Groundwaters from shallow surficial sediments of southern Ontario regularly contain total dissolved solids in excess of the maximum desirable level of 500 mg/L. In such cases a major difficulty lies in determining whether the elevated concentrations reflect contamination of the groundwaters by pollutants or are simply the product of natural rock-water chemical reactions.

A comprehensive hydrochemical study involving major and minor ion analysis of over 260 groundwater samples was carried out in the Duffins Creek - Rouge River drainage basins to the north and east of Metropolitan Toronto. As an aid to interpreting the wealth of data, a specially modified form of trilinear diagram was used to identify uncontaminated waters and determine their degree of chemical variability under naturally evolving conditions. This allowed chemical anomalies to be recognized at concentrations only marginally above background and considerably below present water quality standards. Significantly, most of the anomalies were found to occur either in groundwaters from wells adjacent to major highways, where road de-icing chemicals cause elevated calcium (through ion exchange) and chloride, or in rural areas, where high nitrate concentrations are associated with agricultural practices.

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