Abstract

Characterization studies of a potential low-level radioactive waste site were conducted in an area 65 km southeast of El Paso. The study area was instrumented with neutron-probe access tubes to monitor moisture content and thermocouple psychrometers to monitor water potential and temperature. The absence of temporal variations in moisture content monitored in deep (41 m) profiles indicated that, within the accuracy of the neutron-probe measurements, water 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. Laboratory-measured water potentials ranged from-15.6 to 0 MPa. After precipitation events, water potentials were highest at the surface and decreased sharply at the base of the wetting front. Water potentials monitored by the in situ psychrometers were out of range (≤–8 MPa) in the top 0.8 m during most of the monitoring period. Seasonal fluctuations in water potential were recorded at depths of 1.1 and 1.4 m. Water potentials at greater depths remained fairly uniform with time. Except in the shallow subsurface after precipitation events, water potentials generally decreased upward; this trend indicated an upward driving force for liquid and isothermal vapor movement, probably controlled by evapotranspiration. Soil temperatures displayed large seasonal and diurnal fluctuations. Temperatures generally increased with depth in the winter and decreased with depth in the summer. Driving forces for thermal vapor movement are downward in the summer and upward in the winter. The net vapor flux depends on the magnitude of water potential versus temperature gradients and D_Φv versus D_Tv. It is difficult to estimate the net liquid flux because of the limited time period monitored relative to long-term climatic variability in this desert system.