OCCURRENCE AND SOURCE OF PERCHLORATE IN THE HIGH PLAINS AQUIFER SYSTEM OF TEXAS AND NEW MEXICO

In the spring of 2002, representatives of the Texas Commission on Environmental Quality (TCEQ) as part of the Unregulated Chemical Monitoring Rule determined that perchlorate (ClO₄^-) was present in some wells located in the McMillan and Paul Davis well fields that supply potable water for the City of Midland. A large-scale sampling program was instituted to determine the source and distribution of perchlorate in the area’s groundwater. This sampling program included public water system (PWS) wells and private wells over an area of 52,900 mi² covering fifty four counties in Northwest Texas and Eastern New Mexico largely in the High Plains Aquifer system. Over 560 PWS and 76 private wells were augmented by wells from USGS and local water districts. In addition, three nested wells systems were installed in Bailey (2 wells), Gaines (2 wells), and Martin (3 wells) Counties including unsaturated zone sampling to investigate the vertical distribution of perchlorate above and within the saturated zone in order to determine whether surface or deeper brine water was the contributing source. All soil and water samples were analyzed for perchlorate and a suite of major ions. Results to be presented include the distribution of perchlorate throughout the study area, vertical distribution in the unsaturated and saturated zone, total mass of perchlorate and implications of this on potential sources, as well as experimental results from experiments designed to investigate natural production mechanisms of perchlorate generation. Results from this study suggest that perchlorate may be more widespread than previously believed especially in semi-arid and arid environments and that atmospheric generation and surface evaporative weathering phenomenon may be responsible for much of the low level detections not contributable to industrial sources.