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y"S • Technical Guidance Document: Construction Quality Management for Remedial Action and Remedial Design Waste Containment Systems (EPA/540/R-92/073), October 1992. 4.2.2 Solidification/Stabilization Technologies Solidification and stabilization(S/S) methods of treating contaminated wastes are applied to change the physical or leaching characteristics of the waste or to decrease Its toxicity. In the solidification process, waste constituents are physically locked within a solidified matrix in the form of a granular soil-tike mixture or a monolithic block. Stabilization converts waste contaminants to a more immobile form, typically by chemical reaction. S/S refers to treatment processes that mix or inject treatment agents into the contaminated material to accomplish one or more of the following objectives: Improve the physical characteristics of the waste, without necessarily reducing aqueous mobility of the contaminant, by producing a solid from liquid or semiliquid wastes Reduce the contaminant solubility Decrease the exposed surface area across which mass transfer loss of contaminants may occur Limit the contact of transport fluids and contaminants S/S treatment improves the waste handling or other physical characteristics of the waste and can reduce the mobility of contaminants. S/S treatment can be accomplished by treatment with inorganic binders such as cement, fly ash, and/or blast furnace slag or by organic binders such as bitumen. S/S technology usually is applied by mixing'contaminated soils or treatment residuals with a physical binding agent to form a crystalline,glassy, or polymeric framework surrounding the waste particles. In addition to the microencapsulation, some chemical mechanisms also may improve waste leach resistance. Other forms of S/S treatment rely on macroencapsulation in which the waste is unaltered but macroscopic particles are encased in a relatively impermeable coating. 4.2.2.1 In Situ and Ex Situ S/S S/S can be either an in situ or ex situ process. The following descriptions of in situ and ex situ S/S are derived primarily from the Engineering Bulletin: Solidification and Stabilization of Organics and Inorganics, EPA/540/S-92/015, and the Engineering Forum Issue, Considerations in Deciding to Treat Contaminated Unsaturated Soils In Situ, EPA/540/S-94/500. Ex situ processing involves: (1) excavation to remove the contaminated waste from the subsurface; (2) classification to remove oversize debris; (3) mixing;(4) off-gas treatment(if volatile,or dusts are present); and (5) a system for delivering the treated wastes to molds, surface trenches, or subsurface injection. In situ treatment processing has only two steps: (1) mixing, and (2) off-gas treatment. The most significant challenge in applying S/S in situ for contaminated soils is achieving complete and uniform mixing of the binder with the contaminated matrix (U.S. EPA, 1990, EPA/540/2-90/002). Three basic approaches are used for in situ mixing of the binder with the matrix: (1) vertical auger mixing; (2) in-place mixing of binder reagents with waste by conventional earthmoving equipment, such as draglines, backhoes, or clamshell buckets; and (3) injection grouting, which involves forcing a binder containing dissolved or suspended treatment agents into the subsurface, allowing it to permeate the soil. Grout injection can be applied to contaminated formations lying well below the ground surface. The injected grout cures in place to produce an in situ treated mass. 4-6