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July 8, 2020 Mr. Jeffrey Swartz Vice-President, Land Development Empire Communities 125 Villarboit Crescent Vaughan, ON L4K 4K2 Re: Water Balance Study, Empire Legacy (Port Robinson Estates), Thorold, ON Dear Mr. Swartz, 1.0 Introduction and Background Information Terra-Dynamics Consulting Inc. (Terra-Dynamics) respectfully submits this water balance study of the proposed Phase 2 of the Empire Legacy (Port Robinson Estates) Site in Thorold, Ontario (Figure 1). Phase 2 development includes townhouse, semi-detached, single residential development, open space and a park (Upper Canada Consultants, 2020, Appendix A). The purpose of this study was to complete a water balance to the satisfaction of Niagara Region. The water balance predicts the future hydrologic function of the Niagara Street Cataract Road Woodlot Wetland Complex Provincially Significant Wetland (Wetland) under Phase 2 post-development conditions (Figure 2). 2.0 Methodology The following methodologies were used to complete the water balance study: A. Characterization of the physical setting using published government agency information (e.g. Ontario Geological Survey and Niagara Peninsula Conservation Authority) and field investigations of local conditions (e.g. groundwater monitoring well and surface water monitoring). B. Modelling of pre-development and post-development water balance conditions through consideration of surface water catchments, land cover, soils, climate normals and the Wetland hydroperiod. 3.0 Physical Setting The Phase 2 area upgradient of the Wetland is fairly flat with topography ranging from 181 to 177 m above mean sea level (m ASL) (NPCA, 2010). The land slopes from northwest to southeast within the surface water catchment upgradient to the Wetland (the Catchment) (Figure 2). This Catchment was delineated for this study using a GIS tool (Watershed Segmentation) based upon a topographic survey completed of the Phase 2 lands and the implied overland flow direction of surface water. The Catchment is 5.4 hectares in size, and as shown does not include the 15 m buffer of the Wetland. Empire Communities July 8, 2020 Page 2 3.1 Surface Water and Wetlands An Un-named watercourse is present along the eastern boundary of Phase 2 which flows northeasterly through the Niagara Street Cataract Road Woodlot Wetland Complex Provincially Significant Wetland (Figure 2). NPCA has classified the reach as “constructed open, with intermittent or ephemeral flow” (NPCA, 2017). Beacon Environmental (2019) has further reported it only “supports ephemeral flows, with flow only occurring during the spring (March/April) freshet …Flow or standing water is not present from end of May through October and the tributary does not support fish or fish habitat…” The Ministry of Natural Resources and Forestry (MNRF) has classified the wetland as swamp (MNRF, 2008). Beacon Environmental has further assigned the ELC Code and Community of SWD2-2, Green Ash Mineral Deciduous Swamp Type (Beacon Environmental, 2019) and reported: 1. “no ephemeral pools or ponds are found within the wetland… and no annual spring flooding of the wetland occurs” 2. “adjacent lands to the wetland within the Phase 2 lands provide only minor sheet flow inputs to the wetland area.” These adjacent lands mentioned, providing surface water flow to the Wetland, are within the Catchment (Figure 2) upgradient of the Wetland. In 2019, Terra-Dynamics installed a staff gauge (SG-3) and a datalogging pressure transducer, in this wetland for long-term monitoring (Figure 2). Surface water levels at SG-3 showed responses to precipitation events reported at the Environment Canada Welland-Pelham Weather Station (Environment Canada, 2020a). Dry conditions were regularly recorded between July and October 2019 (Figure 3). These results correspond with (i) the NPCA surface water classification of intermittent flow for the Un-named watercourse, and (ii) the reporting of dry conditions by Beacon Environmental (2019). 3.2 Overburden Aquitard and Water Table The surficial geology within the Catchment is mapped as glaciolacustrine clay and silt (Ontario Geological Survey (OGS), 2003). The regional thickness of the clay and silt is between 30 to 35 m (NPSPA, 2013). Multiple authors have mapped this surficial silty clay as an aquitard (Burt, 2016 and Gartner Lee Limited (GLL), 1987). In May 2019, a borehole, and associated monitoring well (MW-3, Appendix B) was completed to a depth of 7.9 metres below ground surface. This location is at the downgradient end of the Catchment, approximately 30 metres upgradient of the wetland. The hydraulic conductivity of the aquitard was determined as fairly low (3x10-8 m/s at MW-3), consistent with silty clay values in Ontario (MECP, 2006) and Niagara (GLL, 1987). The limited groundwater flow in the overburden aquitard is expected to follow topography (Haitjema and Mitchell-Bruker, 2005), being limited in velocity by the low hydraulic conductivity. Gartner Lee Limited (1987) provides a good description of the expected water table conditions within this overburden aquitard: “Detailed studies indicate that the water table fluctuates over the weathered/fractured upper two to three metres of the glaciolacustrine silts and clays comprising the overburden aquitard…flow in this shallow zone responds to daily climatic changes such that, during precipitation, the open fractures from Empire Communities July 8, 2020 Page 3 weathering will quickly fill with water. The bulk of the discharge will then occur locally in swales that carry intermittent surface water …. The remainder will go to depth to recharge the ground water system.” Groundwater levels at MW-3 ranged from 0.2-2.4 mBGS at MW-3 (Figure 4). The 2019-2020 Pre-development MW-3 groundwater levels showed an expected decline during the summer, with limited groundwater recharge occurring in late fall. No groundwater level response to short-term precipitation events was noted. 3.3 Surface Water and Groundwater Interaction A component of groundwater discharge could be inferred at the Un-named watercourse in the spring and fall because: 1. The ground surface at the staff gauge was lower than associated monitoring well groundwater elevations for a portion of the year (Figure 5); and 2. The groundwater level elevations were generally higher than the surface water levels. However due to the low hydraulic conductivity of the silty clay, the groundwater contribution to the Wetland would be undetectable. For example, across the 250 metres of wetland boundary along the east of Phase 2, it is calculated that less than 42 litres/day of groundwater contribution could occur in June 2019, and less than 41 litres/day in November 2019. These values were determined using Darcy’s Law; the hydraulic gradient between monitoring well MW-3 and SG-3 (from the water levels collected by the datalogging pressure transducers), the height of surface water above the ground surface and the hydraulic conductivity of MW-3 (Fetter, 1994). 3.4 Wetland Hydroperiod A hydroperiod is defined as “the seasonal pattern of the water level of a wetland…It characterizes each type of wetland, and the constancy of its pattern from year to year ensures a reasonable stability for that wetland. It defines the rise and fall of a wetland’s surface and subsurface water by integrating all of the inflows and outflows” (Mitsch and Gosselink, 2007). Based upon the Section 3 information, the hydroperiod for the Wetland suits characterization as a Canadian mineral-soil Swamp (Mitsch and Gosselink, 2007) (Figure 6). The shaded lower portion of the hydroperiod graph corresponds with the wetland ground surface and the months of the year are listed along the x-axis. Mitsch and Gosselink (2007) state this type of wetland may “have distinct periods of surface flooding in the winter and early spring due to snow and ice conditions followed by spring floods but otherwise have a water table that can be a meter or more below the surface”, similar to as seen at SG-3 (Figure 3). Empire Communities July 8, 2020 Page 4 Figure 6 – Hydroperiod for a mineral soil swamp, Ontario, Canada (Mitsch and Gosselink, 2007) 4.0 Water Balance A water balance study for the Phase 2 area upgradient of the wetland complex (the Catchment, Figure 2) was completed and informed by the Conservation Authority Guidelines for Development Applications (Conservation Ontario, 2013). It is noted that the Conservation Ontario water balance approach (and the Ministry of the Environment, Conservation and Parks, 2003 approach it borrows from) is typically concerned with the evaluation of post-development to prevent (i) increased runoff, and/or (ii) reduction in groundwater recharge. However, the Catchment’s hydrologic function with respect to the Wetland is providing additional surface water flow, not groundwater discharge, therefore maintenance of sufficient runoff to maintain the wetland hydroperiod is the criteria for the water balance assessment. 4.1 Pre-development Phase 2 A pre-development water balance for the upstream Catchment (and 15 metre buffer) was completed using the U.S. Geological Survey (USGS) Monthly Water Balance Model (McCabe and Markstrom, 2007). The Thornthwaite Monthly Water Balance model adjustable parameters include: a runoff factor, a direct runoff factor, soil moisture storage capacity, latitude of location, rain temperature threshold, snow temperature threshold and a maximum melt rate (Appendix C). After some sensitivity analysis, the runoff factor in the USGS model was increased to best match water balance infiltration rates