COMPLEX GEOLOGY AND WATER-USE EFFECTS ON THE BARKA SLOUGH IN SAN ANTONIO CREEK VALLEY, SANTA BARBARA COUNTY, CALIFORNIA,

Groundwater is the primary source of water supply in the San Antonio Creek Valley groundwater basin (SACVGB) that supports agricultural irrigation, the town of Los Alamos, Vandenberg Air Force Base, and critical wetland habitat for several endangered species in Barka Slough. Stakeholders are concerned that continued groundwater withdrawals will cause groundwater levels to decline to a point where pumping groundwater will be uneconomical and the viability of the habitat in Barka Slough will be affected. The SACVGB is a synclinal, sedimentary basin with uplifted consolidated bedrock at the downgradient end, which forms a choke point that causes groundwater to discharge at Barka Slough. A detailed hydrogeologic study, including analysis of geologic, geophysical, and hydrologic data from oil and water wells and two recently drilled multiple-depth monitoring-well sites has identified a confining unit that controls groundwater flow and areas where the more productive Paso Robles Formation is thicker or where the less productive underlying Careaga Sandstone will be the primary resource. The confining unit inhibits vertical groundwater flow at the eastern end of the basin and causes differential groundwater-levels above and below the unit. The effects of groundwater-level declines are evident throughout the SACVGB but are particularly noticeable at Barka Slough where there are observed reductions in baseflow, upward groundwater flow, and groundwater evapotranspiration, and there is a corresponding reduction in riparian habitat. Climatic conditions have punctuated effects on baseflow and other hydrologic metrics at Barka Slough, but the reduced hydrologic conditions are largely the result of long-term increases in groundwater pumping, which is primarily driven by the expansion of irrigated agriculture. The San Antonio Creek Integrated Hydrologic Model simulates the hydrologic conditions in the basin and allows for numerical analysis of the aquifer system. This study enhances the understanding of the SACVGB aquifer system and provides a tool that stakeholders can apply to management of water resources in the future.