In situ chemical oxidation (ISCO) has proven effective in reducing potential Department of Defense (DoD) liability and health risks by oxidation of organic contaminants in situ. ISCO has been applied widely at DoD and non-DoD sites, and while it is effective for organic contaminant removal, it has also resulted in significant release of metals and metalloids in groundwater. The release of metals and metalloids is not fully understood nor is it predictable; elevated levels of metals in groundwater have recently prohibited a number of site closures and represent significant uncertainty in the remedial selection process. For ISCO to be a cost-effective and predictable remediation method, the unintended reactions that release metals and metalloids need to be understood and mitigation methods need to be developed and demonstrated.
The objectives of this project were to: (1) develop a fundamental and predictive understanding of metals release as a result of three common ISCO treatments; (2) develop a mechanistic geochemical model to describe release and to design mitigation measures; (3) experimentally evaluate the fate of redox and pH perturbations and elevated metals released; (4) demonstrate efficacy of pre-, co-, or post-treatment metals mitigation measures; and (5) develop guidance for the design community to predict release, understand mechanisms, and design mitigation strategies, as well as maximize the utility of ISCO, reduce impacts to groundwater, and minimize life-cycle costs of the remedial technology.
A database of historical ISCO actions was developed using 89 field implementation case studies. Metals were monitored in 21% of the case studies (19 out of the 89 case studies). The database includes a compilation of chemistry, soils, and site data relevant to metals release. Leaching studies were conducted on 10 DoD site soils using standard protocols, and geochemical speciation modeling was conducted using LeachXS to determine factors controlling solid phase speciation and enable prediction of metals release issues for a site. Laboratory batch experiments were performed to evaluate potential mitigation measures to reduce metals release and to describe and predict propagation of redox and pH fronts and metals migration. Column experiments were used with site soils to measure migration of geochemical perturbations. A guidance document was produced to aid site managers and design engineers to predict metals release at a site, understand how and why it may occur, and design strategies to minimize or eliminate the release.
An increase in metals concentration was observed in 63% of the field implementation case studies (12 out of 19 case studies) that were included in the historical ISCO applications database. Chromium (total and hexavalent), iron, manganese, and arsenic were the four most frequently observed mobilized metals in these 12 sites. Results of the metal mobilization screening experiments showed that the leaching of metals depended on soil type, ISCO treatment applied, and the metal in question. Several metals were released at elevated levels in each oxidant system compared to control batches, but more metals were mobilized in the catalyzed hydrogen peroxide and persulfate systems than in the presence of permanganate. Differences in metals mobilization were also observed within the various catalyzed hydrogen peroxide and persulfate chemistries; the citric acid chelating agent contributed to metals release. In general, dissolved metal concentrations increased with the addition of higher oxidant dose. Subsequent pH-dependent leaching studies revealed that while the release of most metals was controlled by pH, the mobilization of select metals was determined by ISCO chemistry. In the latter case, metals mobilization either increased or decreased in the presence of the oxidant. Geochemical speciation modeling revealed that secondary effects such as the formation of organic complexes and the dissolution of iron oxides and other minerals also contributed to the release of select metals.
Column studies also verified that metals release due to contact with an ISCO regent is site-specific (aquifer specific) and metal-specific. This observation reinforces the recommendation that all sites should be initially screened for the presence of metals, especially if there is knowledge of historical uses and/or suspected prior metal releases that could have a legacy in the subsurface.
Mitigation studies determined that mobilized metal concentrations from ISCO treatment applications consistently occurred, but returned to near baseline concentrations within 90 days without any additional treatment. This indicates that mobilized metals may be a temporary phenomenon and that measures may need to be taken to monitor and help mitigate the potential for metals mobilization in the transition period immediately following application.
Testing found that citrate and acetate were effective in mitigating the mobilization of metals during the application of iron-activated persulfate (IAP). The addition of acetate with IAP consistently helped maintain metal concentrations close to those observed in the control reactors. The addition of acetate and citrate were similarly beneficial during the application of catalyzed hydrogen peroxide, with slower kinetics. The addition of lactate to permanganate treatments was also observed to reduce the mobilization of metals. No additive intended to stimulate biotic activity was deemed successful enough to warrant additional evaluation for alkaline-activated persulfate.
The apparent trade off to mitigating the mobilization of metals in ISCO systems with electron donors such as lactate, acetate, and citrate is an observed general tendency toward a decrease in treatment efficacy of the target contaminant of concern. In many instances the impact was minimal, and in other instances the decrease in treatment efficacy may still be acceptable to the ISCO designer and associated stakeholders.
This project improved the understanding and knowledge of the impacts to groundwater quality following the application of ISCO technologies. The research helped identify the factors contributing to the release and fate of mobilized metals that had been documented to occur during remedial activities. In addition, modifications to ISCO technology that minimize or eliminate the degree to which metals are mobilized were designed and evaluated. This understanding and knowledge will aid site owners, design engineers, and scientists during site characterization, remedial selection, and the design and implementation of ISCO.