SELENIUM SPECIATION ALONG A GROUNDWATER FLOW PATH IN THE CARRIZO SAND AQUIFER, TEXAS, USA

Selenium concentration and speciation [Se(VI), Se(IV) and Se(-II)] was determined along a flow path in the Carrizo Sand aquifer (southeastern Texas). Groundwater sampling was carried out in March 2006, from ten wells located along the flow path. The collected samples were filtered through 0.45 µm filters and then acidified to pH <2 using ultra pure HCl. Analysis for Se concentration and speciation was done using selective hydride generation – Atomic Absorption spectrometry (HG-AAS). Additional in situ measurements from each well included determination of pH, alkalinity, oxidation-reduction potential (Eh in mV), dissolved iron speciation, dissolved sulfide concentrations and dissolved manganese concentrations. Selenium concentrations in groundwaters of the Carrizo Sand aquifer are low (i.e., nanomolal concentrations). Variations in redox condition and geochemical processes appear exert important controls on the distribution of Se species in Carrizo groundwaters. Redox conditions fluctuate between oxic to mildly reducing environment, in regions proximal to the recharge zone (i.e.~ 0-15 km). Downgradient at a distance of 15-41 km, Fe(III) reduction is the dominant process, which is followed by sulfate reduction predominating at a distance >41 km along the flow path. Se(VI) and Se(-II) show variable behavior in regions near the recharge zone, whereas Se(IV) dominates in this region. Beyond 26 km, Se(VI) becomes the dominant species. The data are consistent with Fe(II) oxidation, reduction of Fe(III)/ Mn(IV) oxides/oxyhydroxides, and pH induced adsorption/desorption as important controlling factors of Se concentrations and speciation within the first 41 km of the groundwater flow path. Further downgradient at a distance >41km (sulfate reduction zone) along the flow path, pH induced desorption appears to lead to the dominance of Se(VI) species in solution. Changes observed in Se concentration and speciation in the Carrizo Sand aquifer are co-relative with the distribution of pH, alkalinity, Eh, and dissolved concentrations of iron, sulfide and manganese, studied along the flow path.