Mapping Shoreline Groundwater Interactions in Southern California Using Geophysical Methods

The interface between fresh groundwater and saline water in coastal areas is frequently complex. The natural discharge of groundwater to the near shore zone is controlled by multiple factors, including the permeability of the aquifer system and the hydraulic gradient in the aquifer. The natural balance is often disturbed by heavy pumping on shore that can cause significant intrusion of salt water into the onshore portion of the aquifer. The shape of the salt water wedge is affected by the pattern of pumping, the properties of the aquifer, and hydraulic boundaries like faults and confining units.

Generally it is impossible to accurately map the pattern of the salt water/fresh water interface with boreholes and monitoring wells due to the cost of drilling, the limited three-dimensional resolution that can be provided by wells with long screened intervals, and the expense of drilling offshore. Several electrical geophysical methods can be used onshore and offshore to map the location of the interface. These methods are faster, much less expensive, and can provide a higher level of three-dimensional detail on the position of the salt water plume. The methods can be used to select optimal locations for monitoring wells, confirm the performance of salt water intrusion barriers, or calibrate solute transport models.

This presentation uses three case histories from southern California to demonstrate how electrical resistivity and time domain electromagnetic induction methods have been used to map salt water intrusion as it moves onshore in a faulted and layered multiple aquifer system, map the migration of remnant salt water plumes behind barrier wells systems, and map the differences in offshore discharge of groundwater in undisturbed aquifer systems as a function of aquifer properties.