CSEM Exploration for Submarine Groundwater: Paths for Encroachment or Exploitation?

Abstract:

Groundwater under coastal communities is especially vulnerable to overdraft and saltwater intrusion. These effects can lead to high total dissolved solids in the drinking water which, if not treated, can result in health issues such as hypertension. Most commonly, reverse osmosis (RO) is used to treat this brackish to saline water for safe human use. However, management of a RO plant can be complicated by unexpected changes in influent water quality; thus, safe and effective use of a RO system requires a thorough understanding of the groundwater. In San Diego, 3.6 billion gallons of drinking water a year comes from a RO plant that extracts brackish water from the coastal San Diego Formation (SDF). The SDF has been extensively studied onshore, but little is known of the geology or groundwater quality extending offshore under the San Diego Bight. As these groundwater systems can be complex and interconnected, the offshore geology needs further analysis to identify possible sequestration of freshwater in the shelf and potential pathways for saltwater encroachment. Controlled-source electromagnetic (CSEM) methods are uniquely suited toward this investigation as they are sensitive to changes in pore fluids, such as those found in the transition from fresh to brackish groundwater. Here, we present preliminary results from a series of surface-towed CSEM surveys designed to better constrain the pore fluids and possible fluid pathways within the San Diego Bight. The preliminary results indicate a considerable volume of fresh to brackish water is sequestered in the shelf; however, it is still unknown if this feature could be used as a resource or what effect it would have on saltwater intrusion as pumping continues onshore.