DESATURATION AND FLOW DYNAMICS BENEATH AN AQUITARD NEAR EXCESSIVELY PUMPED WELLS

As confined aquifers are increasingly heavily exploited, particularly for municipal water supplies, hydraulic head can be drawn down below the bottom elevation of the overlying aquitard. Results of regional numerical flow models show that this situation has recently occurred in the vicinity of Green Bay and Milwaukee, WI, and probably also near Chicago. Little field data is available to investigate such a process, and commonly-used computer models are not well suited to examining this transition between saturated and unsaturated conditions. Using a scaled sand-tank model to represent the confined groundwater system in southeastern Wisconsin, we present data on the dynamics of flow in space and time as head is drawn down below the aquitard.

Sand-tank models have been very useful for public outreach about hydrogeology and groundwater contamination processes. The model used here was refined from the "Wisconsin" version, originally developed by educators from the University of Wisconsin-Extension, now in common use across the U.S. It consists of layers of glass beads sized to specification, and mixtures of sand and bentonite powder between vertical plexiglas plates an inch apart, to represent the aquifers and aquitards in cross-section. Pumping wells are embedded in the sediment and, in this research model, manometer tubes allow measurement of head below the elevation of numerous points on either side.

Results indicate that as elevation head becomes zero and pressure head becomes increasingly negative, air penetrates down well bores causing partially saturated areas to develop near pumping wells. Hydraulic head measurements show fluctuation at different points around the pumping wells, and variation along the length of the open intervals. Preliminary analysis suggests that as hydraulic conductivity varies with degree of saturation, flow to the wells is blocked or shifted to different, more saturated areas. Further investigation is planned to elucidate the pumping-drawdown relationship between this quasi-2D model and a pumped aquifer system with radial flow.