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

Abstract

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.

Electrical Resistivity Anomalies Offshore a Carbonate Coastline: Evidence for Freshened Groundwater?

“…Several studies have demonstrated the effectiveness of controlled-source electromagnetic (CSEM) methods to image the electrical resistivity distributions associated with OFG along continental margins and volcanic islands (e.g., Attias et al, 2020Attias et al, , 2021Gustafson et al, 2019;Haroon, Lippert, et al, 2018;Micallef et al, 2020). The bulk electrical resistivity is sensitive to both changes in pore fluid salinity and sediment matrix porosity (Archie, 1942).…”

Nitrogen stable isotopes (δ15N) and tissue nitrogen in shallow‐water and mesophotic macroalgae differ between the Main Hawaiian Islands and the Northwestern Hawaiian Islands

“…2a, 3a) at 70–95 m depths, suggesting the possibility that novel inputs of sewage or products from other biogeochemical processes may reach mesophotic depths (Dailer et al 2010, 2012a). Recent studies of groundwater flow in and around Hawaiʻi island describe a newly found transport mechanism of fresh groundwater from onshore to offshore depths in volcanic islands with multilayer basaltic formations (Attias et al 2020, 2021). It is conceivable that somewhat older, adjacent high islands of Oʻahu and Maui may have similar processes underway (Attias et al 2020).…”

“…Knowledge of the nutrient regimes from shallow to mesophotic reefs throughout the Hawaiian Archipelago, informed by known land‐use practices and their recent changes, could be used to identify areas that are likely to be influenced by point and nonpoint pollution around the Main Hawaiian Islands via increased nitrogen loading in macroalgal tissue (Lapointe and Bedford 2011; Dailer et al 2012a,b; Amato et al 2016). SGD clearly plays a role in supplying enhanced nutrient loads to shallow‐water depths around the Main Hawaiian Islands (Dailer et al 2012a,b; Amato et al 2018; Dulai et al 2021), and may also fuel the growth of mesophotic invasive or bloom‐forming macroalgae in volcanic islands via submarine vents (Attias et al 2020, 2021). Conversely, upwelling or internal waves from deeper water (200 to 350+ m depths) may introduce nitrogen inputs with altered nitrate δ 15 N in the range of 6–7‰ (Casciotti et al 2008; Knapp et al 2011; Wilson et al 2019).…”

Identification of fresh submarine groundwater off the coast of San Diego, USA, using electromagnetic methods

“…Historically, electromagnetic (EM) surveying has been used on land to map groundwater resources with great success (Knight et al 2018;McNeill 1988). Also, EM methods have been proposed by many authors to identify salinity of submarine groundwater (Cohen et al 2010;Haroon et al 2021;Micallef et al 2020Micallef et al , 2021Post et al 2013) and several controlled-source electromagnetic (CSEM) surveys have been successful at identifying groundwater offshore, most notably offshore Martha's Vineyard (Gustafson et al 2019) and Hawaii (Attias et al 2021). The geologic setting of San Diego varies from these previous studies in that the presence of offshore groundwater has not yet been confirmed, and the SDF is heavily faulted, possibly isolating pockets of freshwater offshore.…”