The U.S. Army Fort Irwin National Training Center (NTC), approximately 35 miles north-northeast of Barstow, California, obtains all of its potable water supply from three groundwater basins (Irwin, Langford, and Bicycle Basins) within the NTC boundaries. In these basins, groundwater withdrawals exceed natural recharge, resulting in water-level declines. However, managed aquifer recharge using treated wastewater has offset water-level declines in Irwin Basin. Additionally, localized water-quality changes have occurred in some parts of Irwin Basin as a result of human activities (for example, wastewater disposal practices, landscape irrigation, and (or) leaking pipes). As part of a research study in cooperation with the U.S. Environmental Protection Agency (EPA) and the U.S. Army, the U.S. Geological Survey (USGS) collected eight electrical resistivity tomography (ERT) surveys on irrigated fields in the Irwin Basin in July 2019 and February 2020. The ERT geophysical technique injects direct-current electricity with known voltage and current into the earth using a series of electrodes and measures the resulting resistivity. This technique is generally limited to investigations of aquifer properties that are less than 100 meters below land surface.