Marine electromagnetic imaging and volumetric estimation of large-scale freshwater plumes offshore Hawai'i

Submarine groundwater discharge (SGD) is a flow of cold and buoyant freshwater from the seafloor the ocean surface. Because SGD contains carbon, nutrients, metals, and green-house gases, it changes the oceanographical and biochemical properties of coastal waters. Therefore, SGD is an important phenomenon that governs hydrological cycles at the land-to-ocean transition zone. Due to the high spatial distribution and variability of SGD at the ocean surface, it is nontrivial to map SGD seep location and fluxes using traditional oceanographic methods. Here, we present electromagnetic imaging of large freshwater plumes in high-resolution, offshore west of Hawai‘i island. Our electrical resistivity models detect multiple vertical freshwater plumes (SGD point-sources) as well as spatially distributed surface freshwater, extending to a distance of ~3 km offshore Hawai‘i. Plume-scale salinity distribution indicates that these plumes contain up to 87% of freshwater. Thus, a substantial volume of freshwater occupies Hawaiian water column plumes. Our findings provide valuable information to elucidate hydrogeologic and oceanographic processes affecting biogeochemical cycles in coastal waters worldwide. This is the first study to demonstrate the marine electromagnetic method’s capability to image and delineate freshwater plumes from the seafloor to the ocean surface.

Keywords: Freshwater Plumes, SGD, Hawai'i, Surface-towed CSEM, high-resolution 2D electrical imaging.