Hydrogeology and Geochemistry of Arsenic Contaminated Shallow Alluvial Aquifers in Florida and Alabama

Abstract

This study integrates groundwater geochemical data and modeling with hydrogeological modeling to study the geochemistry and hydrogeology of arsenic contaminated shallow alluvial aquifers in Florida and Macon County, Alabama. Geochemical data collected for the Florida site showed levels of arsenic (As) in groundwater from 0.0002 to 0.577 ppm, above the US EPA drinking standard for As concentration. Geochemical data also suggest a high degree of mixing of meteoric and carbonate groundwater in the surficial aquifer. The main hydrochemical facies of groundwater in the surficial aquifer is characterized as a Ca-HCO3-Na-Cl type. Groundwater is enriched in Ca, Mg, and HCO3- relative to the conservative mixing line of seawater. Groundwater geochemistry data indicate that reduced ferrous iron (Fe2+) and arsenite (As(OH)3) are the dominated species under moderately reducing conditions (Eh = -77 to 130.4 mV). Geochemical modeling utilizing reaction path models and Eh-pH diagrams predict that a further drop in current redox conditions will lead to the precipitation of Fe-sulfides (i.e. pyrite) and arsenic sequestration.

XRF and XRD data from both sites indicate that biogenic pyrite is naturally forming at the site, and had removed arsenic, presumably by co-precipitation and sorption. XRF analyses of dark sediment slurry recovered from monitoring wells indicate elevated concentrations of Fe, S, and As. XRD analyses indicate pyrite and perhaps other forms of iron sulfides are currently forming at both sites in reducing environments.

Hydrogeological modeling and historical water table data show a general flow trend from east to west at a velocity of a few to a few tens of meters per year across the Florida industrial site and the main factors controlling to arsenic transport at this site include advection, dispersion, and adsorption. Varying the Kd values (1 to 10 ml/g) for different adsorption models showed that the higher the degree of sorption (high Kd values) the more arsenic transport is inhibited. Sensitivity numerical analysis shows that adsorption can lower the peak concentration and cause time lag of transport.

Field data and geochemical and hydrogeologic modeling provide the basis for a future bioremediation strategy of the Florida industrial site. The Florida site is sulfate-limited (sulfate concentration < 9 mg/L) and thus should be amended with labile organic carbon and iron sulfate to stimulate metabolism of indigenous sulfate-reducing bacteria. The strategy will utilize the biogeochemical reaction involving natural sulfate reducing bacteria to form iron sulfide solids. With time, perhaps within months, groundwater is expected to become more reducing, and most dissolved arsenic will be removed due to precipitation of iron sulfide once biogenic sulfate-reduction begins.