ZVI Permeable Reactive Barrier CVOC Plume
PROJECT SUMMARY
A groundwater plume with concentrations of CVOCs greater than 10,000 μg/L was causing a vapor plume directly affecting residential properties. The plume emanated from an upgradient source caused by systemic releases from historic chemical manufacturing. The geological setting (Mason Esker and ≈1 foot/day Darcy velocity) contributed to off-site migration of high concentration CVOCs in groundwater affecting a nearby residential neighborhood.
Passive Flux Meters (PFMs) were used to determine mass flux throughout the upper 10 feet of aquifer at two spatial locations within the proposed treatment area. PFM outputs (at left) were used to target design dosages of remediation compounds.
Oxic conditions (>3 mg/L DO) and positive oxidation-reduction potentials (ORP) existed in the aquifer due to its position in the highly permeable Mason Esker. Published research by others and field observations by HMA have demonstrated that oxic conditions may only slightly reduce the effectiveness of Sulfidated ZVI under oxic and positive ORP conditions. ITRC* guidance for PRBs suggests that groundwater environments with less than 4 mg/L of oxygen provides ideal conditions for ZVI remediation.
The design approach included injection of a combination of PlumeStop Liquid Activated Carbon® (PlumeStop), Sulfidated Micro-ZVI® (SMZVI), Extended Hydrogen Release Compound® (HRC), Merichlor Dehalos® (a consortium of dehalococcoides bacterium), and calcium chloride as a parking agent. The PlumeStop provides physical sequestration and adsorption of the CVOCs to the carbon matrix for more efficient chemical reduction and biodegradation, which results in immediate declines of aqueous CVOCs. The ZVI acts as a reducing agent and provides electrons for direct contaminant degradation. The HRC provides extended release of electron donor material to further stimulate anaerobic biodegradation. Significant reductions in CVOCs (and increases in dissolved ethene and ethane) were observed within 2-3 months of delivery, even as far as 15-20 feet downgradient of the PRB. In addition, DOs and ORPs in the treatment area reflect the shift to reduced conditions conducive to the chemical and biological reduction pathways being sought.
PROJECT HIGHLIGHTS
• Challenging residential setting with VI risk reduction goal
• Challenging hydrogeologic setting (Mason Esker) with fast moving (> 1’/d) oxic groundwater (~3 mg/L in treatment area)
• Groundwater sourcing vapor plume in residential area (>10,000 μg/L Total CVOCs)
• Design data acquisition—passive flux meters to optimize design and target zones of greatest contaminant mass-flux
• Utilized a unique combination of chemical reduction, physical sequestration, and biological reduction
• Cut-off plume from upgradient source area (chemical plant)
• PRB deployment in City ROW and on residential property
