Nutrient biogeochemistry in a Gulf of Mexico subterranean estuary and groundwater‐derived fluxes to the coastal ocean

Abstract

We hypothesize that biogeochemical processes in a subterranean estuary in the northeastern Gulf of Mexico significantly alter nutrient concentrations that are discharged into the ocean via submarine groundwater discharge (SGD). This subterranean estuary is a site of steep gradients over small spatial scales, so selecting the appropriate groundwater endmember for flux calculations is difficult. Two-endmember mixing models typically applied to investigate biogeochemical processes in surface estuaries are not useful in this case because of aquifer heterogeneity and complex solid-fluid interactions. Groundwater residence times, redox potential, source strength, and aquifer matrix appear to be the most important drivers for dissolved organic carbon (DOC) and nutrient concentrations in this pristine coastal plain aquifer. The distribution of dissolved species was consistent with a sequence of reactions in a narrow (,2 m) surface layer in which nitrate is initially exhausted (likely because of denitrification), organic nitrogen is remineralized releasing ammonium, and some DOC remains. On the basis of a 222 Rn-derived non-steady state box model, total (terrestrial + marine) SGD rates were estimated to oscillate around 11 cm d 21 . The SGD is composed almost entirely of recirculated seawater (accounting for ,95% of total SGD fluxes). Conservative estimates of groundwater-derived nitrogen inputs into the coastal ocean were 8.2 6 1.2 mmol m 22 d 21 , consisting of ammonium (58%), organic nitrogen (28%), and nitrate (14%). We suggest that these mainly represent recycled rather than new nutrient additions. When extrapolated to the Florida Gulf coast, the nitrogen and DOC fluxes were comparable to the main regional rivers, supporting the proposition that SGD plays a major role in coastal biogeochemistry.

Dynamics of groundwater‐derived nitrate and nitrous oxide in a tidal estuary from radon mass balance modeling

“…For instance, a study by Addy et al (2005) showed that mixing between the organic-matter-rich seawater and the nutrientrich fresh groundwater increased the denitrification capacity in the STE below a Rhode Island fringing salt marsh. Santos et al (2008) found that re-circulated SGD drives oxic re-mineralization and de-nitrification in the STE of the Gulf of Mexico. However, the active biogeochemical transformations in the STE do not always take place.…”

Submarine Groundwater Discharge to a High-Energy Surf Zone at Stinson Beach, California, Estimated Using Radium Isotopes

“…Submarine groundwater discharge (SGD) is well documented as an important source of inorganic, organic, and microbial pollutants to the coastal ocean (Boehm et al 2004;Bone et al 2007;Santos et al 2008). Numerous physical forcing mechanisms including meteoric hydraulic head, tide and wave pumping, seasonal precipitation/evapotranspiration cycles, and variations in groundwater density force the flow of groundwater to the ocean (Burnett et al 2006).…”

Groundwater dynamics in subterranean estuaries of coastal unconfined aquifers: Controls on submarine groundwater discharge and chemical inputs to the ocean

“…In spite of these uncertainties, different local and regional studies demonstrate that SGDassociated fluxes of nutrients into the coastal ocean can be comparable to or even higher than the relatively better quantified river fluxes [Slomp and Van Cappellen, 2004;Santos et al, 2008]. Large scale estimates of SGD based on 228 Ra observations in large ocean basins are now available Kwon et al, 2014].…”