaches and provide for consistency across the province, there are uncertainties in the analyses that should be recognized and considered during the decision making process. This paper discusses some of these key uncertainties related to water use estimates, surface water flow estimates for ungauged catchments and groundwater inflows across subwatershed boundaries.. 2 DESCRIPTION OF THE STUDY AREA This paper focuses on water budget work completed by TRCA and CLOCA. The study area is located within the Greater Toronto Area from Mississauga in the west to Clarington in the east (Figure 3). The northern boundaries of TRCA and CLOCA form the northern study area boundary, while Lake Ontario marks the southern boundary, respectively. The area is highly urbanized along the Lake Ontario shoreline, while land-

3 use in the northern and eastern areas (particularly in CLOCA) is primarily agricultural. 2.1 Geology The geology of the study area comprises Quaternary sediments (overburden) of variable thickness overlying Ordovician bedrock. The Quaternary sediments include a sequence of glacial and interglacial (lacustrine/fluvial) units recording deposition over approximately the last 135,000 years (Kassenaar and Wexler, 2006). There are three important geologic features present within the TRCA/CLOCA watersheds that influence the surface and ground water flow patterns: bedrock and associated valley systems which may contain sand and gravel deposits; the ORM which forms a upland recharge area and headwaters for the larger streams; and areas where low-permeability Quaternary sediments have been eroded and replaced with sands and silt ( tunnel channels ). The study area overburden stratigraphy has been outlined in a number of previous studies (Kassenaar and Wexler, 2006), TRCA and CLOCA Tier 1 Water Budget reports (TRCA, 2010, Earthfx, 2009). 3 MODEL DESCRIPTION AND APPROACH 3.1 Surface Water Model The USGS Precipitation-Runoff-Modelling-System (PRMS) is an open-source code that calculates all components of the hydrologic cycle on a watershed or subwatershed scale (Leavesley, 1983). It is a modular, deterministic, distributed-parameter modelling system developed to evaluate the impacts of various combinations of precipitation, climate, and land use on streamflow and groundwater recharge. The modular design provides a flexible framework for model enhancement. To create a model in PRMS, the watershed is divided into subunits, referred to as homogeneous response units (HRUs) based on such basin characteristics as slope, aspect, elevation, vegetation type, soil type, land use, and precipitation distribution. Water and energy balances are computed daily for each HRU. The sum of the responses of all HRUs, weighted on a unit-area basis, produces the daily system response and streamflow for a basin. A second level of partitioning is available for storm hydrographs. While the storm mode includes more complex moisture routing processes, it must be run on a maximum fiveminute time-step basis. The daily mode was felt to be adequate for the Tier 1 analysis and was more compatible with the types of climate data readily available in the study area (Earthfx, Inc., 2009). The 1991 version of the PRMS source code was limited to a maximum of 50 HRUs. Because there is no real reason, except computer memory limitations, to restrict the number of HRUs, Earthfx Inc. modified the PRMS code to allow each HRU to represent a cell in a rectangular grid similar to PRMS modifications completed by the USGS as part of the development of the GSFLOW model. The ability to compute the water balance on a cell-by-cell basis made PRMS much more compatible with the existing MODFLOW model. For this study, square cells, 25 metres on a side, were found to adequately represent the distribution of land use, topography, and soil properties within the CLOCA areas. The 25 x 25m grid cell size allowed for a more detailed assessment of surface and shallow groundwater interaction and allowed for a level of discretization comparable to polygon based models. A 100 m cell size was required for the TRCA because of its larger land area and computing limitations. Cells that covered areas outside of the watershed boundaries were designated as inactive and were not included in the water balance computations (Earthfx, 2009). 3.2 Groundwater Model The groundwater flow model for this study was developed based on the United States Geological Survey (USGS) MODFLOW code. This code has been extensively tested and is well-suited for modelling regional and local-scale flow in multi-layered aquifer systems and can easily account for irregular boundaries, complex stratigraphy, and spatial variations in hydrogeologic properties. The version of MODFLOW used is documented in McDonald and Harbaugh (1988) and Harbaugh and McDonald (1996). The model included eight layers with 100 m by 100 m cells. 3.3 Input Parameters Both models were constructed using all available data (climate, soils, topography, land-use, and water use) as well as interpreted products (geology/stratigraphy) and calculated parameters (hydraulic conductivity estimated based on the lithology from water well records, and limited pump tests and slug tests, transmissivity, etc). Note that work at the Tier 1 and 2 levels does not include funding for field verification. This is a significant data gap that will be addressed during future work either under the CWA or other Conservation Authority programs. The models were calibrated using stream gauge data where total flow in the streams along with precipitation, are the key measured controls of inputs and outputs. Essentially an over (or under) estimation of recharge (PRMS output, MODFOW input) predicts higher (or lower) water levels than those measured in the streams. The strength of the models, are to some extent dependent on the volume and accuracy of the stream gauge data. A post-processing program, Zone Budget (Harbaugh, 1990), was used to calculate groundwater budgets based on model results: The program determines simulated lateral groundwater inflows and outflows across watershed boundaries as well as simulated groundwater discharge to streams. These lateral inflow and outflow values were used to adjust the calibration of the PRMS model and determine the Q IN component of water supply used in the watershed stress analyses. Simulated groundwater discharge from the MODFLOW model was compared against

4 simulated PRMS values as well as separated baseflows. Table 1. Groundwater Stress Assessment Thresholds 4 WATER BUDGET BENEFITS, UNCERTAINTIES AND PRECAUTIONS 4.1 Water Use Estimates Under the CWA, Q Demand is assessed against Q Supply to determine the level of stress. The purpose of these calculations is primarily to screen out less vulnerable supplies and focus on watersheds where municipal supplies may be at risk. Where municipal supplies do not exist within a subwatershed, the stress estimates highlight subwatersheds where the existing or future water use are or may be unsustainable. In the province of Ontario, a Permit to Take Water (PTTW) is required for takings of or over, 50,000 litres/day (exceptions include fire emergency use, livestock and domestic takings). These data are recorded in a Provincial database and is regularly used for estimates of water use and sustainability of supplies. The publically available database generally provides the name of the permit holder, the location of the permitted taking, the source (ground or surface water), type of use and the maximum permitted annual rate of taking. The water demand component of the water budget considers water taken for anthropogenic uses. These uses include municipal and private water well takings as well as other permitted takers. In the Tier 1 analyses, water demand is estimated from permitted takings. Takings that do not require a permit are estimated from population data and water well record classifications. The demand estimates are tabulated for each watershed and then adjusted for consumptive demand and seasonal (monthly) extraction rates. The PTTW database is the primary source of consumptive demand information. Estimating and verifying actual consumption from this source during the calculation of the water budgets under the SWP program is difficult as the PTTW database often only provides annual maximum permitted rates as versus actual takings on a monthly basis. As well, conditions or restrictions that may be stipulated on the permit are not included in the database and these data are currently only available in hard copy at the MOE s regional offices. The following tables present the provincial thresholds for the assessment of groundwater and surface water stress. Groundwater Quantity Stress Assignment Average Annual Water Demand Monthly Maximum Water Demand Significant > 25% > 50% Moderate > 10% > 25% Low 0 10% 0 25% Surface Water Quantity Stress Level Assignment Monthly Maximum Water Demand Significant > 50% Moderate > 25% Low 0 25% Table 2. Surface Water Quantity Stress Thresholds The incompleteness of the PTTW data is an important factor, as stress calculations under the water budget analysis may be erroneous where demand is overestimated and may result in a calculated water deficit. In the assessment of surface water stress, the water budget analyses determined that for many of the watersheds particularly the smaller ones within the study area, the abstractions were initially calculated to be greater than 100 % of available supply in some watersheds (between 119 and 360% of available supply in some TRCA watersheds). This deficit scenario is often not the case as actual extraction rates are often lower than the permitted rates; rates vary seasonally; and conditions are often placed on the permits by provincial staff to ensure that other takings as well as a reserve for ecosystem needs is considered. Conditions may include summer storage, water level monitoring and triggers as well as drought provisions. This deficit scenario results from the uncertainties associated with the timing of the takings, operational details and the availability of on-site storage reservoirs which may all be utilized to manage the water resources. The Q Supply term may vary drastically in watersheds between winter and summer months shown on Figure 2. When Q Supply term is very low in summer months the PTTW conditions and the use of accurate taking information are both critical to the assessment of stress. This is especially true in some small watersheds such as where groundwater discharge represents 100% of flow during the summer. Figure 2. Monthly Supply and Demand for Significantly Stressed Subwatersheds in the TRCA study area

5 In the assessment of monthly groundwater stress, this is exacerbated by the use of the calculated annual groundwater supply. While detailed monthly recharge estimates are available from PRMS for the study area in the calculation of Q Supply for use in stress assessment, the Technical Rules and provincial guidance, for simplicity and provincial consistency, stipulates that annual supply for groundwater is to be divided by 12 in the calculation of stress. Scientists must be mindful of these uncertainties in the calculation of stress and the intent of these calculations under the CWA when using these results for other program needs. Accurate water taking data is thus critical in the determination of stress and it is essential that decision makers are aware of these potential inaccuracies. While provincial methodologies are satisfactory for the intended purpose of determining risk to municipal drinking water supplies, these uncertainties in the water use data must be recognized and communicated to the decision makers. Recent legislation (O. Reg. 384/04) under the Ontario Water Resources Act (MOE, 1990) requires the submission of actual extraction rates. It is anticipated that as the provincial database is populated, demand estimates will improve. Scientists are also encouraged to liaise with provincial PTTW staff for additional information regarding permits particularly in watersheds where significant and moderate stress has been determined, but these data were not available at the time that the TRCA and CLOCA Tier 1 and Tier 2 Water Budgets were completed. Permit conditions should be used to adjust seasonal demand estimates. It is also suggested that where available modelled monthly supply estimates be used for ground water stress assessment similar to surface water calculations for a more accurate assessment of stress. 4.2 Surface Water Flow Estimates in Ungauged Watersheds As discussed earlier, the TRCA and CLOCA coupled two numerical models in an iterative fashion to solve for water budget components. The primary target for calibration was to match annual average total flows with observed annual long-term surface water flow gauges maintained by Environment Canada. Simulated monthly and daily flows were checked against observed values. A secondary target for model calibration was to match annual average simulated baseflow to estimated baseflow at these Environment Canada gauging stations given that they have long periods of record. Ungauged catchments were interpolated using the results of gauged watersheds correcting for watershed size and acknowledging similar settings. Groundwater/surface water interactions are extremely important in the northern portions of the TRCA and CLOCA watersheds on the Oak Ridges Moraine. This is where the river networks are primarily first and second order streams, and where most of the baseflow originates for the watercourses. Gauging stations, however, are limited in these headwater areas (See Figure 3). Long term stream gauge data are critical to the calibration of the models. However, several stations were discontinued in the 1980s. The source protection studies concluded the need for additional stream gauging stations for future refinement of the water budget estimates. Additional gauging stations are recommended in the headwaters streams of the study area (Etobicoke, Lynde, Oshawa creeks) as well as in in the ungauged watersheds such as Farewell and Black Creeks, These additional stations would assist in a more robust calibration of the model and serve to refine estimates of lateral flow between watersheds 4.3 Groundwater Cross-Boundary Flow and the calculation of Q Supply for Stress Assessment Hydrologic models such a PRMS can provide a reasonable representation of the physical processes in the shallow soil zones but only offer a simplified representation of the process of groundwater flow. This limitation can be important in areas where watersheds gain or lose significant amounts of water across external boundaries or where there is significant cross watershed flow such as in the northeast of the CLOCA jurisdiction in the study area (See Figure 4). These transfers must be accounted for in the calibration of the models before an accurate representation of water supplies in each watershed can be completed. MODFLOW was used in this study to solve for Q Supply including groundwater lateral in-flow from outside of the jurisdictions as well as cross-watershed flow. In the TRCA and CLOCA jurisdictions, lateral inflow and outflow are important considerations. Analyses revealed for example, significant flow from the northeast into the Bowmanville and Soper watersheds (see Figure 4). As the groundwater model extends beyond the CLOCA jurisdictional boundary, it was useful in understanding and reconciliation of stream gauge observed streamflow data. A similar situation was found in the Rouge River watershed in TRCA that receives groundwater from north of the ORM surface water divide. This explained the higher than expected stream flow as compared to other watersheds in the study area where soil and climactic conditions were similar. A simple GIS water budgeting exercise using the watershed boundary would suggest unusually high infiltration rates in the sandy soils within the watershed. By using PRMS to simulate recharge based on soil and land-use within the watershed boundary and MODFLOW with boundaries beyond the watershed, cross boundary flow could be estimated and the gauge information reconciled. Cross-watershed flow in and out of the study area watersheds is significant in the study area and an important factor in calibration of the models and in calculating the Q Supply that is used in the assessment of stress in a watershed. It is an important factor in calculating Q Supply and stress. Initially, TRCA and CLOCA accounted for this cross flow by calculating a Q Net term for each watershed for

6 use in the assessment of stress. Q Supply was solved as follows: Figure 3. Location of stream gauge stations in TRCA/CLOCA Q Supply = Q Recharge + Q Net (Q inflow -Q outflow ) Provincial guidance however required a revision to disregard the Q outflow component. In the Technical Rules, Q Supply is equal to the total of Q Recharge plus Q inflow, and the thresholds were adjusted to match the calculations in the Technical Rules. It is important to note that some models may account for outward cross boundary flux intrinsically in the model code (finite element) while others such as MODFLOW treat this flux in separate accounting. Scientists should be aware of this potential inconsistency as stress may not be equitably assessed across the province depending on the model code and the resulting calculation of the Q Supply term. It should be noted that while Q Net offers a more realistic estimate of Q Supply on an annual basis, during summer months some of the study area catchments exhibit a net loss of groundwater (negative Q Net ). Infiltration during high recharge months is stored within aquifers but continues to flow laterally out of the catchment during the summer months when Q Recharge is low, resulting in a negative Q Supply, This situation may be exacerbated further with a consideration of estimates of Q Reserve and Q Demand. Current provincial guidance does not consider a negative Q Supply scenario as this scenario would result in stress in practically every watershed in the study area (except Bowmanville Creek). There is little benefit or insight gained from assigning all catchments a significant monthly stress. Therefore, a new approach is needed for assessing seasonal water supply. The authors recommend that the monthly model output should be used instead of the annual amount divided by 12. This revised approach may reduce the number of stressed subwatersheds and provide more meaningful results. 5 CONCLUSIONS The Clean Water Act and the Provincial Source Protection initiative has served to raise the bar on the minimum level of analysis required for the assessment of water quantity in Ontario. Minimum standards are outlined in the Technical Rules and even as these analyses are intended to screen for risk to municipal drinking water supplies, these analyses are useful to a number of other purposes including the assessment of ecosystem health, the development of watershed plans, the review of development applications and water taking permits and the modelling of land-use scenarios. For good use of the developed numerical models and the calculated water budgets results, analysts must, however be cognizant of the uncertainties and assumptions associated with the models and the calculation. Understanding the model limitations especially in areas with limited stream gauge data for calibration is important. A comprehensive understanding of the system including lateral inflows and outflows as well as a careful consideration of water

7 taking and use data in the assessment of stress is critical. Figure 4: Reverse particle tracking (1L/s) completed in the CLOCA jurisdiction 6 ACKNOWLEDGEMENTS The writers would like to acknowledge the modelling and analysis completed by Earthfx Inc., and the financial support from the Province of Ontario to undertake the water budget analyses. 7 REFERENCES Earthfx, 2009, Tier 1 Water Budget, Study of the Watersheds in the Central Lake Ontario Conservation Area. Harbaugh, A.W., 1990, A computer program for calculating subregional water budgets using results from the U.S. Geological Survey modular three-dimensional ground-water flow model. U.S. Geological Survey Open-File Report , 46 p. Harbaugh, A.W. and M.G. McDonald User s documentation for MODFLOW-96, an update to the U.S. Geological Survey modular finitedifference ground-water flow model. U.S. Geological Survey Open-File Report , 56p Kassenaar J.D.C. and E.J. Wexler, Groundwater Modelling of the Oak Ridges Moraine Area. CAMC-YPDT Technical Report Nº Leavesley GH, Lichty RW, Troutman BM, Saindon LG Precipitation-runoff modelling system user s manual. U.S. Geological Survey Water Resources Investigation Report ; 207 pp. McDonald and Harbaugh, A Modular Three- Dimensional Finite Difference Ground-Water Flow Model. Techniques of Water Resource Investigations of the US Geological Survey, USA. Ontario Ministry of the Environment (MOE), 1990: Ontario Water Resources Act. Ontario Ministry of the Environment (MOE), 2006: Clean Water Act Ontario Ministry of the Environment (MOE), 2009: Technical Rules: Assessment Report, Clean Water Act, Issued by the MOE December, Toronto and Region Conservation, Tier 1 Water Budget, TRSPA Watersheds, Tier 2 Water Budget, Little Rouge and Stouffville/Reesor Creeks.