Identification of fresh submarine groundwater off the coast of San Diego, USA, using electromagnetic methods

Roslynn B King,Wesley R Danskin,Jillian M Maloney,Steven Constable

doi:10.1007/s10040-022-02463-y

Abstract

Climate change has a pronounced effect on water resources in many semiarid climates, causing populated areas such as San Diego County (USA), to become more vulnerable to water shortages in the coming decades. To prepare for decreased water supply, San Diego County is adopting policies to decrease water use and to develop additional local sources of water. One new local source of freshwater is produced by a desalination facility that purifies brackish groundwater from the coastal San Diego Formation. This formation has been studied extensively onshore, but little is known about the geology or groundwater quality offshore in the adjacent continental shelf. Because most groundwater systems are interconnected and complex, further analysis is needed to identify offshore geology, possible sequestration of freshwater in the shelf, and potential pathways for saltwater intrusion. This comprehensive understanding is important because seawater intrusion may limit use of the San Diego Formation and longevity of desalination facilities. Controlled-source electromagnetic methods are uniquely suited to detecting offshore groundwater as they are sensitive to changes in pore fluids such as the transition from fresh to brackish groundwater. This paper describes results from surface-towed electromagnetic surveys that mapped the pore-fluid salinity and possible fluid pathways in the continental shelf off the coast of San Diego. The results indicate a considerable volume of fresh-to-brackish groundwater sequestered in the shelf, both in continuous lenses and isolated pockets, that appear influenced by fault systems and shallow stratigraphy.

Highlights

Climate change significantly impacts the water resources available in semiarid climates and populations living within these regions are likely to become more vulnerable to water shortages
That water supply agencies can adapt to the changing climate in semiarid regions, groundwater resources could be further developed to account for an expected decrease in overall water supplies
The geologic setting of San Diego varies from these previous studies in that the presence of offshore groundwater has not yet been confirmed, and the San Diego Formation (SDF) is heavily faulted, possibly isolating pockets of freshwater offshore. This current study aims to further the understanding of the onshore groundwater system by characterizing and mapping pore fluids within the offshore part of the SDF using CSEM methods

Summary

Introduction

Climate change significantly impacts the water resources available in semiarid climates and populations living within these regions are likely to become more vulnerable to water shortages. The southwestern United States is a semiarid region hosting several large population centers, including coastal southern California with the three highest population density counties in the state (Los Angeles, Orange, and San Diego). This region is dependent upon water from snowpack in north-. San Diego County is especially vulnerable to these predicted water disruptions as the county currently purchases 85–90% of its water from northern California and the Colorado River. In coastal semiarid areas such as San Diego County, development of groundwater resources is complicated by saltwater intrusion and the potential presence of fresh groundwater reservoirs offshore. Groundwater in the San Diego region is further complicated by its location along the North American-Pacific Plate boundary where several strikeslip fault systems influence coastal geomorphology and subsurface geology

Results

Discussion

Conclusion