Resistivity imaging reveals complex pattern of saltwater intrusion along Monterey coast

Electrical Resistivity Tomography data were acquired along 40 km of the Monterey Bay coast in central California. These data resulted in electrical resistivity images to depths of approximately 280 m.b.s.l., which were used to understand the distribution of freshwater and saltwater in the subsurface, and factors controlling this distribution. The resulting resistivity sections were interpreted in conjunction with existing data sets, including well logs, seismic reflection data, geologic reports, hydrologic reports, and land use maps from the region. Interpretation of these data shows a complex pattern of saltwater intrusion resulting from geology, pumping, and recharge. The resistivity profiles were used to identify geological flow conduits and barriers such as palaeo-channels and faults, localized saltwater intrusion from individual pumping wells, infiltration zones of surface fresh and brackish water, and regions showing improvements in water quality due to management actions. The use of ERT data for characterizing the subsurface in this region has led to an understanding of the spatial distribution of freshwater and saltwater at a level of detail unattainable with the previously deployed traditional well based salinity mapping and monitoring techniques alone. Significant spatial variability in the extent and geometry of intrusion observed in the acquired data highlights the importance of adopting continuous subsurface characterization methods such as this one.