Theme: 04. Addressing Challenging Site Conditions

Topic Area: 4a. Adaptive Site Management: Lessons Learned for Site Characterization and Remedy Implementation

Formatted Title:

Optimizing Operation of an SVE System to Remove CVOCs while Impacted by Regional Natural Gas

Background/Objectives:

A Site located in El Segundo, California is impacted by chlorinated volatile organic compounds (CVOCs) in soil vapor, primarily tetrachloroethene (PCE), trichloroethene (TCE), and their degradation products. An extended soil vapor extraction (SVE) pilot test was conducted to evaluate SVE as a viable remedial technology to remove CVOC mass and reduce the concentrations of CVOCs in soil vapor. During the SVE system operation, discrepancies were observed in the “total volatile organic compound (TVOC) as hexane” values in laboratory analyses, which are part of the monitoring conducted to verify compliance with discharge limits specified in the South Coast Bay Area Air Quality Management District (SCAQMD) permit (Air Permit). Investigations were conducted to understand and rectify the cause of these discrepancies, which led to the discovery of naturally-occurring natural gas (or “stray gas”) from a regional oil and gas field in the SVE system influent. This case study presents the investigational steps taken to identify the stray gas in the soil vapor, impact on TVOC as hexane results, and operational optimization of the SVE system to effectively remove CVOCs from soil vapor while mitigating potential risks associated with the presence of elevated methane levels in the soil vapor.

Approach/Activities:

We reviewed the chromatogram of the EPA Method TO-3 laboratory analyses for TVOC as hexane and sent additional soil vapor samples to a separate lab analyzing for total volatile hydrocarbons (TVH) C2-C10 as hexane and for C1-C4 hydrocarbons. Elevated concentrations of methane, along with ethane and propane, were detected in each sample, and the combination of these three hydrocarbons indicated the presence of diluted natural gas. After ruling out a leaking gas line as a potential source, we reviewed the City of Los Angeles Methane and Methane Buffer Zones Map and a methane assessment report from a property approximately 500 feet from the Site, and determined the elevated methane is likely from naturally-occurring natural gas from an oil and gas field in the Los Angeles region. Additionally, we discussed and clarified with the SCAQMD that the ethane component of the natural gas should not be included in the “TVOC as hexane” analysis, which was not previously defined in the permit or in SCAQMD regulations. Changing the “TVOC as hexane” laboratory analysis to only include VOCs in the C3-C10 hydrocarbon range, which excludes methane and ethane yet still captures the known CVOCs, resolved the permit issue. We added methane monitoring (measured as % LEL) at SVE wells and the collection and treatment system to the weekly SVE system operation and maintenance (O&M) activities to identify and monitor elevated methane concentrations. Fifteen SVE wells were installed at five locations, and at each location three SVE wells were screened in the three unsaturated soil units in the upper 90 feet below the ground surface, i.e., the Stratified Unit, the Older Dune Sand, and the Transition Unit. We adjusted the operations of these 15 SVE wells to optimize the CVOCs removal and maintain combustible gases below 50% LEL. The SVE system safely removed approximately 1,600 pounds of CVOCs over 12 months of operation.

Results/Lessons Learned:

Understanding the details of the analytical results, not just accepting the values presented in lab reports, is important when inconsistency is observed in data sets. Clear communication with regulatory agencies is essential for compliance. Analytical results and field measurements during routine O&M inspections can be used as a diagnostic tool to identify potential sources of unexpected variables, such as presence of methane from sources unrelated to site contamination. By adjusting the operation of the SVE system, it is possible to achieve successful remediation while mitigating the risks posed by the presence of regional natural gas.

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Hui Li