WATER BALANCE ASSESSMENTS WITHIN THE YORK REGION WHPA-Q2

Transcription

1 CTC Source Protection Plan Water Balance Requirements 1 WATER BALANCE ASSESSMENTS WITHIN THE YORK REGION WHPA-Q2 Regulatory Authority under the Clean Water Act, 2006 The approved Source Protection Plan (SPP) developed by the CTC Source Protection Region under the Clean Water Act took effect December 31, One of the policies in the SPP, Policy REC-1, specifies the following requirements: 1. For applications under the Planning Act within the Tier 3 Water Budget WHPA-Q2 identified as having significant water quantity threats, the relevant Planning Approval Authority shall ensure recharge reduction does not become a significant drinking water threat by: a) Requiring new development for lands zoned Low Density Residential (excluding subdivisions) or zoned Agricultural to implement best management practices such as Low Impact Development (LID) with the goal to maintain predevelopment recharge. 2. Requiring that all site plan (excluding an application for one single family dwelling) and subdivision applications for new residential, commercial, industrial and institutional uses provide a water balance assessment for the proposed development to the satisfaction of the Planning Approval Authority which addresses each of the following requirements: a) maintain pre-development recharge to the greatest extent feasible through best management practices such as LID, minimizing impervious surfaces, and lot level infiltration; b) where pre-development recharge cannot be maintained on site, implement and maximize off-site recharge enhancement (within the same WHPA-Q2) to compensate for any predicted loss of recharge from the development TRCA has been asked to implement Policy REC-1 Parts 2 (a) and (b) on behalf of the Planning Approval Authority (lower tier municipalities). This policy, in effect, requires applicants to complete water balance assessments for future activities proposed under the Planning Act where applications were submitted for approval after December 31, It also requires that proponents demonstrate that their projects will maintain the estimated pre-development groundwater recharge. Off-site compensation may be considered for those sites where pre-development recharge cannot be maintained. Policy Objectives WHPA-Q2: A type of Well Head Protection Area (WHPA) comprising the land around a municipal water well where changes in recharge could affect the quantity of water extractable from the well. CTC: One of the 19 Source Protection Regions and Areas in Ontario, comprising the jurisdictions of Credit Valley Conservation, Toronto and Region Conservation and Central Lake Ontario Conservation. The overall objective of the water balance policies under the Clean Water Act, 2006 is to ensure that the municipal drinking water systems are sustainable into the future. The recharge policies, in particular, ensure that the recharge that supplies the municipal aquifer systems is maintained. Efforts to manage water balance may require the incorporation of infrastructure as part of new development.

2 2 WHPA-Q Water Balance Requirements In contrast with surface water quantity control approaches (i.e., 5 mm on-site retention) that focus on event-based simulations, water balance analyses under Policy REC-1 consider total annual precipitation and recharge. However, measures that manage total precipitation can also reduce the extent of flood control infrastructure required to manage major events (i.e., 100 year storms). WHPA-Q1/WHPA-Q2 A Well Head Protection Area (WHPA) is the surface and underground area surrounding a water well through which contaminants are reasonably likely to move so as to eventually reach the water well or wells. In particular, the WHPA-Q1 (area affected by groundwater use) and WHPA-Q2 (area affected by recharge reduction) for the municipal wells in York Region were delineated through the application of a numerical groundwater flow model that considered all known high volume groundwater users. The simulated aquifer water levels with groundwater extraction were subtracted from the simulated aquifer water levels without pumping. This resulted in a drawdown map, which was then clipped to remove any theoretical drawdowns less than 0.5 m (assumed to be within seasonal aquifer water level variability). For York Region, this WHPA-Q1 area extends from Richmond Hill/Markham in the south to north of Queensville in the north and from Maple in the west to beyond Uxbridge to the east. To map the extent of the WHPA-Q2, the same numeric model was used to simulate aquifer water levels with and without future urban development inside and outside of the WHPA-Q1 boundary. In the case of York Region, no effects were predicted from development outside of the WHPA-Q1, so the WHPA-Q1 and Q2 happen to be the same. Figure 1 shows the location and extent of the WHPA-Q2 within which Policy REC-1 applies. Figure 1: Location and Extent of the WHPA-Q2 for the TRCA Jurisdiction March 2016

3 CTC Source Protection Plan Water Balance Requirements 3 Water Balance Defined In simple terms, precipitation that lands on the ground surface is distributed to the natural environment through three pathways. Some of the water infiltrates the ground (infiltration); some runs off the surface (runoff); and much of the remainder either evaporates or is consumed by plants (evapotranspiration). These processes are shown graphically on Figure 2. The distribution of water among these pathways is referred to as the water balance. In natural settings, most of the precipitation follows the infiltration and evapotranspiration pathways, which leaves a relatively small portion that becomes runoff. In built communities, the introduction of hard surfaces and the reduction in vegetated cover routes more water to the runoff pathway and less to evapotranspiration and infiltration. The resulting imbalance causes flooding and erosion on the land surface and lower groundwater levels that support our streams, wetlands, and groundwater resources. Figure 2: Water Balance Components Graphic courtesy of Conservation Ontario Water Balance Primer The basic water balance for a region can be expressed as: Where, P = RO + ET + RE + ΔS (Thornthwaite and Mather, 1957) P = Precipitation (rain and snow) RO = Runoff ET = Evapotranspiration RE = Recharge ΔS = Change in Storage (assumed to be zero under steady state conditions)

4 4 WHPA-Q Water Balance Requirements Total precipitation and runoff can be easily measured, but evapotranspiration depends on many variables and is usually estimated by means of empirical equations or through numerical modelling. The purpose of the long term or steady-state water balance is to estimate some of these quantities via a method of accounting for each component of the budget over a long term in order to arrive at mean values for each of the components. Modeling undertaken by TRCA and York Region has yielded a consistent understanding and mapping of water balance parameters throughout TRCA s jurisdiction as well as northwards into the jurisdictions of Nottawasaga Valley Conservation Authority (NVCA) and Lake Simcoe and Region Conservation Authority. These calculated distributions of recharge, precipitation, evapotranspiration, and runoff are based on numerical modelling that was subject to extensive peer review and acceptance by the Ministry of Natural Resources and Forestry (MNRF) and the Ministry of the Environment and Climate Change (MOECC). Water Balance Criteria The criteria described below summarize TRCA s requirements with respect to estimation of water balance parameters and calculation of overall water balances for development sites. It is important to note that addressing these criteria may in part also address the requirements associated with TRCA s policies for erosion control and to a lesser extent the requirements associated with water quantity and quality control. While these Stormwater Management Criteria are required to be met through TRCA s Living City Policies, the requirements under the CTC Source Protection Plan are separate and distinct in that they only apply to recharge within the WHPA-Q2. Water Balance Analysis Methodology Completing a water balance requires the expertise of a multi-disciplinary team of qualified professionals, typically including a terrestrial or aquatic ecologist, water resources engineer, and hydrogeologist. The input and analysis of appropriate professionals need to be integrated into the final water balance submission. To assist with site-specific water balance assessments, TRCA has released maps of precipitation, evapotranspiration (actual), runoff, and recharge based on outputs from the numerical models developed under the Drinking Water Source Protection Program. This mapping is provided in Maps 1 to 5 in Attachment A, and is available to consultants in electronic form upon request. Note that these values are based on a regional assessment of the water balance, and may be supplanted by site-specific data or modelling. Another option for proponents is to use the Ministry of the Environment (MOE) hydrologic cycle component values for southern Ontario basins as found in Table 3.1 on page 3-4 of the Stormwater Management Planning and Design Manual (MOE, 2003). Note that the MOE Manual can be used to estimate an infiltration factor for the site based on topography, soils and cover factors, but precipitation and evapotranspiration should be site-specific. Land owners should note that when development is considered within the catchments of natural features containing species at risk (SAR), proponents are responsible for meeting the requirements of the relevant federal and provincial agencies. For example, the Ministry of Natural Resources and March 2016

5 CTC Source Protection Plan Water Balance Requirements 5 Forestry is the regulatory agency responsible for SAR under the Endangered Species Act and may have requirements that are beyond the details described within these guidelines (e.g. water quality). If SAR occur on or near the property, the appropriate agencies must be consulted; this includes monitoring activities. Precipitation Precipitation is the easiest water balance component to measure. Liquid (rainfall) and solid (snowfall) precipitation are monitored across the TRCA watersheds on a continuous basis. Average annual precipitation across TRCA watersheds is approximately 830 mm/year ( Precipitation data have a multitude of uses for TRCA, our stakeholders, and our clients. These data allow for the development of design storms, storm event reporting and complex hydrological modelling. Environment Canada operates a number of additional stations (i.e., Pearson Airport, Buttonville Airport) and our municipal partners have their own networks. Consultants must use appropriate station data to complete the water balance analysis based on the location of the proposed site, since precipitation varies across our jurisdiction (see Maps 1 and 2 in Attachment A). Evapotranspiration Evapotranspiration includes the water lost to the atmosphere from both evaporation of water directly from soil and water surfaces and transpiration from vegetation. It represents a significant portion of the water balance, but is challenging to either measure or model, and exhibits high spatial variability. In their literature review of evapotranspiration studies, Bella and So (2009) summarize the issues with this important, but poorly understood parameter: Despite its dominant role in the Earth's water balance, evaporation is poorly understood. This is because the meteorological measurement of evaporation does not form a component of routine monitoring programs anywhere. Due to the highly technical equipment and sophisticated data analysis required, the measurement of actual evaporation has remained in the realm of the specialist. Consequently data availability is restricted to specific habitats and isolated time periods of experimental studies. This has created an unacceptable level of uncertainty surrounding our future water resources owing to both the absence of long-term time series and data which could provide evaporation model parameterization or model validation. The simplest application of the water balance for measuring evaporation is the Class A evaporation pan (Bella and So, 2009). This tool comprises an elevated stainless steel tank filled with water. The change in water level provides an estimate of the rate of evaporation. Although this tool provides a reasonable estimation for evaporation from standing water, it does not account for water lost through transpiration from plants. Complex equipment and software exist for more comprehensive estimates of evapotranspiration (TRCA, 2014), but given the cost of establishing an evapotranspiration monitoring station, proponents will not be expected to provide site-specific data.

6 6 WHPA-Q Water Balance Requirements Numerical modelling can also be used to estimate evapotranspiration (Hwang et al., 2015), but this is not practical at the scale of most development projects. Therefore, for the purposes of implementing the REC-1 policy, TRCA will accept the use of our regional model output as shown in Map 3 in Attachment A (TRCA, 2010) for estimation of evapotranspiration at the site scale. Digital data in GIS format will be provided to proponents or their agents upon request. Alternatively, consultants may use their own site-specific numeric model output, if available. Runoff Runoff is the portion of precipitation that neither returns directly to the atmosphere via evapotranspiration nor infiltrates into the ground (recharge). This water flows over the land surface as either sheet flow or within a channel as a watercourse. Under most normal rainfall events, runoff is not an issue; it becomes a challenge only with fast flowing water at high water levels from major storm events. High water levels and flows can cause damage to private and public property, as well as public infrastructure (photograph at right). Maintaining the predevelopment water budget reduces the risk of erosion downstream, but its contribution in reduction of the risk of flooding is not significant. Average estimates of runoff in southern Ontario based on land cover and soil type have been developed by the MOE (2003), and TRCA has estimated runoff rates across our jurisdiction (Map 4 in Attachment 1) but for complex sites, hydrologic models may also be used. Groundwater Recharge Groundwater recharge is a term that is widely used to describe the replenishment of the groundwater system from precipitation. Urbanization and land use change introduce hard surfaces to the landscape that reduce the degree to which precipitation enters the groundwater system. The groundwater system is the primary source of baseflow for many of our watercourses, and the nature of this source water yields the conditions necessary to support many sensitive ecosystems. The sensitivity of watersheds to changes in the groundwater regime has been established as part of the subwatershed studies completed throughout TRCA s jurisdiction. Within this document, the terms recharge and infiltration are used interchangeably, with both generally referring to the entry of surface water into the subsurface. With respect to water balance analyses and the design of related infrastructure, emphasis is placed on promoting infiltration as a means to mimicking the natural water cycle. The physical properties of the landscape that determine the proportions of precipitation that partition into recharge/infiltration, evapotranspiration, and runoff include soil permeability, soil moisture, depth to groundwater table, slope, and type of vegetation. In areas where soils have high permeability and relatively flat topography, infiltration rates are higher than run-off. The highest infiltration occurs in areas of hummocky topography with internally drained areas (i.e. no surface water outlet). In contrast, for areas with steep slopes and less permeable soils, run-off usually exceeds infiltration. Other factors which influence infiltration rates include the amount of available water, intensity of precipitation, air temperature, and sunshine duration. March 2016

7 CTC Source Protection Plan Water Balance Requirements 7 One of the challenges in any development project is the determination of the pre-development infiltration rate. Most modeling studies provide detailed analysis of precipitation and runoff, and allocate evapotranspiration based on published literature. Infiltration is then estimated by subtraction. What is known is that infiltration rates vary considerably across TRCA s jurisdiction (Map 5 in Attachment A). Consultant studies (Kassenaar and Wexler, 2006, TRCA, 2015) have estimated infiltration rates as high as 400 mm per annum in the northern portions of TRCA s watersheds, where the topography is hummocky, and the surficial soils comprise permeable sand and gravel. However, areas on the south slope of the Oak Ridges Moraine covered by Halton Till may have infiltration rates below 50 mm per year. Water Balance Mitigation Measures Water balance can be achieved by a number of the SWM practices listed in the MOE Stormwater Management Planning and Design Manual (MOE, 2003) and the CVC/TRCA Low Impact Development Stormwater Water Management Planning and Design Guide (TRCA and CVC, 2010). However, the principal and most effective mechanisms for achieving water balance fall under the category of low impact development. Low impact development practices are generally designed to manage rainfall and the resulting runoff at the source, which is more effective at reducing runoff volumes during frequent rainfall events than the larger centralized facilities known as End of Pipe solutions. TRCA staff recognizes that not all areas are suitable for recharge mitigation measures. Unsuitable conditions for recharge may include areas with: Slopes >20%; Bedrock within 1 metre of ground surface; Lands within 250 metres of a landfill site; and Wetlands and associated hydric soils. Although TRCA does not recommend engineered recharge facilities in areas listed above, the proponent must make every effort to maintain overall infiltration across the site based on the noted requirements. The vulnerability of an aquifer to contamination from surface sources must also be considered, and consultation with TRCA is necessary to determine whether a proposed development is in proximity to areas that may impact an aquifer. Infiltration of potentially contaminated water (i.e. parking lots, roadways) would not be promoted in these areas. Considerations for infiltration in these areas include the volume and toxicity of chemicals used or stored, livestock density, and contaminant management plans. Mitigation Measures A number of mitigation strategies have been developed, tested, and proven to reduce postdevelopment water balance imbalances. A short discussion of some alternatives is presented below and detailed descriptions are provided in the MOE Stormwater Management Planning and Design Manual (MOE, 2003) and the Low Impact Development Stormwater Management Planning and Design Guide (TRCA and CVC, 2010). Proponents can also obtain information from TRCA s Sustainable Technologies website at: Some of the potential measures include: Rooftop runoff to pervious areas;

8 8 WHPA-Q Water Balance Requirements Roof top runoff to infiltration trench(es); Green roofs; Permeable pavement; and Additional topsoil depth. Rooftop Runoff to Pervious Areas It was once common for rooftop downspouts to be connected to the sanitary or storm sewer systems. However, most municipalities require downspouts to be disconnected, with downspout drainage being directed to lawns. In theory, this strategy should increase infiltration, but depending on the soil type and degree of compaction during construction, infiltration may be minimal. The amount of roof water that reaches the storm sewer system is reduced, but homeowners have been known to regrade drainage swales to make lawn maintenance easier. Such regrading may reduce the effectiveness of this strategy and cause undesirable surface ponding during larger storm events. Rooftop Runoff to Infiltration Gallery This approach collects roof water and directs it to an engineered infiltration trench. The trench surface area will depend on the depth to the water table, the infiltration capacity of the soil and the drawdown time for the trench. Depths should be no more than 1.5 m deep as per MOE criteria (MOE, 2003) to avoid compaction of the soil medium under the trench and a potential reduction in the infiltration rate across the bottom of the trench. Green Roofs Rooftop gardens have been widely recognized as providing significant private and public benefits to urban environments. These benefits include improved stormwater control, better air quality, lower energy use, moderated summer air temperatures, increased biodiversity, and healthier, more beautiful cityscapes. Broader recognition and appreciation of these values has been one of the drivers behind significant growth of the greenroof industry in the Greater Toronto Area (GTA) over the past decade. Permeable Pavements Permeable pavements (PP) are one of several Low Impact Development practices that are being used to treat runoff and help increase infiltration in an effort to reproduce the pre-development hydrologic regime. Since PPs replace conventional asphalt, they are ideally suited to older built-up areas that lack stormwater management and have little to no space for conventional stormwater facilities. They can also be cost effective in new development areas where runoff reductions from infiltration can reduce or eliminate the need for sewer infrastructure beneath the pavement (TRCA, 2012). Additional Topsoil Depth Placement of additional topsoil provides additional storage across a development site. This approach is usually easily implemented, since most properties generate surplus topsoil during the site grading process. This measure increases evapotranspiration, since the additional topsoil can hold more water that can then be utilized during periods of drought. Refer to TRCA's Topsoil Management Guideline for more information. It is noted that grading or placing of topsoil in a TRCA regulated area may require a permit from TRCA. March 2016

9 CTC Source Protection Plan Water Balance Requirements 9 Water Balance Reviews by TRCA Staff As mentioned at the beginning of this guide, TRCA s hydrogeology team within the Water Resources Engineering Group will review water balance assessments under the SPP Policy REC-1, Part 2. These reviews will be completed in accordance with TRCA s service delivery standards at rates included in TRCA s approved fee schedule. The typical review will consist of looking at each of the following components of the water balance to ensure that the intent of Policy REC-1, Part 2 is met: Pre-development infiltration and runoff volumes; Post-development infiltration and runoff volumes; Post-development estimates of recharge deficit or excess; and Proposed mitigation measures and design. Reviews will be conducted based on the principles documented in the MOE Stormwater Management Planning and Design Manual (MOE, 2003), the Low Impact Development Stormwater Management Planning and Design Guide (TRCA and CVC, 2010), as well as taking into account The Living City Policies and appropriate stormwater management criteria. TRCA hydrogeologists will also be available to advise proponents on both the water balance calculations and mitigation strategies. Mapping of the water balance components in electronic form can be obtained through the TRCA GIS department. Please note that all reviews will be facilitated through the TRCA Planner who will circulate the necessary material to the TRCA hydrogeological team.

10 10 WHPA-Q Water Balance Requirements REFERENCES Bello, R. and So, D., Literature and Database Evaluation of Evapotranspiration in Ontario, Canada. Prepared for: Toronto and Region Conservation Authority, Final Report #E-W-GM-00235, July Hwang, H.T., Y.J. Park, S.K. Frey, S.J. Berg, and E.A. Sudicky, A Simple Iterative Method for Estimating Evapotranspiration with Integrated Surface/Subsurface Flow Models. Published in the Journal of Hydrology, Volume 531, Part 3, December 2015, Pages MOE, Stormwater Management Planning and Design Manual. Toronto, ON. TRCA, Approved Updated Assessment Report - Toronto and Region Source Protection Area. Toronto, ON. TRCA, Measurement of Evapotranspiration Across Different Land Cover Types in the Greater Toronto Area. Toronto, ON. Available online at: Evapotranspiration-Report-Executive-Summary.pdf TRCA, Evaluation of Permeable Pavements in Cold Climates Kortright Centre, Vaughan. Available online at: TRCA and CVC, Low Impact Development Stormwater Management Planning and Design Guide. TRCA, Evaluation of an Extensive Greenroof, York University, Toronto, ON. Available online at: March 2016

11 ATTACHMENT A Regional Water Balance Estimates from TRCA s Assessment Report (Approved by MOECC, July 2015)

12 Map 1: Measured Average Annual Precipitation (mm/year)

13 Map 2: Modelled Average Annual Precipitation (mm/year)

14 Map 3: Modelled Average Annual Evapotranspiration (mm/year)

15 Map 4: Modelled Average Annual Runoff (mm/year)

16 Map 5: Modelled Average Annual Recharge (mm/year)