STATISTICAL ANALYSIS OF REGIONAL AQUIFER BACKGROUND, PAJARITO PLATEAU, NEW MEXICO

The New Mexico Environment Department (NMED) initiated a geochemical investigation in 2011 to determine low-level background concentrations of 23 EPA Target Analyte List metals (excluding Hg) and 19 other trace metals naturally present in the regional aquifer at Los Alamos, New Mexico. The purpose of the project is to update and supplement background chemical data for the regional aquifer collected by Los Alamos National Laboratory (LANL) and increase the protection of groundwater resources beneath and downgradient from the facility. High resolution-inductively coupled plasma mass spectrometry (HR-ICPMS) is used to quantify background solute concentrations at low levels of detection and quantitation. Instrument detection and reporting limits for the various dissolved trace elements are below 0.001 mg/L. The NMED collected 102 filtered groundwater samples for HR-ICPMS analyses at 34 well screens located on the Pajarito Plateau and eight springs discharging in White Rock Canyon southeast of Los Alamos, New Mexico. The sampling stations represent each of the five major Pliocene and Miocene age regional-aquifer lithologies, including the Puye Formation, Tschicoma Formation, Totavi Lentil, basaltic rocks of the Cerros del Rio volcanic field, and Chamita Formation. Statistical analyses for each background solute, including calculating upper tolerance limits (UTL), were conducted using the US EPA software package ProUCL, Version 4.1. For each constituent, the selected 95% UTL was based on 95% coverage. Solute concentrations above instrument detection limits and below reporting limits (J values) were considered in calculating UTL values. Updated UTL values for dissolved Cr, Ba, Fe, and Mn are lower than UTL values provided by LANL in 2011. UTL values calculated by both the NMED and LANL are very similar for B, Ca, Mg, silica, Na, U, and V. An updated UTL for dissolved Sb was slightly higher than the UTL value provided by LANL. Differences in UTL values are most likely related to a higher percentage of detectable results and associated lower detection and quantitation limits for the analytes, as well as the quality and increased number of background sampling locations.