VAPOR EXTRACTION SYSTEMS: AIR QUALITY ANALYSIS FOR A SUB-SLAB VENTILATION SYSTEM

A recent environmental study was performed to evaluate the potential for vapor migration of volatile organic compounds (VOCs) from soil contaminated with manufactured gas plant (MGP) waste locateddirectly underneath a building at a retail shopping center in New York. Air sampling for potentialVOCs had been conducted on the soil vapor located underneath and immediately adjacent to the building, and the compounds detected included benzene, toluene, and m,p-xylene, having average soil vapor concentrations of 4300, 2000 and 1100 micrograms per cubic meter (μg/m3), respectively. The soil vapor sampling results indicated that soil vapor intrusion pathways cannot be ruled out. A sub-slab ventilation system was designed to remove the VOCs from the soil under the retail store to prevent potential intrusion of the vapor inside the building. This ventilation process(or vapor extraction system) consists of perforated polyvinyl chloride (PVC) insert pipes that penetrate the rear building foundation. A PVC header pipe connects the insert pipes to a blower system that moves air from the header pipe to a PVC exhaust riser. The exhaust riser (ventilation stack) discharges the soil vapor to the ambient air above the roof line of the building. The system is permanent and will operate continuously.To avoid the appearances of taking VOCs of concern from one environment and discharging them freely to the ambient air, an air dispersion modeling analysis was performed to demonstrate that ambient air impacts due to the emissions from the ventilation stack would comply with all the appropriate air quality criteria. A basic land use analysis, including population density, was performed, and it was determined that the site is located in an urban setting. Air quality impacts at potentialreceptors were evaluated. Impacts in ambient air were determined using the United States Environmental Protection Agency (USEPA) Industrial Source Complex Short Term (ISCST3) dispersion model. These receptor impacts determined by the dispersion modeling analysis were compared to New York StateDepartment of Environmental Conservation (NYSDEC) air toxics criteria (annual guideline concentrations (AGCs) and short-term guideline concentrations (SGCs)). The results demonstrated that all VOCs complied with the AGCs and SGCs.