Analysis of Unsaturated Flow Based on Physical Data Related to Low Level Radioactive Waste Disposal Chihuahuan Desert, Texas

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Scanlon, Bridget R.
Wang, Fred P.  
Richter, Brend C.

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Unsaturated flow in arroyos and fissures in the Hueco Bolson of West Texas was studied to determine if downward fluxes are higher beneath these features relative to other geomorphic settings. Soil samples collected from five boreholes in two arroyos and in and adjacent to a fissure were analyzed for moisture content, water potential, and chloride content to evaluate moisture flux. In addition, three neutron-probe access tubes were installed in and adjacent to a fissure to monitor moisture content over time. A ponding test with CaBr2 as a tracer was conducted at a fissure to compare moisture movement in the fracture fill with movement in the surrounding sediments. The arroyos are floored by a shallow (~2-m [~6.6-ft]) layer of surficial gravel underlain by clays. Moisture content was low in the gravel (~4%) and much higher in the clay (21% to 33%). Water potentials decreased with depth, which indicates a potential for downward flux. A sharp increase in chloride concentrations below the contact between the shallow gravels and deeper clays is attributed to a larger mass of water influenced by evaporation from the low-permeability clay material. Soil texture and moisture content in the fissure were similar to that in surrounding sediments. Water potentials were close to 0 at depth in the fissure as well as in surrounding sediments and decreased toward land surface, which suggests a potential for upward liquid water flow. Chloride concentrations were approximately two orders of magnitude lower in the fissure than in the clays of the arroyos. These low chloride concentrations indicate that chloride is being flushed downward in the region of the fissures. The ponding test showed that the downward flux of chloride and bromide was greater in the fracture fill than in surrounding sediments. Water potentials in the fissures and arroyos are higher than those found in the ephemeral stream and interstream settings, and the higher potentials are attributed to wetter conditions in the former geomorphic settings. The upward decrease in water potentials in the fissures is similar to that found in the stream/interstream settings and suggests upward liquid water movement. These water-potential gradients contrast with those found in the arroyos, which are downward and indicate a potential for downward water movement. Chloride concentrations in the arroyos are similar to those found in the stream/interstream sediments; however, chloride concentrations in the fissure sediments are an order of magnitude lower than those in other geomorphic settings. These comparisons suggest that the downward flux of water is greater in the fissures than in other geomorphic settings.


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