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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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