Groundwater in Operable Unit 1 (OU-1) has been impacted by volatile organic compound (VOC)-contaminated materials in the former solid waste landfill. The remedy for controlling contamination from residual VOCs in OU-1 is groundwater extraction, treatment, and disposal.

Monitored natural attenuation (MNA) is being considered a viable alternative to the current remedy, which uses hydraulic containment to prevent migration of residual contaminants from the source term originating from the former landfill. The former landfill has been removed, resulting in decreased concentrations of VOCs in groundwater (concentrations are still above drinking-water standards). Also observed in OU-1 groundwater are attenuation mechanisms such as dilution, dispersion, and reductive dechlorination of trichloroethene (TCE).

Historical investigations and recent studies led to a recommendation to the U.S. Environmental Protection Agency (EPA) and Ohio EPA to perform a field demonstration showing that enhanced attenuation through structured treatment zones can lead to MNA being a viable alternative for addressing the current VOC effects in OU-1 groundwater. The recommended approach focuses on reducing VOC concentrations in portions of the soil or groundwater and creating an environment more conducive to destroying VOCs. Within the affected areas, reductive dechlorination of perchloroethene (PCE) to TCE occurs; however, subsequent reductive dechlorination of TCE to cis-1,2-Dichloroethene (DCE) is limited. Overall, aerobic conditions dominate the OU1 groundwater system indicating cometabolic aerobic oxidation of TCE and cis-1,2-DCE is possible.

Figure 1. Neat oil injection.
Figure 2. EVO injection.



The field demonstration is addressing VOC-impacted groundwater using a two-part deployment: (1) neat (pure) vegetable oil at the water table beneath areas with the highest residual VOC concentrations in soil, and (2) emulsified vegetable oil (EVO) in the areas with highest VOC concentrations in the groundwater plume. In the first part, neat oil (Figure 1) spreads laterally, forming a thin layer on the water table beneath residually contaminated soil sources in the vadose zone, to intercept and reduce future VOC loading (via partitioning) and reduce oxygen inputs to the local groundwater (via biostimulation). In the second part, EVO (Figure 2) injected below the water table forms active bioremediation reactor zones within the plume footprint to degrade existing groundwater contaminants (via reductive dechlorination and/or cometabolism) and stimulates long-term attenuation capacity in the distal plume (via cometabolism).

Key factors considered in the implementation of the field demonstration were:

  1. Former Source Area – Strategic deployment of neat oil into the lower portion of the vadose zone in areas with elevated measured soil concentrations of PCE or TCE greater than 1 mg/kg.
     
  2. Former Source Area – Groundwater: Strategic emulsified oil injection in the groundwater to form treatment zones that address key flow lines in the aquifer beneath the former landfill area.
     
  3. Downgradient of Former OU-1 Landfill – Groundwater: Intensive emulsified oil injection in multiple locations to address VOC-impacted groundwater downgradient of the former landfill.
    Figure 3. Injection point locations and PCE/TCE reduction zones.

     

Deployment consisted of injecting neat oil at 6 locations within the OU-1 landfill footprint, and injecting emulsified oil at 19 locations throughout the OU-1 area (Figure 3). Initially, operating the pump and treatment (P&T) system during the injection was to provide blending water and mitigate the contaminated groundwater from spreading downgradient. However, EVO breakthrough in the extraction wells was observed and the regulators approved turning off the P&T system.

Emulsified oil and blending water injection began August 25 and ended September 22, 2014. A total of 198,263 gallons of emulsion (EVO blended with water) was injected into the aquifer. The EVO blend was approximately 45 percent soybean oil with surfactants and amendments (lactate, yeast extract, and vitamin B-12) blended with treated water obtained from the P&T system at a ratio of 50:1 (water to EVO). Neat oil injection began on September 22 and ended December 7, 2014. A total of 4,590 gallons of neat oil was deployed into the vadose zone under gravity flow.

Post-deployment groundwater sampling of the monitoring wells was performed monthly from November 2014 through April 2015. The wells were sampled for VOCs and a suite of geochemical parameters. During this same period, the EVO injection points were sampled every 2 weeks for geochemical parameters. Data from the sampling events were used to monitor the treatment zone formation progress. Microbial sampling was performed in August and November 2014.

Totes containing emulsified oil are staged and ready for injection during the OU-1 field demonstration.
Injection of oil during the OU-1 field demonstration.

To date, VOC data and geochemical indicators—as well as microbial type and abundance—indicate the formation of discrete zones conducive to the reduction dechlorination of PCE and TCE and support increased microbial activity (Figure 3). These zones display:

  • Decreased concentrations of PCEand TCE
     
  • Reduced footprint of elevated PCE- and TCE-impacted groundwater areas (Figures 4 and 5)
     
  • Increased concentrations of cis-1,2-DCE
     
  • Negative oxidation-reduction potential values and declining dissolved oxygen concentrations
     
  • Increased metabolic by-product (acetone, 2-butanone, and alkalinity)
     
  • Foul odor and changes in well water color that are indicative of reduced conditions
     
  • Increased bacterial count

The field demonstration will continue for 3 years. When demonstrated to the regulators that deployment of EVO has created and maintained structured geochemical zones, resulting in the reductive dechlorination of PCE and TCE, it will be proposed to change the remedy to MNA.

 
Figure 4. TCE Distribution – August 2014. Figure 5. TCE Distribution – February 2015.