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SWMU B-20 Investigation Report Addendum Appendix G - Preliminary Evaluation of Remedial Alternatives
Section 2 - Preliminary Screening of Technologies and Alternatives
Table 2.1 lists technologies that Parsons ES has identified and their applicability to the B-20 site. These technologies were identified to provide a range of protection for human health and the environment. Innovative technologies which have not been evaluated here, but may be appropriate for the site, include electrokinetics and phytoremediation.
No action is always retained as a baseline for comparison with other technologies.
Two institutional control technologies have been identified. Deed Restriction uses legal procedures to limit future land use, such as no excavation. Fencing typically uses a barbed metal wire fence to physically prevent unauthorized entry or trespassing. Both these technologies would be effective for access control and are therefore retained.
Three capping technologies have been identified for potential onsite containment of contaminated or treated soils. Caps are typically used as a hydraulic and/or physical barrier. As site groundwater is not contaminated, a low permeability cap (i.e., hydraulic barrier) would not be necessary. The physical barrier function is more important to meet the RAO of eliminating direct contact. Therefore, the low permeability concrete/asphalt and multi-layered caps are eliminated from further consideration based on cost-effectiveness. The soil cap is retained based on its equally effective physical barrier function and lower cost.
Excavation has been retained for removal of the contaminated soil. Since the soils are generally less than 2 feet deep, a backhoe or other common earth-moving equipment will be adequate.
Four treatment technologies have been identified: thermal, soil flushing, soil washing, and in-situ stabilization. A thermal treatment such as incineration is typically used ofr treating organic contamination and is not effective for removal of heavy metals.
Soil flushing circulates a chemical fluis, such as a detergent, through the contaminated soil to mobilize and transport contaminants to a collection point or well. The treatment is conducted in-site and no soil excavation is required. Soil flushing would not be effective for surface soils located far above the groundwater table, as circulating the flushing fluid would be difficult. In addition, the high percentage of fines (e.g., >40%) in the soil indicates that (1) the permeability of the soil will be too low to allow any meaningful flushing and (2) it will be difficult to mobilize contaminants from within the fines.
Soil washing mizes excavated soil with a chemical fluid in a vessel to loosen and eventually concentrate the contaminants in the fines that are separated and removed from the soil being washed. This results in a reduction in the volume of contaminated material to be handled. Soil washing is not cost-effective for soils with a high percentage of fines, as there would be a large quantity of fines to separate and dispose.
In-situ stabilization mixes soil in-place with a chemical binding agent such as cement to immobilize the contaminants in the soil. A stabilized soil will have little contaminant leaching and can have a high strength similar to a concrete. This treatment makes the soil inert chemically and physically, which minimizes human ingestion, inhalation and the potential impact on groundwater and surface water. In-situ stabilization has been successfully used at many Superfund sites with metals contamination and therefore is a proved technology. This technology is therefore retained for formulation of remedial alternatives.
Two disposal technologies have been identified and are retained for formulation of alternatives. On-site disposal consists of leaving materials on site after excavation and/or treatment. It would be effective for site soils after satisfactory treatment such as in-situ stabilization. Off-site disposal consists of placing soils in permitted landfill(s). Off-site disposal would be effective for site soils with or without treatment, as site soils are not restricted from landfilling.
For the technologies retained in Table 2.1, six alternatives have been developed and are preliminarily screened in this section. Three of these alternatives merit further evaluation in Section 3.
Alternative 1: No Action is a no action alternative, presented only as a basis of comparison for the other alternatives. Under this alternative, the B-20 site would remain as it currently exists, with no controls to restrict human exposure to contamination.
Although not effective, the No Action alternative is retained as a baseline for comparison with other alternatives.
Alternative 2: Fencing and Monitoring includes fencing the entire B-20 site. In addition, quarterly inspections and maintenance, if necessary, would be conducted to assess the extent to which unauthorized entry is being limited. It does not, however, include significant construction to directly reduce exposure to contamination. Long-term groundwater monitoring would not be required as no contamination has been found in the groundwater. This alternative would not satisfy any of the Texas Risk Reduction Standards (RRSs) as significant human health and environmental risks would still exist.
Since Alternative 2 would not reduce the level of contamination, resulting in no attainment of RRS1 or RRS2, Alternative 2 is eliminated from further evaluation.
Alternative 3: Consolidation and Capping is a covering alternative where the metals-contaminated shallow soils would be consolidated and capped at a small portion of the site. The cap would be constructed of clean soil to provide a physical barrier. Existing craters would be filled with clean fill to eliminate ponding of surface water and its associated ingestion risks. Long-term operation and maintenance (O&M) would include semi-annual inspections and erosion repair, if necessary. This alternative would not satisfy any of the Texas RRSs as significant human health and environmental risks would still exist.
Although Alternative 3 would provide more protection to human exposure than Alternatives 1 or 2, it still would not meet any of the RRSs. In addition, its limited land use and requirement for long-term maintenance make it less favorable than Alternatives 4 and 5. Alternative 3 is therefore eliminated from further evaluation.
Alternative 4: "Hot Spot" Excavation and Offsite Disposal is an offsite disposal alternative where soil with contamination levels above RRS2 (i.e., "hot spot") would be excavated and disposed offsite in permitted landfill(s). The excavation areas would be filled with clean fill, graded and revegetated. Existing craters would also be filled with clean fill to eliminate ponding of surface water and its associated ingestion risks. Although no long-term O&M would be required, there would be restrictions on future land use due to residual contamination left. This alternative would meet the Texas RRS2, but not RRS1.
Per the RI Report, there were three "hot spots" located within the surface soil. The estimated "hot spot" volume was 560 cubic yards.
Based on its significant reduction is risks by meeting RRS2 with limited soil removal, Alternative 4 is retained for further evaluation.
Alternative 5: Excavation and Offsite Disposal is an offsite disposal alternative where all of the soils with contamination levels above background levels (RRS1) would be excavated and disposed offsite in permitted landfill(s). The excavation areas would be filled with clean fill, graded and revegetated. Existing craters would also be filled with clean fill to eliminate ponding of surface water and its associated ingestion risks. Upon completion of this remediation, no long-term O&M would be required. there would be no restrictions on future land use. This alternative would meet the Texas RRS1.
Per the RI Report, soils with contamination above the background level occupied approximately 30 percent of the site area or about 10 acres. The depth of this contamination was generally limited to less than two feet. The estimated volume of contaminated soils was 34,300 cubic yards.
Based on its proved effectiveness at other similar sites and meeting RRS1, Alternative 5 is retained for further evaluation.
Alternative 6: In-Situ Stabilization is a treatment alternative where soils contaminated with heavy metals would be stabilized in-place with a chemical binding agent. The stabilized soil area would be covered with a soil cap to promote establishment of vegetation and to improve landscape. Existing craters would be filled with clean fill to eliminate ponding of surface water and its associated ingestion risks. The area and volume of soils requiring such treatment would be the same as in Alternative 5. Although no long-term O&M would be required, use of the land would be restricted due to existence of the stabilized waste soil. Alternative 5 would not meet either RRS1 or RRS2 as the stabilization treatment would not reduce the total concentrations of the heavy metals.
Based on not meeting RRS1 or RRS2 and the deed restriction requirement, Alternative 6 is eliminated from further evaluation.