| 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 mum 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 very low (i.e., nanomolal concentrations). Variations in redox condition and geochemical processes appear to 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 dominating 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. (Abstract shortened by UMI.)... |
