Assessing the Effects
of Soil-Forming Processes on Surface Caps
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.
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.
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
Environmental Systems Research and Analysis (ESRA), Environmental Management