Processes Governing Rapid Recharge Events in a Shallow Fractured Rock Aquifer Having Minimal Overburden Cover
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The goal of this work is to investigate the influence of overburden cover on controlling recharge to a bedrock aquifer, specifically focusing on rapid recharge events. Rapid recharge events have been observed in a fractured rock site in eastern Ontario and fractured rock sites around the world. The mechanisms that cause these events are poorly understood. At a field site near Perth, Ontario, measurements of hydraulic head were obtained in the spring and summer of 2012 using 18 different monitoring wells. Rainfall and weather data were also collected. Infiltration experiments were performed in the summer period using a 10 m by 10 m rainfall simulator. A ground penetrating radar (GPR) survey was conducted around a piezometer to determine depth to bedrock. Permeameter tests were performed in the overburden layer. A piezometer which responds rapidly to rainfall was identified and field measurements and observations were used to numerically model the piezometer on an outcrop. Three-dimensional numerical simulations reproduced the response in the piezometer for both short (24 hour) and long (one month) timescales. An equivalent porous media (EPM) approach was taken to numerically model fractured rock. The numerical simulations for a month-long period required that evapotranspiration was accounted for and this was achieved by limiting applied rainfall to the area above the outcrop in the model. Numerical simulations were also used to determine what parameters have the greatest effect on controlling rapid recharge. Based on this study it was concluded that large magnitude head rises recorded in this piezometer are a result of recharge to the shallow aquifer. Hydraulic head rises rapidly because of transmissive vertical fractures connecting the low specific yield rock to the surface. A thin layer of overburden (0.4 m) can completely eliminate response in the well especially during times when evapotranspiration is high.