Project Description

Background

Elgin Mills Crossing is a large shopping centre at Elgin Mills Road East and Leslie St. in Richmond Hill, just north of Toronto. The centre opened in November 2007, with an impressive tenant mix that now includes Costco, Home Depot and Staples. Eighteen months before, the site was a field beside a big bend in the Rouge River.

For installation of on-site storm and sanitary sewers, temporary dewatering (by pumping) of shallow groundwater would be necessary to keep the construction trenches dry. Ontario Regulation 387/04 requires a Permit to Take Water (PTTW) from the Ministry of the Environment (MOE), to pump groundwater 50,000 L/d or more. Toronto and Region Conservation Authority (TRCA) would also have criteria to be met, regarding potential impacts of the development on groundwater discharge to the Rouge River. So the Application for a PTTW by Trinity Development Group to the MOE would include a hydrogeology study to estimate the necessary dewatering rates, and to provide an impact assessment. I was part of a team in Dillon Consulting Limited hired by Trinity Development Group to do such a hydrogeological study, and to facilitate the environmental approvals process.

Hydrogeology

Even before drilling, our team knew from regional maps that glacial till underlies most of the site. A huge technical bonus for our study, however, was that 200 geotechnical boreholes had already been drilled on the site in a dense grid, for engineering purposes. Cross-sections using this drilling data showed that a subsurface, lens-shaped, sand layer is continuous across the site. These boreholes turned out to be crucial to the success of our study, because they allowed us to accurately map the geometry and extent of the sand lens.

Although the sand lens mostly pinches out near the site boundaries, it outcrops in the Rouge River streambed along a reach on the north side. The sections also showed that 700 m of sewer alignment would lie within the saturated portion of this lens. Only limited dewatering would be necessary in the low-permeability till, but the sand could potentially deliver much higher rates of groundwater seepage to the construction trenches.

We designed a robust hydrogeological site characterization program, aided by our on-site sections. Groundwater levels measured in 9 monitoring well nests demonstrated that shallow groundwater flows northeastward, and discharges to the Rouge River. An estimate of groundwater discharge to the river, using seepage meters and mini-piezometers, was supported by stream flow measurements and a water budget analysis. Recharge to the groundwater system under the site, and also groundwater discharge to the river, is limited by the low till permeability. Hydraulic testing showed that the hydraulic conductivity (K) of the sand lens is a thousand times higher than that of the till.

Results

Our hydrogeological report, submitted in June 2006, contained 3 main conclusions: 1) that the pumping rate for continuous dewatering in the sand portions of the sewer alignment would be around 470 L/min, equivalent to 680,000 L/d, and well above MOE’s criterion of 50,000 L/d; 2) that temporary dewatering would not measurably reduce discharge to the Rouge River; and 3) that an enhanced infiltration system, based partly on the mapped subsurface sand lens distribution, could be used to mitigate the water budget deficit caused by the development.