Pesticides in Ontario drinking water - 1987 groundwater sampling program

The 1987 pesticide monitoring program was a continuation of pesticide monitoring of domestic and municipal wells located in hydrogeological environments which are sensitive to groundwater contamination; namely shallow groundwater conditions in sandy permeable soils. Monitoring was conducted in predominantly corn growing areas since corn herbicides are the group of pesticides which are used in the greatest amount in Ontario. As well, an area of intense market gardening, the Thedford Marsh was also included in the 1987 survey. A total of 41 domestic wells and 1 municipal well were sampled weekly for pesticides and bacteria and weekly or monthly for major ion chemistry in Southeastern, Central and Southwestern Regions of the province. Pesticides of interest included the triazine group and the acetanilide group. In Southwestern Region organophosphate and chlorophenoxy group pesticides were analyzed in samples from Thedford Marsh area, and samples from the Bowmanville area of Central Region were also analysed for chlorophenoxy pesticides. The results indicated that detectable levels of pesticides were found in 6 of 23 wells from Southwestern Region, none of 11 wells from Central Region and 8 of 8 wells from Southeastern Region. Eighty-eight per cent of the 108 pesticide occurrences reported were for atrazine and/or d-ethyl atrazine, 7% for cyanazine, 4% for metolachlor and 1% for simazine. Concentrations for atrazine were all below 1.5 ppb. which is considerably below the Interim Maximum Acceptable Concentration (IMAC) of 60 ppb set by the Federal-Provincial Subcommittee on Drinking Water and reported in the Guidelines for Canadian Drinking Water Quality. Concentrations of cyanazine, metolachlor and simazine were all below 1.0 ppb, and their respective IMAC's of 10, 50 and 10 ppb. Persistent but low level pesticide occurrence in wells in the Frankford area of Southeastern Region, including the Frankford municipal well indicate the sensitivity of this aquifer to land use. Sixty-six percent of all the wells sampled also exceeded provincial drinking water quality objectives for nitrate-nitrogen and/or fecal coliform. The results of this program indicate that shallow domestic wells and shallow groundwater are susceptible to contamination and underscore the importance of proper well installation and maintainance, and the protection of groundwater supplies.