Hydrogeologic Characterization of an Aquitard Using Poroelastic Responses and Near Surface Geophysics

Abstract:

In central Wisconsin, irrigation use has dramatically increased over the last several decades. This increase has given rise to concerns over impacts to surface waters from groundwater pumping. We instrumented an irrigated field and a nearby prairie to assess those impacts and compare hydrogeologic responses. The hydrogeology of the two sites is similar, a layered system of 10 meters of sand over 2 to 3 meters of clay and silt over 30 meters of sand. The clay and silt is a recognized geologic unit, the New Rome Formation. Although the New Rome Formation is found over an area of 160 km², little is known about its hydrogeologic properties. We used poroelastic responses and near-surface geophysics, in addition to traditional hydrogeologic tools such as pumping tests and sediment characterization, to assess the hydrogeologic parameters of the New Rome and the upper and lower sands.

The instrumentation consisted of piezometers in all three layers and pumping wells above and below the New Rome. We observed a head decrease of 1.1 meters across the New Rome. We conducted pumping tests using the wells above and below the New Rome. When pumping from below the New Rome, we observed the Noordbergum effect, a small transitory head increase of 4 cm in the New Rome and 1.5 cm in the shallow piezometer above. The piezometer in the deep sand aquifer showed around 30 cm of drawdown during this pumping. We also conducted a loading test on the New Rome. We backed a truck mounted drill rig over the piezometer and monitored the response. We saw a head increase of 1.3 cm that dissipated in around 3 minutes. These responses were modeling using a coupled poroelastic model. We also collected ground penetrating radar data that can be used to identify the depositional model for the New Rome and the overlying sand aquifer. These results all suggest that the New Rome is behaving like an aquitard at this site with a vertical hydraulic conductivity that varies with depth and location.

This new understanding of the hydrogeologic characteristics of a regional aquitard in an area of increased water use is essential for estimating and communicating the impacts of the increased water use. For example, we can better estimate the impacts to surface waters from groundwater pumping for irrigation if we know the role that the New Rome aquitard plays in the regional flow system.