CHARACTERIZATION OF WATER MOVEMENT IN A RECONSTRUCTED SLOPE IN KEOKUK, IOWA, USING ADVANCED GEOPHYSICAL TECHNIQUES

Despite being widely studied, ground water movement in hills in not fully understood. In order to more completely model movement, this project tests the utility of integrating advanced geophysical techniques in hillslope hydrology studies with conventional methods. Piezometers, Ground Penetrating Radar (GPR), and electrical resistivity surveys were used to provide discrete measurements of the subsurface water level in a reengineered slope near Keokuk, Iowa. Estimated water table depth variation based on GPR and resistivity data was calibrated using an array of recording pressure transducers. This information was then integrated with on-site rainfall data in order to evaluate rainfall-groundwater response dynamics. There were two hypotheses: 1) The depth and fluctuation of the water table can be accurately sensed with both GPR and resistivity surveys; and 2) Integration of data from the instrumentation array and the geophysical surveys would enable characterization of water movement in the slope in response to rainfall events. Preliminary results suggest that this approach holds considerable potential, which could lead to improved monitoring techniques. While piezometer arrays remain invaluable for studies requiring frequent data points, periodic water level monitoring with the geophysical techniques can allow for the interpolation of water levels between the point measurement locations, and also allow for the development of a more complete 3D view of changes taking place. While data from GPR and resistivity surveys requires more rigorous processing than conventional methods, the ability to develop a 3D image for a slope seems well worth the additional input of time and field labor. The geophysical techniques also provide information on other subsurface features and characteristics not available from conventional piezometer readings. This project demonstrates that geophysical methods, when integrated with continuous in-situ point measurements of water level, can provide additional important data for monitoring water movement in slopes and the response of a slope to water movement. This will ultimately help us to better understand and study the factors in hillslopes that contribute to slope failures.