The non-invasive characterization of pumping-induced dewatering using ground penetrating radar

Abstract

Ground penetrating radar (GPR) profiling is a non-invasive geophysical technique that has been used by Endres et al. [Ground Water 38 (2000) 566] to successfully image pumping-induced drainage in an unconfined aquifer. However, the drained water volume calculated from the GPR data was significantly less than the actual pumped volume. To investigate the reasons for this discrepancy, a seven-day pumping test and five-day recovery test was performed at Canadian Forces Base Borden in Ontario, Canada. A dense spatial coverage of GPR profiles was used to better quantify variations in drainage due to small-scale aquifer heterogeneity. In addition, a neutron moisture content probe was used to directly observe drainage and the soil moisture profile at a sealed well near the pumping well. Neutron logging indicated that the transition zone translated downward during pumping without undergoing significant extension. Comparison of the GPR- and neutron-derived transition zone drawdowns show nearly equal responses. Both of these observations support the hypothesis that the behaviour of the GPR reflection is an accurate measure of the transition zone response. In contrast, transition zone drawdown obtained from both GPR and neutron logging are significantly delayed relative to potentiometric drawdown, resulting in an extended capillary fringe. The drained water volume was determined from the GPR-derived transition zone drawdown data using a number of different approaches. Methods that incorporated information about spatial variations in drainage gave larger estimates of drained water volume; however, those estimates were still lower than the actual pumped volume. The unaccountable volume of water could be a result of several factors—aquifer heterogeneity may still not be adequately represented by the increased GPR coverage, and/or leakage from the underlying aquitard may be providing a significant volume of water.