Electrical Resistivity Imaging of Saltwater and Freshwater Along the Coast of Monterey Bay

Abstract:

A coastal region represents a dynamic interface where the processes of saltwater intrusion and freshwater flow create complex spatial and temporal changes in water chemistry. These changes in water chemistry affect both human use of coastal groundwater aquifers and the functioning of coastal ecosystems. Mapping out the subsurface distribution of saltwater and freshwater is a critical step in predicting, and managing, changes in water chemistry in coastal regions. Our research is focused on California’s Monterey Bay region where agriculturally-intensive land meets the sensitive marine environment of the Monterey Bay National Marine Sanctuary. Along the coast of Monterey Bay extensive groundwater extraction (groundwater provides more than 80% of the area’s water supply) has led to saltwater intrusion into aquifers at various locations. To date, the mapping of saltwater intrusion has relied on measurements of changing water chemistry in monitoring wells. But it is challenging with wells to capture the spatially complex hydrostratigraphy resulting from changing depositional environments and numerous faulting events. We suggest that geophysical methods be used to map and monitor the distribution of saltwater and freshwater by acquiring non-invasive, high-resolution continuous images of the subsurface. In a pilot study conducted over the past four years, we used electrical resistivity imaging to successfully identify regions of saltwater and freshwater 150 m below sea level along a 7 km stretch of the southern Monterey Bay coast. We employed large-offset electrical resistance tomography using a 96-electrode system with an overall array length of 860 m. The results showed excellent agreement with measurements in nearby monitoring wells. The large-scale image provided by the geophysical measurements revealed the hydrostratigraphic controls on the spatial distribution of the saltwater/freshwater interface. In October 2014 we will expand this study, using large-offset electrical resistance tomography to image to a depth of 300 m along a 40 km stretch of the Monterey Bay coast. The acquisition of this continuous dataset will provide an improved understanding of the biophysical and human factors controlling the processes of saltwater intrusion and freshwater flow in this coastal region.