Formatted Title
Sustainable Remediation Case Study: Transitioning from Pump and Treat to ISS and ISBR to Address DNAPL and Dissolved-Phase VOCs
Background/Objectives
A remedy consisting of groundwater extraction and treatment (GWET) and an asphalt cap was operating at a site since 1986 to protect human health and the environment from dissolved-phase volatile organic compounds (VOCs) and dense nonaqueous-phase liquid (DNAPL). The 5-acre site manufactured pesticides from 1929 to 1968, resulting in halogenated VOC impacts (primarily ethylene dibromide [EDB] and 1,2-dichloroethane [DCA]) to soil and groundwater. The existing GWET remedy had high operation and maintenance (O&M) costs, high natural resource consumption, poor compatibility with proposed site redevelopment, was not aligned with sea-level rise resiliency, and had low sustainability and long-term effectiveness because it was not projected to meet cleanup goals in a reasonable timeframe.
A new remedy consisting of in situ solidification/stabilization (ISS) and in situ biological reduction (ISBR) was implemented that allowed shutdown of the GWET systems, resulting in lower O&M costs, more effective treatment, and improved sustainability and compatibility with site redevelopment. Initial assessment of the revised remediation approach began in 2015 with a treatability study for ISS and ISBR technologies for soil and groundwater, respectively. Full-scale implementation of the combined ISS/ISBR remedy is projected to be completed in 2023. This presentation outlines the process to transition to an integrated, in situ remedy with minimal long-term O&M burden and discusses how the remediation project team overcame the challenges of stakeholder and regulatory acceptance, complex site geology, and recalcitrant halogenated VOCs.
Approach/Activities
A groundwater fate and transport model was developed to assist with remedial alternatives evaluation in the Feasibility Study, provide projections to the various stakeholders about the potential long-term effects of ISS and GWET system shutdown, and selection of the final combined ISS/ISBR remedy.
ISS was implemented in 2020 and 2022 to address DNAPL in shallow soil and groundwater to a maximum depth of 30 feet below ground surface (bgs). ISS was selected for field pilot testing following the treatability study performed with collected site soil, DNAPL, and groundwater. The large-scale field pilot test was performed to verify the treatability study results, evaluate construction means/methods, and optimize the ISS mix design. ISS-treated soil was collected and cast into molds for quality assurance/quality control testing and post-ISS in situ core samples were collected and tested to demonstrate that strength and hydraulic conductivity performance criteria were achieved. ISS leachability testing using modified U.S. Environmental Protection Agency LEAF Method 1315 demonstrated significant reductions in the mass flux of EDB and 1,2-DCA from the ISS treated soil over time.
ISBR was used to remediate dissolved VOCs in shallow groundwater (10 to 20 feet bgs) and deeper (40 to 70 feet bgs) groundwater. Pilot studies were performed to confirm ISBR was effective for the deeper groundwater, which was also impacted with sea water intrusion. Up to 99 percent VOC mass destruction was observed with no rebound over 1 year following ISBR treatment.
Performance groundwater monitoring will continue to confirm residual VOC concentrations are stable or declining. A contingency plan will be implemented if results indicate additional assessment or remedial actions are necessary.
Results/Lessons Learned
ISS was successful in solidifying and stabilizing chlorinated DNAPL cost-effectively and sustainably; and addressing future effects of sea level rise. Following in situ remediation treatments, the GWET systems operating in the shallow and deep groundwater bearing zones were shut down in 2019 and 2023, respectively. A multiple-lines-of-evidence approach was necessary to demonstrate long-term performance and effectiveness to stakeholders. The ISS treatment area of the site will be redeveloped into a community park in 2025 with nearby residential housing.