AGRICULTURAL IMPACTS ON GROUNDWATER IN FRACTURED ROCK - TAY RIVER WATERSHED

Water stored in fractured bedrock aquifers is widely utilized for water supply in many areas of the world. Often these aquifers are susceptible to contamination from anthropogenic activity such as industrialized agriculture. The sensitivity to contamination arises in bedrock aquifer settings as a result of high groundwater velocities and minimal overburden protection. The objective of this research is to investigate the potential impact on the groundwater quality in a bedrock aquifer as a result of local agricultural activities, focusing on nitrate contamination and pathogen transport. To conduct this investigation, a research site has been developed in a farmer's field in eastern Ontario, near the town of Perth, adjacent to the Tay River. At this location, the gneissic fractured bedrock, which acts as the local aquifer, is overlain by a thin veneer of unconsolidated glacial material, ranging in thickness from 0 to 2 m. Eleven bedrock wells (30 m to 45 m in depth) were installed in and nearby the 50-hectare field. Extensive hydraulic testing was completed in the summers of 2005 and 2006. Pumping tests, hydraulic tests (1.77 m packer spacing) and borehole videotaping were conducted in order to characterize the significant fracture features and the hydraulic properties of the bulk rock. The wells were then completed as multilevel piezometers to isolate specific fracture zones in the bedrock aquifer. Beginning in October 2005 samples for nitrate, ammonia, dissolved organic carbon, total coliform, and E. coli were obtained on a monthly basis. Week long intensive sampling rounds under baseflow and recharge conditions were conducted. Results show that permeable features have elevated nitrate and bacterial concentrations (in exceedance of drinking water guidelines) at some locations. Seasonal trends have also been observed; there is a lag time between periods of maximum recharge and high concentrations in the aquifer. Denitrification is limited due to low values of DOC. The location of the impacted monitoring wells can be correlated to a potential upgradient source in adjacent farmers' fields.