USING δ¹⁸O AND δD TO INVESTIGATE COMPLEX GROUNDWATER-SURFACE WATER INTERACTION AND RECHARGE IN A GNEISSIC TERRAIN, PERTH, ONTARIO

Many studies have been performed on the topic of groundwater-surface water interaction in porous media. Groundwater-surface water interaction in fractured bedrock aquifers, however, is a poorly understood phenomenon. The objective of this study is to explore recharge and discharge in a complex gneissic terrain having only minimal overburden. The study was conducted in a well-instrumented field site and the principal investigative tools are δD and δ¹⁸O. The study area includes a 1.2 km stretch of the Tay River, near Perth, Ontario. Seven monitoring wells were drilled in a hay field adjacent to the reach. Hydraulic testing of the wells using a straddle packer system indicates transmissivity values between 4 x10^-2 to 1 x10^-7 m²/s. The wells were instrumented with multi-level piezometers giving a total of 18 isolated intervals. Fracture mapping and electrical conductivity surveys were conducted in the river to identify potential discharge points. Groundwater, surface water and precipitation (rain and snow) samples were collected to measure δD and δ¹⁸O during and following several storm events, and during baseflow conditions. Results of the hydraulic testing showed that groundwater flow is dominated by a few isolated horizontal fractures which are connected by sparsely-arranged vertical fractures. δD and δ¹⁸O results indicate that precipitation does not directly influence the isotopic signature of surface water and deep groundwater during a rain event. Recharge is localized and very rapid where overburden is thin, however, it does not penetrate deeply into the flow system at this site. There is very little, if any, groundwater discharge in the 1.2 km section of the river. Flow in this reach, and likely in the bulk of the river, is controlled by an upstream dam and by surface water run-off. No discharge locations were identified from the fracture mapping and conductivity surveys. There is no isotopic evidence that groundwater discharges into the river, thus most of the groundwater underflows the study reach. Empirical isotopic evidence from a well close to the river shows that surface water may enter the shallow groundwater zone when the river stage is high.