ELECTROMAGNETIC GEOPHYSICAL MAPPING OF OFFSHORE FRESHENED GROUNDWATER ON THE U.S. ATLANTIC MARGIN (Invited Presentation)

Offshore aquifers within continental shelves are a global phenomenon, yet the distribution and volume of these groundwater systems are poorly constrained due to limited observations. Improved characterization of the extent and geometry of offshore aquifers is necessary in order to better understand the mechanisms of offshore groundwater emplacement, potential hydrologic connections to onshore systems, the influence of submarine groundwater on biogeochemical cycles, and the feasibility of sustainable extraction. Electromagnetic methods are well suited for this characterization as the salinity difference between fresh groundwater and seawater results in a strong electrical resistivity contrast. Here, we use shallow water electromagnetic geophysical methods to image laterally continuous submarine aquifers extending 90 km offshore New Jersey and Martha’s Vineyard, Massachusetts, on the U.S. Atlantic margin. In both surveys, the aquifer thickness is about 200 m and ranges from 50 m to 400 m below sea level and is underlain by briny pore water from 400 m to at least 800 m below sea level. Our electromagnetic imaging shows the lowest aquifer salinities exist closest to shore, implying an active connection to the onshore groundwater system. Our results suggest a continuous aquifer system may span the distance between our two surveys and contains about 2800 km³ of low-salinity groundwater, if similar aquifer geometry is present along the margin. Our results are consistent with pore water salinity observations from previous drilling campaigns offshore New Jersey and onshore Martha’s Vineyard. We identify clinoforms as a previously unknown structural control on the seaward extent of low-salinity groundwater and potentially a control on where low-salinity water rises into the seafloor. Our study highlights the effectiveness of using electromagnetic data to image offshore groundwater. We also demonstrate how model resolution is improved when combining surface towed controlled source electromagnetic data and magnetotelluric soundings.