Characterizing the fresh-saltwater interface of submarine aquifers
PI: A. Wilson (Uni. of South Carolina)
co-PI's: R. Evans (WHOI), E. Attias (UTIG), A. Haroon (HIGP)
The freshwater-saltwater interface is a key feature in coastal aquifers. From a water supply perspective, this interface marks the seaward boundary of fresh groundwater resources, now known to extend tens of kilometers offshore in many world regions. From an oceanographic perspective, the freshwater-saltwater interface can be viewed instead as the landward boundary of subseafloor seawater-groundwater exchange, which exports nutrient-rich saline groundwater to the ocean in volumes similar to river discharge. The freshwater-saltwater interface thus ties together such seemingly diverse interests as water supply and nutrient cycling in continental shelf ecosystems. Still, the location of the freshwater-saltwater interface is very poorly known in all but the shallowest aquifers. Recently, a new generation of geophysical surveys has demonstrated the ability of CSEM and MT surveys to detect freshened groundwater below the seafloor at significant depths in four locations where large supplies of fresh groundwater offshore had already been identified. Integration between geophysical and hydrogeological approaches is still evolving.
Conceptual models for coastal groundwater flow.
INTERFACE will perform a joint analysis combining marine EM (MT and CSEM) and multi-channel seismic data with numerical models offshore of South Carolina. This work will test the overarching hypothesis that current models (conceptual, analytical, and numerical) seriously underestimate the degree of saltwater intrusion below the seafloor because short-term (daily, seasonal, interannual) sea-level variations have been neglected. The South Carolina shelf is ideal for this coupled geophysical-hydrogeological investigation because of extensive knowledge of the hydrostratigraphy onshore, including the USGS regional groundwater model for the Atlantic coastal plain of North and South Carolina. This location is also unique as a nexus of research in offshore submarine groundwater discharge (SGD) and seawater-groundwater exchange. This work will significantly advance scientific understanding of the controls on the location of the freshwater-saltwater interface and seawater-groundwater interactions while connecting directly with coastal water supply for such cities as Charleston, Georgetown, and Myrtle Beach, SC.
INTERFACE survey layout: Joint multi-channel seismic (MCS), Ocean-Bottom Electromagnetic (OBEM) receivers, and the SWAN near-surface towed CSEM system.
SWAN: Near-surface towed CSEM system (Pastoressa et al., 2023)
Mapping offshore fresh groundwater reserves is a high priority for a world that supports large and growing coastal populations. However, the mapping process is still young and expensive, so each target must be chosen to bring maximum returns. Whereas prior CSEM projects focused primarily on mapping the volume of fresh groundwater, this project will make equal use of the volume of saline groundwater below the seafloor to interpret the combined effects of saltwater intrusion during sea level rise and major seawater-groundwater exchange offshore. The same models that allow us to assess freshwater flow and aquifer salinization will allow us to assess SGD fluxes across a broad seafloor swath. This project will bring together geophysicists, hydrogeologists, USGS scientists, and state agency personnel to integrate interpretations of the stratigraphy, hydrostratigraphy, and offshore geophysical survey results fully.