Extremely rapid and localized recharge to a fractured rock aquifer

Abstract

The factors that govern recharge to fractured rock aquifers with overlying soil are poorly understood. The objective of this study is to determine if recharge to a fractured crystalline aquifer in a humid climate is governed by heterogeneous fracture networks and/or overlying soil characteristics. A 10 km2 study area is instrumented with a network of 15 bedrock wells completed to observe conditions in the shallow bedrock aquifer. Hydrogeological characterization, detailed observation of the 2007 snowmelt freshet and numerical simulations are used. Snowmelt is a valuable natural tracer because it has a distinct thermal and isotopic signature and is applied simultaneously and evenly to the entire ground surface. Results indicate that soil thickness and bedrock transmissivity are both highly heterogeneous at the site scale but that much of the study area is underlain by silty sand with a thickness of >1 m. Cold, δ2H depleted snowmelt locally recharged the bedrock aquifer to depths of at least 20 m within two days. Since recharge is typically quantified and discussed as an annual flux, the snowmelt event is extremely rapid. However, hydraulic, isotopic, and thermal data also indicate that most wells did not rapidly recharge. Numerical simulations indicate that soil thickness and hydraulic conductivity are critical parameters that controls whether the underlying bedrock aquifer rapidly recharges. The vertical fracture aperture, number of vertical fractures, the amount of snowmelt, water-table gradient, depth to water table and pressure–saturation relations are all less significant controls on the rate and amount of rapid recharge. Both field results and numerical simulations indicate that the rapid recharge process is localized to areas where the soil is very thin, such as the fringes of outcrops. Outcrops are exposed in <0.1% of the study area indicating that the process of rapid recharge is also extremely localized. This poorly documented hydrogeologic phenomenon has broad implications for groundwater management and protection, as well as our understanding of recharge processes in fractured rock aquifers.