Road-impacted sediment and water in a Lake Ontario watershed and lagoon, City of Pickering, Ontario, Canada: An example of urban basin analysis

摘要

The world is increasingly urban but there are few studies of how contaminated water and sediment move through urban basins with their built landscapes and complexly disturbed geology. The central Canadian city of Pickering, Ontario sprawls across a small (27 km~2) densely urbanized (pop: 53,000) watershed and is underlain by Pleistocene glacial sediments and thick artificial fill deposits. Almost 80% of the area is hardened by impervious cover; road and rail lines cover 40% and include Canada's busiest highway (12-lane Highway 401:177,000 vehicles per day in 2003). The basin discharges to Lake Ontario through a small (85 ha) shallow (<3.5 m) lagoon (Frenchman's Bay). A 3-D steady state finite element groundwater numerical model (FEFLOW) was applied to 200 cored and geophysically-logged (gamma and resistivity) boreholes and 3400 digital water wells. It identifies the subsurface stratigraphy and hydrostratigraphic function of deposits and the rates of groundwater flow. Year-round monitoring of groundwater, creek and lagoon water quality shows that transportation infrastructure is the primary source of contaminated water and sediment. Some 7600 tonnes of de-icing salt are applied to watershed roads each year; 52% accumulates in groundwater where it continues to be released as brackish baseflow to creeks in summer. The remainder is rapidly delivered by surface runoff to Frenchman's Bay where chloride contents are more than double the average values in waters across the Great Lakes. Highway 401 is the largest single source of salt contamination to the lagoon; it receives 26% of all road salt applied to the watershed but covers just 1.3% of its area. Prominent spikes in chloride content (>2000 mg L~(-1)) occur during winter thaws in creeks downstream of the highway. Enhanced stream bank erosion as a consequence of flashy storm runoff from road surfaces moves ～ 100 tonnes of contaminated sediment to Frenchman's Bay each year. Instantaneous suspended sediment concentrations in storm runoff are as high as 1600gm~(-3) and loadings of sulphate (SO_4~(-2)) nutrients reach 594 kg h~(-1). Metals and E. coli in road runoff are all elevated well above Canadian water quality standards. These findings underscore findings from other urban areas regardless of regional climate, that transportation corridors are a major global source of contaminated water and sediment.