Remediation of Large Groundwater Plumes through Optimized Extraction System and Monitored Natural Attenuation

A site-wide groundwater extraction and optimization process has been ongoing at a former large munition manufacturing process facility in the United States. The site contains five large RDX plumes with combined surface area of 4,500 acres. The other smaller comingled plumes are largely covered within the footprint of the RDX plumes. A formal dispute between the government agencies was resolved in 2008. An adaptive groundwater extraction system has been implemented to contain the plumes and mass reduction of RDX. The final Record of Decision (ROD) directs the relocation of extraction wells over time to maximize mass reduction of RDX. According to the ROD, the optimized groundwater extraction phase will continue for 15 years or more until a specific target concentration is reached. Once reached, the extraction system will be discontinued and monitoring natural attenuation (MNA) will be followed for another 45 years. Based on the preliminary remedial goal (PRG) for RDX of 2 µg/L and the site-specific half-life of 8 years, the ideal target concentration at the end of extraction phase is estimated to be 100 µg/L. The optimization process of the extraction phase is limited to the maximum treatment capacity of 2,400 gpm from the two on-site treatment plants. An extensive groundwater model has been utilized to optimize the relocation of the extraction wells annually. Additional challenge of the remedial action is to obtain the desired attenuation rate throughout the plumes at the end of the extraction phase. Following the completion of the remedial action in 60 years, the groundwater should restore to drinking water standard and allow unrestricted use. The total cost of the groundwater extraction system to date is approximately $70 million, including capital and O&M (operation and maintenance) costs.