Constructed wetland treatment of streams flowing into Lakes Rotoehu and Okaro Preliminary assessment.

Transcription

1 Constructed wetland treatment of streams flowing into Lakes Rotoehu and Okaro Preliminary assessment. NIWA Client Report: HAM April 2003 NIWA Project: BOP03210

2 Constructed wetland treatment of streams flowing into Lakes Rotoehu and Okaro Preliminary assessment. Chris C Tanner Prepared for Environment B O P NIWA Client Report: HAM April 2003 NIWA Project: BOP03210 National Institute of Water & Atmospheric Research Ltd Gate 10, Silverdale Road, Hamilton P O Box 11115, Hamilton, New Zealand Phone , Fax All rights reserved. This publication may not be reproduced or copied in any form without the permission of the client. Such permission is to be given only in accordance with the terms of the client's contract with NIWA. This copyright extends to all forms of copying and any storage of material in any kind of information retrieval system.

3 Contents Executive Summary iv 1. Introduction 1 2. Study sites 1 3. Assessing treatment performance 2 4. Predicted performance 4 5. General wetland treatment principles 5 6. Recommendations 8 7. References 9 Reviewed by: Approved for release by: David Ray Dr Robert Davies-Colley Formatting checked

4 Executive Summary This report provides a preliminary assessment of the suitability for wetland treatment of outflows from three streams (with predominantly agricultural catchments) draining into Lakes Rotoehu and Okaro in the Rotorua Region. Nitrate-N and total phosphorus (TP) removal is calculated for a range of wetland areas, and general guidance on wetland design outlined and potential management issues identified. Under the baseflow and water quality conditions assumed, Te Pohue (L. Rotoehu) and Okaro (L. Okaro) Streams showed similar nitrate-n and TP exports of ~1900 kg N y -1 and 47 kg P y -1. In comparison, the Raukaumakere Stream (L. Rotoehu), with lower recorded nitrate-n and similar TP concentrations, but substantially higher flows, showed export rates of ~3,150 kg N y -1 and 158 kg P y -1. Readily useable wetland areas were restricted to ~0.25 ha at Te Pohue and estimated to be only capable of providing ~ 4-6 % nitrate-n removal and <2% TP removal (Table 2). Wetland areas of ~3 ha were predicted to be required to achieve 50% nitrate-n removal, and ~5.2 ha to achieve 70% removal, at this site during summer. Corresponding estimates for TP removal efficiency were lower at ~16 and 24%, respectively, for these wetland areas. Because of area constraints this stream appears unsuitable for significant wetland treatment. Larger readily usable lakeside land areas of ~3 ha at Raukaumakere and ~2 ha at Okaro are potentially available and are predicted to be able to achieve nitrate-n removal of and %, and TP removals of ~5 and 11 %, respectively. Wetland areas ~ 3-fold larger at Raukaumakere and 1.5-fold larger at Okaro would be required to achieve ~50 % removal of nitrate-n during summer. It is recommended that estimated wetland nutrient removal efficiencies be compared with nutrient budgets and reduction targets for the affected lakes to determine what level of performance is desired. Wetland areas capable of achieving these nutrient reductions can then be estimated, and available land areas and associated costs compared against expected benefits. Monitoring of stream flow variability and water quality will provide improved information for wetland design. Demonstration wetland systems would provide valuable information on the true costs and performance of these systems under local environmental conditions, and illustrate the concept to farmers, agricultural industries, interest groups and the general public. Natural and restored wetland areas around the lakes should be integrated into catchment management plans. Preliminary assessment for constructed wetland treatment of streams flowing into Lakes Rotoehu and Okaro iv

5 1. Introduction The Environment B O P Water and Land Regional Plan identifies five Rotorua Lakes that require remediation. Environment B O P plans to produce action plans that outline potential remedial approaches, and are interested in identifying and assessing practical means that could be used to reduce diffuse nutrient inputs to these lakes. Constructed and restored wetlands are one of the key options available to intercept and remove nutrients entering the lakes via drains, streams and lakeside seeps. Environment B O P asked NIWA to: 1. Undertake a preliminary assessment of the feasibility and expected nutrient removal performance of constructed wetlands, using selected streams entering Lakes Rotoehu and Okaro as practical examples; and 2. Outline principles that should be considered to adapt retirement areas for nutrient stripping by constructed wetlands. This report provides a preliminary assessment of the suitability for wetland treatment of three sites associated with Lakes Rotoehu and Okaro. Based on the available stream monitoring data for these sites, nitrate-n was selected as the main focus of investigation. Nitrate-N removals possible using relatively small wetlands that would fit readily into the existing landscape are assessed, and an estimate made of wetland sizes required to achieve 50 and 70% reductions in nitrate-n load under summer conditions. Total phosphorus (TP) removal is also assessed for the same sized wetlands. General guidance on wetland treatment for nitrate removal is outlined, and potential effects and management issues identified. 2. Study sites The three sites proposed by Environment B O P were visually assessed in association with John McIntosh (Manager, Environmental Investigations) on 12 November 2002, and potential wetland sites identified (Figures 1 3). All of the streams drained predominantly agricultural catchments and had relatively consistent base-flows in the range of L s -1. Available monitoring data showed dissolved N in the form of nitrate to be the key nutrient of concern in these streams. The limited flow and water quality data available was used to estimate generalised mass loadings of nitrate-n and TP from these streams and predict potential removal using surface-flow constructed wetlands of different size (Tables 1 and 2). Preliminary assessment for constructed wetland treatment of streams flowing into Lakes Rotoehu and Okaro 1

6 Figure 1: Approximate path of Te Pohue Stream (in dark blue) draining into Te Pohue Bay on the southwestern side of Lake Rotoehu. The area considered readily usable for wetland creation within the existing landscape is delineated in green. 3. Assessing treatment performance Wetland treatment performance was assessed using a first-order, tanks-in-series kinetic approach (Kadlec (unpublished manuscript, in prep.), Kadlec & Knight 1996, Reddy et al. 1999), represented by: C C o i = 1 + k Nq N where: C i = inlet concentration (g m -3 ) C o = outlet concentration (g m -3 ) k = temperature dependant first order removal rate constant (m y -1 ) N = hydraulic efficiency parameter q = hydraulic loading (m y -1 ) Preliminary assessment for constructed wetland treatment of streams flowing into Lakes Rotoehu and Okaro 2

7 Mean k rates and modified Arrhenius temperature coefficients for nitrate-n removal were derived from a comprehensive recent review of available international data (65 systems; Kadlec; unpublished manuscript, in prep.), and for TP from Kadlec & Knight (1996) and Kadlec (1999). Seasonal variations in Nitrate-N removal for the sites were predicted assuming mean water temperatures of 15 C for summer and 10 C for winter. Flow into the wetlands was assumed to be constant at the rates specified in Table 1. The hydraulic efficiency of the wetlands was assumed to be equivalent to that of well-designed and vegetated surface-flow constructed wetlands with approximate length to width ratios between 4:1 and 6:1. It should, however, be noted that different wetland systems show a range of performance depending on their specific flow and loading regime, design, age, vegetation type and cover, and local climate and site conditions. (Kadlec 1999, Kadlec; unpublished manuscript, in prep.). Figure 2: Approximate path of Rakaumakere Stream and associated surface drains (in dark blue) draining to west of Rakaumakere Bay on the mid-southern side of Lake Rotoehu. The area considered readily usable for wetland creation within the existing landscape is delineated in green. Preliminary assessment for constructed wetland treatment of streams flowing into Lakes Rotoehu and Okaro 3

8 Figure 3: Approximate path of streams (in dark blue) draining into the northwestern side of Lake Okaro. The area considered readily usable for wetland creation within the existing landscape is delineated in green. 4. Predicted performance Under the baseflow and water quality conditions assumed, Te Pohue and Okaro Streams showed similar nitrate-n and TP exports of ~1900 kg N y -1 and 47 kg P y -1. In comparison, the Raukaumakere Stream, with lower recorded nitrate-n and similar TP concentrations, but substantially higher flows, showed export rates of ~3,150 kg N y -1 and 158 kg P y -1. Readily useable wetland areas were restricted to ~0.25 ha at Te Pohue (see Fig. 1) and estimated to be only capable of providing ~ 4-6 % nitrate-n removal (Table 1) and <2% TP removal (Table 2). Wetland areas of ~3 ha were predicted to be required to achieve 50% nitrate-n removal, and ~5.2 ha to achieve 70% removal, at this site during summer. Corresponding estimates for TP removal efficiency were lower at ~16 Preliminary assessment for constructed wetland treatment of streams flowing into Lakes Rotoehu and Okaro 4

9 and 24%, respectively, for these wetland areas. The use of areas nearer the lake is not recommended because trout use these areas for spawning. The deeply incised nature of the upstream channel would also constrain wetland establishment above the potential area identified. Larger readily usable lakeside land areas of ~3 ha at Raukaumakere and ~2 ha at Okaro were predicted to be able to achieve nitrate-n removal of and % (Table 1), and TP removals of ~5 and 11 % (Table 2), respectively. Wetland areas around 3-fold larger at Raukaumakere and 1.5-fold larger at Okaro would be required to achieve ~50 % removal from these inflows during summer. Corresponding TP removal was ~16 % for such wetland areas. Additional areas suitable for wetland creation may be available in these areas. 5. General wetland treatment principles The use of constructed wetlands has become increasingly common for treatment of a wide range of wastewaters and stormwaters. Design information on wetland performance under relatively constant flows (e.g., wastewaters), and for removal of particulate-associated contaminants (e.g., urban stormwaters) is now reasonably advanced, but performance treating soluble nutrients and contaminants associated with very fine particulates is less well known under the variable flow condition generally associated with agricultural run-off and drainage (Tanner & Nguyen 2003). Surfaceflow wetlands vegetated with emergent macrophytes, can provide effective nitrate-n removal via denitrification and, to a lesser extent, plant uptake and accretion in sediments (Kadlec (unpublished manuscript, in prep.), Mitsch et al. 2000). Key regulators of denitrification rates include temperature, organic carbon availability, alkalinity and contact time. Wetlands can generally provide only low level P removal. Particulate P removal occurs predominantly by settling, which is promoted in quiescent conditions such as occur in vegetated zones. Soluble P removal occurs via reversible soil sorption (which eventually becomes saturated) and uptake by bacteria, algae and macrophytes. Cycling through growth, death and decomposition returns much of the biotic uptake, but an important residual contributes to long-term accretion of P in newly formed sediments and soils (Kadlec 1997, Reddy et al. 1999). P removal may also be promoted by the use of P-sorbing media, including iron and calcium-rich materials (Nguyen et al. 1998, Sakedevan & Bavor 1998), but such materials generally have a finite life, after which they must be replaced. Preliminary assessment for constructed wetland treatment of streams flowing into Lakes Rotoehu and Okaro 5

10 From a practical point of view, optimal wetland treatment conditions for both N and P removal are created through provision of wetland areas, depths and length to width ratios that provide sufficient wetland assimilative area, efficient hydraulic characteristics and conditions suitable for establishment of dense growths of desirable vegetation. For systems constructed to treat stream flows, provision must also be made for management of storm and low flows, siltation, and fish passage. Wetlands built off-stream (Fig 3) have significant advantages in this respect, because the original stream channel remains intact. However, this is not always practically achievable, requiring provision for routing of flood-flows around (or through a armoured floodway within) the wetland. Wetlands receiving flood flows may require more frequent maintenance and specific rehabilitation after large flood events. Figure 4: Comparison of off-stream (in parallel) and on-stream (in-channel) treatment wetlands (from Bendoricchio et al. 2000). Preliminary assessment for constructed wetland treatment of streams flowing into Lakes Rotoehu and Okaro 6

11 It is outside the scope of this report to provide detailed design advice, but general principles are listed briefly below: Hydrology and hydraulics are crucial to wetland treatment performance and sustainable functioning. Flow must be dispersed across the wetland crosssection, minimising short-circuiting and preferential flow which markedly reduce performance. This is best achieved through use of inflow dispersal pipes, even vegetative cover and careful attention to hydraulic design. As noted above, provision must also be made to protect the wetland from flood flows which could cause scouring and/or sedimentation resulting in channelisation and damage to vegetation. Wetland area should be sufficient to receive and sustainably process the contaminant loads. In general wetland flow velocities should be kept low (<0.1 m s -1 ), and residence times at least one day to achieve reasonable removal of dissolved and fine particulate contaminants. Water depths should be to maintain good growths of emergent wetland plants under sustained flooding. Open water zones will generally provide poorer nitrate removal performance per unit area than vegetated zones. Deeper, open-water zones are useful in the inlet zones of wetlands for removal and retention of coarse sediment loads. Provision should be made for periodic mechanical silt removal from these areas. Open water areas in the wetland can also improve wildlife habitat values, although this may compromise performance in terms of water quality and microbiological safety. Nitrate removal via denitrification is promoted by close contact with organic sediments and wetland plants that provide anoxic conditions and organic matter (decomposing plant litter) for denitrifiers. Such conditions may also be created or supplemented through the addition of organic amendments such as cereal straws or wood chips/sawdust. Nitrate removal is temperature sensitive, and will generally be poorer during winter than summer. To facilitate any required maintenance or vegetation management provision should be made to enable temporary drainage of the wetlands and diversion of the inflow. Treatment wetlands are low maintenance, not no maintenance. Basic problems need to be identified and remedied before they multiply. Preliminary assessment for constructed wetland treatment of streams flowing into Lakes Rotoehu and Okaro 7

12 6. Recommendations The estimated wetland nutrient removal efficiencies outlined in this report should be compared with nutrient budgets and reduction targets for the affected lakes to determine what level of performance is desired. Wetland areas required to achieve or contribute to these nutrient reductions can then be estimated, and available land areas and associated costs compared against expected benefits. Wetland treatment performance can vary for different situations, and treatment performance could be up to 20-30% better (or worse) than predicted here. Suitably designed and constructed systems should be able to provide treatment performance at least as good as estimated in the present study. Reliable information on stream flows and variability (in particular) and water quality greatly assists in development of appropriate wetland designs and achievement of optimum performance. Flow and water quality monitoring programmes should be developed for these and other proposed sites in the region. Use of natural or rehabilitated wetland areas may provide the most cost effective and appropriate solutions at many sites around these lakes. Existing, degraded and drained wetland areas around the lakes should be delineated and where possible incorporated into catchment management plans. Demonstration wetland systems at 2-3 sites would provide valuable information on the true costs and performance of treatment wetland systems under local environmental conditions, and illustrate the concept to farmers, agricultural industries, interest groups and the general public. Monitoring of performance and practical management costs of such treatment wetlands should continue for at least 3-5 years to quantify performance and management costs during system maturation. Preliminary assessment for constructed wetland treatment of streams flowing into Lakes Rotoehu and Okaro 8

13 7. References Bendoricchio, G.; Cin, L.D.; Persson, J. (2000). Guidelines for free water surface wetland design. EcoSys Bd. 8: Kadlec, R.H. (1997). An autobiotic wetland phosphorus model. Ecological Engineering 8: Kadlec, R.H. (1999). The limits of phosphorus removal in wetlands. Wetlands Ecology and Management 7: Kadlec, R.H. (unpublished manuscript, in prep.). Nitrogen farming for pollution control. 26 p. Kadlec, R.H.; Knight, R.L. (1996). Treatment wetlands. CRC Press, Boca Raton, FL. Mitsch, W.J.; Horne, A.J.; Nairn, R.W. (eds). (2000). Nitrogen and phosphorus retention in wetlands. Special issue of Ecological Engineering 14: Nguyen, M.L.; Monaghan, R.; Tanner, C.C. (1998). Zeolites and constructed wetlands: possible technologies for nutrient recapture and drainage pollution control. In: Currie, L.D.; Loganathan, P. (eds). Presented at a workshop on Longterm nutrient needs for New Zealand's primary industries, Palmerston North, NZ, February. Reddy, K.R.; Kadlec, R.H.; Flaig, E.; Gale, P.M. (1999). Phosphorus retention in streams and wetlands: a review. Critical Reviews in Environmental Science and Technology 29: pp Sakedevan, K.; Bavor, H.J. (1998). Phosphorus adsorption characteristics of soils and zeolite to be used as substrates in constructed wetland systems. Water Research 32: Tanner, C.C.; Nguyen, M.L. (2001). Intercepting the riparian short-circuit: constructed wetland treatment of agricultural drainage. Proceedings of the International Ecological Engineering Conference, Lincoln University, NZ, November. Preliminary assessment for constructed wetland treatment of streams flowing into Lakes Rotoehu and Okaro 9

14 Table 1: Estimated summer and winter wetland nitrate-n loadings and removal for selected streams draining into lakes Rotoehu and Okaro. Season 1 Wetland area (ha) Stream Flow (L s -1 ) Wetland inflow (m 3 d -1 ) Wetland nitrate-n mass loading (g m -2 y -1 ) Wetland nitrate-n mass removal (g m -2 y -1 ) Assumed influent nitrate-n concentration (g m -3 ) Estimated outlet nitrate-n concentration (g m -3 ) Te Pohue Stream, Lake Rotoehu Summer * , % Summer , % Summer , % Winter * , % Winter , % Winter , % Raukaumakere Stream, Lake Rotoehu Summer * , % Summer , % Summer , % Winter * , % Winter , % Winter , % Northern Okaro Stream, Lake Okaro Summer *2 30 2, % Summer , % Summer , % Winter *2 30 2, % Winter , % Winter , % * Estimated readily available area; further areas given are those estimated to provide 50 and 70% nitrate removal during summer. 1 Assumed seasonal temperatures; Summer 15 C; Winter 10 C. Estimated percentage reduction Preliminary assessment for constructed wetland treatment of streams flowing into Lakes Rotoehu and Okaro 10

15 Table 2: Estimated annual wetland total phosphorus loadings and removal for selected streams draining into lakes Rotoehu and Okaro. Wetland area (ha) Stream flow (L s -1 ) Wetland inflow (m 3 d -1 ) Te Pohue Stream, Lake Rotoehu Wetland TP mass loading (g m -2 y -1 ) Wetland TP mass removal (g m -2 y -1 ) Assumed influent TP concentration (g m -3 ) Estimated outlet TP concentration (g m -3 ) * , % , % , % Raukaumakere Stream, Lake Rotoehu * , % , % , % Northern Okaro Stream, Lake Okaro *2 30 2, % , % , % * Estimated readily available area; further areas given are those estimated to provide 50 and 70% nitrate removal during summer (see Table 1). Estimated percentage reduction Preliminary assessment for constructed wetland treatment of streams flowing into Lakes Rotoehu and Okaro 11