Soil Vapor Mitigation System Design And Construction
CATEGORY OF SERVICE, LOCATION:
Technical design, pilot testing pre-construction, installation, construction, permitting, remote telemetry monitoring, and operation, monitoring and maintenance (OM&M) of an active Sub-Slab Depressurization System (SSDS). Commercial property tenant improvement project, Mountain View, California
TYPE OF CLIENT:
Commercial Property Owner - Tenant Improvement Project
BACKGROUND:
The former use of the property as a dry cleaning facility resulted in localized impacts to the soil matrix, which has contributed to the volatile organic compound (VOC) tetrachloroethene (PCE) in soil gas and indoor air at concentrations above applicable commercial/industrial screening levels. The primary exposure pathway is vapor intrusion from the sub-surface to inside the building. The property owner renovated the existing building for commercial use in 2021 during which Trinity installed a vapor intrusion mitigation system (VIMS). The VIMS consisted of an active sub-slab depressurization system (SSDS) with five (5) riser pipes connected to a single manifold on the roof top for VOC abatement.
CLIENT GOALS:
The primary goals for this project were to mitigate vapor intrusion to below commercial Environmental Screening Level (ESLs), obtain safe building occupancy with the City of Mountain View Building Department and the Department of Toxic Substance Control within a short time frame, and stay within the pre-determined client budgetary constraints.
PROJECT APPROACH:
Trinity provided the client and their bank with estimated environmental cleanup and liability costs to meet commercial property use cleanup goals, and obtain approval for occupancy from the City and governing regulatory oversight agency.
Trinity conducted a remedial investigation (RI) assessment and SSDS/VIMS pilot study to delineate areas of impacted soil above clean-up criteria, characterize the strength of the PCE vapor sources, and evaluate Site conditions to allow for the design of the SSDS/VIMS.
Trinity submitted a Remedial Action Workplan to the California Department of Toxic Substances Control (DTSC) for review under the Standard Voluntary Agreement (SVA) between the DTSC and the property owner.
Trinity installed active SSDS with permits through Bay Area Air Quality Management District (BAAQMD). The SSDS conveyance piping was trenched through the sub-slab and walls to not interfere with the building occupant’s business.
The SSDS was installed with five (5) separate riser pipes plumbed into a single manifold on the roof with two (2) 55-gallon drums of approximately 200 lbs. of vapor phase granular activated carbon for VOC discharge abatement.
The SSDS was equipped with remotely monitored Sensaphone Sentinel Alarm System designed to alert the owner or operator if any system parameters fail preset thresholds or if power to the system is interrupted, ensuring tenants are protected from potential PCE exposure.
Trinity installed a network of Cox-Colvin sub-slab vapor probes for soil gas testing and SSDS vacuum monitoring. Negative pressure differential readings have been measured across the entire sub-surface of the concrete slab, indicating the SSDS is working effectively at mitigating vapor intrusion.
RESULTS, RESOLUTION, NEXT STEPS:
Trinity self-directed the installation of the VIMS with DTSC approval to avoid the tedious regulatory approval process and meet the client's expeditious redevelopment project timeline. Trinity obtained the necessary building permits from the City of Mountain View, completed an SSDS pilot test, conducted a remedial excavation inside the building, repaired and replaced the sewer lines, and installed the full-scale SSDS below the concrete slab within approximately 10 months, ahead of the clients redevelopment schedule. All of the work was completed during the COVID-19 pandemic further complicating the permitting approval process and construction worker safety.
After the implementation of the SSDS, the indoor air PCE concentrations within the building fell below the commercial ESLs set by the San Francisco Bay Regional Water Quality Control Board (SFBRWQCB) allowing tenants to safely occupy the building. Occupancy was granted by the DTSC and City of Mountain View ahead of schedule, prior to the redevelopment project being completed.
Vapor discharge requirements regulated under a Permit to Operate (PTO) from BAAQMD were met and continue to be monitored by Trinity to assure regulatory compliance. Due to the decrease in VOC concentrations in the influent SSDS airstream overtime, Trinity has requested BAAQMD update the PTO to allow for unabated discharge. This will decrease the monitoring frequency from quarterly to annual, lower the amount of discharge samples collected and analyzed from three (3) to one (1), and terminate the use of VOC abatement.
Trinity performs quarterly OM&M on the active SSDS to ensure a safe breathing space for commercial tenants.
CATEGORY OF SERVICE, LOCATION:
Certification of new mixed-use building vapor intrusion mitigation barrier (VIMB) installation for a sub-slab depressurization system (SSDS), Oakland, California (Vapor Barrier Leak Testing and Certification).
TYPE OF CLIENT:
New Construction Commercial Developer.
BACKGROUND:
The subject property was formerly a dry cleaning facility with the confirmed presence of tetrachloroethene (PCE) in groundwater . During building foundation construction, a SSDS was designed and installed by others to address the primary exposure pathway of vapor intrusion into the future building from PCE observed in the groundwater during construction dewatering. The SSDS consisted of an appropriate solvent-tolerant plastic sheeting sealed to the surrounding building foundation with double-sided tape and mastic sealant.
CLIENT GOALS FOR PROJECT:
Primary goal for this project was to redevelop the site for commercial use and address the PCE impacts identified in soil vapor at the site. The property developers chose to install and test the SSDS to mitigate potential vapor intrusion into the building post construction.
PROJECT APPROACH:
Trinity assisted the developer by reviewing and providing modifications to the SSDS design, including selection of materials for the VIMB and the selection of locations of horizontal extraction piping within each foundation cell created by the building’s grade beams.
Trinity conducted leak testing of the VIMB to find and seal any leaks. Indicator smoke was injected under the VIMB, and leaks were patched until no smoke was visible.
The smoke testing and patching was followed up with applying a vacuum (negative pressure) under the VIMB, and then assessing the presence of additional leaks using a smoke pen.
Trinity prepared a vapor barrier testing report signed and stamped by a registered engineer documenting VIMB third party testing and certification for submittal to regulatory agencies and other stakeholders.
RESULTS, RESOLUTION, NEXT STEPS:
After seal testing and patching, Trinity did not observe any gaps in the VIMB which allowed the contractor to pour the concrete slab building floor.
Trinity’s report provided assurance to owner, regulators and future building occupants that the risk to human health from subsurface contamination had been mitigated.
CATEGORY OF SERVICE, LOCATION:
Sub-Slab Depressurization System (SSDS), New Residential Construction, Los Gatos, California
TYPE OF CLIENT:
Residential Property Owner
BACKGROUND:
The Property is located in the Santa Cruz Mountains near Lexington Reservoir, the Client purchased the property to construct two residential structures, a Guest and Main House. During preliminary grading work at the Property, discolored soils with strong petroleum hydrocarbon odors were exposed, excavated and stockpiled by the general contractor. Trinity was subsequently contacted for assistance in delineating and mitigating the apparent contamination. Through historical document research it was determined the previous property owner operated a tractor and machine/equipment rental and repair business and apparently dumped waste directly to the soil.
Trinity conducted a preliminary soil and soil vapor investigation at the site, which revealed petroleum hydrocarbon concentrations in soils and soil vapor above San Francisco Bay Regional Water Quality Control Board (SFBRWQCB) Tier 1 Environmental Screening Levels (ESLs).
CLIENT GOALS FOR PROJECT:
The client requested an expedited site mitigation approach that did not impede the progress of the ongoing development.
PROJECT APPROACH:
Trinity proposed a sub-slab depressurization system for both proposed residential building structures to mitigate potential vapor intrusion into residential spaces. The SSDS was installed concurrently with grading and was installed in ¾ inch to 1 inch rock.
Trinity collaborated with the site general and grading contractors to design a versatile SSDS that (1) was easily integrated within the proposed construction designs and (2) allowed for either passive (electrically powered) or active soil vapor extraction.
The SSDS consists of a network of horizontal PVC piping beneath the building foundations. At designated locations, these pipes are diverted vertically and will penetrate the roof surface, where they may be fitted onto passive wind ventilators or active soil vapor extraction blower units above each roof surface. The SSDS was designed for an active system in anticipation of updated and more stringent Department of Toxic Substances Control vapor intrusion guidelines for residential property use.
The SSDS were installed beneath both residential building structures on schedule, in coordination with the general contractor.
As an additional safety factor, Trinity installed vapor barrier sheeting above the subgrade soils and baserock and conducted a series of leak detection tests to demonstrate that the sub-slab area was properly sealed.
Sub-slab monitoring points were installed in the base rock with ¼ inch Teflon tubing ran to a central manifold to avoid installing sub-slab probes after the concrete slab installation and possibly impacting the vapor barrier – see photo above. The manifold was installed with Magnehelic gauges to measure, riser pipe and sub-slab pressure.
Preliminary vacuum tests were performed for the SSDS, wherein vacuum measurements were collected from sub-slab vapor probes that were installed with the system. Results from these were used to appropriately size the vacuum source for the system and confirmed that adequate vacuum was generated beneath the structures.
It was determined that a passive SSDS would work at the Site and provide a wide enough negative pressure differential field below the concrete slab and vapor barrier.
An elementary school was adjacent to the Site so sampling and understanding discharge concentrations were critical. Trinity sampled the vapor discharge from the SSDS and determined the concentrations were well below Bay Area Air Quality Management District (BAAQMD) discharge criteria. As such, Trinity applied for an exemption with BAAQMD, saving the client time and money. The BAAQMD permitting process can take months to even a year at times.
NEXT STEPS:
Trinity will continue to operate, maintain and monitor (OM&M) the two SSDSs. Trinity performed semi-annual OM&M events for 2-years. Currently the OM&M events are conducted annually. All work is self-directed, there is no regulatory agency to report to.
CATEGORY OF SERVICE, LOCATION:
Design, pilot testing pre-construction, installation, construction, post-construction startup testing, and operation and management of a Sub-Slab Depressurization System (SSDS). Commercial property transaction due diligence, Mountain View, California.
TYPE OF CLIENT:
Commercial Property Developer - Tenant Improvement Project.
BACKGROUND:
The Subject Property was formerly a dry cleaning facility. Soil, groundwater, indoor air and sub‑slab vapor investigations at the property have confirmed the presence of the halogenated volatile organic compound (HVOC) tetrachloroethene (PCE) above currently published environmental screening levels. The primary exposure pathway is vapor intrusion from the sub-surface to inside the building. The client recently purchased the Property as a bona fide purchaser under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA, commonly known as Superfund), and plans to redevelop the property for commercial use.
CLIENT GOALS FOR PROJECT:
The primary goal for this project was to address the PCE impacts so the Client could redevelop and lease the site to a commercial tenant as quickly as possible.
PROJECT APPROACH:
Trinity provided the client with estimated environmental cleanup and liability costs, which the client used for purchase negotiations.
Trinity met with DTSC and developed a regulatory strategy under California Land Reuse and Revitalization Act of 2004 (CLRRA) to make the property suitable for the proposed redevelopment and beneficial use.
Trinity worked with attorneys to indemnify the site from groundwater cleanup as there are several other sites contributing to groundwater impacts in the local area.
Trinity conducted pre-emptive response actions (excavation, SSDS installation and pilot test) before entering the CLRRA agreement to save time and money. Excavation of impacted soils in the former dry cleaning area was performed concurrently with redevelopment and tenant improvement activities to meet the client’s timeline.
Trinity installed active SSDS with permits through Bay Area Air Quality Management District (BAAQMD). The SSDS was equipped with Sensaphone Sentinel Alarm System designed to alert the owner or operator if any system parameters fail preset thresholds or if power to the system is interrupted, ensuring tenants are protected from potential PCE exposure.
Trinity’s engineer designed a special carbon drum to prevent backflow issues identified during the pilot study.
RESULTS, RESOLUTION, NEXT STEPS:
Following completion of a test pilot study and several rounds of sub-slab soil vapor sampling and analyses, an active SSDS system was designed for the site and a permit to operate the active SSDS was obtained from the BAAQMD.
Trinity shall submit all necessary documents per DTCS guidance protocol such as Response Plan for public review, Operation and Management (O&M) and Monitoring Plan for long-term operation, Financial Assurance Cost Estimate, Land Use Covenant and O&M Agreement, and Response Action Completion Report.
Obtain a Certificate of Completion from DTSC.
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:
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).
CATEGORY OF SERVICE, LOCATION:
Indoor Air Mitigation, San Mateo, California
TYPE OF CLIENT:
Former Dry Cleaners Property Owner
BACKGROUND:
The former dry cleaner operated for approximately 35 years and was previously shown to have impacted nearby soil, groundwater, and soil vapor with volatile organic compounds (VOCs), primarily tetrachloroethene (PCE). Due to health risk concerns with regard to vapor intrusion (VI) from the historical release of VOCs to the subsurface at the Site, the San Francisco Bay Regional Water Quality Control Board (SFRWQCB), requested that VI mitigation occur at the Site. As recommended by the United States Environmental Protection Agency (USEPA), a Sub-Slab Depressurization System SSDS is considered an active engineering control to address VI.
CLIENT GOALS FOR PROJECT:
The property owner seeks to work with the SFRWQCB and State Water Resources Control Board (SWRCB) to ensure the immediate health and safety of occupants around the Site in regard to possible vapor intrusion into indoor air.
PROJECT APPROACH:
Trinity placed IQAir GC Multigas™ Room Air Purifiers inside the site building where levels of COCs were above the residential environmental screening levels to bring them below acceptable levels as an interim mitigation measure.
Trinity excavated 16 cubic yards of PCE-impacted in the site building at the location of former dry cleaning operations.
Zero valent iron (ZVI) and powdered activated carbon (PAC) amendment materials were added to the excavation prior to backfilling to absorb and facilitate the abiotic breakdown of PCE in shallow groundwater.
The excavation was backfilled with 3/8” pea gravel and controlled density fill slurry and a (SSDS) was installed with a 40-foot long horizontal extraction well constructed of a 4-inch diameter Schedule 40 polyvinyl chloride (PVC) pipe with 0.040-inch milled slots.
Trinity performed a passive SSDS pilot test to evaluate the performance of the system; and the airflow and VOC mass discharge to the atmosphere. Based on the airflow measurements and laboratory analytical testing of the air stream, total VOCs and no individual VOC exceeded the Bay Area Air Quality Management District (BAAQMD) Table 2-5-1 criteria. Trinity determined that the passive SSDS system qualifies for a certificate of exemption due to the low emission rates and does not need a permit to operate from BAAQMD.
The passive SSDS system is operated using a 6” Edmonds Hurricane wind turbine attached to the end of the air discharge pipe which extends approximately 5‑feet above the roof line of the former dry cleaner building. The spinning wind turbine provides the source energy (no electrical power required) to initiate and maintain airflow and a sub-slab vacuum for the passive SSDS.
Investigative, monitoring, indoor air mitigation, and passive SSDS installation, operation/maintenance costs for this project are 100% funded (at no cost to the client) by the Site Cleanup Subaccount Program (SCAP), a funding program established by Health & Safety Code Section 25299.50.6 administered by the State Water Resources Control Board.
RESULTS, RESOLUTIONS, NEXT STEPS:
The field investigation is still in progress; next steps include:
Following operation of the passive SSDS system, indoor air monitoring at the site building and neighboring building show significantly decreased concentrations of VOCs and they are below commercial Environmental Screening Levels (ESLs).
Trinity will continue operation and maintenance of the passive SSDS system and IQAir units within the site building. Additional indoor air monitoring will be performed at the site building to ensure mitigation measures continue to reduce all VOC levels below commercial ESLs.
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