Pearson International: Moore Creek Stormwater Facility

The Greater Toronto Airports Authority developed a master plan to address environmental concerns about stormwater runoff from the new developments. Discharges from the infield threatened local tributaries of Etobicoke Creek, which is a valuable recreational zone between Toronto and Mississauga. Run-off might contain pollutants from jet fuel, de-icing and airport construction.

To mitigate the problem, AWS Engineers (formerly Thorburn Penny) and Hatch Mott MacDonald of Mississauga, Ontario in joint venture designed a stormwater quality control facility to handle the first 25 mm of run-off from three adjacent catchment areas in the infield area. The catchment area totals 441 hectares and represents 25% of the Pearson site.

The engineering team’s first challenge was to locate a place to store 84,000 cubic metres of water in an area that has cargo and aircraft de-icing stations in the immediate vicinity — both of which require taxiways and object-free areas.

Instead of pumping large volumes of water away to a distant treatment plant, the engineers used a combination of surface detention ponds and a large underground storage tank. The first half of the stormwater run-off, which typically contains most of the contamination, is stored in the 42,000 m3 tank, while the remainder is directed to the two adjacent surface ponds which store an additional 42,000 m3. The surface ponds discharge the water back into Moore Creek via perforated pipes.

The storage tank and oil/water separator is an underground concrete complex roughly equal to two football fields in area. A large part of the tank is located under a planned apron so it must support the weight of aircraft plus three metres of backfill. Designed to withstand 140 kPa, the tank roof is an 825-mm thick slab, increasing to 1,500 mm at beam locations. The walls are up to 800 mm thick and the base 1,500 mm thick. In all, 44,000 m3 of concrete and 6,000 tonnes of reinforcing steel were used, accounting for 70% of the $29 million construction value.

The tank is separated into four cells, which can remove up to 70% of total suspended solids. The majority of heavy sediment and coarse debris is captured within the first cell, and since the cells are connected by weirs that allow them to operate in series, the number of cells used can be tailored to the rainfall event, thus saving operation and maintenance.

The facility is fully automated using a SCADA system. Two oil/water separator channels in a gallery adjacent to the storage tank are designed to drain the tank in a 24-hour period and produce an effluent containing less than 10 mg/L of oil droplets 60 microns or larger. Effluent is continuously monitored using an on-line biochemical oxygen demand (BOD) analyzer, and depending on its quality is either discharged by gravity into the creek, or diverted to the pumping station for disposal in the Region of Peel sanitary sewer system.

The tank was constructed within the existing Moore Creek watercourse to take advantage of the topography, but it complicated the construction. The contractors also had to cope with winter storms that exceeded the 50-year anticipated precipitation level. Nonetheless, the project was completed on time by December 2001 and within the $30 million budget.CCE

Consultants: Hatch Mott MacDonald and American Water Services (AWS) in joint venture (David MacDonald, P.Eng.; Rob Poisson, P.Eng.; Chris Tattersall, P.Eng.; Bill Andrews, P.Eng.)

SCADA system: Summa Engineering

Construction: Kenaidan Contracting