ASSESSING GROUNDWATER RECHARGE THROUGH THE PAJARITO FAULT ZONE, UPPER PAJARITO CANYON, LOS ALAMOS, NEW MEXICO

Limited knowledge exists concerning the role of the Pajarito fault zone (PFZ) as a groundwater recharge mechanism (conduit and/or barrier) within the Los Alamos region. Consequently, the NMED DOE Oversight Bureau initiated a study to determine if surface water loss in upper Pajarito Canyon is occurring within the PFZ. General hydrologic conditions include a perennial surface-water reach flowing west to east for approximately 2.4 km upstream of the PFZ. This reach is supplied by seeps and springs at about 2,740 m. Approximately 1.9 km downstream of PA-10.6 (surface water at the PFZ), the perennial springs Homestead, Starmer, and Bulldog discharge from horizontal fractures and/or surge beds at about 2,330 m. Recharge and discharge (water balance) measurements were nearly equivalent. From 1997 through 2003, hydrogeochemical parameters were obtained at PA-10.6 and the springs. Isotopic data suggest that Homestead and Starmer Springs are chemically and isotopically similar to the PA-10.6 waters. Bulldog Spring contains elevated concentrations of tritium and major ions, and is slightly enriched in δD and δ18O compared to water at PA-10.6, suggesting a different recharge source. Solutes produced from the Cerro Grande fire (May 2000) introduced tracers that show direct links between water at PA-10.6 and the downgradient springs. Subsequent monsoon flooding in the upper reach of Pajarito Canyon impacted water quality at PA-10.6. The combined data indicate that Homestead and Starmer Springs are supplied by waters that percolate into the PFZ while Bulldog Spring is recharged independently of the PFZ.