The role of the overburden-bedrock contact and upper bedrock properties in the recharge and contamination of shallow bedrock aquifers

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Groundwater obtained from fractured bedrock aquifers is an important source of water supply in many parts of the world. In some areas such as eastern Ontario, most of the Maritimes, a good part of northern Europe, much of Africa, and Australia, bedrock aquifers are the only source of water for rural residents. Typical bedrock aquifers are highly variable in permeability, of very low storage capacity and often overlain by only a minimal thickness of overburden cover. As a consequence, these water supplies tend to be vulnerable to contamination as a result of short residence times and minimal attenuation. The principal pathway for infiltrating rainwater and/or contamination is the vertical or sub-vertical fractures that intercept the overburden-bedrock contact. As little is known about the transport mechanisms here, the objective of the proposed work is to conduct detailed field studies to explore the exchange of fluid and contaminants between unsaturated drift material and the underlying bedrock aquifer. A total of five studies involving two MASc and three PhD students are proposed. The studies will include a detailed hydraulic assessment of the properties of the upper bedrock in a crystalline rock setting, and several tracer migration experiments from ground surface to the upper bedrock in both crystalline and carbonate rock settings. Analysis of the results will be conducted using a finite-element model which accommodates porous media, discretely fractured rock and unsaturated-saturated flow and transport. The proposed work will be focused on three general field sites, which have only a thin veneer of overburden, are well instrumented with boreholes in the bedrock, and are well characterized, hydraulically. The results of this study will be of significant importance to the management of both municipal water supplies and rural wells that utilize groundwater from bedrock aquifers. The Province of Ontario is presently undertaking the establishment of well head protection areas at all municipal wells in the province, including those utilizing bedrock aquifers. The results from this study can be directly incorporated into the approach taken for these sensitive aquifer settings. The results will also contribute to the approach taken in other international jurisdictions.