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Title: In Situ Dechlorination of TCE During Aquifer Heating

Abstract

Laboratory and field efforts were undertaken to examine trichloroethene (TCE) dechlorination as a function of temperature as an aquifer is heated to temperatures approaching boiling. Experiments were conducted using sediment samples during electrical resistance heating (ERH) treatment at the Fort Lewis East Gate Disposal Yard, which contains non-aqueous phase TCE and hydrocarbon contamination. Laboratory microcosms with these sediments showed TCE dechlorination at 70oC with measured products of acetylene, ethene, and ethane, indicating an abiotic component of the degradation. In contrast, TCE was dechlorinated to cis-1,2-dichloroethene in experiments at 10oC, likely by biological reductive dechlorination. The observed products at 70oC suggest dechlorination catalyzed by reduced sediment iron. Indications of in situ dechlorination were observed in periodic groundwater samples collected during field-scale electrical resistance heating from an average ambient temperature of about 19oC to near boiling. Dechlorination indicators included an increase in chloride concentration at the onset of heating and observation of acetylene, ethene, and methane at elevated temperatures. The data collected in this study suggest that dechlorination can occur during ERH. The overall cost-effectiveness of ERH may be enhanced by fortuitous in situ dechlorination and, potentially, can be further enhanced by specifically designing and operating ERH to maximize in situ dechlorination.

Authors:
; ;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
918459
Report Number(s):
PNNL-SA-48752
Journal ID: ISSN 1069-3629; GWMREV; 400403209; TRN: US200819%%286
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Ground Water Monitoring and Remediation, 27(2):96-105; Journal Volume: 27; Journal Issue: 2
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; AMBIENT TEMPERATURE; AQUIFERS; DECHLORINATION; HEATING

Citation Formats

Truex, Michael J., Powell, Thomas, and Lynch, Kira P. In Situ Dechlorination of TCE During Aquifer Heating. United States: N. p., 2007. Web. doi:10.1111/j.1745-6592.2007.00141.x.
Truex, Michael J., Powell, Thomas, & Lynch, Kira P. In Situ Dechlorination of TCE During Aquifer Heating. United States. doi:10.1111/j.1745-6592.2007.00141.x.
Truex, Michael J., Powell, Thomas, and Lynch, Kira P. Sun . "In Situ Dechlorination of TCE During Aquifer Heating". United States. doi:10.1111/j.1745-6592.2007.00141.x.
@article{osti_918459,
title = {In Situ Dechlorination of TCE During Aquifer Heating},
author = {Truex, Michael J. and Powell, Thomas and Lynch, Kira P.},
abstractNote = {Laboratory and field efforts were undertaken to examine trichloroethene (TCE) dechlorination as a function of temperature as an aquifer is heated to temperatures approaching boiling. Experiments were conducted using sediment samples during electrical resistance heating (ERH) treatment at the Fort Lewis East Gate Disposal Yard, which contains non-aqueous phase TCE and hydrocarbon contamination. Laboratory microcosms with these sediments showed TCE dechlorination at 70oC with measured products of acetylene, ethene, and ethane, indicating an abiotic component of the degradation. In contrast, TCE was dechlorinated to cis-1,2-dichloroethene in experiments at 10oC, likely by biological reductive dechlorination. The observed products at 70oC suggest dechlorination catalyzed by reduced sediment iron. Indications of in situ dechlorination were observed in periodic groundwater samples collected during field-scale electrical resistance heating from an average ambient temperature of about 19oC to near boiling. Dechlorination indicators included an increase in chloride concentration at the onset of heating and observation of acetylene, ethene, and methane at elevated temperatures. The data collected in this study suggest that dechlorination can occur during ERH. The overall cost-effectiveness of ERH may be enhanced by fortuitous in situ dechlorination and, potentially, can be further enhanced by specifically designing and operating ERH to maximize in situ dechlorination.},
doi = {10.1111/j.1745-6592.2007.00141.x},
journal = {Ground Water Monitoring and Remediation, 27(2):96-105},
number = 2,
volume = 27,
place = {United States},
year = {Sun Apr 01 00:00:00 EDT 2007},
month = {Sun Apr 01 00:00:00 EDT 2007}
}
  • In situ chemical reduction of aquifer sediments is currently being used for chromate and TCE remediation by forming a permeable reactive barrier. The chemical and physical processes that occur during abiotic reduction of natural sediments by sodium dithionite were investigated. In different aquifer sediments, 15 to 25% of Fe{sup III} -oxides were dissolved/reduced, which produces primarily adsorbed Fe{sup II}, and some siderite. The sediment reduction rate ({approx} 5h) was the chemically controlled (58 kJ/mole) reduction of a minor phase (<20%). It was necessary to maintain neutral to high pH to maintain reduction efficiency and prevent iron mobilization, as reduction generatedmore » H{sup +}. Sequential extractions on reduced sediment showed that adsorbed ferrous iron and iron oxides on the clay size fraction controlled TCE reactivity, and not structural ferrous iron in clay. The mass and rate of field-scale reduction of aquifer sediments were generally predicted with laboratory data using a single reduction reaction.« less
  • Sodium lactate additions to a trichloroethene (TCE) residual source area in deep, fractured basalt at a U.S. Department of Energy site have resulted in the enrichment of the indigenous microbial community, the complete dechlorination of nearly all aqueous-phase TCE to ethene, and the continued depletion of the residual source since 1999. The bacterial and archaeal consortia in groundwater obtained from the residual source were assessed by using PCR-amplified 16S rRNA genes. A clone library of bacterial amplicons was predominated by those from members of the class Clostridia (57 of 93 clones), of which a phylotype most similar to that ofmore » the homoacetogen Acetobacterium sp. strain HAAP-1 was most abundant (32 of 93 clones). The remaining Bacteria consisted of phylotypes affiliated with Sphingobacteria, Bacteroides, Spirochaetes, Mollicutes, and Proteobacteria and candidate divisions OP11 and OP3. The two proteobacterial phylotypes were most similar to those of the known dechlorinators Trichlorobacter thiogenes and Sulfurospirillum multivorans. Although not represented by the bacterial clones generated with broad-specificity bacterial primers, a Dehalococcoides-like phylotype was identified with genus-specific primers. Only four distinct phylotypes were detected in the groundwater archaeal library, including predominantly a clone affiliated with the strictly acetoclastic methanogen Methanosaeta concilii (24 of 43 clones). A mixed culture that completely dechlorinates TCE to ethene was enriched from this groundwater, and both communities were characterized by terminal restriction fragment length polymorphism (T-RFLP). According to T-RFLP, the laboratory enrichment community was less diverse overall than the groundwater community, with 22 unique phylotypes as opposed to 43 and a higher percentage of Clostridia, including the Acetobacterium population. Bioreactor archaeal structure was very similar to that of the groundwater community, suggesting that methane is generated primarily via the acetoclastic pathway, using acetate generated by lactate fermentation and acetogenesis in both systems.« less
  • An expert knowledge site-screening methodology has been developed to evaluate naturally occurring reductive dechlorination as a remedial option for sites with TCE-contaminated groundwater. This methodology combines a causative model for the reductive dechlorination of TCE and expert knowledge within a Bayesian Belief Network. The knowledge base for this expert system was obtained from 22 experts via an expert elicitation protocol The resulting expert system can be used to aid environmental decision making by evaluating the adequacy of reductive dechlorination at TCE-contaminated sites. Comparisons between this expert system and a commonly used screening tool show that this expert system reproduces predictivemore » models that may better discriminate between locations that were sampled. The 22 elicitations revealed different beliefs and assumptions among experts about the biochemical processes involved in reductive dechlorination. The decision-making value of some types of evidence is a matter of dispute; however, findings about biodegradation daughter and/or end products have high decision-making value for all of the experts. The methodology demonstrated herein can provide insights for other environmental decision-making challenges.« less
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  • A field evaluation of enhanced reductive dechlorination of trichloroethene (TCE) in ground water has been in progress since November 1998 to determine whether in situ biodegradation can be significantly enhanced through the addition of an electron donor (lactate). An in situ treatment cell was established in the residual source area of a large TCE plume in a fractured basalt aquifer utilizing continuous ground water extraction approximately 150 meters downgradient of the injection location. After a 1-month tracer test and baseline sampling period, the pulsed injection of lactate was begun. Ground water samples were collected from 11 sampling points on amore » biweekly basis and in situ water quality parameters were recorded every 4 hours at two locations. Within 2 weeks after the initial lactate injection, dissolved oxygen and redox potential were observed to decrease substantially at all sampling locations within 40 m of the injection well. Decreases in nitrate and sulfate concentrations were also observed. Both quantitative in situ rate estimation methods and qualitative measures such as changes in redox conditions, decreases in chlorine number, and changes in biomass indicator parameters are being used throughout the test to evaluate the extent to which biodegradation of TCE is enhanced.« less