Sensitivity of long‐term bare soil infiltration simulations to hydraulic properties in an arid environment

Abstract

The suitability of Yucca Mountain, Nevada, for emplacement of a high‐level nuclear waste geologic repository is currently being evaluated. Assessments of the repository performance suggest that the uncertainty in infiltration rates strongly affects predicted repository performance. Most of the ground surface over the potential repository footprint is characterized by shallow to deep colluvium/alluvium overlying densely fractured, welded tuffs. In order to identify characteristic behavior of infiltration that might be expected at the site, two idealizations of this situation are examined: an effectively semi‐infinite column of alluvium and a two‐layer column of alluvium over a fractured impermeable matrix. For each idealization the impact of hydraulic properties is assessed. Examining the sensitivity of bare soil simulator predictions for an effectively semi‐infinite column, it is found that decreasing the air entry pressure while holding all other parameters at a fixed level tends to increase both the long‐term average moisture content and the long‐term average net infiltration flux for homogeneous media. In contrast, increasing the van Genuchten scale parameter (m=1 − 17sol;n) or decreasing the porosity tends to decrease the average soil moisture but increase the infiltration. Most interestingly, three regimes are found for permeability. For relatively high permeabilities, there is a trend toward increasing average infiltration and increasing average moisture content with decreasing permeability. For relatively low permeabilities, vapor transport dominates over liquid transport, runoff and evaporation overwhelm infiltration, and the soil becomes very dry with essentially no infiltration flux. Between the extreme cases of high and low permeability, there is a zone where decreasing permeability results in decreased infiltration but increased moisture content, which is explained by the capacity of more permeable media to maintain surface wetness for longer periods of time after precipitation, thus allowing longer periods of evaporation at the potential rate. When there is a highly permeable fractured matrix underlying the alluvium, the depth of the alluvial cover has a dominant impact on infiltration. With little or no cover, more than half of the precipitation results in net infiltration. When alluvial cover is between depths of 25–50 cm and roughly 5 m, there is little or no net infiltration. Once the alluvial cover increases beyond 10 m, the semi‐infinite infiltration rate is recovered. The hydraulic properties of the fracture continuum have little impact on the infiltration behavior, as long as the equivalent fracture porosity is above a minimum value.