ESTABLISHING GROUNDWATER BACKGROUND VALUES FOR THE PAJARITO PLATEAU, LOS ALAMOS, NEW MEXICO

Groundwater monitoring at Los Alamos National Laboratory necessitates the ability to detect when constituents are measured at concentrations above natural background. Background values for groundwater are typically defined as constituent concentrations in waters directly upstream of a known source. These values are then compared to values directly downgradient of the source. In the case of the Laboratory, however, the complex hydrogeology, natural variability of groundwater, geographic setting, and multiple historic releases make this background model impractical. Geographically the Laboratory is in the upward reaches of the Sierra de los Valles watershed which limits upgradient sampling locations of the regional aquifer. Groundwater in the regional aquifer beneath the Pajarito Plateau moves generally from northwest to southeast, where it discharges at or near the Rio Grande. Along the flow path the water interacts with rocks and mineral phases to produce more evolved water with relatively high total dissolved solids. Groundwater is also found in isolated but substantial perched water bodies in the vadose zone.

Background must account for natural variability within the vadose zone and the regional aquifer. Potential background locations were selected based on length of sampling record, quality of well, consistency of sampling location for springs, and lack of impact from Laboratory activities. Locations were grouped using a cluster analysis and principal component analysis of geochemical constituents. The groups established through this statistical approach fit well with the current understanding of the groundwater flow system, which reflects groundwaters of varying ages, and displaying various degrees of rock/water interaction. The Pajarito Plateau background group consists of 29 locations. A suite of statistical values was calculated for perched intermediate groundwater and the regional aquifer which is now being used to screen analytical data to identify potential contamination. Monitoring data can be screened more appropriately based on association with a background characteristic of the Pajarito Plateau group. As additional background locations are identified over time, background may be refined using the same statistical approach.