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

Abstract

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. During the spring and summer of 2012, frequent measurements of hydraulic head were obtained in gneissic terrain covered by a thin veneer of drift materials using 21 multi-level monitoring wells installed in the bedrock. Each of the wells was hydraulically tested from the water table to total depth using a straddle-packer system and fractures intersecting the wells were identified using a borehole camera prior to the construction of the multi-level piezometers. Rainfall and weather data were also collected over the same time period. A piezometer located on a bedrock outcrop which responded rapidly to rainfall was identified and used as a focus for numerical simulations. To determine the properties of the drift materials in the vicinity of the outcrop, a ground penetrating radar (GPR) survey was conducted over a 40×40m area to map depth to bedrock and five in-situ permeameter tests were performed to estimate the hydraulic conductivity. Three-dimensional numerical simulations were conducted to reproduce the response in the piezometer for both short (24h) and long (one month) timescales. 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 hydraulic head responds rapidly because of the low specific yield of the rock to which the transmissive features are connected. The modelling also showed that as little as 0.4m of drift material can completely eliminate the response in the well especially during times when evapotranspiration is high.