Feasibility And Remedial Pilot Studies

 
 

CATEGORY OF SERVICE, LOCATION:

Soil and Groundwater Remediation, Sunnyvale, California

TYPE OF CLIENT:

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Major Oil Company

BACKGROUND:

  • Site is a former service station, redeveloped as a strip mall.
  • Soil and groundwater impacts have been defined.
  • Residual impacts in shallow water and capillary fringe remain
  • CAP recommended dual-phase extraction/air sparging (DPE/AS) utilizing three extraction and 30 sparge wells, but at an estimated cost of $750,000 to implement to achieve closure

CLIENT GOALS FOR PROJECT:

Cost-effective remediation, maintaining regulatory compliance.

PROJECT APPROACH:

  • Conducted 11-day DPE/AS pilot test to gain design parameters for SVE/AS system
  • Evaluate additional alternatives including risk assessment to achieve closure.

RESULTS, RESOLUTION, NEXT STEPS:

  • DPE/AS shown to be an effective remedial option based on pilot test results.  However, full-scale implementation may not be cost-effective, because most residual mass is in fine-grained soils.
  • Continued groundwater monitoring is underway to evaluate the longer-term effect of the DPE/AS test.
  • Site may be suitable for risk-based closure in view of low residual concentrations and lack of nearby receptors.
 
 

CATEGORY OF SERVICE, LOCATION:

Active Sub-Slab Vapor Depressurization System (design, construction, O&M), Alameda, CA

TYPE OF CLIENT:

Property Development/Management Company

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BACKGROUND:

While reviewing the site for closure, Alameda County requested the collection of sub-slab vapor samples for volatile organic compound (VOC) and Stoddard Solvent analysis to determine if the former dry cleaning operations at the site affected soil beneath the building and resulted in the potential for vapors to migrate into the building.  A sampling program was designed and implemented and sub-slab vapor results indicated elevated levels tetrachlorethene, vinyl chloride, carbon tetrachloride and Stoddard solvent were present beneath the building.  Mitigation of the vapor intrusion risk to indoor air for the commercial building was warranted.  A sub-slab depressurization pilot test was conducted, followed by the design and installation of an active sub-slab depressurization system.

CLIENT GOALS FOR PROJECT:

Mitigate the vapor intrusion risk to indoor air for the commercial building in the most cost-effective manner without significantly disturbing the building tenants or their business operations.

PROJECT APPROACH:

Trinity conducted a sub-slab depressurization pilot test during the weekend after business hours to determine if an active or passive system was needed to mitigate the vapor intrusion concern.  Pilot test results indicated that an active sub-slab depressurization system (SSDS) with carbon abatement was needed for the site.  The system was designed, permitted, installed and operated and maintained by Trinity.

RESULTS, RESOLUTION, NEXT STEPS:

  • The SSDS startup and continued operation was successful at mitigating the vapor intrusion pathway beneath the building.  The SSDS system influent concentrations reduced significantly enough to cease carbon abatement before discharge to the atmosphere and modify the air permit from monthly to quarterly system monitoring.
  • The next step is to shut the SSDS down to evaluate rebound effects and determine if sub-slab VOC and Stoddard solvent concentrations are below closure action levels.  A component of this phase will be to determine a site-specific attenuation factor to apply to the sub-slab vapor concentrations.  Radon gas testing in the sub-slab vapor and inside the building are proposed in order to develop a suitable attenuation factor.