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.

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The spatial variations in Quaternary sediments on the inner continental shelf are produced by the progression of depositional environments during the latest sea-level rise, and this sedimentary architecture plays a fundamental role in controlling groundwater discharge. However, coincident seismic mapping, sediment cores, and hydrological studies are rare. Here, we combine high-resolution, 0.5–10 kHz, high-frequency seismic profiles with sediment cores to examine the nature of the sediment deposits, including paleochannels, where submarine groundwater discharge has also been studied in a 150 km2 area of the inner shelf north of Charleston, South Carolina. We used high-frequency seismic reflection to interpret seismic facies boundaries, including 16 paleochannel crossings, to 20 km offshore. From 13 vibracores taken at the intersections of the seismic lines, we defined seven lithofacies representative of specific depositional environments. The paleochannels that we cored contain thick layers of structureless mud sometimes interbedded with silt, and mud is common in several of the nearshore cores. Our results indicate that paleochannels are often mud-lined or filled in this area and were most likely former estuarine channels. Neither the paleochannels nor a mud layer were found farther than 11 km off the present shoreline. This offshore distance coincides with the strongest pulses of groundwater discharge, emerging just beyond the paleochannels. This suggests that the muddy paleochannel system acts as a confining layer for submarine groundwater flow.