New Mexico Geological Society Annual Spring Meeting — Abstracts

Limited knowledge exists concerning the role of the Pajarito fault zone (PFZ) as a ground-water recharge mechanism (conduit and/or barrier) within the Los Alamos region. Consequently, the NMED DOE Oversight Bureau indicated 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. The reach is supplied by seeps and springs at about 2,740 m. Approximately 1.9 km downstream of PA-10.6, three perennial springs 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 two of the three aforementioned springs (Homestead and Starmer Springs) are chemically and isotopically similar to the PA-10.6 waters. The third spring (Bulldog Spring) contains elevated concentrations of 3H 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 summer0monsoon flooding in the upper reach of Pajarito Canyon impacted water quality at PA-10.6. The combined data indicate that some springs are supplied by waters that percolate inti the PFZ (Homestead and Starmer) while other nearby springs are recharged independent pf the PFZ (Bulldog).