DEVELOPMENT OF AN INFILTRATION MAP FOR THE LOS ALAMOS AREA, NEW MEXICO

A map of net infiltration for the Los Alamos area, New Mexico, was created for the pre-Cerro Grande fire period using new and previously published estimates of point infiltration in upland areas and estimates of stream-flow losses and gains along canyon bottoms. The point infiltration estimates are based on a combination of techniques that include the use of the Richards equation, the chloride mass-balance method, and numerical modeling. The infiltration rates estimated with these techniques were extrapolated to uncharacterized parts of the study area using maps of environmental variables that are correlated with infiltration (such as topography, vegetation cover, and surficial geology and structure) and spatial algorithms implemented with GIS software that use the mapped variables. The map indicates that infiltration rates on mesas of the Pajarito Plateau are generally less than 2 mm/yr, except near faults, where infiltration rates may be several tens to hundreds of mm/yr. Infiltration rates at higher elevations in the Sierra de los Valles are typically greater than 25 mm/yr in mixed conifer areas and greater than 200 mm/yr in areas vegetated by aspen. An irregular transition zone with infiltration rates between 2 to 25 mm/yr exists near the western edge of the Pajarito Plateau adjacent to the Sierra de los Valles. This transition zone extends to lower elevations on the north-facing slopes of deeply incised canyons. Canyon-bottom infiltration rates are highly variable, ranging from several hundred mm/yr in canyons with large watersheds that have their headwaters in the Sierra de los Valles or in canyons that receive effluent from Laboratory operations, to several mm/yr in canyons that have their headwaters on the Pajarito Plateau but do not receive Laboratory effluent. The total net infiltration of approximately 10.6 x 106 m3/yr (8,600 acre-ft/yr) is consistent with estimates of the steady-state groundwater discharge to perennial streams in the study area, whereas the relative rates of infiltration within the study area are consistent with the distribution of natural and anthropogenic tracers such as tritium in perched and regional groundwaters.