Reactive transport of trace elements and isotopes in the Eutaw coastal plain aquifer, Alabama

Abstract

[1] We integrate groundwater geochemistry, mineralogy, and numerical modeling techniques to study the reactive transport of heavy metals and isotopes in the Eutaw coastal plain aquifer, Alabama. Geochemical data show that the elevated concentrations of Fe, Mn, and Sr can be correlated with high pH and alkalinity. These geochemical correlations suggest that that elevated metal concentrations may be derived from bacterial iron and manganese reduction. Geochemical modeling of bacterial Fe(III) and Mn(IV) reduction shows that the biotransformation of iron and manganese minerals could control mobility and concentrations of Fe and Mn in coastal plain aquifers. Petrographic, SEM, and EDAX studies of sediments show the formation of biogenic siderite, rhodochrosite, and pyrite in sediments associated with Fe-and Mn-rich groundwater. Rhodochrosite and siderite occur together as spheroids (%1.0 mm) in which rhodochrosite forms the center and siderite forms an outer rind, which is consistent with the redox sequence of mineral precipitation predicted by the geochemical modeling. The low d 13 C ratios of siderite and rhodochrosite (À14.4 to À15.4%, PDB) and groundwater DIC (À20.6 to À14.1%, PDB) imply carbon of biogenic origin. Higher DIC-d 13 C levels are found to be correlated with elevated Fe and Mn concentrations and high pH values of groundwater. This unexpected result implies novel carbon isotopic fractionation processes associated with bacterial Fe(III) reduction. We used 36 Cl/Cl ratios of groundwater and isotope transport modeling to calculate the residence time of regional groundwater in the Eutaw aquifer. The calculation considering the natural decay only would yield 36 Cl levels that are significantly higher than field data, suggesting that a significant mixing with Cl-rich, older groundwater at depth is an important reason for substantial 36 Cl depletion along flow path.

Abundant Fe(III) Oxide‐Bound Arsenic and Depleted Mn Oxides Facilitate Arsenic Enrichment in Groundwater From a Sand‐Gravel Confined Aquifer

“…The trace amount of Mn oxides, one of the important oxidants in aquifers (Gao, Guo, et al., 2020; Owings et al., 2019), favors groundwater As remaining as As(III) species, which is less adsorbed and thus is more mobile than As(V) (Dixit & Hering, 2003). On the other hand, the low abundant Mn oxides facilitate reduction of As‐bearing Fe(III) oxides based on the classical redox sequence (Lee et al., 2007), which thus leads to As release into groundwater. Similar conditions have also been observed in a Pleistocene aquifer from Cambodia with limited total Mn contents in sediments (around 0.47 mmol/kg), where high‐As groundwater (1.13–4.61 μmol/L) was observed (Gillispie et al., 2019).…”

Geochemical bio-signatures in Martian analogue basaltic environments using laboratory experiments and thermochemical modelling

“…Huntite, could be particularly difficult to detect on Mars (especially if present in low amounts) since it is metastable at surface temperature (Garrels et al, 1960;Kinsman, 1967) and it is replaced with time by magnesite (Kinsman, 1967;Spotl and Burns, 1994;Marini, 2007), a more stable Mg-carbonate that is present on the Martian ground together with siderite, calcite, rhodochrosite and other Mg-and Fe-bearing alteration minerals (Ehlmann et al, 2008;Niles et al, 2013;Goudge et al, 2015). Finally, alteration phases predicted to form in the biotic system (siderite, rhodochrosite, quartz, pyrite, hydroxyapatite) are all minerals that naturally can precipitate in near-neutral pH aqueous environments at ambient temperature following the action of microbes (Lee et al, 2007;Ehrlich and Newman, 2009;Sun et al, 2014;Duverger et al, 2020).…”

Geochemical reactivity of subsurface sediments as potential buffer to anthropogenic inputs: a strategy for regional characterization in the Netherlands

“…In some studies, qualitative analysis of the aquifer sediments is performed to corroborate inferences from groundwater analyses and modelling (e.g. Lee et al 2007). Quantitative studies of sediment reactivity are found mainly in connection with local contaminant plumes, with some recent studies addressing the problem of As mobilisation in drinking water aquifers (Swartz et al 2004;Shamsudduha et al 2008).…”