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
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;
Comparing the hydrologic performance of four ET cap
Examining the effects of biobarriers on water movement
throughout the soil proﬁle of ET caps
Assessing the performance of alternative ET cap designs
under current and future climatic scenarios, and
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.
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
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
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
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
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.
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.
A.G., and E.F. Redente. 1998. Biointrusion of protective
barriers at hazardous waste sites. Journal of Environmental
and B.A. Gross. 1995. Caps. National Technical Information
Service, U.S. Department of Commerece, Springﬁ led, Virginia.
1986. Hydrogeologic factors in the selection of shallow land
burial for the disposal of low-level radioactive waste.
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,
1991. Radioactive Waste Isolation in Arid Zones. Journal of Arid
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.
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.