Groundwater quality and contaminant pathways in the Raisin River agricultural watershed: Susceptibility to groundwater contamination, Cornwall, Ontario.

The Raisin River watershed, located approximately 130 kilometres east of Ottawa, is the site of an environmental project examining the groundwater quality and contamination potential of a fractured bedrock aquifer in an agricultural basin discharging to the St. Lawrence River. Groundwater quality was monitored in five tile drains, four piezometers, and thirteen domestic wells over a period of 14 months (May-94 to June-95). Temporal variations in total dissolved solids (TDS), water level and major element concentrations, as well as fluctuations in dissolved organic carbon (DOC), $\delta\sp2$H and $\delta\sp $C were used to distinguish between regionally recharged groundwaters and recharge occurring by local infiltration through agricultural fields. Nitrate, agricultural pesticides, and heavy metals were analyzed as the principal components of groundwater quality. Water level, dissolved solids, $\delta\sp2$H, and $\rm\delta\sp C\sb{DIC}$ fluctuated according to seasonal change. Springtime showed high water levels coupled with depleted $\delta\sp2$H values. In addition, nitrate and DOC values were elevated due to local infiltration through the soil zone into the bedrock aquifer. The $\delta\sp $C values show that recharge contributions from (i) bedrock recharge areas, and (ii) infiltration through cultivated fields, vary on a seasonal basis. Spring and summer periods are dominated by recharge through cultivated fields where $\delta\sp $C values are governed by infiltration of C$\sb4$ plant (corn) carbon. Contributions of local recharge during the fall and winter months are reduced. The watershed's susceptibility to groundwater contamination is considered high since the aquifer responds to seasonal climatic variations. The mean residence time of these groundwaters is in the range of several months, based on cross-correlations between air temperature and $\delta\sp2$ values of Raisin River waters. The most potentially harmful times of year occur during rapid groundwater infiltration during the spring and periods of high precipitation. The effect of tile drains in diverting agro-contaminants to the Raisin River contributes to overall good bedrock groundwater quality as tile drainage effluent shows elevated concentrations of pesticides, nitrate, and trace metals