Road salt contamination of groundwater in a major metropolitan area and development of a biological index to monitor its impact

A survey of 23 springs in the Greater Toronto Area (GTA) of southern Ontario recorded chloride contamination levels, resulting from the winter application of road de-icing salt, ranging from < 2 to > 1200 mg l^-1. Chloride level measured in spring water was far more seasonally stable than that measured in surface (river) water, and thus the spatial pattern of Cl contamination indicated by the former was judged to be more reliable. Chloride contamination of groundwater in the GTA was strongly related to urbanisation, and at the four most affected springs increases of between 21 and 34% were detected over the period November 1996 to November 1997. The response of macroinvertebrates living in these springs to increasing salinity was examined with the aim of creating a biological index of contamination: the Chloride Contamination Index (CCI). A power function yielded a significant correlation between this index and the mean Cl concentration measured at each spring. Taxa were able to be categorised as either 'tolerant' or 'non-tolerant' of high Cl although none was unique to either end of the scale. However, from both field observations and salinity tolerance trials in the laboratory, the amphipod Gammarus pseudolimnaeus was found to be associated with source aquifers only mildly contaminated with Cl. Absence of this species from a spring, particularly if nymphs of the stonefly Nemoura trispinosa are present should indicate moderate to high contamination.