Ground Penetrating Radar Monitoring Of Unconfined Aquifer Pumping Tests: Results From New Experiments

Abstract

Ground penetrating radar (GPR) imaging of transient water content changes during pumping tests in unconfined aquifers has been performed with varying degrees of success. A problem encountered at the CFB Borden site is the drained water volume determined from GPR data significantly underestimated the actual water volume pumped from the aquifer. A series of pumping test experiments are currently being performed to determine the source of this discrepancy and evaluate the use of GPR information in pumping test analyses. Two seven-day pumping tests were performed and monitored with traditional water level measurements, GPR and thermal neutron logging of moisture content. Spatial variation in the GPR data set indicates aquifer heterogeneity that would be difficult to identify through monitoring well data. As well, direct observations of the water content profile drawdown by neutron logging indicate that the transition zone does not significantly change shape or become extended as a result of pumping (i.e., no excess water is stored in this zone.) The movement of the water content profile closely followed the drawdown of the GPR reflection correlated with the transition zone. This result suggests that drainage is accurately imaged using GPR profiling. Drawdown of the transition zone as monitored through GPR and neutron logging is delayed relative to that of the potentiometric surface. Since the transition zone does not change shape, this delay causes an extension of the capillary fringe. As the duration of pumping increased, the difference between transition zone and potentiometric drawdown decreased to about 0.10m at a distance of 15m from the pumping well. It is expected that as quasi-equilibrium is reestablished the drawdowns from different sources would be become equal.