Evidence of rock matrix back-diffusion and abiotic dechlorination using a field testing approach

Schaefer, Charles E.; Lippincott, David R.; Klammler, Harald; Hatfield, Kirk

doi:10.1016/j.jconhyd.2018.01.004

Cited by 9 publications

(6 citation statements)

References 29 publications

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“…To assess the extent to which complete dechlorination of TCE is occurring at the site, the net isotopic enrichment for TCE, DCE, and VC is calculated as follows:

δ^{13} normalC = ((), χ_{TCE} δ^{13} C_{TCE} + χ_{DCE} δ^{13} C_{DCE} + χ_{VC} δ^{13} C_{VC}),

where δ 13 C is the molar weighted isotopic carbon enrichment, χ i is the mole fraction of compound i , and δ 13 is the 13 C isotopic level in either the TCE, DCE, or VC (‰). Use of an isotopic mass balance similar to Equation for multiple transforming species has been previously employed for chlorinated ethenes and ethene (Mundle et al ; Schaefer et al ). Figure a shows δ 13 C as a function of total chlorinated ethene molar concentration emanating from the DNAPL sources in the lower‐permeability materials; Figure b shows the corresponding values for the underlying higher‐permeability material.…”

Section: Resultsmentioning

confidence: 99%

Long‐Term Impacts on Groundwater and Reductive Dechlorination Following Bioremediation in a Highly Characterized Trichloroethene DNAPL Source Area

Schaefer¹,

Lavorgna²,

Haluska³

et al. 2018

Groundwater Monitoring Rem

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High‐resolution soil and groundwater monitoring was performed to assess the long‐term impacts of bioremediation using bioaugmentation with a dechlorinating microbial consortium (and sodium lactate as the electron donor) in a well‐characterized trichloroethene (TCE) dense nonaqueous phase liquid (DNAPL) source area. Monitoring was performed up to 3.7 years following active bioremediation using a high‐density monitoring network that included several discrete interval multi‐level sampling wells. Results showed that despite the absence of lactate, lactate fermentation transformation products, or hydrogen, biogeochemical conditions remained favorable for the reductive dechlorination of chlorinated ethenes. In locations where soil data showed that TCE DNAPL sources persisted, local contaminant rebound was observed in groundwater, whereas no rebound or continuous decreases in chlorinated ethenes were observed in locations where DNAPL sources were treated. While ethene levels measured 3.7 years after active treatment suggested relatively low (2 to 30%) dechlorination of the parent TCE and daughter products, carbon stable isotope analysis showed that the extent of complete dechlorination was much greater than indicated by ethene generation and that the estimated first‐order rate constant describing the complete dechlorination of TCE at 3.7 years following active bioremediation was approximately 3.6 y–1. Overall, results of this study suggest that biological processes may persist to treat TCE for years after cessation of active bioremediation, thereby serving as an important component of remedial treatment design and long‐term attenuation.

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“…To assess the extent to which complete dechlorination of TCE is occurring at the site, the net isotopic enrichment for TCE, DCE, and VC is calculated as follows:

δ^{13} normalC = ((), χ_{TCE} δ^{13} C_{TCE} + χ_{DCE} δ^{13} C_{DCE} + χ_{VC} δ^{13} C_{VC}),

Section: Resultsmentioning

confidence: 99%

Long‐Term Impacts on Groundwater and Reductive Dechlorination Following Bioremediation in a Highly Characterized Trichloroethene DNAPL Source Area

Schaefer¹,

Lavorgna²,

Haluska³

et al. 2018

Groundwater Monitoring Rem

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“…This is particularly true at DNAPL sites where it is likely the case that both DNAPL dissolution and back diffusion are relevant and concurrent features over some finite duration as the site ages. As noted by Schaefer et al [ 58 ], without sufficient data it can be difficult to determine which mechanisms are responsible for plume persistence. Schaefer et al [ 58 ] also noted that contaminant migration from upgradient contaminant sources after treatment too often remains a source of uncertainty that impedes assessment of conditions in the treated area, and the case studies at Jacksonville NAS and Brandywine serve as examples of that herein.…”

Section: Discussionmentioning

confidence: 99%

“…As noted by Schaefer et al [ 58 ], without sufficient data it can be difficult to determine which mechanisms are responsible for plume persistence. Schaefer et al [ 58 ] also noted that contaminant migration from upgradient contaminant sources after treatment too often remains a source of uncertainty that impedes assessment of conditions in the treated area, and the case studies at Jacksonville NAS and Brandywine serve as examples of that herein. Because of these challenges, there are limited well-documented field sites where treatment of back diffusion can be studied in detail.…”

Section: Discussionmentioning

confidence: 99%

Contaminant Back Diffusion from Low-Conductivity Matrices: Case Studies of Remedial Strategies

Blue

Boving

Tuccillo

et al. 2023

Water

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Recalcitrant groundwater contamination is a common problem at hazardous waste sites worldwide. Groundwater contamination persists despite decades of remediation efforts at many sites because contaminants sorbed or dissolved within low-conductivity zones can back diffuse into high-conductivity zones, and therefore act as a continuing source of contamination to flowing groundwater. A review of the available literature on remediation of plume persistence due to back diffusion was conducted, and four sites were selected as case studies. Remediation at the sites included pump and treat, enhanced bioremediation, and thermal treatment. Our review highlights that a relatively small number of sites have been studied in sufficient detail to fully evaluate remediation of back diffusion; however, three general conclusions can be made based on the review. First, it is difficult to assess the significance of back diffusion without sufficient data to distinguish between multiple factors contributing to contaminant rebound and plume persistence. Second, high-resolution vertical samples are decidedly valuable for back diffusion assessment but are generally lacking in post-treatment assessments. Third, complete contaminant mass removal from back diffusion sources may not always be possible. Partial contaminant mass removal may nonetheless have potential benefits, similar to partial mass removal from primary DNAPL source zones.

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“…A relatively new cryogenic coring method that freezes the soil core in place prior to extraction has emerged as an alternative to preserve in-situ conditions and improve detection of these chemicals (Kiaalhosseini et al 2016). Prior approaches to estimating insitu TCE reactions rates in LPZs may have underestimated TCE reaction rates if these volatile reduction products were lost during sampling (Schaefer et al 2018b;Harte and Brandon 2020;Allen-King et al 2022). This is important to address because accurate subsurface depth profiles of TCE and volatile reaction products can be used to enhance conceptual site models, identify relevant reaction pathways, and possibly estimate natural attenuation rates within LPZs.…”

Section: Introductionmentioning

confidence: 99%

Assessment of Cryogenic Coring to Preserve Vertical Distributions of Trichloroethylene and Reaction Products in an Aquitard

Kearney,

Blount,

Palmer

et al. 2024

Groundwater Monitoring Rem

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There is not a clear understanding of the extent by which naturally occurring reactions can attenuate trichloroethene (TCE) and its daughter products within low permeability zones (LPZs), and addressing this knowledge gap requires advancement of methods to accurately measure in situ volatile chemical concentrations. In this study, a soil coring method that freezes the soil in‐situ (a.k.a., cryogenic coring) was utilized to measure depth‐discrete distributions of TCE and its volatile reaction products through a TCE‐impacted silty clay aquitard, and results were compared with those from adjacent soil cores taken using a conventional coring approach. Vertical concentration profiles of TCE, cis‐1,2‐dichloroethylene (DCE), vinyl chloride (VC), ethane, and methane were all compared between the two coring methods, and results indicate the two coring methods recovered statistically equivalent concentrations of volatiles across most depths of the fine‐grained cohesive clayey soil at the study site. Biotic reductive dechlorination was the dominant TCE reaction pathway at the site; several reduced gasses that are possible markers for abiotic reduction were detected, but their concentrations and intervals of occurrence were not sufficiently consistent to indicate whether they were from abiotic TCE reduction or unrelated biological processes. Overall, cryogenic coring yielded improved recovery of sand lenses compared to conventional coring, but offered no apparent benefits for improved recovery of TCE and its volatile reaction products in the low permeability aquitard material at the site.

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Evidence of rock matrix back-diffusion and abiotic dechlorination using a field testing approach

Cited by 9 publications

References 29 publications

Long‐Term Impacts on Groundwater and Reductive Dechlorination Following Bioremediation in a Highly Characterized Trichloroethene DNAPL Source Area

Long‐Term Impacts on Groundwater and Reductive Dechlorination Following Bioremediation in a Highly Characterized Trichloroethene DNAPL Source Area

Contaminant Back Diffusion from Low-Conductivity Matrices: Case Studies of Remedial Strategies

Assessment of Cryogenic Coring to Preserve Vertical Distributions of Trichloroethylene and Reaction Products in an Aquitard

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