Geochemistry of Groundwater and Naturally Occurring Pyrite in the Holocene Fluvial Aquifers in Uphapee Watershed, Macon County, Alabama

Abstract

Naturally occurring biogenic pyrite has been found in Holocene fluvial aquifers in Uphapee watershed, Macon County, Alabama. The electron microprobe (EMP) analysis showed that the pyrite grains contain 0.20-0.92 weight% of arsenic (As). The scanning electron microscope and energy dispersive spectroscopy (SEM-EDS) analysis confirmed a similar level of As concentration in pyrite that was consistent with the EMP analysis. The SEM analysis also confirmed the presence of additional trace elements such as cobalt (0.19 wt.%), and nickel (0.15 wt.%), indicative of pyrite’s capacity to sequester arsenic and other metals. Pyrite grains were naturally formed and developed as large (20-200 μm) euhedral (i.e. cubes, truncated octahedron) crystals and none-framboid aggregates. The XRF analysis of the collected lignitic wood (from the wells) also showed the presence of As. However, the ICP-MS analysis showed that As level in the groundwater was not high and it was within the EPA drinking water standards (<10 ppb). These results indicate that dissolved arsenic is sequestered in naturally formed pyrite found in the fluvial sediments. The groundwater was moderately reducing to slightly oxidizing (Eh= 46 to173 mV), and nearly neutral to slightly acidic (pH = 5.53 to 6.51). Groundwater geochemistry data indicate a redox sequence of oxidation, Mn(IV) reduction, Fe(III) reduction, and sulfate reduction along the flow path in fluvial aquifers. The down-gradient increases in dissolved Mn and then Fe concentrations reflect increased Mn(II) and Fe(II) production via microbial competition as the aquifer becomes progressively more reduced. Bacterial sulfate reduction seems to dominate near the end of the groundwater flow path as the availability of Mn- and Fe-oxyhydroxides becomes limited in sediments rich in lignitic wood where increased sulfate-reducing activities, leading to the formation of biogenic pyrite. The groundwater is a Ca-SO4 type, is not SO4-limited, as compared to most Holocene groundwater systems in Bangladesh, thus sulfate may serve as an electron acceptor for the bacterial sulfate-reducing reactions that sequester As into pyrite, which in turn results in very low groundwater As concentration (≈1-2 ppb). This result implies that groundwater in Holocene alluvial aquifers in Bangladesh (a sulfate-limited system) must be amended with an electron acceptor (e.g., iron sulfate) and labile organic carbon to stimulate the metabolism of indigenous sulfate-reducing bacteria (SRB). This study compared the size and morphology of biogenic pyrite precipitated naturally in the Uphapee fluvial sediments with those formed via biostimulation at a Florida industrial site, where groundwater was severely contaminated with an elevated level of As (300-500 ppb). Arsenian-pyrite grains formed via a biostimulation process appear either as well-defined euhedral nano-crystals or as spherical aggregates (framboids) of 1-50 μm in diameter, smaller than those formed naturally in the Uphapee watershed. The biogenic pyrites formed at the Florida site contained between 0.05-0.40 wt.% of As, indicating similar As adsorbing capability.