Application of a pump and treat system to addressing potential vapor intrusion risk in a karst terrane: design parameter optimization and performance evaluation

Vapor intrusion has become a great concern in residential areas where shallow groundwater is contaminated with volatile organic compounds. Although active depressurization technologies such as sub-slab depressurization is the most frequently selected technology to reduce or eliminate exposure risks, such mitigation methods can hardly constitute the permanent remedy that eliminates or substantially reduces the level of contamination in groundwater that functions as the subsurface vapor source to acceptable-risk levels. This paper presents a case study in which a pump and treat (P&T) system is used as the long-term response to eliminate vapor intrusion risk in an area where contaminated groundwater in a non-karst terrain flows into a karst terrane. The underlying karst aquifer functions as an underdrain that limits the lateral and downgradient movement of the contaminated groundwater. Aquifer testing and a groundwater availability assessment helped optimize the groundwater extraction design so that the contaminated groundwater is captured in the contamination source area and prevented from flowing into the neighboring and hydraulically downgradient residential area. Performance of the installed P&T system was evaluated by comparing groundwater levels between the contamination source area and residential area and a groundwater capture analysis. An optimized pumping rate of 7.7 gallons per minute from three extraction wells resulted in the reversal of the groundwater flow in the study area. Because P&T systems are typically installed to either reduce contaminant concentration in groundwater or prevent contamination from spreading to uncontaminated areas, this P&T system expanded the P&T applications to include vapor intrusion risk mitigation.