Mapping Shoreline Groundwater Interactions in Southern California Using Geophysical Methods
Presented on Tuesday, May 6, 2014
John Jansen, Ph.D., PG1 and Leslie Dumas, P.E., D.WRE2, (1)Leggette, Brashears and Graham, West Bend, WI, (2)RMC Water and Environment, Walnut Creek, CA
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.
John Jansen, Ph.D., PG
Leggette, Brashears and Graham, West Bend, WI
John Jansen, Ph.D., is a Senior Associate with Collier Consulting and a hydrogeologist and geophysicist. He has modeled a variety of unconventional wells to compare relative yield and developed a new horizontal cryogenic drilling method.
Leslie Dumas, P.E., D.WRE
RMC Water and Environment, Walnut Creek, CA
Leslie Dumas is a hydrologist and senior project manager with RMC Water and Environment. She graduated with a B.S. in Civil Engineering from Virginia Tech, and an M.S. in Civil Engineering from the University of California at Berkeley. In her 15+ years of practicing engineering, Dumas has provided hydrogeologic, hydrologic, environmental, and scientific consultation for projects throughout the United States. She has managed multidisciplinary teams on a wide variety of projects including water resources planning, groundwater investigation and modeling, resource planning, environmental permitting, and site remediation.
