Formatted Title
Optimizing Post-Remediation Groundwater Performance with Enhanced Microbiological Analysis
Background/Objectives
A former specialty chemical manufacturing facility located in New Jersey has been investigated since the late 1990s and a pump and treat system ran from 2001 to 2006. Primary contaminants of concern were 1,1,1–trichloroethane (TCA), its respective degradation products, and other chlorinated compounds. Bench-scale testing identified a unique strain of Dehalococcoides that naturally were degrading chlorinated volatile organic compounds (CVOCs) at a less optimum pH range of 5.8 to 6.0 known at the time. Significant concentrations of both biotic and abiotic degradation products had been observed in groundwater samples collected from the Site, suggesting that both degradation processes were occurring. In August 2002, a microbiological bench-scale study was performed to evaluate the potential for anaerobic biodegradation, specifically reductive dechlorination, to occur at the Site. The results of this study concluded:
- Strong anaerobic conditions along with high chloride concentrations existed at select monitoring well locations and suggested prior CVOC reduction had occurred. The relatively low concentrations of chloroethane and a lack of preferable organic acids indicated that the microbial degradation of TCA was limited by a lack of nutrients.
- Abiotic breakdown products of TCA, which have degraded to vinyl chloride, were observed. It was hypothesized, if sufficient nutrients were provided, it was likely that the abiotic degradation of TCA would decrease, and the anaerobic biodegradation of TCA would increase.
- High concentrations of 1,1-DCE, the abiotic breakdown product, were also observed. However, there are only low concentrations of vinyl chloride, which is the biotic byproduct of 1,1-DCE. This suggests that nutrient limitation may exist.
As a result, multiple field pilot studies were designed and successfully implemented to evaluate in situ chemical reduction technologies and delivery methods for enhanced dechlorination biodegradation with the co-application of a zero-valent iron (ZVI) and emulsified vegetable oil (EVO).
Approach/Activities
Results of pneumatic injection pilot tests during 2003 to 2007 yielded significant positive results for injection delivery design, contaminant mass treatment, and that resulted in permanent shutdown of the pump-and-treat system. Accessible source areas were subsequently removed (2011) by soil excavation and amended with EVO and ZVI to accelerate treatment of impacted groundwater in overburden and weathered fractured bedrock. Post pilot testing and post remediation groundwater monitoring have included analyses of CVOCs, organic fatty acids, dissolved gases and QuantArray® -Chlor to quantify key microorganisms (e.g., Dehalococcoides, Dehalobacter, etc.) and functional genes (e.g., vinyl chloride reductase, methane monooxygenase, etc.) to assess potential for reductive dechlorination and aerobic co-metabolism of CVOCs.
In 2022, the first commercial application of MetaArray™ was performed at the site. MetaArray™ utilizes statistical analysis, such as principal component analysis and multivariate analysis to provide evidence that reductive dechlorination is active or even that it is slowing. This creates actionable data allowing users to save money by making important site management decisions earlier.
Results/Lessons Learned
The results of the MetaArray™ analysis identified groundwater monitoring wells with a 80% confidence that were characterized as either limited for reductive dechlorination or had a high reductive reduction dechlorination potential. The results of MetaArray™ will be used to further optimize the site’s post remediation monitoring program for monitored natural attenuation.