Abstract

Using new and previously published estimates of point infiltration in upland areas and estimates of stream-flow losses and gains along canyon bottoms, we created a map of net infiltration for the Los Alamos area, New Mexico for the pre-Cerro Grande fire period. 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 <2 mm yr−1, except near faults, where infiltration rates may be several tens to hundreds of millimeters per year. Infiltration rates at higher elevations in the Sierra de los Valles are typically >25 mm yr−1 in mixed conifer areas and >200 mm yr−1 in areas vegetated by aspen. An irregular transition zone with infiltration rates between 2 to 25 mm yr−1 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 millimeters per year 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 millimeters per year in canyons that have their headwaters on the Pajarito Plateau but do not receive Laboratory effluent. The total net infiltration of approximately 10.6 × 106 m3 yr−1 (8600 acre-ft yr−1) 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. Limitations of the study are that it does not address the effects of the Cerro Grande fire on the hydrology of the study area, nor does it completely capture the complex and sometimes incompletely documented history of Laboratory generated discharges during its 60-yr history.

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