SITE CHARACTERISTICS

The engineered covers are located in the Chihuahan Desert of Texas in northwest Eagle Flat basin, 10 km east of Sierra Blanca and about 120 km southeast of El Paso (Figure 1). The regional climate is subtropical arid (Larkin and Bomar, 1983). Long-term meteorologic data were obtained at Sierra Blanca, situated on the western edge of the regional study area. Mean annual precipitation was 321 mm for a 30-yr period characterized by large interannual variation (516 mm in 1974 to 180 mm in 1994). Most precipitation (70 percent) falls as local, intense, short-duration convective storms during the summer when temperature and potential evaporation are highest. Minor winter frontal storms are generally of lower intensity and longer duration.

Detailed characterization studies were conducted from 1993 to 1996. Northwest Eagle Flat basin is a closed topographic depression ~ 500 km2 in area that drains through the ephemeral Blanca Draw to Grayton Lake playa. Characterization studies included a comprehensive evaluation of the surface geomorphology, subsurface geology, and unsaturated and saturated zone hydrology. The basin-fill sediments consist predominantly of fine-grained eolian and fluvial material. The surface geomorphic settings guided the unsaturated zone studies as variations in geomorphology reflect differences in topography, texture, and vegetation, which in turn affect unsaturated flow. Geomorphic settings (Figure 1) include drainage areas (Blanca Draw and Grayton Lake playa) and interdrainage areas (alluvial fan, basin-fill deposits, and eolian sheets).

Figure 1. Regional location map and geomorphic settings in the study area. The prototype engineered barriers are located in the older sand sheet setting.

 

The prototype engineered cover studies are located in the eolian setting (Figure 2). Mean water contents in 17 profiles (10 to 25 m depth) ranged from 0.07 to 0.09 g g-1. Water potentials are low near the surface and increase with depth, suggesting an upward driving force for water movement. Chloride has been flushed out of the sediments in the upper 0.5 to 2-m depth and increases with depth to maximum values of 1,700 to 7,800 mg L-1 in different profiles. Water flux estimated from the chloride data below the upper 2 m is about 1 mm yr-1. Water potential monitoring since 1993 in the natural sand-sheet setting shows that wetting fronts do not penetrate below the upper 0.4 m, despite several large rainfall events and snow melts that occurred during this period (e.g., 116 mm in September 1995).

Figure 2. Typical view of the eolian setting. The climate is subtropical arid and the generally sparse native vegetation includes soap tree yuccas, mesquite, creosote, and various grasses. (Photo by John Gamble)

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Environmental Quality