Mapping Saltwater Intrusion in an Offshore Coastal Aquifer Using Airborne Electromagnetic Data

Link to the published paper

The Place

Along the coast of the Monterey Bay in California, groundwater is the primary source of freshwater, supporting agricultural, industrial, domestic, and ecosystem needs. Groundwater extraction over the past century has resulted in significant saltwater intrusion along some parts of bay, while others still observe freshwater at the coast.

The Problem

Saltwater intrusion is the process by which saltwater migrates into a portion of a coastal aquifer that previously contained freshwater, often as a result of anthropogenic changes such as increased groundwater pumping or diversion of recharge sources. Where aquifers extend offshore, the freshwater/saltwater interface can exist in the offshore portion of the aquifer, making it very difficult to map or monitor using traditional approaches. There is a need for understanding and monitoring any movement of these interfaces.

Our Approach

We use airborne electromagnetic (AEM) imaging to map saltwater intrusion in the offshore environment. This method utilized a helicopter towed instrument, which acquires the data to generate models of the electrical resistivity of the subsurface with depth. Because of the large contrast in the electrical resistivity of saltwater saturated sediment and freshwater saturated sediments, this approach is well suited to mapping out saltwater intrusion. Because the system is towed with a helicopter, data can be acquired over open water, where it is very difficult or impossible to acquire data with other methods.

Results

Three-hundred and twenty line-kilometers of airborne electromagnetic (AEM) data were acquired offshore, near Santa Cruz, CA. 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.