PERCHLORATE IN THE SOUTHERN HIGH PLAINS OF TEXAS AND NEW MEXICO

Perchlorate (ClO4-) has become the focus of research since it was discovered as a ground-water contaminant in California in 1997. The Texas Commission on Environmental Quality funded a large-scale sampling program to determine the source(s) and distribution of ClO4- in ground water in the southern High Plains of Texas. More than 1,000 wells over a 59,000 sq.mi area that covers 56 counties in northwestern Texas and eastern New Mexico, most in the High Plains Aquifer System were sampled. Well types that were sampled included public supply, irrigation, domestic, and water-table monitor wells. In addition, five vertically-nested monitor well clusters in Bailey (2 wells), Gaines (3 wells), Martin (3 wells), Hale (4 wells) and Castro Counties (4 wells), and three unsaturated zone boreholes in Bailey, Cochran, and Carson Counties were sampled to investigate the vertical distribution of ClO4- above and within the saturated zone. Soil and water samples were analyzed for ClO4- and selected major ions. The frequency of detection in soil samples was 19 percent and the range of measured concentrations was from 0.5 to 113 ug/kg. The frequency of detection in water samples was 56 percent and the range of measured concentrations in water samples was from 0.19 to 179 ug/L. Results from this study indicate that ClO4- in the southern High Plains aquifer is more widespread than previously known. In semi-arid and arid environments, atmospheric generation and surface evaporative weathering processes might explain the widespread occurrence of low-level (less than 1 ug/L) detections that are not attributable to industrial or agricultural sources.