GEOCHEMICAL PROCESSES AFFECTING ARSENIC IN PRIVATE WELLS OF THE NORTH CAROLINA PIEDMONT

Arsenic concentration and speciation along with selected elemental concentrations were measured in 16 residential water supply wells located in the fractured bedrock aquifer of the North Carolina Piedmont. Groundwater samples were collected several times from residential wells using a disposable anion exchange resin cartridge, allowing field speciation of As. In addition, a multi-probe was used to measure the oxidation-reduction potential (ORP), dissolved oxygen (DO), and pH. Eight wells are located in the metasedimentary rocks of the Carolina terrane (CT) underlying Union and Stanly counties and seven of the other eight wells are located in gneiss and schists of the Inner Piedmont (IP) underlying Lincoln and Gaston counties. In addition, samples analyzed by the NC Dept of Health and Human Services were evaluated to assess the variations in As concentrations.

Total As concentrations range from < 5 to 190 ug/L. The distribution of arsenic species indicate that As(III) is more prevalent in the CT compared to the IP. Total As levels for individual wells are fairly stable over the period of measurement. Previous work has established that arsenic is most likely derived from geologic materials. Different geochemical processes may, however, affect As concentrations in the two study regions. There is a consistent relationship between pH, ORP, Fe and As concentrations. In both the regions, groundwater with elevated As concentrations (>50 ug/L) have lower Fe concentrations (<50 ug/L). In the CT, higher As levels are associated with higher pH (>7) and lower DO (<2 mg/L) levels, whereas higher As levels in the IP are associated with lower pH and higher DO. The ORP, in particular, shows a strong positive relation with As in the IP. Higher Fe levels present naturally in the IP groundwater may constrain the As levels through adsorption or precipitation onto Fe oxyhydroxides. In contrast, reductive dissolution of As or desorption of As from oxyhydroxides may control the As levels noted in the CT.