IMPROVED HYDROGEOLOGIC FRAMEWORK FROM INTEGRATED GEOLOGIC MAPPING, BOREHOLE LOGGING, AND REGIONAL RESISTIVITY REVEAL HYDROGEOLOGIC INFLUENCE OF FAULTS IN LARGE-SCALE AQUIFER TEST, NORTHERN MOJAVE DESERT, FORT IRWIN, CALIFORNIA

The U.S. Army National Training Center at Fort Irwin, in the Mojave Desert of California, relies entirely on groundwater resources, so hydrogeologic resource assessment is critical. Nelson basin, in the northwest part of the base, has seven wells that are up to 274 m deep in a 2x18 km² area. The basin includes two (20 and 33 km long) mapped E-W faults and other shorter faults cutting the primary aquifer units in Miocene volcaniclastic rocks and lava flows. Initial geologic and hydrogeologic frameworks for the basin (2016) were based on available geologic maps, borehole lithostratigraphic and geophysical logs (LGUs), borehole hydrogeologic properties (HGUs), and preliminary evaluation of airborne electromagnetic (AEM) derived resistivity models. In 2017, a 2-month long aquifer test was conducted with a single pumping well (NELT3) and 5 monitoring wells. During the aquifer test, four monitoring wells (NELT1, 2, 5, and 7) that are 4.2-8.9 km from NELT3 responded to pumping, but one well (NELT6) 2.5 km away did not respond. The 2016 models could not account for these variations. Borehole LGU and HGU properties provide rock matrix properties for the models, and they were re-examined with a focus on resistivity logs for comparison with AEM results. AEM depth-dependent resistivity maps from the ground surface to ~550 m depth resolve the geometry of (1) high-resistivity lava flows and rocks in the shallow unsaturated zone, (2) moderate to low resistivity volcaniclastic rocks, and (3) discontinuities in resistivity from faults mapped at the ground surface. These resistivity maps enable tracing of faults and potential fracture zones under areas of young surficial deposits, resulting in longer trace lengths at depth and identification of previously unknown fault intersections. The newly modeled 3D fault network indicates a mechanism for hydrogeologic communication along faults through hypothesized fracture zones between NELT3 and monitoring wells NELT1, 2, and 5, whereas NELT6 is across three faults and structural blocks and lacks such communication. These integrated results from Nelson Basin highlight that detailed models of subsurface structural complexity are required to understand the broader hydraulic response to pumping and to develop more accurate groundwater models, especially in faulted intermountain basins.