Field observation of the response to pumping and recovery in the water table region of an unconfined aquifer

A 24-h pumping test was conducted in the shallow water table portion of an unconfined aquifer located at Canadian Forces Base Borden near Alliston, Ontario, Canada. This test was designed to monitor the dynamic nature of the vertical gradients that form within the zone of tension saturation above the water table during pumping and recovery. During the test, pressure head was monitored throughout the saturated zone, both above (tensiometers) and below (piezometers) the water table; soil water content was monitored using both neutron moisture probes and time domain reflectometry (TDR). Following pumping, recovery was monitored in the same manner as the pumping portion of the test. The hydraulic head drawdown observed above the water table in the tension saturated zone during pumping and recovery was very similar to the drawdown observed below the water table. Vertical gradients throughout the saturated zone appeared to be uniform and low, and did not change significantly in the transition across the water table. The magnitude of the vertical gradients peaked early in the test, and subsequently decreased, with the reduction being more significant at greater radial distances from the pumping well. Neutron measurements indicate that only a minor degree of drainage occurred within the measurement interval for the duration of the test, a result of the low magnitude of drawdown generated during pumping and influence of hysteresis on the moisture profile. Although drainage was negligible from within the tension saturated zone during pumping, the hydraulic head drawdown below the water table displayed reduced rates of drawdown during the intermediate period of the test. TDR data was only collected during recovery; however, the data set displays hysteretic behavior in the pressure-saturation profiles, with all profiles plotting on scanning curves. Based on these observations it appears that a conceptual model in which vertical gradients below the water table are of the same order of magnitude as those within the capillary fringe is appropriate for the analysis of vadose zone response to pumping. (C) 2011 Elsevier B.V. All rights reserved.