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Assessing the Effects of Soil-Forming Processes on Surface Caps

Background

Vegetative surface caps for the disposal of radioactive or hazardous wastes are often constructed of homogenized subsoil material collected from the local area. In arid regions, these caps rely on ET to prevent water from percolating into the waste. Over time, these materials are subjected to natural soil-forming processes, eventually resulting in the development of strata within the soil material that may ultimately influence cap performance. Organic carbon and available phosphorus play an important role in the structure and function of the soil ecosystem by influencing the growth of plants at the site which, through transpiration, help to prevent precipitation from moving downward through the cap and ultimately reaching the buried wastes.

The PCBE was established at the INEEL EFS in 1993 to examine four different simulated surface cap designs under two different vegetation types and three different moisture regimes. Because these caps have been in place for ten years, they represent an example of accelerated soil forming processes, providing insight into plant-soil interactions within the surface cap.

Objectives

Specific objectives for this study include:

  • Compare the vertical distribution of carbon and phosphorus concentrations in soil cores from the PCBE site with those from an undisturbed site with mature soil development.
  • Evaluate cation nutrients (potassium, calcium, and magnesium) for a subset of the soil cores.
  • Determine cation exchange capacity, base saturation, and soil texture on a subset of cores.

Accomplishments through 2003

Soil cores were collected from the PCBE site and from an undisturbed site located nearby. In each sampling location, soil cores were collected from beneath a sagebrush and a bunchgrass, as well as from an open area adjacent to each plant. Six depth intervals within the top 12.5 cm (5 in.) were evaluated by cutting each core into segments. Organic carbon concentrations were determined according to the tube digestion/heating block method, a modification of the Walkley-Black method. For plant-available soil phosphorus, samples were extracted with a buffered alkaline solution of sodium bicarbonate, and the solution analyzed using an inductively coupled plasma atomic emission spectroscopy (ICP/AES) analyzer. Concentrations of individual cation nutrients and measurements of cation exchange capacity, base saturation, and soil texture were determined using standard soil analytical techniques.

Results

Vertical distribution of carbon and phosphorus at the PCBE site was pronounced; indicating that development of soils on the surface caps is progressing (Figure 9-9). A strong interaction between vegetation, location, and depth was observed for both carbon and phosphorus, which reflects the particularly strong organic enrichment under vegetation. There is also an interaction between irrigation and depth, which indicates surface enrichment following irrigation. Cap design effects are mostly absent. For cation exchange capacity and base saturation data, results differed between the PCBE cores and those collected from the undisturbed site. In particular, the potassium data show increased concentrations in the upper soil layers, decreasing with depth. Significant differences are also apparent between samples collected beneath shrub canopies versus in the open, again pointing out the influence of vegetation on the chemistry of the cap soil.

Collectively, these data show evidence of discernible development of the upper soil profile in caps after eight years. However, the data also indicate that additional time is needed to approach the accumulation seen in natural soils of sagebrush steppe ecosystems.

The carbon and phosphorus results were presented at the Soil Science Society of America Annual Meeting in Denver in 2003, and a formal journal article has been submitted to the Soil Science Society of America Journal. A second paper on the cation results is in preparation.

Investigators and Affiliations

  • Terence McGonigle, Research Assistant Professor, Department of Biological Sciences, Idaho State University, Pocatello, ID
  • M. Lala Chambers, Staff Scientist, Idaho National Engineering and Environmental Laboratory, Idaho Falls, ID
  • Gregory White, Consulting Scientist, Idaho National Engineering and Environmental Laboratory, Idaho Falls, ID

Funding Sources

Environmental Systems Research and Analysis (ESRA), Environmental Management

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