DEVELOPING A THREE-DIMENSIONAL HYDROGEOLOGIC FRAMEWORK MODEL OF A FRACTURED HARD-ROCK AQUIFER USING AIRBORNE ELECTROMAGNETIC SURVEY DATA, GEOLOGIC DATA, AND HYDROLOGIC DATA, ADELAIDA AREA, SAN LUIS OBISPO COUNTY, CALIFORNIA

Climate change and high demand for water have exacerbated California’s limited water supplies, but watershed-scale groundwater availability assessments can provide information needed to help achieve sustainable resource management goals. This project is a pilot study for the use of airborne electromagnetic (AEM) data with geologic and hydrologic data for sustainable groundwater assessments for consolidated, fractured sedimentary rock aquifers. Groundwater is the sole source of water supply in the Adelaida area, yet there has been no comprehensive evaluation of groundwater resources. As a result, relatively little is known about the groundwater system and hydrogeologic setting and there is concern that increased water demand will affect groundwater availability. This project integrates new AEM data with geologic and hydrologic data to develop a three-dimensional hydrogeologic framework model (HFM) of the Adelaida area aquifer system. The HFM is a computer model that quantifies the geometry of the geologic units of the aquifer system and the spatial and vertical variations of hydrogeologic properties within and between those units. In August 2021, 152 miles of AEM data were collected by the California Department of Water Resources. The aquifer system is comprised of fractured, consolidated sedimentary rocks with thin layers of alluvium present in ephemeral stream channels. Preliminary resistivity models of the AEM data show abrupt variations in resistivity at mapped geologic unit contacts. The resistivity models will be used along with lithologic and geophysical data to interpret subsurface geologic contacts and provide insights into the complex structural geometry of the area. Hydrogeologic and structural variations within the interpreted geologic units will be inferred by evaluating high-resolution changes in the resistivity models, the percentage of coarse-grained material recorded in water-well logs, groundwater-level measurements, and measurements of the frequency and direction of fractures and folds in outcrops. The HFM can be used to evaluate locations of groundwater recharge, groundwater flow within the aquifer, provide a tool for local stakeholders to make water management decisions, and can be informative for other studies in similar fractured, consolidated rock aquifers.