Californian aquifers located near coastal regions face stress due to saltwater intrusion, which has contaminated many coastal aquifers, rendered municipal drinking water supplies unpotable, and has reduced vital freshwater discharge to coastal ecosystems.
A delicate balance must be struck between human groundwater demand and the strain on coastal environmental health. Without effective evaluation and management of coastal groundwater, the sustainability of coastal aquifers is in jeopardy, which will have negative ramifications for human use and the broader environment alike. Field scale studies using direct methods to resolve spatial saltwater intrusion are plagued by a lack of data, since many expensive and deep wells must be drilled to collect data in even a small—often biased—fraction of the area of interest.
We are learning more about the process of saltwater intrusion by integrating geophysical methods with numerical saltwater intrusion models. In contrast to point measurements from wells, indirect geophysical methods provide near-continuous measurements over a sampled domain. We have collected airborne electromagnetic (AEM) data in the Northern Salinas Valley, in partnership with a local groundwater agency, Marina Coast Water District. These data have been used to produce images of the subsurface resistivity, revealing a complex distribution of both saltwater and sources of drinking water in the area.
Resistivity measurements contain uncertainty, since changes in the measured resistivity can often be attributed to either heterogeneity of the sediment type or to changes in the groundwater salinity. To quantify and constrain uncertainty, we are integrating our airborne electromagnetic data with numerical simulations of saltwater intrusion. We are leveraging geostatistical methods to simulate the distribution of sediment types, consistent with data we have collected, for use in our simulations of saltwater intrusion.
Through this approach, we are investigating new ways to predict the salinity distribution and heterogeneity of sediment type in coastal margins. Through this research we will develop a new methodology that will build on the use of airborne electromagnetic data to predict the intrusion of saltwater into coastal aquifers.
Airborne Geophysical Method Maps Fresh Water and Salt Water Along the Monterey Coast
Using AEM Data to Interpret Hydrostratigraphy and Water Quality
Marina Coast Water District (http://mcwd.org/)
The Zeitgeist Foundation
The S. D. Bechtel, Jr. Foundation (http://sdbjrfoundation.org/)