Full-scale SVE system operates in tandem with groundwater treatment

Since 1993, SVE has been implemented using two designs at the TP Industrial, Inc. (TPI) site in Gardena, CA, to treat soil contaminated with chlorinated solvents. The SVE system was modified in 1997 to improve emissions control efficiency by using carbon adsorption units that treat effluent from an air-stripping system for treating site ground water. By the end of 2007, SVE had removed a cumulative halogenated organic compound (HOX) mass of approximately 25,000 pounds.

The former TPI facility was used from 1964 through 1986 for solvent reprocessing, which involved storage of chemicals in aboveground and underground tanks. Investigations by the California Regional Water Quality Control Board in the early 1980s identified 1,1-dichlorothene, tetrachloroethene (PCE), 1,1,1-trichloroethane (TCA), methylene chloride, trichloroethene (TCE), and 1,1,2-trichlorotrifluorethane. All were found in both the saturated and unsaturated zones of two areas, with representative high concentrations reaching 1,790 mg/kg for PCE and 725 mg/kg for TCA. Site characterization indicated that the vadose zone consists of well-stratified layers of silty sand, clean sand, sandy and/or clayey silt, and silty clay. Ground water is approximately 75 feet below ground surface (bgs).

In 1986, the California Department of Toxic Substances Control (CA DTSC) began managing the site’s corrective actions. Ground water treatment began in 1988 through use of an air stripping system. Site closure activities included removal of three underground tanks and offsite disposal of approximately 2,000 yd3 of soil excavated from depths extending to 30 feet bgs. CA DTSC determined that additional excavation was infeasible, and in 1990 tested and selected in situ SVE as the most feasible remedial technology for removing the remaining HOX.

SVE operation began in 1993. The SVE network encompasses eight 2-inch-diameter vapor extraction wells screened between 16 and 66 feet bgs in two of the previously excavated areas. The wells are connected to a knock-out pot and a single 7.5-hp rotary blower that extracts soil gas from the well network at 100-130 scfm. Initial design of the system relied on refrigeration technology to condense and control VOC emissions.

After removing approximately 10,000 pounds of HOX, operation was suspended in April 1996 due to failure of the refrigeration system compressor and difficulties in attaining an emissions control efficiency of 90%, in accordance with South Coast Air Quality Management District requirements. Efficiency reduction was attributed directly to the significant VOC concentration reductions in extracted soil gas. Over the three years of operation, the system had extracted 9,200-10,300 pounds of HOX and recovered 6,700 pounds of liquid solvents that were drummed and delivered to an offsite reclamation facility.

As a consequence, the SVE system was reconfigured in July 1996 to reroute SVE gases to the existing carbon adsorption units used to treat effluent from the ground water air stripper. Soil gas is delivered to a series of three carbon adsorbers, each containing approximately 5,500 pounds of activated carbon. Air from the carbon adsorbers is exhausted to the atmosphere through the facility’s 60-foot stack.

SVE performance monitoring is conducted through 50 shallow semi-permanent vapor probes and eight deep multi-level nested vapor probes to measure soil gas concentrations across the entire site. Performance tests rely on use of a flame ionization detector (FID) for field detection of VOCs in soil gas extracted by each well. The FID sample line is connected directly to a sample port on the well while isolating the well from the rest of the system. Due to consistent field screening results, fixed-laboratory analysis is not required.

Over 10 years of operation, the modified SVE system successfully removed an additional 15,000 pounds of HOX (Figure 1). In general, measured HOX concentrations decreased as much as 80% in soil and as much as 50% in ground water. Performance monitoring in the first half of 2008 indicated that the system was operating at a flow rate averaging 110 scfm. VOCs were being removed at an average rate of 0.07 lbs/hr, slightly higher than the previous six-month period. The system had performed without interruption for 4,368 hours, with soil gas extracted continually from five of the eight SVE wells. An estimated 292 pounds of HOX were removed from the subsurface in the first half of 2008, the majority of which were captured by the carbon adsorption Installation costs for the project are estimated at $312,00. Operation of the combined treatment system costs approximately $33,000 each year, including $3,000 for electricity plus an annual change-out of carbon in the air stripping system (at a material cost of $5 per pound).

Contamination in the capillary fringe and ground water continues to serve as a source of VOCs in the soil gas. As a result, CA DTSC is negotiating an interim measure using in situ chemical oxidation (ISCO) to attack the most recalcitrant HOX contaminants, in this case chlorinated compounds such as PCE and TCE. The SVE system will continue operating until ISCO is implemented and possibly afterward if ISCO mobilizes additional contamination. Rebound testing and risk-based soil and soil gas remediation levels may be considered if mobilization does not occur.

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