1,4-Dioxane, a cyclic diether and an additive in the chlorinated solvent 1,1,1-trichloroethane (TCA), has proven to be a persistent groundwater contaminant. Conventional soil vapor extraction (SVE) can remove some 1,4-dioxane, but a substantial residual source is left behind causing long-term groundwater contamination. Although 1,4-dioxane’s vapor pressure is in the range of trichloroethene or benzene, it is totally miscible in water soluble. As a result, 1,4-dioxane becomes sequestered in vadose zone pore water which serves as a long-term source of groundwater contamination. Extreme soil vapor extraction (XSVE), an enhancement of SVE, specifically addresses 1,4-dioxane-contaminated soil by incorporating enhancements such as decreased infiltration, increased air flow, focused vapor extraction, and injection of heated air.
The XSVE field demonstration site was at the former McClellan Air Force Base near Sacramento, California, adjacent to an SVE well with high 1,4-dioxane concentrations. Pneulog® was used to determine vertical profiles of 1,4-dioxane vapor concentrations and effective permeabilities in the SVE well. Field analysis of soil boring samples for 1,4-dioxane during drilling operations was conducted to insure suitable placement of injection and extraction wells for the demonstration. The XSVE system consisted of four two-inch steel-cased injection wells forming a 20-foot square with a central four-inch steel-cased extraction well (38–68 ft below ground surface (bgs) screened interval each). The treatment zone and soil beneath were instrumented with thermocouples, soil moisture sensors, and soil vapor monitoring probes. 1,4-Dioxane and soil moisture distributions prior to XSVE were determined using five soil borings.
The system operated for about 13 months with about 98% uptime. Injection temperatures were maintained in the 100 to 130 ̊C range (mid-screen) for the bulk of system operation, with flow rates generally in the 70 to 90 scfm range for each injection well. Extraction well flow rate was generally in the 70 to 110 scfm range. Observed treatment zone temperatures reached as high as 90 ̊C near the injection wells, however extraction well temperatures did not exceed 40 ̊C. Soil heating costs were about $25/CY for this demonstration. Soil moisture readings decreased significantly in the sensors closest to the injection wells, whereas those near the extraction well generally remained stable. Treatment zone and extraction well 1,4-dioxane vapor concentrations were determined using a vapor/condensate sampling apparatus due to elevated temperature soil gas having the potential to condense water vapor in ambient temperature vapor sampling canisters. Water condensation has the potential to removing 1,4-dioxane from the vapor. Approximately 13 kg 1,4-dioxane was removed from the treatment zone over the course of the demonstration.
1,4-Dioxane in the treatment zone decreased by approximately 94% and soil moisture decreased by approximately 45%. Downward migration of 1,4-dioxane due to condensation was not observed. A screening-level mass and energy balance model, HyperVentilate (HypeVent) XSVE, was developed to simulate the remediation of 1,4-dioxane by XSVE. HypeVent XSVE adequately simulated 1,4-dioxane removal, soil moisture, and soil temperatures observed during the demonstration—proving itself a useful feasibility assessment and design tool for XSVE of 1,4-dioxane. Sensitivity analyses showed that 1,4-dioxane removal benefited considerably from heated air injection. XSVE has been demonstrated to be a cost-effective remediation approach for vadose zone 1,4-dioxane.