Marine Electromagnetic Imaging and Volumetric Estimation of Freshwater Plumes Offshore Hawai'i.

Submarine groundwater discharge (SGD) is an important phenomenon that governs hydrological cycles at the land‐to‐ocean transition zone. SGD manifests as cold and buoyant freshwaters influx from the seafloor to the water column that contains carbon, nutrients, metals, and greenhouse gases, altering coastal areas' oceanographical and biochemical properties. Here, we present electromagnetic imaging of large‐scale freshwater plumes in high‐resolution, offshore west Hawai'i. Electrical resistivity models detect multiple vertical freshwater plumes extending from the seafloor to the ocean surface. Additionally, our models image extensive spatially distributed surface freshwater. The resistivity of these plumes and surface freshwater ranges from ∼1 to 30 Ωm. Resistivity‐to‐salinity calculation indicates a plume‐scale salinity range of ∼0.3–9.9, containing up to 87% of freshwater. Our results imply that substantial volumes of freshwater occupy water column plumes in Hawai'i. This study offers a new and effective method to elucidate hydrogeologic and ocean processes affecting biogeochemical cycles in coastal waters worldwide. Plain Language Summary: Submarine groundwater discharge (SGD) is a flow of cold and buoyant freshwater from the seafloor to the ocean surface. Because SGD contains carbon, nutrients, metals, and greenhouse gases, it changes coastal waters' oceanographical and biochemical properties. 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‐scale freshwater plumes in high‐resolution, offshore west Hawai'i. 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. 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. We offer a new and effective method to elucidate hydrogeologic and ocean processes that affect biogeochemical cycles in coastal waters worldwide. Key Points: Surface‐towed marine‐controlled source electromagnetic technique is capable of imaging freshwater plumes in high‐resolutionMultiple large‐scale freshwater plumes and surface freshwater bodies were detected offshore the island of Hawai'iSubstantial volumes of freshwater occupy seafloor to ocean surface plumes in west Hawai'i [ABSTRACT FROM AUTHOR]