The importance of groundwater discharge to the methane budgets of nearshore and continental shelf waters of the northeastern Gulf of Mexico

Methane concentrations in groundwater (wells, sinkholes, and springs) averaged 61 ± 9 μM, while concentrations in nearshore and continental shelf waters within the study area averaged 62 ± 7 nM and 27 ± 5x nM, respectively. We tested the hypothesis that the three orders of magnitude difference between groundwater and seawater concentration would make CH, an indicator of submarine groundwater discharge to surficial waters. Methane budgets for nearshore and continental shelf water columns were consistent with the hypothesis that groundwater seepage or seawater recirculation through the seabed is the dominant source of CH4 relative to benthic diffusive flux, riverine flux, and in situ water column production. Seepage/recirculation appears to account for approximately 83–99% of the total CH4 input into the water column within the study area. Utilizing measured porewater CH4 concentrations, the calculated amounts of seepage required to support the observed benthic fluxes were comparable to seepage rates measured in the field. Nearshore seepage meter transects showed a strong and direct correlation between the integrated quantity of groundwater seepage along a shoreline and the inventory of CH4 in those waters. Our study further showed a similar correlation between 222Rn (another potential groundwater tracer) and CH4 in offshore waters supporting the hypothesis of a common benthic source for these constituents.