3.1  Summary of Activities

This section summarizes activities performed by Parsons ES and Brice during the treatability study. A detailed description of methods and techniques used for the study is presented in Appendix A.

3.2  Background

Soil washing involves incorporating several separation techniques, both physical and chemical, in order to segregate contaminants from soils. Typical separation techniques include sieving, gravity separation, magnetic separation, and chemical leaching. Methods incorporated depend largely upon contaminant particle size, density, and the soil fraction in which the contamination is associated.

Typically, larger particle sized contaminants are removed in the primary stages of remediation through physical separation techniques. Physical separation methods, which depend upon density differences between the soil and the contaminant, are a relatively inexpensive and efficient removal method for larger contaminants. However, inorganic and organic contaminants tend to bind and sorb to clay, silt, and organic soil particles. Chemical leaching follows physical separation methods to break chemical bonds and recover contaminants associated with smaller soil fractions. Used in combination, multiple removal techniques have the capacity to remove contaminants from all soil fractions. Recovered metals from soil fractions may be recycled at a smelter, while the recovered soils are returned to the site.

Although soil washing can be quite effective, it oftentimes results in a reduction of soil volume returned to the site, since a percentage of the original soils must be disposed as hazardous waste or remediated using another method.

3.3  Technical Approach

3.3.1  Soil Sample Collection

Parsons ES personnel collected approximately 40 pounds of soil from the sifted soils and the projectile contaminated sand found at SWMU B-20 in March 1998. Two 5-gallon buckets containing soil samples were placed in separate mixers for 24 hours then sent to the subcontractor for treatability analysis.

Sifted soils were used for the treatability study because site characterization results indicated significantly higher lead levels than those observed within the 10-acre area in the central portion of the site. Although these analytical data provided by ITS laboratory were eventually used as screening data, the soils were considered to represent "worst case" scenario for lead at SWMU B-20.

A sample of the sand contaminated with projectiles within the small arms ammunition disposal areas (B-20) was also collected for the treatability study. This sand contained a large amount of particulate lead. At the time of the study, it was planned that this material would be disposed of under the RL33 contract; however, the disposal method had not yet been identified. In April 1999, TNRCC requested that this material be disposed as soon as possible. Due to the schedule restraints, soil washing treatment of these soils was not pursued further. Instead this portion of the B-20 soils (approximately 100 tons) was disposed offsite as hazardous waste in July, 1999.

3.3.2  Brice Soil Washing Design

Upon arrival at the Brice facility in Fairbanks, Alaska, soil samples taken by Parsons ES were measured for moisture content by Brice personnel. The samples were then mixed and dried in preparation for soil washing.

For this treatability study, the subcontractor employed density separation as the physical separation technique for the two soil samples (i.e., sifted soils and projectile-contamined sand) taken from SWMU B-20. Physical separation techniques rely on the size and density differences between lead and soil particles. Size separation includes processes such as screening or classification that separate whole projectiles and larger fragments from soil particles. Density separation uses the large density difference between lead and soil particles. With density separation, water pulsates alternatively up and down in an open top, screened bottom "jig". When the soil lead mixture is fed from the top, the lead particles sink to the bottom and the soil overflows from the top.

After discrete lead particles are separated from each other, there may still be lead absorbed on the ionic sites within the soil particles. The ionic adsorbed lead is typically extracted from the soil matrix by bringing them in contact with a leaching solution, such as an acid (referred to as chemical treatment). Chemical treatment was to be used following physical separation to maximize contaminant removal. However, due to the calcareous nature of the soils, the leaching agents proved unsuitable for use at these sites. Initial treatment screening tests showed that the lead contamination found in the form of lead carbonate in B-20 soils is less soluble that the soil itself. Therefore, a significant quantity of the soil must be dissolved to remove a small amount of ionic lead. The complications resulting from acid leaching of calcareous soils were verified during electrokinetic testing performed on SWMU O-1. The soil at SWMU O-1 showed a very high buffering capacity due to the high limestone content. In order to achieve the desired results, the electrokinetic process cost was dominated by the cost of chemicals used in the process.

The vigorous release of carbon dioxide resulting from the mixture of the acid and calcarcous soils also limits the use of chemical leaching. The froth resulting from this reaction requires additional plant componentry, costly emulsion breaker reagents, or both.

3.3.3  Treatability Study Activities

Brice provided benchscale testing of two composite soil samples from SWMU B-20. The assessment of soil washing to attain cleanup levels was the objective of Brice�s soil washing study. The benchscale testing was conducted in a manner simulating field scale process steps. These steps are identified in the following paragraphs.

The soil washing testing efforts began with a visual inspection of the composite samples followed by size gradation analysis to predict the physical behavior of the soil within process equipment and the usefulness of a soil classification step. Wet sieving was used to separate the soil into its constituent particles of gravel, sand, and fines for an accurate determination of soil gradation. Sieve analysis consists of shaking the soil sample through a set of sieves with progressively smaller openings. After the completion of the sieving, the mass of soil retained on each sieve is determined. Portions retained on each sieve are collected separately and oven dried before the mass retained on each sieve is weighed to determine the distribution of particle sizes in the bulk soil. Sieves of 4 mesh; 10 mesh; 40 mesh; and 200 mesh (0.187 inches, 0.079 inches, 0.0167 inches, 0.003 inches) were used to generate the soil fractions available for sizing the density treatment processing equipment. Soil analyses occurred both prior to and following the soil washing process in order to determine the effectiveness of soil washing. Analysis consisted of the following tests:

TCLP Analysis. A reduction in contaminant concentrations alone is not acceptable, a reduction in contaminant leaching must also be achieved. Following the benchscale study of B-20 soils, Brice performed TCLP analysis for lead for all soil fractions. TCLP was determined for the composite soil, as well as the plus 200 mesh and the minus 200 mesh soil fractions. Soils classified as plus 200 mesh equate to soil particles that did not pass through the 200 mesh sieve, therefore having particle diameters greater than 0.003 inches. All other soil is classified as minus 200 mesh. Brice performed the TCLP analysis.

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