CONCENTRATIONS AND SPECIATION OF ARSENIC ALONG A GROUNDWATER FLOW PATH IN THE CARRIZO SAND AQUIFER, TEXAS, USA

Groundwater samples were collected from ten wells located along a flow path in the Carrizo Sand aquifer (south Texas) as part of a study designed to characterize biogeochemical processes in the deep subsurface that affect trace element concentrations, speciation, and cycling. Here, we collected samples for determination of As concentrations and speciation [i.e., As(III) and As(V)] along the flow path, and supplemented these samples with in situ measurements of pH, alkalinity, temperature, dissolved oxygen, oxidation reduction potential (i.e., Eh), dissolved iron speciation, and dissolved sulfide concentrations. Separate groundwater samples were collected from each well for major solute analysis and to measure redox sensitive trace element concentrations (U, Re). Arsenic species were separated on site during sample collection using anion-exchange chromatography, and subsequently quantified by ICPMS. The 2004 measured concentrations of total dissolved Fe range from ~1.25 to 184 umol kg-1 with Fe(II) dominating in all but 2 sampling locations. Iron speciation, sulfide concentrations, Eh, U, and Re provided an understanding of the local variability of the groundwaters’ redox conditions, which change along the flow path from aerobic conditions, to Fe(III) reduction, and then to sulfate reduction. Total dissolved arsenic concentrations for the Carrizo groundwaters range from 2.48 to 7.91 nmol kg-1. Arsenic speciation data are pending at the time of our abstract submission. Our combined analyses indicate that As concentrations change systematically along the groundwater flow path as a function of adsorption onto iron oxhydroxides and owing to microbially mediated reduction, likely fueled by organic matter within the aquifer. Upcoming research plans involving As speciation, and microbial studies of the subject groundwater, are expected to provide more insight into the relative importance of adsorption/desorption versus microbial metabolism on As concentrations and speciation along the flow path.