ESER manages a program of innovative
research to expand the INL's status as a center for state-of-the-art
ecological and environmental activities.
Summaries
of current ESER research projects
Natural
and Assisted Recovery of Sagebrush in Idaho’s Big Desert:
Effects of Seeding Treatments on Successional Trajectories
of Sagebrush Communities.
Protective Cap/Biobarrier Experiment
The Protective Cap/Biobarrier Experiment (PCBE) was established in 1993 at the Experimental Field Station (EFS), INL to test the efficacy of four protective landfill cap designs. The ultimate objective of the PCBE is to design a low maintenance, cost effective cap that uses local and readily available materials and natural ecosystem processes to isolate interred wastes from water received as precipitation. Four evapotranspiration (ET) cap designs, planted in two vegetation types, under three precipitation regimes have been monitored for soil moisture dynamics, changes in vegetative cover, and plant rooting depth in this replicated field experiment.
Background
In semiarid regions, where potential evapotranspiration greatly exceeds precipitation, it is theoretically possible to preclude water from reaching interred wastes by:
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Providing a sufficient cap of soil to store precipitation that falls while plants are dormant.
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Establishing sufficient plant cover to deplete soil moisture during the growing season, thereby emptying the water storage reservoir of the soil.
Additionally,
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Previous studies at the INL have demonstrated that an ET cap consisting of 2 m of homogenous soil supporting a healthy stand of perennial plants should prevent water from percolating into buried wastes.
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Concern over burrowing animals prompted many landfill managers to include biointrusion barriers in their cover designs.
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The effects of biointrusion barriers that limit burrowing of small mammals and ants on soil moisture dynamics must be fully understood before final recommendations can be made for protective caps at the INL.
Two
of the four ET caps studied in the PCBE consist of 2.0 m (6.6 ft) of soil
interrupted by a 0.5 m (1.6 ft) gravel/cobble biointrusion barrier. The
biointrusion barrier is located 0.5 m (1.6 ft) below the soil surface in the
shallow biobarrier cap design, and 1.0 m (3.3 ft) below the soil surface in
the deep biobarrier cap design. In addition, a cap based on RCRA
recommendations and a soil only cap have been monitored. The RCRA cap
consists of 0.6 (2 ft) of compacted clay overlain by a flexible membrane liner
(FML), which, in turn, is overlain by 1 m (3.3 ft) of topsoil. The soil
only cap consists of 2 (6.6 ft) of topsoil. All cap designs are
vegetated.
Vegetation
Waste caps at the INL typically have been planted in crested wheatgrass, a non-native species; however, native species are likely to invade these crested wheatgrass monocultures over the one thousand years many caps are mandated to function effectively. In addition, the Department of Energy (DOE) favors replanting native species when possible. Thus, both vegetation types were included in the PCBE, because understanding water extraction patterns for crested wheatgrass and a mix of native species is key to determining the effectiveness of current and future cap designs.
Precipitation
Models of global climate change predict increased precipitation for sagebrush steppe ecosystems, such as the INL.
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Some models predict an increase during summer, whereas others predict an increase in winter or early spring.
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To investigate the influence of increased precipitation on cap performance, two supplemental irrigation treatments were included in the PCBE.
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One irrigation treatment augments summer precipitation by 200 mm and the other augments fall/spring precipitation by the same amount.
Results
Preliminary results indicate that the soil only cap and the biobarrier caps generally performed similarly throughout the study period under ambient precipitation, and all moisture received as precipitation was returned to the atmosphere annually via evapotranspiration.
The RCRA cap occasionally drained, even under ambient precipitation, as a result of water infiltrating to and running off the flexible membrane liner.
Water extraction patterns were similar between vegetation types under ambient precipitation. However, in many years the amount of water in the soil profile at the end of the growing season did significantly differ in response to vegetation type under augmented precipitation treatments. For example, from 1996 - 2000, end of season average percent volumetric water content was significantly higher for crested wheatgrass plots than for native vegetation plots in deep biobarrier caps and RCRA caps under augmented summer precipitation. Nevertheless, cap failure on the PCBE has been a rare occurrence under any cap design, vegetation type, precipitation regime combination.
Conclusion
The Protective Cap/Biobarrier experiment has confirmed that low maintenance, cost effective ET caps can be used to effectively isolate buried waste at the INL. This project has also generated a large amount of data useful for making decisions related to specific capping projects at the INL. Recommended research for the PCBE includes
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Long-term maintenance
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Response to fire
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Response to invasive plant
species
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Erosion in response to
disturbarnce
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Role of soil microbiota in cap
function
Full Report - The Protective
Cap/Biobarrier Experiment: A Study of Alternative Evaportranspiration
Caps for the Idaho National Laboratory
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