Borehole geophysical logs near an irrigated field in Fort Irwin National Training Center, San Bernardino County, California, 2018-2020

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), the U.S. Geological Survey (USGS) collected geophysical data to do a site characterization of an irrigated field (Four-plex baseball field) in the Irwin Basin. To aid in the understanding of the subsurface near the Four-plex baseball fields the USGS collected borehole geophysical data during 2018–2020. Natural gamma and neutron geophysical logs were collected in two monitoring wells that were installed by the EPA during 2018–19. Fluid resistivity, natural gamma, and electromagnetic induction logs were repeated in one of the EPA monitoring wells in February 2020; these types of geophysical logs also were collected at a nearby monitoring well on the eastern side of the Four-plex baseball field.