Outfalls from high explosives (HE) production facilities at Los Alamos National Laboratory discharged HE-contaminated waters onto a semiarid mesa on the Pajarito Plateau for about 50 yr. As part of an initial hydrogeologic characterization to examine the impact of HE contamination, four boreholes were drilled into the vadose zone to depths between 38.1 and 63.3 m. The study objectives were to characterize contaminant nature and extent and to identify potential source areas and transport pathways. Besides providing an example of HE transport in the vadose zone, this study shows the value of integrating chloride and stable isotope tracer approaches with contaminant distribution information, and it provides insights on semiarid vadose zone behavior in the little studied, but widespread ponderosa pine forests of the American Southwest. Chloride-based vadose zone residence time estimates (1950–6080 yr) suggest that downward flow and transport over much of the mesa is limited. However, the presence of HE-contaminated transient saturated zones in two boreholes indicates that localized fast pathways also occur. Stable isotope data (δ18O and δD) suggest that the source areas for contamination are former HE outfall discharge ponds that provide focused recharge to the transient saturated zones.

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