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Coupled Effects of Biointrusion and Precipitation on Soil Caps
Background
The National Research Council's characterization of infrastructure material
clearly indicates the need to understand, and to be able to predict over the
long term, how integrated processes impact the performance of caps used to
isolate hazardous or radioactive wastes. It is recognized that biointrusion and
the processes it affects comprises a complex network of interactions; however,
two pieces of information seem to be missing from previous studies. First, it is
not clear what processes and how much time is needed for long-term exposure to
biointrusion to actually affect the performance of the cap. Second, to construct
more realistic models of cap performance, it is necessary to be able to
understand and evaluate cap performance as a function of coupled processes as
opposed to single processes.
Engineered barriers are designed to isolate hazardous waste from moving to
the environment and ideally, they are expected to sustain functionality well
beyond the breakdown of the materials they contain. Current barrier designs are
not invulnerable to environmental and biological assaults and, to date, it is
difficult to determine the significance of these intrusions on the long-term
performance and effectiveness of the barrier. Therefore, it is important to
elucidate the interactions between geophysical and biological processes, and how
these processes ultimately act on the long-term performance of caps.
Objectives
This study evaluated the coupled effects of geophysical, environmental, and
biological intrusions and how those factors ultimately affect the performance of
the cap. This project identified and evaluated time and cost-effective early
warning methods for detecting biointrusion. The tests were conducted at the
Engineered Barrier Test Facility (EBTF) near the RWMC on the INEEL. The
objectives of this study were:
- Test coupled effects for a natural material system, especially increased
rainfall and the effects of animal and plant intrusion.
- Induce animals to create worst-case (deepest) animal intrusion.
- Test colored sand tracers (emplaced as layers) to show depth of animal
intrusion.
- Evaluate capillary barrier performance.
Accomplishments through 2003
The experimental setting was designed to test a series of interactive
conditions: burrowing, plant evapotranspiration, and water percolation through
the barrier. Mockups of an evapotranspiration-storage type soil cap were
constructed in 12 test cells at the EBTF (Figure
9-10).
Cells 1 to 3 had rodents (mice) and no vegetation. These cells differ only in
the level of precipitation that each cell received (normal, 2X normal, and 3X
normal, respectively). Cells 4 to 6 were similar to Cells 1 to 3 with respect to
precipitation, the presence of rodents, and the presence of colored sand in the
soil. Unlike Cells 1 to 3, however, Cells 4 to 6 were vegetated. The same type
and density of vegetation was established on these cells at the start of the
experiment. Cells 7 to 9 were similar to Cells 1 to 3 with respect to
precipitation and lack of vegetation, but lacked burrowing mammals. Cells 10 to
12 were similar to Cells 4 to 6 with respect to precipitation and vegetation.
Again although, Cells 10 to 12 lacked rodents and colored sand in the soil.
Cells 1 through 6 had the same number of rodents introduced to each of these
cells.
Considered in total, the 12 treatments enabled us to evaluate the coupled
effects and interactions between accelerated precipitation, animal burrowing,
vegetated/bare surfaces, soil microbiology, and soil cap hydrologic performance.
The caps were comprised of (from top to bottom) 1.6 m (5.2 ft) of silt loam
soil, a geotextile fabric, 0.15 m (approximately 5 ft) of gravel, 0.75 m (2.5
ft) of cobbles, and 0.5 m (1.6 ft) of silt loam soil. The surface 0.15 m
(approximately 5 ft) of soil was mixed with gravel (25 percent by volume) as a
wind erosion preventative.
Each test cap was constructed in lifts to enable precise control of soil
density and facilitate the installation of soil moisture monitoring
instrumentation and soil tracers for detection of burrowing. Time domain
reflectometry (TDR) probes for monitoring soil moisture, heat dissipation
sensors (HDS) for monitoring soil moisture tension, and thermocouples (TC) for
monitoring soil temperature were installed at various depths. Instrument cables
were routed horizontally to a cable tower installed within the test cell.
Horizontal installation precluded the creation of vertical preferred pathways
for water infiltration at the soil surface. Snowfall accumulating on the test
plots was measured using an ultrasonic sensor. Data collection from all soil and
snow instruments was automated to provide an uninterrupted time series of data
and to reduce manpower requirements. Meteorological parameters were obtained
from the National Oceanic and Atmospheric Administration weather station located
near the EBTF.
To prevent introduced rodents from escaping and wild fauna from invading the
cells (e.g., predators such as snakes or other carnivores, as well as other
rodents), 1.5 m (approximately 5 ft) lexan walls were used in the construction
of the plots. The walls were buried 20 cm (approximately 8 in.) deep and in
direct contact with a concrete lip inside the walls, creating a tight seal and a
structural barrier in the event of potential attempts of mice to dig out of the
cells. Because the cells were open to the environment (no lids), predation by
raptors was prevented using a bird chase ultrasonic model UB43 (Bird-B-Gone,
Inc.) that emits a 20 to 25 KHz tone in a variety of mode combinations (i.e.:
steady, burst, sweep, and random). This frequency range does not harm the birds
and keeps them away from the facility.
Vegetation was incorporated inside plots 4, 5, 6, 10, 11, and 12, following a
distribution of vegetation from an area randomly selected in the vicinity of the
experimental site. All of the plots were vegetated with the same distribution
and type of plants: four sagebrush; four green rabbitbrush; two bluebunch
wheatgrass; three prickly phlox, and two forbs. Surveys of test plot vegetation
will be conducted at the conclusion of the project to determine the survivorship
of plant species and biomass. Test plot soils will be excavated to determine
root distributions and biomass. The distribution of animal burrows will be
mapped at the conclusion of the project by injecting hardening foam into the
burrows and carefully excavating the surrounding soil.
Results
Data collection has just recently been completed and data currently being
compiled and interpreted on precipitation, vegetation, and burrowing effects on
the cap. Results will be provided in next year's annual report.
Investigators and Affiliations
- Angela Stormberg, Principal Scientist, Idaho National Engineering and
Environmental Laboratory, Idaho Falls, ID
- Roelof Versteeg, Senior Advisory Scientist, Idaho National Engineering and
Environmental Laboratory, Idaho Falls, ID
- Kristine Baker, Principal Scientist, Idaho National Engineering and
Environmental Laboratory, Idaho Falls, ID
- Carlan McDaniel, Idaho National Engineering and Environmental Laboratory, Idaho
Falls, ID
- Indrek Porro, Idaho National Engineering and Environmental Laboratory, Idaho
Falls, ID
Funding Sources
Environmental Systems Research and Analysis (ESRA), Environmental Management
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