The Redwood City Case Study involves the remediation of a former dry cleaner site located in Redwood City, California, which was contaminated by tetrachloroethylene (PCE) and its degradation byproducts, such as trichloroethylene (TCE). The contaminated zone covered an area of 384 square feet with PCE concentrations reaching up to 7,100 μg/kg in soils at a depth of 5 feet below ground surface (bgs). The remediation goals were driven by the need to reduce the risk of vapor intrusion, ensuring that contaminant levels were brought below commercial soil gas screening criteria. The target depth for treatment was 10 feet beneath the building(Redwood City).
The GTR© system developed by GEO, in combination with the C3 Technology refrigerated vapor condensation process, was used for In Situ Thermal Remediation (ISTR). The GTR system effectively heats the contaminated soil and groundwater to target temperatures of 100°C, which allows for the volatilization of PCE. The C3 Technology was then used to condense the vaporized contaminants, achieving >99.9% efficiency in PCE and TCE removal from the vapor stream. This technology helped condense contaminants above ground, avoiding direct release into the atmosphere or absorption by activated carbon, which would eventually require landfill disposal(Redwood City).
Technology and Approach:
Thermal Conductive Heating (TCH): The GTR system utilized 9 thermal conductive heating wells placed in a triangular grid pattern across the 384 square feet treatment zone. The wells were powered by 5 GTR+F© heaters, which were fueled by a temporary propane tank system.
Auxiliary Fuel Recycling: The GTR+F units have the capability to recycle captured vapors into the GTR unit, utilizing them as auxiliary fuel if the contaminants have fuel value. This design helped reduce overall energy consumption by relying partially on the contaminants as a fuel source.
Temperature Control and Monitoring: The system was equipped with 10 thermocouples strategically placed throughout the treatment area to monitor temperature at various distances and depths from the heating wells. A central PLC system was used to monitor and control each heater, ensuring optimal in-situ heating results throughout the 6.5-month treatment period(Redwood City).
Results:
The remediation successfully achieved the reduction of >99% of PCE contamination in the subsurface soils, meeting all air quality management standards for discharge.
The entire process took 6.5 months, after which the remediation goals were met, significantly reducing the potential for vapor intrusion.
A notable challenge encountered during the remediation was contamination from a neighboring railroad line, which migrated into the treatment area and necessitated extended heating times. This issue underscored the importance of high-resolution site characterization and establishing clear treatment boundaries(Redwood City).
Full Scale Implementation:
The GTR system was installed in three overlapping stages to ensure comprehensive coverage of the target zone. The wells were placed in a grid that ensured thorough heating and vapor extraction from the soil. The co-located vapor extraction wells were critical in capturing volatilized contaminants as they were heated, preventing them from escaping into the atmosphere.
Conclusion:
The Redwood City remediation project demonstrated the effectiveness of the GTR system and C3 Technology in addressing complex PCE contamination at a former dry cleaning site. The use of thermal conductive heating paired with vapor condensation not only achieved the target remediation goals but also minimized secondary pollution by capturing and treating vapors above ground. The recycling of vapors as fuel represents an innovative step toward reducing energy needs and improving overall system efficiency. The lessons learned from managing migrating contamination emphasize the need for thorough site assessments and adaptable treatment plans in dynamic environmental conditions.
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