GEOCHEMISTRY OF GROUNDWATER AND NATURALLY OCCURRING BIOGENIC PYRITE IN THE HOLOCENE FLUVIAL AQUIFERS IN UPHAPEE WATERSHED, MACON COUNTY, ALABAMA

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. The scanning electron microscope and energy dispersive spectroscopy (SEM-EDS) analysis confirmed a similar level of arsenic 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 trace elements. Pyrite grains were naturally formed and developed as large (20–200 μm) euhedral (i.e. cubes, octahedron) crystals and none-framboid aggregates. However, the ICP-MS analysis showed that arsenic level in the groundwater was not high and it was within the EPA drinking water standards (<10 µg/kg). 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, denitrification, Mn(IV) reduction, Fe(III) reduction, and sulfate reduction along the flow path in the fluvial aquifer. 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, thus sulfate may serve as an electron acceptor for the bacterial sulfate-reducing reactions that sequester arsenic into pyrite, which in turn results in very low groundwater arsenic concentration (<2 µg/kg).