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 μg/kg, and was associated with plumes
of dissolved Fe, Mn, and P in the groundwater. Sedimentary
As, ranging from 360 to 7500 μg/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.