The challenge at the Department of Energy (DOE) Pantex Plant was to provide a technology solution to treat a variety of constituents, including trichloroethylene (TCE), high explosives, hexavalent chromium, and perchlorate (an emerging contaminant) located in a relatively deep saturated zone, approximately 300 feet below grade, where conventional ex situ technologies were inefficient and not an option.
Trihydro installed an in situ bioremediation (ISB) system at multiple locations at the DOE Pantex plant, designed to remove a variety of constituents and contain the plume. The ISB system treats constituents at a depth of approximately 280 feet below grade where the impacts are located in a complex geologic setting of perched groundwater on top of a thin fine-grained aquitard, which effectively inhibits contaminant migration downward to the Ogallala Aquifer (a regional aquifer).
Project implementation required phased work with field efforts related to well inspection and testing, well maintenance, and scheduled amendment injection. Prior to injection, each well was evaluated with a hydraulic ‘injected water’ test. The data collected were used to calculate specific capacity values for each well; this data was used to determine the effect of biofouling in each well and overall well efficiency to maintain injection operations. The water used during well testing and for injection blending was obtained from the onsite pump and treat system; using treated water provided a cost-effective, sustainable solution while achieving the overall site goal to decrease the amount of water in the perched zone.
The ISB system was maintained by injecting various carbon substrates, such as emulsified vegetable oil and molasses, into the subsurface. The conceptual site model (CSM) was revisited every year to apply a new understanding of subsurface conditions observed in monitoring wells, modifying the injection design accordingly. The project also included large-scale injection of deoxygenated water to facilitate the bioaugmentation of the natural biota with a species-specific bacteria known to fully degrade TCE and its daughter products. Trihydro performed statistical analysis of the site data to optimize the system. For example, data analysis provided insight into where and when well injections could be discontinued or paused, leading to cost savings for the client.
Trihydro also leveraged data to evaluate innovative aspects of the technology, such as performing a carbon isotope study, molecular diagnostics, decision tree analysis, abiotic degradation, and biotic degradation modeling. Following each injection event, Trihydro prepared reports documenting the results of the well maintenance and injection events. These reports focused on changes observed in the system and recommended optimization actions. Through Trihydro’s continual system optimization process, the team maximized mass reduction, reduced treatment costs, and gained efficiencies while maintaining compliance with the Record of Decision (ROD).
The project resulted in the following outcomes:
- Sampling results indicated complete removal of high explosives, hexavalent chromium, and perchlorate, as well as TCE degradation into daughter products
- Saved client over $1M due to targeted injection locations and optimizing injection volumes
- Completed value-added system optimization by completing a pH study, flux zone analysis, and molecular diagnostics such as a carbon isotope study
- Completed a decision tree study with cost-benefit for determination of future remediation scenarios
- Experienced zero health and safety incidents or lost work time over 25,000 manhours