Susceptibility of Groundwater to Pesticide and Nitrate Contamination in Predisposed Areas of Southwestern Ontario
Abstract Agricultural practices today employ a vast array of chemicals in large volumes in order to improve both the quantity and quality of our agricultural products. While it has long been recognized that runoff from agricultural land has the potential to degrade surface water quality, only recently has attention been focused on the effect of agricultural usage on groundwater. In order to study the effects of pesticides and nitrate usage on the quality of groundwater, in 1985 the Ontario Ministry of Environment and Energy began operating a groundwater monitoring program in southwestern Ontario. Data generated from this program, which utilized sample data collected from both wells and piezometers, indicate that in areas of heavy pesticide and nitrate usage, shallow groundwater is continuously testing positive for nitrate and a variety of pesticides. Factors which influence the number of positive incidents for pesticides are directly related to the persistence of the chemical, its method of application, and the amounts utilized. Soil types and depth to groundwater, although influencing the time of detection, do not govern the number of detection events. Changes in agricultural practices are also monitored to see if pesticide reduction, a variation in the method of application, crop rotations and an increase in soil organic matter could influence the levels of pesticide It was found that when chemicals of a low persistence were applied post emergent at the minimum recommended rate, pesticides were not detected in the groundwater. Crop rotations were also effective in reducing the level of pesticides in groundwater. Tillage practices and increases in soil organic matter were also effective in reducing pesticide contamination. It was found that when chemicals of a low persistence were applied post emergent at the minimum recommended rate, pesticides were not detected in the groundwater. Crop rotation and reduction in nitrate loadings were found to be the only effective methods to reduce nitrate loading to groundwater. It was also found that elevated levels of potassium and/or nitrate in groundwater serve as a reliable indicator of the groundwater susceptibility to pesticide contamination. Remedial action to alleviate the impact of pesticides and nitrates in groundwater must focus on the chemical usage patterns employed on the farm site and an overall reduction of the quantities of pesticides and nitrates utilized. These patterns must incorporate a well-designed program of crop rotation with the proper utilization of these chemicals on site.