GROUNDWATER RECHARGE IN BEDROCK AQUIFERS: RAPID, LOCALIZED AND LIMITED

In porous media aquifers, recharge typically consists 20-40% of precipitation and is considered to be slow and spatially predictable. In contrast, bedrock recharge studies document large and rapid rises in hydraulic head during large precipitation events, interpreted to represent loading rather than recharge. The study area is a low gradient, crystalline aquifer in the Canadian Shield. The objective is to evaluate recharge rates and patterns in fractured crystalline bedrock, using hydraulic head data and natural and artificial tracers, such as δD, δ¹⁸O, tritium, Lissamine, and temperature. The tritium data and a multi-year record of hydraulic head and stable isotopes (δD and δ¹⁸O) are augmented by detailed data from specific recharge events, a forced gradient tracer experiment conducted in August 2006, and the hydraulic response to the spring 2007 freshet. The freshet event is numerically modeled using HydroGeoSphere, an integrated surface-subsurface model. Tritium results and long-term means of δD and δ¹⁸O suggest young groundwater preferentially recharged by winter snow melt is found to at least 50 m depth. The tracer experiment reveals that water ponded at the surface on shallow overburden can travel into bedrock piezometers in under 4 hours. Thermal and δD data confirm that cold, depleted snow melt rapidly recharges the vertical fractures in the aquifer during freshet causing a disproportionate rise in hydraulic head. The freshet response is highly localized to wells with shallow overburden and surface fractures. Numerical modeling and water table fluctuation calculations indicate that the rate of recharge is likely limited to less than 5% of precipitation. Like previous studies, we observe large hydraulic head rises during precipitation events but interpret these rises as recharge rather than loading based on natural tracer data. Recharge to crystalline bedrock aquifers seems to be fundamentally different than porous media aquifers: rapid, localized to recharge windows, and extremely limited.