HGS Research Highlight - Large water-table response to rainfall in a shallow bedrock aquifer having minimal overburden cover

Authors: Owen W. Miles and Kent S. Novakowski

Model domain showing a hydraulic head rise during an infiltration event.

Rapid recharge events manifested as significant increases in hydraulic head have been observed in many fractured bedrock aquifers around the world. Often the response in hydraulic head exceeds what would be observed in an equivalent porous media by more than an order of magnitude. As the mechanisms that cause these events are poorly understood particularly under highly-transient conditions, a detailed investigation was conducted at a well-characterized field site in eastern Canada.

Using HydroGeoSphere, three-dimensional numerical simulations were conducted to reproduce the response in the piezometer for both short (24 h) and long (one month) timescales. Modeling was performed with both a discrete fracture network, and equivalent porous media approaches, though the best calibrations were achieved with an equivalent porous media approach. The numerical simulations were used to determine what parameters have the greatest impact on controlling rapid recharge. Based on this study it was concluded that the large magnitude head rises recorded in this piezometer are a result of recharge to steeply inclined fractures exposed on or immediately adjacent to the outcrop. The modelling also showed that as little as 0.4 m of drift material can completely eliminate the response in the well especially during times when evapotranspiration is high.

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