IMAGING SUBSURFACE GROUNDWATER AND LITHOLOGY VARIATIONS NEAR THE GREAT SALT LAKE USING ELECTRICAL RESISTIVITY TOMOGRAPHY

The Great Salt Lake (GSL) is experiencing unprecedented declines in water levels, posing severe environmental, economic, and public health risks. As a critical habitat for migratory birds and a significant contributor to Utah’s economy, the lake's preservation is essential. However, the role of groundwater seepage in replenishing the lake remains poorly understood, limiting the effectiveness of water management policies. In this study, we investigate the applicability of Electrical Resistivity Tomography (ERT) to assist in deciphering groundwater flow patterns in the vicinity of the GSL. ERT is a non-invasive geophysical method that is sensitive to changes in electrical properties of lithology as well as the presence of groundwater and its associated salinity. Between August and November, 2024, we collected 29 2-D ERT transects using a Syscal Pro 10 Switch electrical resistivity meter in dipole-dipole configuration. We focused our measurements on the playa at (1) the south shore of the GSL, (2) In Farmington Bay, and (3) North of the Antelope Island causeway. For each 2-D transect, we collected approximately 3,000 measurements along lengths of 72, 180, or 360 m depending on electrode spacing, with depth of investigation ranging from 15 to 70 m. Three ERT transects were collocated with wells in which the pore-water conductivity was measured as a function of depth. We compare our measured resistivity values to these pore-water conductivities to directly relate our measurements to groundwater salinity across the study region. Our results show the presence of fresh groundwater beneath the Farmington Bay, Howard Slough WMA, and Ogden Bay regions, although Ogden Bay contains a larger proportion of brackish water. No fresh water is observed in the subsurface at sites along the southern shore of GSL. Initial ERT surveys near circular phragmite mounds in Farmington Bay and Howard Slough WMA are suggestive that these features may be indicative of freshwater seepage at the surface of the playa.