Groundwater discharge impacts marine isotope budgets of Li, Mg, Ca, Sr, and Ba.

Groundwater-derived solute fluxes to the ocean have long been assumed static and subordinate to riverine fluxes, if not neglected entirely, in marine isotope budgets. Here we present concentration and isotope data for Li, Mg, Ca, Sr, and Ba in coastal groundwaters to constrain the importance of groundwater discharge in mediating the magnitude and isotopic composition of terrestrially derived solute fluxes to the ocean. Data were extrapolated globally using three independent volumetric estimates of groundwater discharge to coastal waters, from which we estimate that groundwater-derived solute fluxes represent, at a minimum, 5% of riverine fluxes for Li, Mg, Ca, Sr, and Ba. The isotopic compositions of the groundwater-derived Mg, Ca, and Sr fluxes are distinct from global riverine averages, while Li and Ba fluxes are isotopically indistinguishable from rivers. These differences reflect a strong dependence on coastal lithology that should be considered a priority for parameterization in Earth-system models.

Hydrologic controls on nutrient cycling in an unconfined coastal aquifer.

Groundwater is an important pathway for terrestrially derived nutrients to enter the coastal ocean. In coastal aquifers, groundwater transits the subterranean estuary, a region of sharp gradients in redox conditions and the availability of reactants. In one such system (Waquoit Bay, MA, USA), we observed more than a doubling of the groundwater-associated nitrogen flux to surface water during the summer compared to winter due primarily to a reduction in nitrogen attenuation within the subterranean estuary. Because marine groundwater intrusion has been shown to increase during the summer, we calculate a greater contribution of recycled nutrients from the coastal ocean to the subterranean estuary. We posit that the longer residence times within the subterranean estuary during the winter, which would result from reduced marine groundwater circulation, allow oxygen depletion of the groundwater, creating a favorable environment for important nutrient transformations such as nitrification, denitrification, and anammox. The timing of nutrient delivery to the coastal ocean has important implications for coastal marine ecology including the potential development of harmful algal blooms.

Has submarine groundwater discharge been overlooked as a source of mercury to coastal waters?

We measured the mercury (Hg) in groundwater, aquifer sediments, and surface water in Waquoit Bay (Massachusetts) and found that this toxic metal (range: <3.2-262 pM) was being released within the subterranean estuary, with similarly high levels (range: 18-256 pM) found in the surface waters of the bay. None of the dissolved species (DOC, chloride, and Fe) normally observed to influence Hg partitioning correlated well with the observed Hg concentrations. It was hypothesized that this was in part due to the variable loading in time and space of Hg onto the aquifer sands in combination with the seasonality of groundwater flow through the aquifer. Aquifer sediment samples from the study site ranged from <1 to 12.5 pmol of Hg/g of sediment, suggesting log Kd values on the order of 1. We hypothesize that this was due to the low organic carbon content typical of the aquifer sediments. Last, itwas estimated that submarine groundwater discharge supplied 0.47-1.9 nmol of Hg m(-2) day(-1) to the bay, which is an order of magnitude higher than the atmospheric deposition rate for the northeastern U.S.

Geochemical cycling of arsenic in a coastal aquifer.

Biogeochemically modified pore waters from subterranean estuaries, defined as the mixing zone between freshwater and saltwater in a coastal aquifer, are transported to coastal waters through submarine groundwater discharge (SGD). SGD has been shown to impact coastal and perhaps global trace metal budgets. The focus of this study was to investigate the biogeochemical processes that control arsenic cycling in subterranean estuaries. Total dissolved As, as well as a suite of other trace metals and nutrients, were measured in a series of wells and sediment cores at the head of Waquoit Bay, MA. Dissolved As ranged from below detection to 9.5 microg/kg, and was associated with plumes of dissolved Fe, Mn, and P in the groundwater. Sedimentary As, ranging from 360 to 7500 microg/kg, was highly correlated with sedimentary Fe, Mn, and P. In addition, amorphous Fe (hydr)oxides were more efficient scavengers of dissolved As than the more crystalline forms of solid-phase Fe. Given that dissolved As in the surface bay water was lower than within the subterranean estuary, our results indicate that the distribution and type of Fe and Mn (hydr)oxides in coastal aquifers exert a major influence on the biogeochemical cycling of As in subterranean estuaries and, ultimately, the fate of groundwater-derived As in marine systems influenced by SGD.