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1. Transport of Radium and Nutrients Through Eastern South African Beaches

   https://doi.org/10.1029/2018JC014772  

   Moore, Willard S. ; Humphries, Marc S. ; Benitez‐Nelson, Claudia R. ; Pillay, Letitia ; Higgs, Caldin ( March 2019 , Journal of Geophysical Research: Oceans)

   Submarine groundwater discharge (SGD) plays a critical role in coastal and ocean biogeochemistry. Elucidating spatially and temporally heterogeneous SGD fluxes is difficult. Here we use radium isotopes to explore the external sources and mixing regime along the eastern coast of South Africa. We demonstrate that the long‐lived radium isotope compositions are controlled by low inputs of low‐ and high‐salinity terrestrial groundwater. While activities of²²⁸Ra and²²⁶Ra in beach porewaters are similar to coastal waters,²²⁴Ra is enriched by inputs of²²⁸Th from coastal seawater. Porewater ages, based on the production of²²⁴Ra from²²⁸Th, range from 0.3 to 2.3 days, indicating rapid flushing of the beach system. Unlike radium, however, nutrients follow a more complex pathway. We hypothesize that high total dissolved nitrogen (TDN) and phosphorus concentrations in beach porewaters (TDN ranges from 1 to >700 μM) and the coastal ocean (TDN ranges from 1 to >40 μM) are derived from a source not enriched in radium. We speculate that this source is terrestrial water flowing below the dune barrier at depths exceeding our beach sampling depths. This water likely flows upward through breaches in the confining layer into the beach or enters the ocean directly through paleochannels. The presence of high nutrient concentrations in the coastal ocean unaccompanied by high²²⁸Ra activities leads to the hypothesis of this additional nutrient source. These combined inputs may be of considerable importance to the coastal ecology of southeastern Africa, an oligotrophic ecosystem dominated by the nutrient‐poor Agulhas Current.

    

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2. Temporal and Spatial Variations in Subterranean Estuary Geochemical Gradients and Nutrient Cycling Rates: Impacts on Groundwater Nutrient Export to Estuaries

   https://doi.org/10.1029/2022JG007132  

   Wilson, Stephanie J. ; Anderson, Iris C. ; Song, Bongkeun ; Tobias, Craig R. ( June 2023 , Journal of Geophysical Research: Biogeosciences)

   Subterranean estuaries (STEs) form at the land‐sea boundary where groundwater and seawater mix. These biogeochemically reactive zones influence groundwater‐borne nutrient concentrations and speciation prior to export via submarine groundwater discharge (SGD). We examined a STE located along the York River Estuary (YRE) to determine if SGD delivers dissolved inorganic nitrogen (DIN) and phosphorus (DIP) to the overlying water. We assessed variations in STE geochemical profiles with depth across locations, times, and tidal stages, estimated N removal along the STE flow path, measured hydraulic gradients to estimate SGD, and calculated potential nutrient fluxes. Salinity, dissolved oxygen (DO), DIN, and DIP varied significantly with depth and season (p < 0.05), but not location or tidal stage. Ammonium dominated the DIN pool deep in the STE. Moving toward the sediment surface, ammonium concentrations decreased as nitrate and DO concentrations increased, suggesting nitrification. Potential sediment N removal rates mediated by denitrification were <8 mmoles N m⁻² d⁻¹. The total groundwater discharge rate was 38 ± 11 L m⁻² d⁻¹; discharge followed tidal and seasonal patterns. Net SGD nutrient fluxes were 0.065–3.2 and 0.019–0.093 mmoles m⁻² d⁻¹for DIN and DIP, respectively. However, microbial N removal in the STE may attenuate 0.58% to >100% of groundwater DIN. SGD fluxes were on the same order of magnitude as diffusive benthic fluxes but accounted for <10% of the nutrients delivered by fluvial advection in the YRE. Our results indicate the importance of STE biogeochemical transformations to SGD flux estimations and their role in coastal eutrophication and nutrient dynamics.

    

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3. A New Mechanism for Submarine Groundwater Discharge From Continental Shelves

   https://doi.org/10.1029/2019WR026866  

   George, Camaron ; Moore, Willard S. ; White, Scott M. ; Smoak, Erin ; Joye, Samantha B. ; Leier, Andrew ; Wilson, Alicia M. ( October 2020 , Water Resources Research)

   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.

    

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4. Groundwater‐Driven Methane Export Reduces Salt Marsh Blue Carbon Potential

   https://doi.org/10.1029/2020GB006587  

   Schutte, C. A. ; Moore, W. S. ; Wilson, A. M. ; Joye, S. B. ( October 2020 , Global Biogeochemical Cycles)

   The burial of “blue carbon” in coastal marsh soils is partially offset by marsh‐atmosphere methane (CH₄) fluxes, but this offset may be greater if other pathways of CH₄export exist. Here we report that salt marshes also export dissolved CH₄via submarine groundwater discharge (SGD). The volumetric fluxes of salt marsh groundwater into adjacent tidal creeks were calculated from mass balances of the conservative tracer²²⁶Ra at four study sites in coastal Georgia, USA. Over the 2‐year study period, volumetric groundwater fluxes across all sites ranged between 1,700 and 105,000 m³ day⁻¹. Dissolved CH₄fluxes of 27–1,200 μmol CH₄m⁻² day⁻¹were calculated by multiplying the volumetric groundwater flux by the groundwater CH₄concentration and normalizing to the intertidal salt marsh area estimated from satellite images. On a mass basis, the cross‐site range in CH₄fluxes was 1.3–5.5 g CH₄ m⁻² year⁻¹with a cross‐site mean of 2.8 g CH₄ m⁻² year⁻¹. This is equivalent to 125 (56–245) g CO₂ m⁻² year⁻¹assuming that CH₄is 45 times more potent than CO₂as a greenhouse gas over a 100‐year time frame. This sustained‐flux global warming potential is similar to the 138 (1.1–260) g CO₂ m⁻² year⁻¹average calculated across other studies of the direct marsh soil to atmosphere CH₄flux. Therefore, SGD drives an effective doubling of salt marsh CH₄export that offsets a combined total of ~30% of the global cooling potential derived from soil carbon sequestration.

    

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5. Radon and Salinity Mass Balance Constraints on Groundwater Recharge on a Semi-Arid Island (Catalina, California)

   https://doi.org/10.3390/w14071068  

   Hagedorn, Benjamin ; Tsuda, Mitsuyo ( April 2022 , Water)

   Quantifying the freshwater component of submarine groundwater discharge (SGD) is critical in the analysis of terrestrial influences on marine ecosystems and in assessing the water budget and groundwater recharge of coastal aquifers. In semi-arid to arid settings, this quantification is difficult because low SGD rates translate into low concentrations of groundwater solutes in coastal waters. In this study, fresh SGD (FSGD) was quantified for Toyon Bay on Catalina Island, California, for wet and dry seasons using a combination of radon and salinity mass balance models, and the results were compared to watershed-specific groundwater recharge rates obtained from soil water balance (SWB) modeling. Calculated FSGD rates vary only slightly with season and are remarkably similar to the recharge estimates from the SWB model. While sensitivity analyses revealed FSGD estimates to be significantly influenced by uncertainties in geochemical variability of the groundwater end-member and fluctuations of water depth, the results of this study support the SWB-model-based recharge rates. The findings of this study highlight the utility of the radon-and-salinity-mass-balance-based FSGD estimates as groundwater recharge calibration targets, which may aid in establishing more refined sustainable groundwater yields. 

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