Theme: 02. Assessing Remediation Effectiveness

Topic Area: 02b. Remedy Implementation: Assessing Performance and Costs

Formatted Title:

Abiotic and Biotic Dechlorination of Carbon Tetrachloride and Chloroform by In Situ Chemical Reduction

Background/Objectives:

A commingled plume composed primarily by chlorinated methanes is present in groundwater at a former chemical manufacturing facility in Brazil. To address these impacts, biostimulation via direct injection of molasses was historically performed in the source area for 5 years. However, molasses is soluble and relatively short-lived, so this approach required multiple injection events that disrupted site operations. Additionally, the high concentrations of chlorinated compounds (mainly chloroform [CF] between 40 and 60 mg/L but also lower levels of trichloroethylene [TCE]) raised the concern of possible inhibition of the in situ degrading population rendering the biostimulation efforts less effective. For these reasons, the remedial strategy was reassessed, and in situ chemical reduction (ISCR) was chosen. This approach was selected because better longevity of the ZVI product would result in reduced injection events, as well as the fact that ISCR allows the combination of abiotic and biotic strategies that can optimize dechlorination rates. The main objectives of the project are to i) implement an effective remedial technology with facilitated operation and maintenance; ii) to offset potential toxicity of chlorinated methanes (carbon tetrachloride [CT] and CF) to microbes responsible for degradation of TCE and iii) prevent off-site migration.

Approach/Activities:

The full-scale ISCR project started in 2020 and the performance monitoring program is still ongoing. Two injection events were performed over a three-year period. The product injected is composed of organic matter and ZVI (ISCR-IR^®) to support both biological and chemical reductive reactions. In the first event (Oct/2020) ZVI was injected into 15 locations. In the second event (Oct-Dec/ 2022) the product was injected in 33 locations. Direct push technology (DPT) was used as the delivery method during both injection events. A groundwater performance monitoring program was conducted before and after the injections and included the monthly monitoring of physico-chemical parameters (pH, oxidation-reduction potential [ORP], dissolved oxygen [DO], electrical conductivity, and temperature) and quarterly monitoring of geochemical (total alkalinity, total organic carbon, ethene, ethane, methane, nitrate, sulfate and sulfide), analytical (carbon tetrachloride, chloroform, dichloromethane, chloromethane, tetrachloroethene, trichloroethene, 1,2-dichloroethene, 1,1,2,2-tetrachloroethene, 1,2- dichloroethane and dissolved metals) and microbial parameters (Quantitative Polymerase Chain Reaction [qPCR] and Next Generation Sequencing [NGS]). The monitoring program is scheduled to be concluded by December 2023.

Results/Lessons Learned:

Geochemical parameters indicated that reducing conditions were established and sustained after the injection of ZVI, as observed by the predominantly negative ORP values, low DO, nitrate and sulfate concentrations and production of sulfide and methane. After the second injection event CT dissolved concentrations were non-detect and CF concentrations decreased to lower than Brazilian intervention levels in the local shallow aquifer. These reductions coincided with the transient formation of CF degradation byproducts (dichloromethane [DCM] and chloromethane). qPCR results indicated increasing concentrations of Dehalobater (up to 10⁵ gene copies/mL) after the injection events which is consistent with the detection of functional genes associated with chlorinated methanes degradation (cfrA/dcrA/tmrA). Next generation sequencing indicated the presence of the genera Candidatus Dichloromethanomonas, a known DCM degrader, as well as other genera related to anaerobic biogeochemical processes. Overall results demonstrated successful degradation of chlorinated methanes via ISCR within a three-year period.

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Marcelo Silveira