TRANSIENT ELECTRICAL RESISTIVITY IMAGING OF A KARST AQUIFER THROUGH DROUGHT

The Eastern Arbuckle-Simpson Aquifer (ASA), located on the Hunton Anticline in South Central Oklahoma, is a sole source karst water supply for Pontotoc County, the City of Ada, and the Chickasaw Nation. Discharge at Byrd Mill Springs (BMS), the state’s largest spring, has a drainage area in the Ordovician Arbuckle Group and runs from the escarpment immediately southwest of BMS to the Blue River in the west. The lithology is principally a karstified dolomite that is approximately a kilometer thick. A managed aquifer recharge research site exists in the spring basin approximately a mile west of the spring along a regional fault. The site includes an installed temporal electrical resistivity imaging (TERI) monitoring station and several monitoring wells at different depths. Over the past year, quarterly and then monthly subsurface images were collected to create a series of TERI images. The images show a consistent drawdown in groundwater level from late summer of 2021 through spring of 2022 and the subsequent recharging during the summer months. As expected, changes in resistivity of the subsurface were noted with increasing resistivity in the vadose zone down to the water table during the drying period and the phreatic zone largely increased in conductivity. Unexpectedly, the largest increases in conductance during the drying period were in the vadose zone. The correlation with available well and meteorology data will be discussed.