wAbstract
@Submarine groundwater discharge (SGD) is an important source
of dissolved elements to the ocean, yet little is known regarding
the chemical reactions that control their flux from sandy coastal
aquifers. The net flux of elements from SGD to the coastal ocean
is dependent on biogeochemical reactions in the groundwater-seawater
mixing zone, recently termed the gsubterranean estuary.h This
paper is the second in a two part series on the biogeochemistry
of the Waquoit Nay coastal aquifer/subterranean estuary. The first
paper addressed the biogeochemistry of Fe, Mn, P, Ba, and Th from
the perspective of the sediment composition of cores Charette
et al. [Charette,M.A., Sholkovitz,E.R., Hansell,C.M., 2005. Trace
element cycling in a subterranean estuary: Part 1. Geochemistry
of the permeable sediments. Geochim. Cosmochim. Acta, 69,
2095-2109]. This paper uses pore water data from the subterranean
estuary, along with Bay surface water data, to establish a more
detailed view into the estuarine chemistry and the chemical diagenesis
of Fe, Mn, U, Ba and Sr in coastal aquifers. Nine high-resolution
pore water (groundwater) profiles were collected from the head
of the Bay during July 2002. There were non-conservative additions
of both Ba and Sr in the salinity transition zone of the subterranean
estuary. However, the extent of Sr release was significantly less
than that of its alkaline earth neighbor Ba. Pore water Ba concentrations
approached 3000 nM compared with 25-50 nM in the surface waters
of the Bay; the pore water Sr-salinity distribution suggests a
26 elevation in the amount of Sr added to the subterranean estuary.
The release of dissolved Ba to the mixing zone of surface estuaries
is frequently attributed to an ion-exchange process whereby seawater
cations react with Ba from river suspended clay mineral particles
at low to intermediate salinity. Results presented here suggest
that reductive dissolution of Mn oxides, in conjunction with changes
in salinity, may also be an important process in maintaining high
concentrations of Ba in the pore water of subterranean estuaries.
In contrast, pore water U was significantly depleted in the subterranean
estuary, a result of SGD-driven circulation of seawater through
reducing permeable sediments. This finding is supported by surface
water concentrations of U in the Bay, which were significantly
depleted in U compared with adjacent coastal waters. Using a global
estimate of SGD, we calculate U removal in subterranean estuaries
at 20~106 mol U y-1, which is the same order
of magnitude as the other major U sinks for the ocean. Our results
suggest a need to revisit and reevaluate the oceanic budgets for
elements that are likely influenced by SGD-associated processes.x
1. Introduction
2. Experimental methods and procedures
@2.1. Field methods
@2.2. Laboratory methods and analysis
3. Results and discussion
@3.1. Salinity distribution in the Bay and its subterranean
estuary
@3.2. REDOX framework of the subterranean estuary: Fe and Mn
@3.3. Barium in the subterranean estuary and the Bay
@3.4. Strontium in subterranean estuary and the Bay
@3.5. Uranium in the Bay and subterranean estuary
4. Biogeochemical processes in the subterranean estuary
@4.1. Barium
@4.2. Biogeochemistry of Sr in the subterranean estuary
@4.3. Geochemistry of uranium in the subterranean estuary
5. Conclusions
Acknowledgments
References