Mapping saltwater intrusion with an airborne electromagnetic method in the offshore coastal environment, Monterey Bay, California

Study Region: The northern coast of the Monterey Bay, California, USA, extending from the coastline to 3.5 km offshore. Study Focus: Three-hundred and twenty line-kilometers of airborne electromagnetic (AEM) data were acquired offshore. These data spanned 20 km of coastline, extending up to 3.5 km offshore in water up to 18 m in depth. Inversion of these data resulted in resistivity models extending to depths between 50 and 200 m below sea level. The data were interpreted in conjunction with onshore monitoring well data, hydrologic and geologic reports, and electrical resistivity tomography (ERT) data to locate the freshwater/saltwater interfaces throughout the region. A resistivity-to-water-quality transform was established using well-based water quality and resistivity logging measurements. For resistivity values that could not be converted to water quality with this transform, local auxiliary information was used on a case-by-case basis to interpret the observed features. Some low resistivity anomalies were identified near the shore at depth, which were interpreted to be artifacts of the 1D assumption in the inversion scheme used for these AEM data. New Hydrogeological Insights for this Region: We found that the acquisition of AEM data provided valuable information about water quality in the offshore extents of the aquifers, a region inaccessible to traditional monitoring methods, but one that plays an important role in the modeling, prediction, and management of saltwater intrusion.