The Protective Cap/Biobarrier Experiment


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 landfi ll 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). Presences 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 reservoir of stored water.

The Protective Cap/Biobarrier Experiment (PCBE) was established in 1993 at the Experimental Field Station, INL Site, 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.


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 2007 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 finalized in February 2007 (Janzen et al. 2007) which focused upon long-term cap performance. The 2007 report built upon the original objectives by adding four additional objectives; (1) comparing plant cover and soil moisture dynamics from the 1994-2000 study period with the relatively drier 2002-2006 study period, (2) assessing the spatial and temporal stability of total vegetation cover, (3) understanding how vulnerable the native and crested wheatgrass communities are to invasion from neighboring communities, and (4) quantifying the relationship between vegetation cover and ET.

During the 2007 field season collection of finer time-scale vegetation cover measurements and direct transpiration measurements began in order to clarify soil-plant water relationships occurring on the PCBE. Specific objectives for these measurements include: (1) identify the relationship between vegetation cover and ET on plots planted with a native seed mix, (2) determine relative contribution by species to plot ET, and (3) determine if community dynamics have been shaped by either cap design or irrigation treatment.

Accomplishments Through 2007

Soil moisture and vegetation cover data from 1994-2006 were analyzed according to the 2007 report objectives listed above and the final report 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

Two supplemental irrigation treatments were completed on the PCBE in 2007. A summer irrigation treatment was applied in fifty millimeter increments on a biweekly basis beginning in late June and ending in early August; totaling 200 millimeters of irrigation. The fall/spring irrigation application of 200 millimeters was completed during late September and early October. Soil moisture data were collected during 2007 beginning in April through mid-October on a biweekly basis. Vegetation cover data were collected throughout the month of July and early August. Fine scale measurements in the form of photographs were taken on a monthly basis for all planted native plots beginning in May and ending in October. Transpiration measurements for selected native species were collected on deep biobarrier caps receiving both fall/spring irrigation and summer irrigation, and Resource Conservation and Recovery Act (RCRA) cap types receiving summer irrigation at the end of July, August, and early October.


Because data collection was initiated in 2007 for the new outlined objectives, limited data analysis has been completed, however, analysis on long-term community dynamics has been completed and results are presented below.

  • Vegetative cover in RCRA cap types was generally lower than in all other cap types. Long-term trends in diversity indices do not differ significantly among cap types when data analysis includes all irrigation treatments.

  • Vegetative cover and Inverse Simpson’s index was lowest in the ambient treatment than in either of the irrigated treatments. Long-term trends in other diversity indices did not differ significantly among irrigation treatments.

  • Species rank abundance was relatively similar among cap types with the exception of the shallow biobarrier cap types which had significantly different species ranks for Ericameria nauseosus and Hedysarum boreale.

  • Species rank abundance varied among irrigation treatments. Plots receiving the ambient treatment generally had a higher species rank for forbs and the lowest species rank for Agropyron cristatum than either of the irrigation treatments.

Plans for Continuation

During the upcoming growing season we will continue to monitor vegetation cover and soil moisture as we continue to assess long-term alternative ET cap performance. Additionally, we will continue to collect fine scale vegetation cover measurements and direct transpiration measurements throughout the growing season in 2008. The measurements taken during the 2007 and 2008 field seasons will be used to better characterize and quantify the soil-plant water relationships on the PCBE, which will be useful for modeling long-term cap performance, as well as improving cap performance through directed revegetation design.

Publication, Reports, Theses, etc.

We anticipate that we will submit two manuscripts to peer reviewed journals in addition to the completion of a M.S. thesis in late 2008 or early 2009.

Investigators and Affiliations

Brandy C. Janzen, Graduate Student, Department of Biological Sciences, Idaho State University, Pocatello, Idaho

Matthew J. Germino, Associate Professor, Department of Biological Sciences, Idaho State University, Pocatello, Idaho

Amy D. Forman, Environmental Surveillance, Education, and Research Program, S.M. Stoller Corporation, Idaho Falls, Idaho

Funding Sources

U.S. Department of Energy, Idaho Operations Office.


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 wasted 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 Commerece, Springfi led, Virginia.

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

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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