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AOC 65 Soil Vapor Extraction System - Operation and Maintenance Plan
Section 2 - Site Conditions
Chlorinated solvents, which are VOCs, were used in Building 90 cleaning processes for more than 30 years. Chlorinated solvent usage was eliminated by pollution prevention initialization that replaced the process with a citrus-based cleaning solvent in 1995.
VOCs were first detected at concentrations above drinking water standards in a CSSA potable well water in 1991. Groundwater samples collected from the monitoring wells installed at AOC-65 and off-post wells have contained VOCs also.
This prompted investigations of the probable source areas of contamination. Source characterization of the Building 90 vicinity, (AOC-65), included a 2001 survey of 319 soil gas samples collected and analyzed for chlorinated and aromatic organics around and inside of Building 90. Perchloroethene (PCE), trichloroethene (TCE), cis-1,2-dichloroethene (DCE) and trans-1,2-DCE were detected. The occurrence of these chlorinated hydrocarbons has implicated Building 90 and its historical processes as the likely source of contaminants encountered in groundwater. Furthermore, the detection of the TCE and DCE indicate that natural degradation of PCE is occurring in the subsurface.
The soil gas survey was followed by 14 soil borings and numerous groundwater samples collected in and around Building 90 and from monitoring wells and piezometers installed in the vicinity. The final Resource Conservation and Recovery Act (RCRA) Facility Investigation (RFI) report for AOC-65 was completed in September 2002 (Parsons, 2002b). An interim removal action was also completed in 2002 included excavation of contaminated soils underlying the pavement and ditch on the west side of the building.
SVE pilot testing was performed at AOC-65 to evaluate removal of VOC contamination from vadose soils and groundwater. SVE was demonstrated to be an effective method for source removal in surface formations at CSSA during a pilot and treatability study at Solid Waste Management Unit (SWMU) B-3. This SVE system at AOC-65 was constructed and installed in late 2002 followed by completion of startup and initial testing in early 2003. The primary objectives of this SVE system are to remove VOC contaminants from the soils, fractured limestone, and groundwater around AOC-65 (both subslab and surrounding Building 90) or at a minimum to stop the migration of contaminants.
Based on the results of the site investigation and recent groundwater results from the Westbay study report (Parsons 2003c), the area around AOC-65 that could be successfully treated for VOCs appears to extend immediately around Building 90 in the apparent down gradient direction to the west/southwest. The total depth of VOC contamination has been encountered at levels above the Safe Drinking Water Act Maximum Contaminant Levels (MCLs) in groundwater intervals measured as deep as 300 feet below grade. The total volume of the treatment area is unknown. The locations of the AOC-65 system SVE wells are shown on Figure 2.1.
The Texas Clean Air Act requires a permit to emit any pollutants to the atmosphere. The Act is codified in 30 Texas Administrative Code (TAC) Chapter 116, �Control of Air Pollutants By Permits for New Construction or Modification�. SVE systems remove contaminants by negative pressure while replenishing oxygen to microorganisms within soils; which, if soil gas is removed to accomplish the oxygenation of contaminated soils, could result in the emission of VOC to the atmosphere. Generally, most soil gas removal systems involve very low air emissions rates. Consequently the systems are generally exempted, under Permit By Rule �106.533, (formerly Standard Exemption 68), as outlined in 30 TAC Chapter 106 Subchapter X.
Rule �106.533 is applicable to �Equipment used to reclaim or destroy chemicals removed from contaminated materials for the purpose of a remedial action�. Its provisions allow air emissions from treatment of groundwater and soils contaminated with petroleum compounds and chemicals other than petroleum products. The emission of chemicals other than petroleum products must also be compliant with the limitations of the Facilities (Emission and Distance Limitations) rule �106.262(2), (3) and (4). �New or increased emissions, including fugitives, of chemicals shall not be emitted in a quantity greater than 5 tons per year (tpy) nor in a quantity greater than E as determined using the equation E=L/K� where K is a parameter corresponding to distance to the nearest receptor and where L (Limit Value) is an emission limit of concentration provided for specific chemicals in Table 262 of �106.262. The maximum emission on an hourly basis of any chemical having an L value in Table 262 is determined by the equation E=L/K. The emission of any chemical not having an L value in Table 262 is one pound per hour (lb/hr), with or without abatement devices. These limitations are applicable only to on-site remediation.
An emissions permit is required since the system could emit air pollutants to the atmosphere. However, based on the contaminant levels detected in soils and the expected flowrates, the site qualifies for a Permit By Rule exemption. Nevertheless, a GAC adsorber was installed to ensure that contaminant levels in exhaust emissions would not exceed health-based standards during operation of the blowers or add additional VOCs to the environment.
An emissions limit formula in 30 TAC Chapter 106 Subchapter K, rule �106.262 (formerly Standard Exemption 118), Section (c), is used to determine maximum allowable emissions rates for chemicals other than petroleum fuels. Part 68(e) provides technical conditions to be met when abatement is required to meet the specific chemical emission limit. The calculated emissions rates for chemicals of concern at this site are presented in the Permit By Rule application prepared for these SVE systems at AOC-65 in August 2002 (Parsons 2002a). The maximum allowable emission rate is 6 lb/hr (5 tpy) for PCE and 1 lb/hr (4.4 tpy) for cis-1,2-DCE, the two most common contaminants found in AOC-65.
Emission samples collected as part of the initial system startup, and contaminant levels in emissions were significantly below the allowable levels. The maximum PCE emission rate measured during initial system startup was 0.08 lb/hr (subslab) and 0.02 lb/hr (exterior), which equate to 0.07 and 0.0175 tpy, respectively. Emission samples will continue to be collected and tested as part of preventive measures for contaminants of concern during routine operations, thereby providing periodic monitoring of contaminant levels in the system exhaust. If the data indicate that contaminant levels in the exhaust from the SVE exceed applicable criteria, then the exemption, GAC adsorber operation as well as other practices will be re-evaluated.