Book Description

Pacific Northwest National Laboratory (PNNL) conducted a proof-of-principle test at the Fort Lewis Logistics Center to determine the feasibility of using the In Situ Redox Manipulation (ISRM) technology for remediating groundwater contaminated with dissolved trichloroethylene (TCE). ISRM creates a permeable treatment zone in the subsurface to remediate redox-sensitive contaminants in groundwater. The permeable treatment zone is formed by injecting a chemical reducing agent (sodium dithionite with pH buffers) into the aquifer through a well to reduce the naturally occurring ferric iron in the sediments to ferrous iron. Once the reducing agent is injected and given sufficient time to react with aquifer sediments, residual chemicals and reaction products are withdrawn from the aquifer through the same well used for the injection. Redox-sensitive contaminants such as TCE, moving through the treatment zone under natural groundwater flow conditions, are destroyed. TCE is degraded via reductive dechlorination within the ISRM treatment zone to benign degradation products (i.e., acetylene, ethylene). Prior to the proof-of-principle field test, the ISRM technology was successfully demonstrated in laboratory experiments for the reductive dechlorination of dissolved TCE using sediments from the Fort Lewis site. The Logistics Center was placed on the National Priorities List in December 1989 because of TCE contamination in groundwater beneath the site. A Federal Facilities Agreement between the Army, the U.S. Environmental Protection Agency, and the Washington State Department of Ecology became effective in January 1990, and a Record of Decision (ROD) was signed in September 1990. The major components of the ROD included installation of two pump-and-treat systems for the upper aquifer and further investigation of the lower aquifer and other potential sources of contamination. The pump-and-treat systems became operational in August 1995. Fort Lewis asked PNNL to provide technical support in accelerating Installation Restoration Program site remediation and significantly reducing site life-cycle costs at the Logistics Center. In support of this program, ISRM was selected as an innovative technology for bench and field-scale demonstration. Emplacement of the ISRM treatment zone was accomplished through a series of four separate dithionite injection tests conducted between November 10, 1998 and March 29,2000. An extensive program of chemical monitoring was also performed before, during, and after each injection to evaluate the performance of ISRM. Prior to emplacement of the ISRM treatment zone, the site was extensively characterized with respect to geologic, hydrologic, and geochemical properties. Sediment core samples collected for the characterization studies were analyzed in bench-scale column tests at PNNL to determine reducible iron content. These site-specific hydrogeologic and geochemical data were used to develop the emplacement design of the pilot-scale (i.e., single injection well) ISRM treatment zone. Performance data obtained from the proof-of-principle test indicate that field-scale reductive dechlorination of TCE using the ISRM technology is feasible. A treatment zone was created in the subsurface that reduced TCE concentrations as much as 92% on the downgradient side of the reduced zone, from a background concentration of approximately 140 ppb to approximately 11 ppb. The appearance of the principal degradation product, acetylene, also confirmed that TCE destruction was occurring. Analysis of sediment samples collected from post-test boreholes showed a high degree of iron reduction, which helped to confirm the effectiveness of the treatment zone emplacement. Another important goal of the testing program was to provide assurances that chemical treatment of the subsurface did not result in undesirable secondary effects, including formation of toxic TCE degradation products, mobilization of trace elements, and degradation of hydraulic performance. Results obtained from the Fort Lewis ISRM proof-of-principle test, which are consistent with results from previous ISRM studies (both bench- and field-scale), indicate that no significant secondary effects were identified that could limit full-scale application of this technology.