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The Protective Cap/Biobarrier Experiment

Investigators and Affiliations

Amy D. Forman, Environmental Surveillance, Education, and Research Program, S.M. Stoller Corporation, Idaho Falls, ID
Brandy C. Janzen, Graduate Student, Department of Biological Sciences, Idaho State University, Pocatello, ID
Matthew J. Germino, Associate Professor, Department of Biological Sciences, Idaho State University, Pocatello, ID

Funding Sources

U.S. Department of Energy Idaho Operations Office

Background

Shallow land burial is the most common method for disposing of industrial, municipal, and low-level radioactive waste, but in recent decades it has become apparent that conventional landfill practices are often inadequate to prevent movement of hazardous materials into ground water or biota (Suter et al. 1993, Daniel and Gross 1995, Bowerman and Redente 1998). Most waste repository problems result from hydrologic processes. When wastes are not adequately isolated, water received as precipitation can move through the landfill cover and into the wastes (Nyhan et al. 1990, Nativ 1991). Presence of water may cause plant roots to grow into the waste zone and transport toxic materials to aboveground foliage (Arthur 1982, Hakonson et al. 1992, Bowerman and Redente 1998). Likewise, percolation of water through the waste zone may transport contaminants into ground water (Fisher 1986, Bengtsson et al. 1994).

In semiarid regions, where potential evapotranspiration greatly exceeds precipitation, it is theoretically possible to preclude water from reaching interred wastes by (1) providing a sufficient cap of soil to store precipitation that falls while plants are dormant and (2) establishing sufficient plant cover to deplete soil moisture during the growing season, thereby emptying the water storage reservoir of the soil.

The Protective Cap/Biobarrier Experiment (PCBE) was established in 1993 at the Experimental Field Station, INL to test the efficacy of four protective landfill cap designs. The ultimate goal 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.

Objectives

From the time it was constructed, the PCBE has had four primary objectives which include; (1) comparing the hydrologic performance of four ET cap designs, (2) examining the effects of biobarriers on water movement throughout the soil profile of ET caps, (3) assessing the performance of alternative ET cap designs under current and future climatic scenarios, and (4) evaluating the performance of ET caps planted with a diverse mix of native species to those planted with a monoculture of crested wheatgrass.

Specific tasks for the PCBE in 2006 included maintenance of the study plots, continuation of the irrigation treatments, and collection of soil moisture and plant cover data. An update to the 2003 PCBE summary report (Anderson and Forman 2003) was also scheduled to be drafted in 2006. Data were analyzed for the updated summary report according to the four major objectives listed above, focusing on long-term cap performance. Four additional objectives, which address emerging landfill-capping issues, were also considered in the summary report. The additional objectives include; (1) comparing plant cover and soil moisture dynamics from the 1994-2000 study period with the relatively more droughty 2002-2006 study period, (2) assessing the stability of total vegetation cover both spatially and temporally, (3) understanding the invasibility of the native and crested wheatgrass plant communities planted on the PCBE, and (4) quantifying the relationship between vegetation cover and evapotranspiration.

Accomplishments through 2006

Three supplemental irrigation treatments were completed on the PCBE in 2006. The fall/spring supplemental irrigation treatment initiated in late September 2005 could not be completed due to a failure of the deep well. Therefore, the deep well was repaired and the balance of the fall/spring irrigation treatment was applied in April of 2006. A summer irrigation treatment was also performed, as scheduled, in 2006. Fifty millimeters of water was applied to the summer irrigated plots once every other week from the end of June through the beginning of August for a total of 200 mm. Finally, the fall/spring 2006 irrigation treatment was completed in mid-October. Soil moisture measurements were collected once every two weeks from beginning of April through mid-October. Vegetation cover data were collected throughout the month of July and into August.

Soil moisture and vegetation cover data from 1994-2006 were analyzed according to the objectives described above. A draft of the updated summary report was completed at the end of 2006 and was published in February 2007. A copy of the report, entitled “PCBE Revisited: Long-Term Performance of Alternative Evapotranspiration Caps for Protecting Shallowly Buried Wastes under Variable Precipitation” (Janzen et al. 2007) is available at www.stoller-eser.com.

Results and Discussion

During the 2002-2006 study period, an alternative ET cap design with a gravel/cobble biobarrier placed at a depth of one meter below the soil surface prevented potential water breakthrough to the simulated waste zone better than the other three designs tested. The capillary break created by the change in substrate texture at the interface of soil and gravel at the top of the biobarrier appears to enhance cap function by forcing the soil above the biobarrier to reach field capacity before water will percolate below the biobarrier, limiting unsaturated flow and preferential flow pathways. These results were similar to those reported for the 1994-2000 study period. In contrast to results reported from the earlier study period, the performance of an alternative design consisting of a two meter soil monolith began declining over the past four years. Two additional cap designs, one based on Resource Conservation and Recovery Act (RCRA) guidelines and the other an alternative ET design with a biobarrier placed at 0.5 m below the soil surface, performed during the second study period much as they had in the first. Water often collected on the flexible membrane liner of the RCRA cap and often percolated below the biobarrier on the design with the shallowly placed biobarrier. In both cases, this percolation didn’t necessarily lead to potential breakthrough at the bottom of a cap, but it does indicate that more soil is needed to prevent water from reaching these physical barriers.

The caps planted with a diverse mix of native vegetation continued to perform better than those planted with a crested wheatgrass monoculture. In fact, crested wheatgrass does not appear to provide adequate transpiration to maintain long-term ET cap function. Poor performance of caps planted with crested wheatgrass may be related to relatively low vegetative cover overall and relatively high variation in vegetation cover spatially and temporally. Caps planted with crested wheatgrass tended to have lower average plant cover that caps planted with native vegetation. The stability of the crested wheatgrass plant community tended to be lower than that of the native plant community as evidenced by the relatively high variability in vegetative cover among caps planted with crested wheatgrass. Additionally, the crested wheatgrass caps had a high incidence of encroachment of species that were not originally planted when compared to encroachment of crested wheatgrass into the native vegetation caps.

When performance of the four cap designs was compared in response to ambient precipitation and two climate change scenarios, all of the cap designs experienced at least one potential breakthrough event under an augmented fall/spring precipitation scenario during the 2002-2006 study period (Figure 9-10). This result was not observed during the 1994-2000 study period and indicates that none of the cap designs would function properly under extreme climate change in which the INL received twice current ambient precipitation during the winter months. As with the first study period, potential breakthroughs were rare under ambient precipitation and augmented summer irrigation. The potential breakthrough events that did occur under those precipitation scenarios occurred only on the caps planted with crested wheatgrass (Figure 9-10). Thus, when planted with native vegetation, all four cap designs precluded water from percolating through the bottom of the cap under current climatic conditions.

Plans for Continuation

Over the next two growing seasons we will monitor vegetation cover and soil moisture as we continue to assess long-term alternative ET cap performance. Weak correlations between vegetation cover and evapotranspiration in analyses conducted for the updated summary report indicate that simple paradigms of soil-plant water relationships may not be adequate to explain the performance of ET caps. Therefore, we will also collect some finer time-scale vegetation cover measurements and direct transpiration measurements throughout the growing season in 2006. These additional measurements will be used to better characterize and quantify the soil-plant water relationship on the PCBE, which will be useful for modeling long-term cap performance, as well as improving cap performance through directed revegetation design.

References

Anderson, J.E., and A.D. Forman. 2003. Evapotranspiration Caps for the Idaho National Engineering and Environmental Laboratory: A Summary of Research and Recommendations. Environmental Surveillance, Education, and Research report, Stoller Corporation and Idaho State University, STOLLER-ESER-56.

Arthur, W.J. 1982. Radionuclide concentrations in vegetation at a solid radioactive waste disposal area in southeastern Idaho. Journal of Environmental Quality 11:394-399.

Bengtsson, L., D. Bendz, W. Hogland, H. Rosqvist, and M. Akesson. 1994. Water balance for landfills of different age. Journal of Hydrology 158:203-217.

Bowerman, A.G., and E.F. Redente. 1998. Biointrusion of protective barriers at hazardous waste sites. Journal of Environmental Quality 27:625-632.

Daniel, D.E., and B.A. Gross. 1995. Caps. National Technical Information Service, U.S. Department of Commerce, Springfield, Virginia.

Fisher, J.N. 1986. Hydrogeologic factors in the selection of shallow land burial for the disposal of low-level radioactive waste.

Grace, J.B. 2006. Structural Equation Modeling and Natural Systems. Cambridge University Press, NY.

Hakonson, T.E., L.J. Lane, and E. P. Springer. 1992. Biotic and abiotic processes. Pages 101-146 in C.C. Reith and B.M. Thomson, editors. Deserts as dumps? The disposal of hazardous materials in arid ecosystems. University of New Mexico Press, Albuquerque, New Mexico.

Janzen, B.C., M.J. Germino, J.E. Anderson, and A.D. Forman. 2007. PCBE revisited: long-term performance of alternative evapotranspiration caps for protecting shallowly buried wastes under variable precipitation. Environmental Surveillance, Education, and Research Program report, Idaho State University and Stoller Corporation, STOLLER-ESER-101.

Nativ, R. 1991. Radioactive Waste Isolation in Arid Zones. Journal of Arid Environments 20:129-140.

Nyhan, J. W., T. E. Hakonson, and B. J. Drennon. 1990. A water balance study of two landfill cover designs for semiarid regions. Journal of Environmental Quality 19:281-288.

Suter, G.W.I.I., R.J. Luxmoore, and E. D. Smith. 1993. Compacted soil barriers at abandoned landfill sites are likely to fail in the long term. Journal of Environmental Quality 22:217-226.

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