Unsaturated Flow Along Arroyos and Fissures in the Hueco Bolson, Texas

Unsaturated flow in the Chihuahuan Desert of Texas was analyzed to evaluate the suitability of this region for low-level radioactive waste disposal because thick unsaturated zones that have low moisture fluxes in arid regions are considered suitable for radioactive waste disposal. The principal study area was instrumented with neutron-probe access tubes and psychrometers to monitor moisture content and water potential, respectively. The absence of temporal variations in moisture content monitored in deep (41 meters [135 feet]) profiles indicated that recharge pulses are not moving through the system. Penetration of moisture after rainfall was restricted to the uppermost meter of the unsaturated zone because of the low degree of saturation of the surficial sediments. Water potentials were as low as -15.6 megapascals (MPa) near the land surface in the summer. Except in the shallow subsurface after precipitation events, water potentials generally decreased upward, indicating primarily upward flow under isothermal conditions, probably controlled by evapotranspiration.

The computer code, TRACRN, was used to evaluate various unsaturated-flow processes in this system. Numerical simulations of upward flow indicated that small variations in water content result in large variations in water potential in dry unsaturated zones. One-dimensional simulation of infiltration was calibrated using chlorine-36 (36Cl) data. Sensitivity analyses suggested that applied flux and initial water potential are the most critical factors in controlling the propagation of the wetting front. Analyses of potential leakage from the base of a possible facility indicated that the direction as well as the net rate of water movement is controlled by lithologic layering in the system. Simulations of downward leakage rates that are similar to the saturated hydraulic conductivity of the deep clays showed that a saturated zone will develop at the contact between the shallow coarse material and underlying clays.