Vadose zone infiltration beneath the Pajarito Plateau at Los Alamos National Laboratory
— David B. Rogers, Bruce M. Gallaher, and Erik K. Vold

We used new Bandelier Tuff core hydraulic properties from seven boreholes to evaluate the direction and amount of flux through the unsaturated zone beneath Los Alamos National Laboratory. The boreholes represent mesa-top and canyon bottom locations, which are the two distinct hydrologic regimes on the Pajarito Plateau. Most head gradients determined for the boreholes are approximately unity, implying that flow is nearly steady state. We use vertical head gradients and unsaturated hydraulic conductivity estimates to approximate infiltration rates for liquid water at several sites. The flux estimates presume that flow is vertical only, thus that no lateral flow is occurring along lithologic interfaces. Apparent fluxes beneath mesa top sites range from 0.006 to 23 cm/yr. High precipitation or surface disturbances, including disposal ponds, lead to higher fluxes beneath some mesas. Hypothesized evaporation resulting from air movement through the mesa apparently creates a barrier to infiltration beneath Mesita del Buey. Apparent canyon bottom infiltration rates are about 0.02 to 0.2 cm/yr beneath two dry canyons, and 0.02 to 0.1 cm/yr beneath Mortandad Canyon, the only relatively wet canyon represented. Canyon bottom infiltration rates beneath wetter canyons such as Los Alamos Canyon are likely much greater, but no data for those sites are currently available.