Mapping and Modeling Saltwater Intrusion Using Airborne Electromagnetic Data
The Problem
Coastal aquifers in California face stress due to saltwater intrusion, where saline water migrates from the ocean into terrestrial aquifers. Saltwater intrusion in the coastal Salinas Valley, CA, has rendered municipal drinking water supplies unpotable and reduced freshwater discharge to coastal ecosystems. Effective management of coastal groundwater is crucial for the sustainability of coastal aquifers.
Our Approach
We are learning more about the process of saltwater intrusion by integrating geophysical methods with numerical saltwater intrusion models. In contrast to traditional measurements, which come from boreholes, geophysical methods provide near-continuous measurements over a sampled area. We focus on is the airborne electromagnetic (AEM) method, which can be used to produce estimates of the subsurface electrical resistivity over a large area, such as the coastal Salinas Valley. Changes in either the lithology or groundwater salinity will affect the resistivity; we leverage this relationship when applying the AEM method for understanding saltwater intrusion. To reduce the uncertainty in our estimates of the salinity distribution derived from AEM data, we integrate the AEM data with numerical simulations of saltwater intrusion, which constrain the predictions of the salinity distribution using the physics of groundwater flow. Through our approach, we are investigating new methods that integrate valuable AEM data with numerical simulations to predict the salinity distribution in coastal margins.
Results
In 2017, we collected airborne electromagnetic (AEM) data in the coastal Salinas Valley, in partnership with a local groundwater agency, Marina Coast Water District. Processing these data resulted in estimates of resistivity extending from the ground surface to between 50 and 300 meters below ground surface. We have used the resistivity estimates derived from these data, along with existing measurements, to understand the complex distribution of both saltwater and fresher water in the area. The estimated resistivity also offered a snapshot of two hydrogeologic processes active in the study area, one of which is shown schematically above. For more information, see Gottschalk et al. (2020) and the videos below.
Airborne Geophysical Method Maps Fresh Water and Salt Water Along the Monterey Coast
Using AEM Data to Interpret Hydrostratigraphy and Water Quality
Project Sponsors
Marina Coast Water District (http://mcwd.org/)
The Zeitgeist Foundation
The S. D. Bechtel, Jr. Foundation (http://sdbjrfoundation.org/)
Seequent