2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 12
Presentation Time: 11:00 AM


GALLAHER, Bruce, Water Quality and Hydrology Group, Los Alamos National Lab, MS K497, P.O. Box 1663, Los Alamos, NM 87544, SULLIVAN, Enid J., Advanced Chemical Diagnostics and Instrumentation Group, Los Alamos National Laboratory, MS J565, TA 46-024, Los Alamos, NM 87545, TEERLINK, Jennifer, Environmental and Remediation Support Services, Los Alamos National Laboratory, Mail Stop M992, Los Alamos, NM 87545 and SIMMONS, Ardyth, Los Alamos National Laboratory, P.O. Box 1663, MS-D446, Los Alamos, NM 87545, ejs@lanl.gov

PCA and CA statistical tools were used with metal and general geochemistry data to delineate geochemical groupings in the regional aquifer beneath the Pajarito Plateau and Rio Grande. Traditional graphical methods of classification typically use only major ions, while the multivariate statistical methods can include trace metal data and other geochemical parameters such as pH. This provides a more complete evaluation of water quality facies. The cluster analysis is a neutral approach to delineation of facies, without the bias of preconceived notions such as flow paths, anthropogenic inputs, or geographical features. From this, a picture of natural, or background, water quality can be extracted. The conceptual model of regional ground water is based on multiple chemistry, numerical modeling, and physical hydrogeologic studies. Regional ground water infiltrates in the Jemez Mountains in the west and Sangre de Cristo Mountains in the east, converging and discharging at the Rio Grande, with a concurrent increase in total dissolved solids and metals with residence time. Different ground water facies exist within this flow regime and the geochemistry is controlled by host rock interactions and altered by dilution and mixing with surface water and alluvial or intermediate depth waters. In addition, 50 years of human residence and industrial activity has added chemical signatures to some areas. PCA and CA methods were used to extract different water groupings or facies and the associated natural background water chemistry from these complicating factors. Water supply wells, monitoring wells, and local springs supplied data on inorganic constituents from 2000-present. Three statistically distinct ground water groups were found. Waters sampled from near the Rio Grande showed higher TDS and levels of some metals, likely indicating older, more evolved regional ground water. Waters from the western Pajarito Plateau showed lower TDS and metals, indicating a less evolved facies, while water sampled from the central Plateau showed intermediate TDS and metals and is likely indicative of mixed waters.