TRACE PERCHLORATE IN GROUNDWATERS OF THE PAJARITO PLATEAU, ESPAÑOLA BASIN, AND THE RIO GRANDE, NORTHERN NEW MEXICO

The New Mexico Environment Department and Los Alamos National Laboratory (LANL) have consistently detected trace levels of perchlorate (0.1 to 0.4 ppb) in groundwaters of northern New Mexico since 2002, including detections in pristine groundwater. Seventy-six perchlorate results, analyzed using an extremely sensitive method, were obtained from 47 well and spring locations representative of groundwater from the Sierra de Los Valles and Pajarito Plateau (LANL area), Española Basin, and springs along the Rio Grande north of Taos. Six distinguishable groundwater zones were analyzed based on location, age, and background water quality. About half the sites represent the regional or drinking-water aquifer beneath the Laboratory. Background water quality was determined by evaluating the major-ion chemistry for each site. Tritium and stable-isotope analyses provide information on recharge, residence time, and flow path. Hydrochemical and hydrogeologic relationships show sites east of Sierra de Los Valles in the mountain-front region to be young or modern in age (<50 y); White Rock Canyon springs and regional well waters are of a sub-modern age/paleo (>50 yr). Tritium data from springs located along the Rio Grande north of Taos possess both modern and sub-modern/paleo ages. Perchlorate concentrations in the LANL area were consistent; however, more variable concentrations were observed between the Los Alamos and Taos sites. This may result from natural variability, hydrogeologic, atmospheric, and/or other unidentified inputs. Nine precipitation samples collected in the LANL area do not contain perchlorate above 0.05 ppb (instrument detection limit). Establishing a background upper tolerance limit (UTL) (mean + 2 σ) for waters within the Northern Rio Grande Basin enables investigators to discern if concentrations of perchlorate above the UTL are anthropogenic. The background UTL of 0.44 ppb was calculated from the 76 analytical results. This UTL is approximately ten times below any perchlorate health standard proposed by EPA or neighboring states.