Integrated Geophysical Investigation of Pauma Groundwater Basin, California

The Pauma groundwater basin in southern California provides a substantial portion of the irrigation water for agricultural operations in the Pauma Valley. As imported water becomes more scarce and more costly, increasing demands are placed upon this local resource. To better understand the basin and maximize the sustainable use of local groundwater, Yuima Municipal Water District (YMWD) commissioned an integrated geophysical investigation of the valley. The first phase of the geophysical program was a detailed gravity survey to identify the depth to bedrock. Large density contrasts between the igneous and metamorphic bedrock and the overlying sediment make gravimetry a practical tool for this purpose. A gravity-based bedrock model was developed to identify locations with greater accumulations of sediment as potential sites for deeper water supply wells. Preliminary evidence indicates that the Pauma basin is heterogeneous. Heterogeneity in the sediments has been assessed via a resistivity survey to delineate conductive clay units which may form a barrier to groundwater flow. Deep bedrock wells in the region can be highly productive if they encounter significant fractures. A magnetotelluric survey was performed to locate and map conductive fracture zones beneath the sediments.