| Abstract: | A field study site was installed in east‐central Pennsylvania to examine processes controlling groundwater recharge. It was instrumented to monitor climatic inputs, soil water dynamics and groundwater response. Characterization of the layered fractured bedrock underlying the site by rock coring, seismic surveys and interval packer testing showed consistencies between layer depths, fracture frequencies, seismic velocities and hydraulic conductivities. Monthly summaries of rainfall and percolate over two years showed that percolate rates were generally high and closely related to precipitation during the dormant season. During the growing season, however, the relationship became erratic with large variabilities occurring between individual lysimeter measurements. Eight dormant season rainfall events were examined in detail. Smaller events produced similar responses from 1 m deep percolate lysimeters. Approximately 10–15 mm of rain was required to initiate percolate, with the time delay in response dependent on how long it took this depth to accumulate; 5 to 6 mm of the rain was retained in storage, with the remainder becoming percolate. Larger rains, from 30–110 mm, caused correspondingly larger depths of percolate and larger water table responses, but generally similar patterns of site response. Groundwater at the site was typically about 6 m below the land surface during the dormant season. It responded 1–2 hours after the onset of percolate, and reached its maximum elevation anywhere from 4 to 16 hours after that, even though percolate was still occurring. Based on causative depth of recharge and amount of water level rise in wells, the specific yield of the aquifer was found to be of the order of 0·01. This value is characteristic of fracture geometry rather than matrix properties of the bedrock. Copyright © 1999 John Wiley & Sons, Ltd. |
