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