REACTIVE TRANSPORT AND IMMOBILIZATION OF ARSENIC IN A SHALLOW ALLUVIAL AQUIFER IN FLORIDA

In this study groundwater geochemistry data are integrated with hydrological and geochemical modelling to investigate reactive transport of arsenic in a shallow alluvial aquifer in Lynn Haven, Florida. Groundwater at this industrial site contains high levels of arsenic (> 0.5 mg/L) derived from herbicides. The aquifer is under moderately reducing conditions, as indicated by relatively low values of Eh and DO, as well as elevated dissolved sulfide and ferrous iron concentrations. The main hydrochemical facies of groundwater is characterized as a Ca-HCO₃-Na-Cl type. Groundwater is strongly enriched in Ca and Mg and slightly enriched in K and SO₄ relative to the seawater dilution trend. The enrichment is attributed to mixing of deeper groundwater from carbonate and clay-rich units as well as ion exchange.

XRF analyses of selected sediments consistently resulted in major peaks for Fe, S, and As. XRD analyses confirmed the presence of iron sulfide (pyrite) and arsenic-bearing sulfide minerals. These results suggest that bacterial sulfate reduction has already led to the sequestration of arsenic in Fe-sulfides. The geochemical modeling results show the sequence of minerals formed during bacterial sulfate reduction with a drop in Eh and amendments of FeSO₄. Ferrous iron in solution reacts with sulfide produced from sulfate reduction to form pyrite as the Eh drops below -0.08V. The precipitation of iron sulfide has the potential to significantly lower the concentration of arsenic in contaminated groundwater via adsorption and co-precipitation processes. Contaminant transport models show that the main factors influencing arsenic transport include advection, dispersion, and adsorption. Sensitivity analysis shows that adsorption tends to lower the peak concentration and cause time lag of arsenic transport. The results suggest that the Lynn Haven industrial site contains hydrogeochemical conditions that may be amenable for immobilizing or sequestration of arsenic in groundwater via precipitation of biogenic pyrite.