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
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Daniel, D.E., and B.A. Gross. 1995. Caps. National Technical
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