pH-Dependent iron oxide precipitation in a subterranean estuary

Author Posting. © The Authors, 2005. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Journal of Geochemical Exploration 88 (2006): 399-403, doi:10.1016/j.gexplo.2005.08.084.Iron-oxide coated sediment particles in subterranean estuaries can act as a geochemical barrier (“iron curtain”) for various chemical species in groundwater (e.g. phosphate), thus limiting their discharge to coastal waters. Little is known about the factors controlling this Fe-oxide precipitation. Here, we implement a simple reaction network in a 1D reactive transport model (RTM), to investigate the effect of O2 and pH gradients along a flow-line in the subterranean estuary of Waquoit Bay (Cape Cod, Massachusetts) on oxidative precipitation of Fe(II) and subsequent PO4 sorption. Results show that the observed O2 gradient is not the main factor controlling precipitation and that it is the pH gradient at the mixing zone of freshwater (pH 5.5) and seawater (pH 7.9) near the beach face that causes a ~7-fold increase in the rate of oxidative precipitation of Fe(II) at ~15 m. Thus, the pH gradient determines the location and magnitude of the observed iron oxide accumulation and the subsequent removal of PO4 in this subterranean estuary.Financial support was provided by the Netherlands Organisation for Scientific Research (NWO) and WHOI Guest Student Program (grants to C. Spiteri), the Royal Netherlands Academy of Arts and Sciences (KNAW) (fellowship to C.P. Slomp) and US National Science Foundation NSF-OCE0095384 and NSF-OCE0425061 (grants to M.A. Charette)

Flow and nutrient dynamics in a subterranean estuary (Waquoit Bay, MA, USA) : field data and reactive transport modeling

Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 72 (2008): 3398-3412, doi:10.1016/j.gca.2008.04.027.A two-dimensional (2D) reactive transport model is used to investigate the controls on nutrient (NO3-, NH4+, PO4) dynamics in a coastal aquifer. The model couples density dependent flow to a reaction network which includes oxic degradation of organic matter, denitrification, iron oxide reduction, nitrification, Fe2+ oxidation and sorption of PO4 onto iron oxides. Porewater measurements from a well transect at Waquoit Bay, MA, USA indicate the presence of a reducing plume with high Fe2+, NH4+, DOC (dissolved organic carbon) and PO4 concentrations overlying a more oxidizing NO3--rich plume. These two plumes travel nearly conservatively until they start to overlap in the intertidal coastal sediments prior to discharge into the bay. In this zone, the aeration of the surface beach sediments drives nitrification and allows the precipitation of iron oxide, which leads to the removal of PO4 through sorption. Model simulations suggest that removal of NO3- through denitrification is inhibited by the limited overlap between the two freshwater plumes, as well as by the refractory nature of terrestrial DOC. Submarine groundwater discharge is a significant source of NO3- to the bay.This research was funded by the Netherlands Organisation for Scientific Research (NWO) and WHOI Guest Student Program (C. Spiteri), the Royal Netherlands Academy of Arts and Sciences (KNAW) and the Netherlands Organization for Scientific Research (NWO VIDI-grant) (C.P. Slomp), the US National Science Foundation NSF-OCE0095384 and NSF-OCE0425061 (M.A. Charette) and the Georgia Sea Grant of the National Sea Grant College Program of the U.S. Department of Commerce’s National Oceanic and Atmospheric Administration under NOAA Grant #NA04OAR4170033 (C. Meile)