Objective
The objective of this project is to test a zero-tension lysimeter designed to be installed in much less time and with less site disturbance than those of the conventional design, which would allow more extensive and therefore more accurate monitoring of colloid transport in contaminated soils. In this project, the lysimeter design is field-tested at two locations: (1) Ames, Iowa, and (2) the Rocky Flats Plant, Golden, Colorado.
Progress
Our main activity during October was completing the chemical, physical, and mineralogical analyses of 15 soils at the Rocky Flats Environmental Technology Site (RFETS). These soil properties have a significant impact on both actinide transport and on the function of zero-tension lysimeters installed in the soils. The information will be used in interpretations of ZTL performance in the project's final report. Complete data may be obtained from our collaborator, Dr. M. L. Litaor, at RFETS. A summary of the data follows.
Soil Classification. Most of the soils in this study were Argiustolls (i.e., soils formed in a semi-arid climate under the influence of prairie vegetation and containing subsurface accumulations of clay). All of the soils except one had mollic epipedons (i.e., relatively thick surficial horizons with abundant organic matter). Mollic epipedons reflect annual below-ground additions of organic matter to the soil by decomposition of the roots of prairie grasses and forbs. In general, mollic epipedons must be at least 18 cm thick and have moist colors with chroma and value < 3.5.
Most of the soils were classified as having argillic horizons, reflecting accumulations of clay in the subsurface. Argillic horizons generally have at least 20% more clay than the soil horizons overlying them. The common occurrence of lithologic discontinuities (see below), however, suggested that not all the clay in the Bt horizons was illuvial. Most of the soils investigated had subsurface accumulations of calcium carbonate as well. In several soils, that accumulation was great enough to formally identify calcic horizons.
Only two of the 14 pedons investigated had no evidence of lithologic discontinuities. This fact suggests the importance of alluvial parent materials to the soils in the study area. Similarly, even though the fine-earth fraction (i.e., < 2-mm material) was dominated by clay particles in most of the soils, there were usually abundant coarse fragments that resulted in classification of the soils in clayey-skeletal particle-size groups. Two soils were classified into torrertic subgroups because they had horizons near the soil surface that had high clay contents, making them susceptible to cracking in summer months.
Clay Mineralogy. Minerals in the clay fractions of the pedons in this
study were identified by standard X-ray diffraction techniques. Smectitic
minerals had d-spacings that expanded to about 1.8 nm upon glyceration.
Vermiculite occurred in trace amounts in some samples, evidenced by a typically
small peak at about 1.4 nm on the glycerated sample's X-ray diffraction
pattern. Clay mica (illite) was identified by its normal reflections at 0.98
nm, 0.5 nm, and 0.33 nm. Kaolinite in the samples provided characteristic
peaks at 0.71 nm and 0.356 nm. Quartz (0.426 nm) occurred ubiquitously in
small amounts in all the samples. The presence or absence of
hydroxy-interlayered 2:1 clay minerals was determined by assessing the degree
of collapse to 1.0 nm of the smectite and vermiculite d-spacings on K-saturated
samples heated to
The majority of soil horizons had clay fractions consisting of a mixture of the clay minerals identified above. Still, in all horizons, smectite was the dominant clay mineral, typically accounting for about 60% of the clay fraction. The abundance of smectite in the soils probably reflects the many potential sources of smectite as well as its characteristic particle size. Smectite in these soils was probably derived partly from Cretaceous-age shale. The shale formed the parent material for many of the soils investigated, either directly or as a source of the colluvium or alluvium in which the soils developed. Smectite may also have formed by neoformation as primary minerals weathered and released Si, Al, Fe, Mg, and Ca. Finally, smectite commonly occurs in fine clay fractions, a size that makes smectite particles susceptible both to transport by wind and water erosion and to accumulation in low-lying landscape positions.
With a few exceptions, clay mica contents were greatest near the soil surface and decreased with depth. This is the opposite trend from what one would expect in moderately to highly weathered soils, and it confirms the hypothesis that the soils have not significantly weathered since deposition of the parent materials.
PI: Michael Thompson, Ames, (515) 294-2415