Vapor intrusion mitigated through solar-powered exhaust systems

EPA Region 6 undertook a Superfund removal action earlier this year at the Delfasco Forge site in Grand Prairie, TX, to address trichloroethene (TCE) vapor migrating from a groundwater plume. Region 6 and the State had determined after extensive site investigations that technologies such as soil vapor extraction or natural attenuation would require several years to remove or reduce the plume significantly. As a result, the time-critical removal now underway involves installing exhaust systems in offsite buildings with TCE vapor concentrations above the action level. Long-term, inexpensive operation of the systems is enhanced through use of solar energy to generate the electricity needed by the exhaust fans.

From 1981 until 1997, the Delaware Forge and Steel Company used less than two acres of the site for metal forging and fabrication that applied degreasing agents containing TCE. Studies in 2003-2005 indicated that degreaser spills and releases had led to contamination of shallow groundwater extending below an adjacent 65-acre area with approximately 500 homes and six light industrial businesses.

Investigations conducted in 2008 for RCRA corrective action involved collection of soil samples, air samples from crawl spaces and indoors, and sub-slab soil vapor samples to assess potential migration of contaminant vapor from the groundwater plume, which is located 18-32 feet below ground surface. Region 6 used EPA's TAGA van, a vehicle equipped with a trace atmospheric gas analyzer and summa-type canisters, to collect and analyze the air. TCE was detected in 18 homes with two showing concentrations above the action level of 14 µg/m3.

Passive air sampling also was conducted in the immediate four-block area to further define the vapor plume. Semi-quantitative samples were collected at 100 points over two weeks, at a unit cost of $18. Results further defined a soil vapor plume that is in the heart of the known groundwater plume and lies under approximately 12 homes.

This past January, Region 6 collaborated with the Texas State Health Services and the Agency for Toxic Substances and Disease Registry to conduct indoor air sampling in forty homes located in four areas of Grand Prairie. The sampling event was scheduled for mid winter, the time of year most likely to present a worst-case scenario of reduced ventilation and low TCE decomposition due to closed-up buildings. Results showed two additional homes with air contamination above the TCE action level, both of which are located in the area of the vapor plume earlier defined through passive soil-gas sampling. The State also collected blood samples from residents of all 40 buildings.

Most of the homes in this neighborhood are 'pier and beam' structures with underlying crawl spaces. This architectural design prompted use of an exhaust system that could evacuate air in the crawl space and prevent TCE buildup and additional migration into the home's interior. The simple mitigation design allows a commercially available fan to be powered by a solar energy unit (Figure 2), which saves the homeowner an estimated $96/year for electricity.

To date, exhaust systems have been installed in the four homes found to have TCE concentrations exceeding action levels. Each system consists of a conventional 6-inch, 200-CFM fan installed in the crawl space and is powered by a 10- by 16-inch, 10-watt solar panel mounted on the building’s roof. Each solar panel can be supplemented by a 24-volt battery with a lifespan of 5-7 years to ensure continuous operation of the exhaust system. System installation was completed in two days, including less than one hour for solar panel installation. Equipment costs for each system included $200 for the fan and the solar panel, and $50 for the battery.

Homes with slab foundations will require a subsurface mitigation system involving lateral pipes between sumps typical of radon fan systems. Project plans estimate that each subsurface exhaust system will employ a 65-watt radon fan operating at a rate of 200 CFM and powered by a 36- by 36-inch solar panel. A subsurface system can be installed within 10 days. Equipment costs are anticipated to include $1,500 for each radon fan and $800 for each panel.

Post-installation sampling of two exhaust systems indicated an immediate 95% reduction in TCE vapor in each building’s interior. Exhaust systems will be offered this fall to 10 additional homes in the area defined by soil-gas sampling. If additional homes are found above the action level, exhaust fan systems will be offered to the homeowners.

Contaminant migration from the Delfasco Forge groundwater plume is expected to continue for up to 30 years. As a result, Region 6 is gathering information to evaluate the site’s potential inclusion on the National Priorities List (NPL).

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