Soil Vapor Extraction Chlorinated VOC Remediation
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Project Objective

The former Athanor Group, Inc. facility in Southern California had a historical release of volatile organic compounds (VOCs), primarily tetrachloroethene (PCE), that impacted underlying soil and groundwater. The site is under the regulatory oversight of the Santa Ana Regional Water Quality Control Board (RWQCB).

Geosyntec’s Scope of Services

Geosyntec assumed responsibility for operation and maintenance (O&M) for an existing remediation system that had been in operation for 18 years by prior consultants. Our team focused on optimization of mass removal rates, and developed and implemented a pathway for site regulatory closure. Two building surveys were completed to assess indoor air and vapor intrusion (VI) mitigation measures were implemented. Potential offsite soil vapor mitigitation and soil vapor extraction (SVE) performance were also evaluated. This included the installation of six dual-nested soil vapor probes and quarterly monitoring events. Soil vapor risk-based screening levels were developed based on California Department of Toxic Substances Control’s guidance (Johson & Ettinger model), regional Environmental Screen Levels, site-specific input parameters, and commercial/industrial occupant exposure scenario.

The monitoring network was optimized based on statistical analyses and defensible vadose zone cleanup goals were developed. Monitoring network optimization has led to $100,000s of cost savings for routine monitoring. Geosyntec completed vertical profiling of VOCs in groundwater per RWQCB’s request and developed a proposed path forward plan for monitoring. Semiannual monitoring reports were prepared and the Conceptual Site Model (CSM) was updated. Multiple groundwater monitoring wells have been installed to depths exceeding 350 ft bgs. and a new SVE infrastructure was added to improve remediation performance.

Notable Accomplishments

After performing system assessments and associated upgrades and optimization measures, mass removal rates from the SVE system significantly increased. In three years of O&M by Geosyntec, soil vapor concentrations have decreased by orders of magnitude and are now approaching asymptotic levels. Vapor intrusion risks have been mitigated, and the quality of underlying groundwater is anticipated to improve due to vadose zone source area removal. A plan to petition the RWQCB for site closure is pending.