Rapid Recharge in a Shallow Bedrock Aquifer Having Minimal Overburden

English Abstract:

Rapid recharge events which occur as significant increases in hydraulic head following rainfall have been observed widely in fractured bedrock aquifers. The response in hydraulic head often exceeds what would be observed in a porous aquifer by more than a factor of ten. These responses have been variously attributed to air entrapment, mechanical loading due to the rainfall, or due to the low specific yield of bedrock aquifers. In this study, a detailed investigation was conducted at a well-characterized field site in eastern Canada, to resolve the possible mechanisms. The bedrock is composed of gneiss covered by a thin veneer of drift materials having variable thickness (0-4 m). During an intensive monitoring period of approximately six months, a multi-level piezometer located on a bedrock outcrop which responded rapidly and in large magnitude to rainfall (> 1 m rise in hydraulic head to less than 20 mm of rainfall) was identified and used as a focus for numerical simulations. Contiguous measurements of transmissivity with depth (total depth equal to 15 m) were collected and a survey of fracture features was obtained via borehole camera, prior to completion as a multi-level piezometer. To determine the thickness of the drift materials in the vicinity of the outcrop, a GPR survey was conducted over a 40×40 m area. A total of eight infiltration experiments were also conducted over the same area using a novel infiltration system which covers an area of 10×10 m. Three-dimensional numerical simulations were conducted using HydroGeoSphere to reproduce the response in the piezometer for both the natural and simulated infiltration events over short (24 hour) and long (one month) timescales. The numerical simulations were used to evaluate the parameters most likely to impart rapid response and 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 hydraulic head responds rapidly because of the low specific yield of the rock to which the transmissive features are connected. The modeling and infiltration experiments also showed that as little as 0.4 m of drift material can completely eliminate response in the well especially during times when evapotranspiration is high.