Managing Soil Moisture on Waste Burial Sites in Arid Regions

Abstract

In semiarid regions, where potential evapotranspiration greatly exceeds precipitation, it is theoretically possible to preclude water form reaching interred wastes by (i) providing a sufficient cap of soil to store precipitation that falls while plants are dormant and (ii) establishing sufficient plant cover to deplete soil moisture during the growing season, thereby emptying the water storage reservoir of the soil. Here we discuss the theory and rationale for such an approach and then present the results of a field study to test its efficacy at the Idaho National Engineering Laboratory (INEL). We examined the capacity of four species of perennial plants to deplete soil moisture on simulated waste trenches and determined the effective water storage capacity of the soil. Those data enabled us to estimate the minimum depth of fill soil required to prevent deep drainage. Any of the species studied can use all of the plant‐available soil water, even during a very wet growing season. The water storage capacity of the soil studied is 17% by volume, so a trench cap of 1.6 m of soil should be adequate to store precipitation received at the INEL while plants are dormant. We recommend a fill soil depth of 2 m to provide a margin of safety in case water accumulates in local areas as a result of heavy snow accumulation, subsidence, or runoff. Fill soil requirements and choice of plant species will vary, but the concepts and general approach are applicable to other shallow land burial sites in arid or semiarid regions.

Enhancements to Natural Attenuation: Selected Case Studies

“…Alternative covers that mimic this natural water conservation may provide long-term hydrologic isolation of subsurface contaminants (Clarke et al 2004). Alternative covers generally consist of thick, finer-textured soil layers that store precipitation in the root zone where it can be removed seasonally by ET (Anderson et al 1993). Capillary barriers consisting of coarse-textured sand and gravel placed below this soil "sponge" can enhance water storage and limit unsaturated flow (Nyhan et al 1990, Ward andGee 1997).…”

“…However, plant community dynamics are complicated and effects are difficult to predict. Even in the absence of large-scale disturbances, seasonal and yearly variability in precipitation and temperature will cause changes in species abundance, diversity, biomass production, and soil water extraction rates on covers (Anderson et al 1993, Link et al 1994. Investigations of natural analogs can provide insights as to how ecological processes may influence the sustainability of alternative covers (Waugh et al 1994).…”

Calculation set for design and optimization of vegetative soil covers Sandia National Laboratories, Albuquerque, New Mexico.

“…Vegetative covers are based upon the overarching premise that the amount of total precipitation refracted back to the atmosphere annually via evapotranspiration is on the order of 80% to 100% in arid and semiarid environments (Anderson et al 1993). The evapotranspiration potential in the southwestern United States far exceeds the amount of water received from precipitation.…”

Geosynthetic capillary barriers: current state of knowledge

“…Also, evapotranspirative covers can be constructed with a reasonably broad range of soils, contributing to cost savings associated with the use of site-specific soils instead of imported material. The performance of evapotranspirative cover systems has been documented by field experimental assessments (Anderson et al 1993;Dwyer 1998), and procedures have been developed for quantitative evaluation of the variables governing their performance (Khire et al 2000;.…”