A New Mechanism for Submarine Groundwater Discharge From Continental Shelves

Abstract

Marine tracer studies indicate that large volumes of saline groundwater discharge to the ocean in passive margin settings. These results have not found widespread recognition because the location and cause(s) of this submarine groundwater discharge (SGD) are unclear. Here we report observations from a new long-term seafloor monitoring network in the South Atlantic Bight that support large-scale SGD far from shore. In the study area near Charleston, South Carolina, we determined hydrostratigraphy via vibracoring and chirp seismic surveys, collected water samples from seafloor wells, and used heat as a tracer to monitor SGD. We detected significant pulses of saline SGD issuing from the seafloor 10-15 km from shore. These pulses coincided with abrupt sea level declines of up to 30 cm. Based on an analysis of marine conditions at the time, we propose that upwelling-favorable winds depressed sea level in the region, causing saline groundwater to discharge from confined coastal aquifers that connect land and ocean. The combination of stacked confined aquifers and variations in sea level are nearly ubiquitous in passive coastal margins. This previously overlooked combination can explain a wide range of other published observations and promotes more dynamic flows than simple tidal fluctuations. This new mechanism may explain Ra tracer signals in the coastal Atlantic Ocean and supports significant nutrient inputs to the ocean. These large natural geochemical fluxes may be sensitive to groundwater usage on land. Plain Language Summary Previous studies based on the chemistry of coastal ocean water show that saline groundwater must discharge to the ocean. In the Atlantic Ocean, the rate of groundwater discharge roughly equals river discharge, and the groundwater delivers more nutrients to the ocean than rivers. Studies of marine ecosystems largely ignore these inputs because it is not clear where or why these flows occur. We installed wells in the seafloor 10-20 km offshore to monitor the composition of groundwater and the temperature of the sediments below the seafloor. We used heat as a tracer to calculate groundwater flow rates. We found that groundwater discharged to the ocean in response to anomalous changes in sea level, likely associated with wind events. This can only happen if permeable sediment layers (aquifers) form a hydraulic connection between the land and the seafloor. Aquifers like this are extremely common in coastal regions, and wind-related variations in sea level are equally common. The combination of stacked coastal aquifers and variations in sea level can explain many different observations. These large groundwater pulses probably occur widely, far beyond the southeastern United States. Groundwater pumping could affect this discharge, which could in turn alter coastal ecosystems.

Coastal Groundwater Flow at the Nearshore and Embayment Scales: A Field and Modeling Study

“…Whereas SGD from the salt marsh declined significantly as the marsh drowned, SGD from the Chicora Member aquifer increased very slightly. Our models relied on simple sinusoidal tidal signals that do not capture the full range of variation in sea level, and these additional variations have the potential to cause significant exchange (George et al, 2020). This additional exchange could increase the salinity in offshore aquifers where they intersect the seafloor, affecting the ability of electrical resistivity surveys to detect the ends of offshore confining units.…”

“…Our models used a simple sinusoidal tide and only extended 15 km from the shoreline. This means that our models did not capture the kind of offshore flow described by George et al (2020), who showed that significant deviations in sea level can drive significant pulses of discharge at least as far as 15 km offshore.…”

Saltwater Intrusion and Submarine Groundwater Discharge: Acceleration of Biogeochemical Reactions in Changing Coastal Aquifers

“…Wahl et al (2014) used seasonal sea level harmonics throughout the 20th and early 21st centuries to document a significant amplification of the annual sea level cycle in the eastern Gulf of Mexico of up to 30% (average 22%) from the 1990s, brought on by both higher summer and lower winter sea levels. Shorter-term changes in sea level result from cyclones and other major storms (Moore and Wilson, 2005;Wilson et al, 2011) or persistent offshore wind events (George et al, 2020). There is growing evidence that cyclones are increasing in intensity and perhaps numbers.…”

“…Though SGD and seawater intrusion may seem intuitively to be opposing processes, Taniguchi et al (2006) recognized that they are, in fact, complementary. Much of the seawater that intrudes into the aquifer actively circulates between the aquifer and the ocean in response to periodic forcing functions, including tides (Moore et al, 2002;Robinson et al, 2007), storms (Wilson et al, 2011), persistent wind events (George et al, 2020), seasonality in precipitation and recharge (Michael et al, 2005), seasonality in sea level (Wahl et al, 2014), and seasonality in freshwater withdrawal (Moore, 1999). The water discharging as SGD has a radically different chemical signature compared to the fresh or saltwater that entered the aquifer (Moore, 1999(Moore, , 2010.…”

The Subterranean Estuary: Technical Term, Simple Analogy, or Source of Confusion?

“…To resolve these sources of confusion, we propose an addition to the original definition in which the subterranean estuary is defined as the part of the coastal aquifer that interacts actively with the ocean. This definition includes fresh groundwater that is influenced by tides or that could discharge to the ocean, the zone of mixing between fresh and saline groundwater, and entirely saline groundwater that may exchange with the ocean far offshore (see George et al, n.d.). The analogy with surface estuaries breaks down somewhat with this definition, because although surface estuaries commonly include freshwater tidal systems, they typically do not extend to purely saline systems.…”