The reactivity profile of “lesser chlorinated” aliphatic hydrocarbons such as 1,2-dichloroethane and 1,2-dichloropropane is relatively less understood and has not been systematically considered when developing remedial approaches for these compounds. The lesser chlorinated hydrocarbon 1,2,3-trichloropropane (TCP) has been identified as an emerging contaminant at more than 45 Department of Defense (DoD) facilities to date. The median detected concentrations have been low but are nonetheless high relative to regulatory levels for TCP.
One treatment process that produces promising rates of TCP degradation is in situ chemical reduction (ISCR) with zero-valent zinc (ZVZ). In 2014, the Navy implemented a pilot study to evaluate the feasibility and performance of in situ ZVZ remediation of TCP in groundwater. The pilot study results showed evidence of TCP degradation, but the pilot study monitoring was not designed to evaluate ZVZ distribution following injections or long-term performance of the zinc media with respect to degradation of TCP. The objective of this project was to collect data to further demonstrate and validate ZVZ, including providing supporting information for engineering design and performance assessment.
Evaluation of ZVZ as a remedial technology for TCP has been ongoing for several years. Proof-of-concept bench-scale testing and field-scale column testing demonstrated that chemical reduction using ZVZ is relatively promising for remediation of TCP. The first field application of ZVZ to remediate TCP in groundwater included pilot-scale injections completed at Marine Corps Base Camp Pendleton (MCBCP) in 2014 to create a permeable treatment zone. The pilot-scale injections included the direct injection of ZVZ into the subsurface via pneumatic fracturing, followed by monitoring for TCP, dissolved zinc, and geochemical parameters in intergrid and downgradient performance monitoring wells. Post-injection results showed evidence of TCP degradation of up to 97% within the pilot study area with no impacts to secondary groundwater quality.
This project was designed to complement and leverage previous Navy-funded ZVZ injection work at MCBCP and provide information to guide the use of this technology at other DoD sites. The project included the collection of soil and groundwater samples within the pilot study area to evaluate the distribution of ZVZ achieved by the pilot study injections and the long-term efficacy of the ZVZ. Major findings include the following:
- Post-treatment groundwater monitoring results from within and downgradient of the ZVZ injection area indicate that TCP reductions exceeding 90% were achieved and maintained for at least four years post-injection.
- Reduced geochemical conditions favorable to ISCR of TCP, including low dissolved oxygen, negative oxidation reduction potential, and decreased sulfate concentrations relative to baseline conditions were secondary indicators of the continued efficacy of the pilot study injections.
- Injected zinc was present in some soil samples at high mass loadings (greater than 0.5%) but not in other samples, suggesting that ZVZ can be delivered to the subsurface but was not delivered in a predictable manner during the pilot study.
Most costs to implement this technology are comparable to implementing a zero valent iron (ZVI) remedy. The primary difference is the higher cost of ZVZ relative to ZVI. Use of ZVZ is most favorable when a stronger reductant is required, either due to the recalcitrant nature of the compound being treated or site-specific conditions which may limit residence time within the treatment area.
The limited number of soil samples within the injection area with elevated zinc concentrations suggest that the ZVZ was not delivered to the subsurface in a predictable manner during the July 2014 pilot study. This is consistent with observed surfacing of ZVZ material during the injections. As such, the project team has developed recommendations for optimizing ZVZ delivery for future injections, including the following:
- Selecting ZVZ material more amenable to subsurface injections; and
- Optimizing injection approaches to improve predictability of material delivery.
While optimal ZVZ delivery may not have been achieved during the pilot study, the observed trends in TCP concentration within and downgradient of the ZVZ injection area during post-injection performance monitoring and as part of this verification project demonstrate that the pilot study was successful in terms of achieving targeted TCP concentration reductions. Based on the TCP concentration trends and the findings of this verification project, the pilot study was considered a success by the Navy and the Navy elected to move forward with optimized ZVZ injections at two areas of MCBCP. Follow-on injections implementing these recommendations were implemented at MCBCP in early 2019. Preliminary results suggest that the optimized injections more predictably distributed ZVZ within the subsurface and that reductions in TCP concentrations are being achieved.
Verification monitoring results clearly demonstrate the importance of verification monitoring in assessing long term viability and performance of the technology. Lessons learned from this work were used to inform the design and implementation of two ongoing full-scale applications of this technology at MCBCP.
Merrill, J.P., E. Suchomel, S. Varadhan, M. Asher, L. Kane, E. Hawley, and R. Deeb. 2019. Development and Validation of Technologies for Remediation of 1,2,3-Trichloropropane in Groundwater. Current Pollution Reports, 5:228–237. https://doi.org/10.1007/s40726-019-00122-7