Neal Durant

Natural and enhanced anaerobic degradation of 1,1,1-trichloroethane and its degradation products in the subsurface – A critical review

1,1,1-Trichloroethane (TCA) in groundwater is susceptible to a variety of natural degradation mechanisms. Evidence of intrinsic decay of TCA in aquifers is commonly observed; however, TCA remains a persistent pollutant at many sites and some of the daughter products that accumulate from intrinsic decay of TCA have been determined to be more toxic than the parent compound. Research advances from the past decade indicate that in situ enhanced reductive dechlorination (ERD) offers promise as a cost-effective solution toward the cleanup of groundwater contaminated with TCA and its transformation daughter products. Laboratory studies have demonstrated that pure or mixed cultures containing certain Dehalobacter (Dhb) bacteria can catalyze respiratory dechlorination of TCA and 1,1-dichloroethane (1,1-DCA) to monochloroethane (CA) in groundwater systems. 16S rRNA Dhb gene probes have been used as biomarkers in groundwater samples to both assess ERD potential and quantify growth of Dhb in ERD applications at TCA sites. Laboratory findings suggest that iron-bearing minerals and methanogenic bacteria that co-occur in reduced aquifers may synergistically affect dechlorination of TCA. Despite these advances, a number of significant challenges remain, including an inability of any known cultures to completely dechlorinate TCA to ethane. CA is commonly observed as a terminal product of the biological reductive dechlorination of TCA and 1,1-DCA. Also important is the lack of rigorous field studies demonstrating the utility of bioaugmentation with Dhb cultures for remediation of TCA in the field. In this paper we review the state-of-the-science of TCA degradation in aquifers, examining results from both laboratory experiments and twenty-two field case studies, focusing on the capabilities and limits of ERD technology, and identifying aspects of the technology that warrant further development.

Field evaluation of biological enhanced reductive dechlorination of chloroethenes in clayey till.

The performance of enhanced reductive dechlorination (ERD) for in situ remediation of cis-1,2-dichloroethene (cDCE) and vinyl chloride in clayey till was investigated in a pilot test. A dilute groundwater solution containing emulsified soybean oil and Dehalococcoides bacteria was injected into a sand-filled hydraulic fracture. Fermentation of the ERD solution caused the establishment of a dechlorinating bioactive zone in the fracture within 1 month of injection. By 148 days, all the cDCE in the fracture was dechlorinated to ethene. Analysis of a clay core from Day 150 indicated that electron donor and fermentation products diffused from the fracture at least 10 cm into clay and that stimulated dechlorination occurred in the clay in the presence of Dehalococcoides (7.9.10(4) cells g(-1)). Comparison of chloroethene profiles in the Day 150 core to modeled diffusion profiles indicated degradation occurred in a bioactive zone extending approximately 5 to 6 cm into the clay matrix. These data suggest that a bioactive zone established in a sand-filled fracture can expand into the adjacent clayey till matrix and facilitate mass transfer from the matrix to the bioactive zone. These findings offer promise for ERD and support further development of methods for deploying ERD in clayey till and other low-permeability deposits.

Microcosm studies of subsurface PAH-degrading bacteria from a former manufactured gas plant

A study was conducted to evaluate the potential for natural in situ biodegradation of polycyclic aromatic hydrocarbons (PAHs) in the subsurface at the site of a former manufactured gas plant. Fifty-seven samples of unconsolidated subsurface sediments were aseptically obtained from five boreholes across the site. Bacteria capable of aerobically degrading PAHs without an acclimation period were detected throughout shallow (2.7 m) and deep (24.7 m) areas of the subsurface in both relatively clean (<20 μg L−1 naphthalene) and contaminated (4400 μg L–1 naphthalene) zones. Significant (p < 0.05) quantities of naphthalene (8±3% to 43±7%) and/or phenanthrene (3±1% to 31±3%) were mineralized in sediment-groundwater microcosms during 4 weeks of aerobic incubation at 22°C. Three samples out of 11 were able to aerobically mineralize significant quantities of benzene (6±2% to 24±1%). Of 11 samples tested for anaerobic mineralization, naphthalene biodegradation (7±1% to 13±2%) in the presence of N03 was observed in two samples. Compound removals were first order with respect to substrate concentration during the first 10–15 days of incubation. Compound biodegradation plateaued in the later stages of incubation (15–40 days), most likely from diminishing bioavailability and nutrient and oxygen depletion. Population densities in the sediments were typically low, with viable aerobic counts ranging from 0 to 105 CFU gdw−1, viable anaerobic counts ranging from 0 to 104 CFU gdw−1, and total counts (AODC) usually 10-fold greater than viable counts. Total counts exhibited a strong (p < 0.01) positive correlation with sample grain size. Viable aerobic and anaerobic populations commonly occurred in the same sample, suggesting the presence of facultative anaerobes. Bacteria were metabolically active in samples from groundwaters with low pH (3.7) and high naphthalene concentrations (11,000 μg L−1). Data from these enumeration and microcosm studies suggest that natural in situ biodegradation is occurring at the site.

Biotransformation of aromatic hydrocarbons in subsurface biofilms

Bioremediation is an emerging in situ treatment technology for soil and groundwater cleanup. Research in the past decade has made significant progress toward understanding how to stimulate microbial growth in the subsurface by optimizing the physical/chemical conditions. Recent laboratory observations and field demonstrations indicate that bioremediation can also be limited by mass transfer processes. In this paper, factors restricting microbial growth are reviewed, and the importance of bioavailability on the performance of in situ bioremediation is discussed by using aromatic hydrocarbons as model contaminants. Successful application of bioremediation relies upon an understanding of interactions among microorganisms, organic contaminants and soil/aquifer materials. Applications of biofilm kinetics toward this goal are addressed. Model simulations and laboratory studies suggest that both low temperature and slow desorption rate could greatly lengthen the time required for effective in situ bioremediation of aromatic hydrocarbons.

Biotreatment of PAH-contaminated Soils/Sedimentsa

The importance of chemical conditions and mass transfer effects to in situ bioremediation of PAHs is presented using a case study. In situ bioremediation is being evaluated as a means for remediating a coal-tar contaminated aquifer at the site of a former manufactured gas plant. Two objectives of this work have been to evaluate the potential for the indigenous bacteria to biodegrade coal tar constituents and to identify factors controlling biodegradation rates. Aquifer sediments collected from a variety of locations across the site contain bacteria capable of aerobically mineralizing some of the principal aromatic compounds in the groundwater plume (benzene, naphthalene, and phenanthrene). Parallel mineralization assays incubated under aerobic and anaerobic conditions strongly suggest that O2 availability is a primary factor controlling the rate and extent of biodegradation. Data indicate that sorption may have also significantly affected biodegradation rates by limiting the bioavailability of the aromatic compounds. A mass transfer-limited numerical model was developed to explore the effect of sorption and bioavailability on biodegradation rates. In this model biodegradation rates are proportional to aqueous concentration, which is directly reduced by sorption. Both biotransformation and bacterial growth are described as being controlled by the rate of desorptive mass transfer. The influence of sorption on biodegradation is quantified by defining a Bioavailability Factor, Bf. A Thiele Modulus which indicates the ratio of characteristic times for sorption and biodegradation is helpful for determining the extent of mass transfer control during biodegradation of the aromatic compounds. This approach is preferred to equilibrium partitioning models, which may overestimate biodegradation rates by failing to consider the effect of rate-limited desorption on bioavailability.

Spatial variability in the naphthalene mineralization response to oxygen, nitrate, and orthophosphate amendments in MGP aquifer sediments

The feasibility of aerobic in situ bioremediation isbeing investigated for use in a strategy to controlsubsurface coal tar contamination at the site of aformer manufactured gas plant. As part of thisinvestigation, anoxic aquifer sands collected between11 and 25 m below ground surface were assayed in batchmicrocosms to measure the singular and combinedeffects of O2, NO3-, andPO43- on 14C-naphthalenemineralization. The influence of these additivesvaried considerably between sediments. A high initialconcentration of O2 (21 mg/L) promoted thegreatest extent of mineralization in the majority ofactive sediments. NO3- (85 mg/L) wasobserved to enhance, inhibit, or have no effect on therate of naphthalene mineralization, althoughsignificant denitrification was observed in nearly allthe active sediments. Data suggest thatPO43- complexation and/or precipitation withsediment cations limited P bioavailability. Thesediments that were incapable of mineralizingnaphthalene were characterized by low pH (< 4.1),high SO42- (> 500 mg/L), and moderate tohigh dissolved Fe(II) (30–265 mg/L) whenequilibrated aerobically with water. Fe(II) likelyexerted a significant O2 demand that reduced theO2 available as an electron acceptor forbiodegradation. These experiments demonstrate thatwhile aeration/oxygenation can be an effectivestrategy for enhancing subsurface bioremediation ofaromatic hydrocarbons, the biodegradation response toaeration/oxygenation and nutrient addition may varyconsiderably within an aquifer.

Biodegradation of Coal Tar Constituents in Aquifer Sediments

In situ bioremediation is an important technology for the cost-effective treatment of contaminated soils and groundwater (NRC 1993). Trial-and-error methods of implementing this complex process at a field scale are inefficient and costly. Therefore, it is important to conduct laboratory studies to establish feasible microbial reactions and to develop reliable engineering models that can analyze in situ options prior to field testing. Specifically, it is important to establish the appropriate chemical conditions required for biodegradation of the contaminants and to assess the relative importance of mass transfer (bioavailability) versus kinetic (biodegradation control) effects.