Soil Vapor Mitigation System Design And Construction

 

CATEGORY OF SERVICE, LOCATION:

Soil Gas/Vapor Mitigation, Operations and Maintenance (O&M), and Annual sampling, Palo Alto, California

TYPE OF CLIENT:

Mixed commercial/residential property owner.

BACKGROUND:

Summa Canister

Currently operating as a multi unit apartment complex/building, previously/historically the site has been used for industrial uses.

Residential development constructed on former industrial site with volatile organic compound (VOCs) chemical impacts.  The new construction incorporated mitigation measures to prevent VOCs from entering indoor living spaces.  The soil vapor mitigation system consists of a geomembrane vapor barrier, vapor extraction system, and sub-slab vapor monitoring system. This is a passive soil vapor mitigation system and operates continuously.

CLIENT GOALS FOR PROJECT:

Primary goal for this project is to maintain continuous operation of the passive sub-slab vapor mitigation system, in part to maintain regulatory compliance.

PROJECT APPROACH:

  • Trinity coordinates and schedules annual O&M and sampling events with the current operator and regulatory agency, San Francisco Bay Regional Water Quality Control Board.
  • Trinity conduct a physical and visual inspection of all the passive sub-slab vapor mitigation/ removal system components located in the sub-floor and on the roof of the structure.
  • Trinity purges and collects soil vapor samples into laboratory supplied stainless steel 1-Liter summa canisters.
  • Once the sub-slab soil vapor samples are collected and properly labelled, they are transported by ground, with completed chain-of-custody document to the laboratory for analysis for tetrachloroethene (PCE), trichloroethene (TCE), and vinyl chloride (VC) by EPA Method TO-15.
  • Once a laboratory report is issued, Trinity staff reviews and analyzes the results and completes a report of the findings.

RESULTS, RESOLUTION, NEXT STEPS:

  • Trinity continues conducting annual O&M events, to maintain regulatory compliance
  • And annual reporting 

 

CATEGORY OF SERVICE, LOCATION:

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

TYPE OF CLIENT:

Property Development/Management Company

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.
 

 

CATEGORY OF SERVICE, LOCATION:

Passive Sub-Slab Vapor Depressurization System (design, construction, O&M), Vallejo, CA

TYPE OF CLIENT:

Property Development/Management Company

BACKGROUND:

The site building was previously used as a grocery store.  The building was recently renovated and is currently being operated as a Dollar General retail store.

The property immediately south of the subject site was historically utilized for dry cleaning activities, doing business as Brewer’s Dry Cleaners.  Previous site assessments of the subject site have discovered dry cleaning chemicals (primarily tetrachloroethene [PCE]) and to a lesser degree toluene, ethylbenzene and total xylenes in soil gas and sub-slab vapor beneath the former grocery store building.  The owner of the adjacent property is responsible for mitigating this condition; however, our client proactively contracted Trinity to construct a mitigation system (as summarized herein) so the property could be sold and redeveloped by a Dollar General.

While reviewing the site for closure, Alameda County Environmental Health (ACEH) 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 of PCE, 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 to ready the property and building for sale and build-out by Dollar General, a national retail grocery chain. Design, construct, operate and maintain and provide post-installation data to show the SSDS is working as designed and the potential vapor intrusion risk to building occupants has been mitigated.

PROJECT APPROACH:

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

RESULTS, RESOLUTION, NEXT STEPS:


  • Following completion of a vent test pilot study and several rounds of sub-slab and 5-foot soil vapor sampling and analyses, a passive SSDS system was designed for the site and a permit to operate the passive SSDS was obtained from the Bay Area Air Quality Management District (BAAQMD).
  • Building permits were obtained from the City of Vallejo Building Department including a sidewalk encroachment permit.
  • A passive SSDS system was installed by Trinity staff inside the 11,000 square foot building when it was vacant.  The SSDS consists of a primary trench with three short laterals, extending across the length of the site building.  The trench contains slotted 6-inch diameter PVC pipe surrounded by pea gravel, and is covered with a plastic liner and concrete.  The slotted PVC is plumbed to solid PVC risers to provide venting to the atmosphere.  Two 6-inch diameter solid PVC risers have wind-driven turbines installed to create a vacuum on the subsurface slotted pipe.  One of the laterals, the middle lateral, is equipped with a stub-up for future connection to a turbine or fan, if needed in future.
  • Post SSDS system inspection, performance monitoring and testing and system optimization was performed.
  • A radon study was performed at the site during construction but before the Dollar General store opening to determine a site-specific attenuation factor (AF) for the concrete building slab.  The site-specific AF increased the sub-slab PCE concentration action level for a commercial building by two orders of magnitude.
  • VOC concentrations in sub-slab vapor have generally decreased after SSDS start-up.
  • VOC source remains in soil and groundwater from the former Brewers Dry Cleaners on the adjacent property, so variable VOC concentrations in sub-slab vapor are possible until such time as the responsible party remediates soil and groundwater impacts.
  • The SSVDS operation provides mitigation of the potential for VOC vapor intrusion by depressurizing the sub-slab area.
  • Based on the recent VOC concentrations measured in the sub-slab vapor probes and the site-specific AF, no VOC concentrations exceeding screening levels are likely to migrate from the subsurface into the occupied portions of the building.
  • As a result of Trinity’s work, the property was sold, the Dollar General store constructed and opened, our client was not named a responsible party for cleanup, and monies held in an escrow account are now ready for distribution back to the seller (Trinity’s Client).