Using an airborne electromagnetic method to map saltwater intrusion in the northern Salinas Valley, California

Abstract

Saltwater intrusion can pose a serious threat to groundwater quality in coastal regions. Estimating the extent of saltwater intrusion is vital for groundwater managers to plan appropriate mitigation strategies. The airborne electromagnetic (AEM) method is commonly used to evaluate groundwater resources, but it is challenging to apply in coastal environments because the low resistivity of saltwater-saturated aquifers attenuates the electromagnetic signal quickly and the relationship between electrical resistivity and pore water salinity is complex. However, if successful, the AEM method can supply information to address questions of critical importance in coastal regions. We investigated the extent of, and controls on, saltwater intrusion using the AEM method in the northern Salinas Valley, CA, USA. We collected 635 line-km of AEM data in the study area, the inversion results of which produced estimates of the electrical resistivity of the subsurface, reaching depths of between 50 and approximately 200 m below the ground surface. We have developed a relationship between the AEM electrical resistivity model and groundwater salinity, calibrated from borehole geophysical and water quality measurements, which allowed us to generate images revealing the distribution of saltwater and fresher groundwater in the study area. This fresher groundwater (defined as “a source of drinking water”) was successfully mapped out in the unconfined aquifer (the Dune Sand Aquifer) and the uppermost confined aquifer (the 180-Foot Aquifer) in the study area, illustrating a groundwater recharge process that helps mitigate saltwater intrusion in the 180-Foot Aquifer. Deep, low-resistivity bodies also were mapped, indicating regions where saltwater likely is migrating vertically from the 180-Foot Aquifer into the lower confined aquifer (the 400-Foot Aquifer). The findings from this case study demonstrate the value of acquiring AEM data for investigating the distribution of salinity in coastal aquifers impacted by saltwater intrusion.