New Mexico Geological Society Annual Spring Meeting — Abstracts

Los Alamos National Laboratory (LANL) is situated on the Pajarito Plateau, a deeply dissected expanse of volcanic ash and underlying fill of the Espanola Basin. The geologic column includes, in descending order, the Bandelier Tuff, the Puye Formation and the Santa Fe Group, as well as basalts and the Tschicoma Formation interbedded with the Puye and Santa Fe. Groundwater has been encountered beneath the Pajarito Plateau in shallow and intermediate-depth perched zones of saturation as well as beneath the regional water table. Perched groundwater occurs in and between tuff subunits in the mesas, in alluvium beneath the canyon floors and in the basalts interbedded with the Puye and Santa Fe in both settings. Groundwater associated with the deep zone of saturation occurs in different rock units making up the regional aquifer, depending on location.

LANL's Groundwater Protection Program calls for the drilling of 32 deep characterization wells across the laboratory area. Observations in wells installed under this program through FY99 (wells R-9, R-12, R-15 and R-25) have already prompted revision of the conceptual hydrogeologic model for the area. These wells not only provide for better definition of the extent and thickness of hydrostratigraphic units, but also suggest that perched groundwater is a more common element of the hydrologic system at LANL than previously thought.

All four of the new deep wells encountered perched zones of saturation. In three of the wells, located in canyons (R-9, R-12, and R-15), the perched water was found in the Cerros del Rio basalt interbedded with the Puye Formation but not various shallower units. Such units include the alluvium at R-9, the alluvium and Guaje Pumice Bed at R12, and the alluvium and Cerro Toledo interval at R-15. Saturated zones in these materials no doubt occur, but must be ephemeral. The lack of shallower perched water at R-15, drilled on the southern edge of Mortandad Canyon, further suggests that such saturation may be restricted to the main axis of canyons and does not spread very far laterally. Static water levels measured at various depths during the drilling of R-25, a mesa-top well on the western edge of the laboratory, confirm a downward vertical gradient in this suspected recharge area.

These findings are of special interest for groundwater modeling and environmentalrestoration efforts in that they provide a better understanding of possible movement of contaminants to the deep regional aquifer that is tapped by water-supply wells in the area.