Nutrients and toxic contaminants in shallow groundwater along Lake Simcoe urban shorelines

Poor water quality associated with eutrophication, attributed largely to phosphorus loading, and toxic contaminants has impaired Lake Simcoe's ecological health. We investigated the potential influence of urban groundwater contamination on lake water quality using a screening approach. In 2010, shallow groundwater (25-120 cm below the lake bed) was collected at 10-15 m spacing along 2 sections of shoreline (about 1 km each) in Barrie, Ontario. Occurrences of elevated groundwater nutrients were widespread, with 24% of samples having soluble reactive phosphorus (SRP) concentrations >0.1 mg L-1 (hypereutrophic water and >= 10 times lake water SRP). Groundwater nitrate or ammonium concentrations were above that of the lake along most of the shoreline sections. Along the north and south shores, 37 and 65%, respectively, of the samples had at least one contaminant over its Canadian aquatic life toxicity guideline, while 11 and 17%, respectively, had 2 or more over guidelines. Exceedances detected at multiple locations included ammonium, nitrate, chloride, chlorinated solvent and petroleum compounds, cadmium, and selenium. In 2011, samples were collected at the same depth range as in 2010 and, additionally, at 5 cm depth (benthic zone) at 22 locations along these same sections. Concentrations of the key contaminants were mostly similar between the 2 sampling depths, suggesting the 2010 results are generally applicable to benthic organism toxicity and nutrient loading to the sediment interface, although fluxes were not determined. Concentrations of SRP, ammonium, and various other contaminants were likely controlled by groundwater redox conditions. Potential sources of key contaminants are also discussed.