『Abstract
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 km2) 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 1600 g m-3 and loadings of sulfate (SO4-2) 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.
Keywords: Urban sedimentology; Canada; Lagoon; Contaminated water
and sediment』
1. Introduction
1.1. Background and objectives of this study
2. Physical setting of study area
2.1. Organization of this paper
3. Regional stratigraphy and groundwater flow across the urban
basin
3.1. Methodology
3.2. Glacial stratigraphy
4. Hydrostratigraphy and hydrogeology
4.1. Aquifers
4.2. Aquitards
4.3. Groundwater flow directions
5. Ground and surface water quality
5.1. Groundwaters
5.2. Surface waters
5.2.1. Winter flows
5.2.2. Summer flows
6. Sediment transport to Frenchman's Bay lagoon
7. Discussion
8. Conclusion
Acknowledgements
References