Temperature dependence of anaerobic TCE-dechlorination in a highly enriched Dehalococcoides-containing culture

Temperature dependence of reductive trichloroethene (TCE) dechlorination was investigated in an enrichment culture (KB-1), using lactate or propionate as electron donors at a temperature interval from 4 to 60 degrees C. Dechlorination was complete to ethene at temperatures between 10 and 30 degrees C (lactate-amended) and between 15 and 30 degrees C (propionate-amended). Dechlorination stalled at cis-1,2-dichloroethene (cDCE) at 4 degrees C (lactate-amended), at and below 10 degrees C (propionate-amended), and at 40 degrees C with both electron donors. No dechlorination of TCE was observed at 50 and 60 degrees C. Concentrations of Dehalococcoides had increased or remained constant after 15 days of incubation at temperatures involving complete dechlorination to ethene. Temperature dependence of dechlorination rates was compared using zero order degradation kinetics and a Monod growth-rate model for multiple electron acceptor usage with competition. Maximum growth rates (mu) and zero order degradation rates were highest for TCE dechlorination at 30 degrees C with lactate as substrate (mu(TCE) of 7.00+/-0.14 days(-1)). In general, maximum growth rates and dechlorination rates of TCE were up to an order of magnitude higher than rates for utilization of cis-dichloroethene (cDCE, mu(c)(DCE) up to 0.17+/-0.02 days(-1)) and vinyl chloride (VC, mu(VC) up to 0.52+/-0.09 days(-1)). Temperature dependence of maximum growth rates and degradation rates of cDCE and VC were similar and highest at 15-30 degrees C, with growth rates on cDCE being lower than on VC. This study demonstrates that bioaugmentation of chlorinated ethene sites may be more efficient at elevated temperatures.     

Evaluation of a laboratory-scale bioreactive in situ sediment cap for the treatment of organic contaminants

The development of bioreactive sediment caps, in which microorganisms capable of contaminant transformation are placed within an in situ cap, provides a potential remedial design that can sustainably treat sediment and groundwater contaminants. The goal of this study was to evaluate the ability and limitations of a mixed, anaerobic dechlorinating consortium to treat chlorinated ethenes within a sand-based cap. Results of batch experiments demonstrate that a tetrachloroethene (PCE)-to-ethene mixed consortium was able to completely dechlorinate dissolved-phase PCE to ethene when supplied only with sediment porewater obtained from a sediment column. To simulate a bioreactive cap, laboratory-scale sand columns inoculated with the mixed culture were placed in series with an upflow sediment column and directly supplied sediment effluent and dissolved-phase chlorinated ethenes. The mixed consortium was not able to sustain dechlorination activity at a retention time of 0.5 days without delivery of amendments to the sediment effluent, evidenced by the loss of cis-1,2-dichloroethene (cis-DCE) dechlorination to vinyl chloride. When soluble electron donor was supplied to the sediment effluent, complete dechlorination of cis-DCE to ethene was observed at retention times of 0.5 days, suggesting that sediment effluent lacked sufficient electron donor to maintain active dechlorination within the sediment cap. Introduction of elevated contaminant concentrations also limited biotransformation performance of the dechlorinating consortium within the cap. These findings indicate that in situ bioreactive capping can be a feasible remedial approach, provided that residence times are adequate and that appropriate levels of electron donor and contaminant exist within the cap.     

Kinetics of trichloroethene dechlorination with iron powder

The dechlorination of trichloroethene (TCE) with metallic iron is an advantageous method for the remediation of contaminated groundwater and soil. The toxic reaction intermediates such as dichloroethenes (DCEs) and vinyl chloride (VC), however, occasionally accumulate in the pathway of the reaction. We have been trying to suppress these intermediates by using metallic iron powder containing impurities. In order to investigate the reaction pathways, we measured the production rates of the intermediates and the final products of the dechlorination of TCE such as DCEs, VC, ethyne or ethene. Ethyne, ethene, ethane and cis-DCE were observed as the major products, and trans-DCE, 1,1-DCE, VC, C3-hydrocarbons (such as propane, propylene), C4-hydrocarbons (such as n-butane) and methane were observed as the minors. Also the rate constants of TCE to ethyne and ethyne to ethene were larger than any other constants. These fact show the production of ethene/ethane via ethyne is the main pathway of the dechlorination of TCE using the metallic iron powder.     

Identification of microorganisms involved in reductive dehalogenation of chlorinated ethenes in an anaerobic microbial community

In this study, we report on phylogenetic and physiological characterization of an anaerobic culture capable of reductive dehalogenation of tetrachloroethene (PCE) obtained from a PCE-contaminated site. The culture was enriched using different combinations of electron donors (hydrogen and acetate) and electron acceptors (PCE, cis-1,2-dichloroethene (cDCE) and controls without chlorinated ethenes). The resulting subcultures were analyzed using three different approaches: chemical analysis to document conversion of chlorinated ethenes; polymerase chain reaction (PCR) of 16S rRNA gene fragments and denaturing gradient gel electrophoresis (DGGE) to compare community compositions; fluorescence in situ hybridization (FISH) to quantify specific groups of microorganisms using oligonucleotide probes previously designed or newly designed based on the sequences retrieved from sequence analysis of specific DGGE bands. Members of two genera which contain bacteria capable of reductive dehalogenation were detected in the culture: Dehalococcoides and Desulfitobacterium. The combined analyses suggested that Dehalococcoides-like bacteria are associated with complete dehalogenation of chlorinated ethenes to ethene with hydrogen as electron donor; and Desulfitobacterium-like bacteria, in contrast, are associated with incomplete PCE dehalogenation to cDCE and appear to be able to use acetate as electron donor. In addition, Sporomusa-like bacteria were identified, which most likely act as homoacetogens. The results demonstrated that combination of culture enrichment with different substrates, DGGE, and FISH allowed a detailed qualitative and quantitative characterization of the dominant microorganisms associated with reductive dehalogenation.     

Field assessment of carboxymethyl cellulose stabilized iron nanoparticles for in situ destruction of chlorinated solvents in source zones

This study pilot-tested carboxymethyl cellulose (CMC) stabilized zero-valent iron (ZVI) nanoparticles (with a trace amount of Pd catalyst) for in situ destruction of chlorinated ethenes such as perchloroethylene (PCE) and trichloroethylene (TCE) and polychlorinated biphenyls (PCBs) that had been in groundwater for decades. The test site was located in a well-characterized secondary source zone of PCBs and chlorinated ethenes. Four test wells were installed along the groundwater flow direction (spaced 5 ft apart), including one injection well (IW), one up-gradient monitoring well (MW-3) and two down-gradient monitoring wells (MW-1 and MW-2). Stabilized nanoparticle suspension was prepared on-site and injected into the 50-ft deep, unconfined aquifer. Approximately 150 gallons of 0.2 g/L Fe-Pd (CMC = 0.1 wt%, Pd/Fe = 0.1 wt%) was gravity-fed through IW-1 over a 4-h period (Injection #1). One month later, another 150 gallons of 1.0 g/L Fe-Pd (CMC = 0.6 wt%, Pd/Fe = 0.1 wt%) was injected into IW-1 at an injection pressure <5 psi (Injection #2). When benchmarked against the tracer, approximately 37.4% and 70.0% of the injected Fe was detected in MW-1 during injection #1 and #2, respectively, confirming the soil mobility of the nanoparticles through the aquifer, and higher mobility of the particles was observed when the injection was performed under higher pressure. Rapid degradation of PCE and TCE was observed in both MW-1 and MW-2 following each injection, with the maximum degradation being observed during the first week of the injections. The chlorinated ethenes concentrations gradually returned to their pre-injection levels after ∼2 weeks, indicating exhaustion of the ZVI's reducing power. However, the injection of CMC-stabilized nanoparticle and the abiotic reductive dechlorination process appeared to have boosted a long-term in situ biological dechlorination thereafter, which was evidenced by the fact that PCE and TCE concentrations showed further reduction after two weeks. After 596 days from the first injection, the total chlorinated ethenes concentration decreased by about 40% and 61% in MW-1 and MW-2, respectively. No significant long-term reduction of PCB 1242 was observed in MW-1, but a reduction of 87% was evident in MW-2. During the 596 days of testing, the total concentrations of cis-DCE (dichloroethylene) and VC (vinyl chloride) decreased by 20% and 38% in MW-1 and MW-2, respectively. However, the combined fraction of cis-DCE and VC in the total chlorinated ethenes (PCE, TCE, cis-DCE and VC) increased from 73% to 98% and from 62% to 98%, respectively, which supports the notion that biological dechlorination of PCE and TCE was active. It is proposed that CMC-stabilized ZVI-Pd nanoparticles facilitated the early stage rapid abiotic degradation. Over the long run, the existing biological degradation process was boosted with CMC as the carbon source and hydrogen from the abiotic/biotic processes as the electron donor, resulting in the sustained enhanced destruction of the chlorinated organic chlorinated ethenes in the subsurface.     

Biotransformation of tetrachloroethylene by anaerobic attached-films at low temperatures

This study was conducted to examine the feasibility of using an anaerobic attached-film expanded-bed (AAFEB) process for the treatment of tetrachloroethylene (PCE) at 15°C. A laboratory-scale continuous-flow reactor, with an expanded-bed volume of 900 ml, was operated at hydraulic retention times of 1.8-4 h and influent PCE concentrations of 8–12 mg/l. Small samples (50 ml) of attached film media were used for batch testing 3–7 mg/l of PCE in a separate 300 ml AAFEB reactor. The attached films were a mixed anaerobic consortium grown on diatomaceous earth support particles under methanogenic conditions. Sucrose was used as an external electron donor and growth substrate. Reductive dechlorination of PCE to trichloroethylene (TCE), cis-1,2-dichloroethylene (DCE), vinyl chloride (VC) and ethylene (ETH) was observed. The conversion efficiency of PCE and TCE to lesser chlorinated compounds and ETH was above 98% during continuous-flow testing. VC accumulated as the major dechlorination product and ETH was produced at very low rates. The maximum PCE dechlorination rate, q_max, was 5.33 mg PCE/g volatile solids-day (32.1 μmol/g VS-d) and the one-half velocity coefficient, K_s, was 0.009 mg PCE/l (0.054 μM) under continuous-flow conditions. Since the AAFEB carried more than 20 g volatile solids per liter of bed, low temperature conversion rates would be expected to exceed 60 mg PCE/l_bed-day. This indicates removal efficiencies greater than 99% could be obtained at hydraulic retention times of less than 1 h at ambient groundwater temperatures with this process.     

Assessment of in situ degradation of chlorinated ethenes and bacterial community structure in a complex contaminated groundwater system

The occurrence of in situ degradation of chlorinated ethenes was investigated using an integrated approach in a complex groundwater system consisting of several geological units. The assessment of hydrogeochemistry and chlorinated ethenes distribution using principal component analysis (PCA) in combination with carbon stable isotope analysis revealed that chlorinated ethenes were subjected to substantial biodegradation. Shifts in isotopic values up to 20.4 per thousand, 13.9 per thousand, 20.1 per thousand and 31.4 per thousand were observed between geological units for tetrachloroethene (PCE), trichloroethene (TCE), cis-dichloroethene (cDCE) and vinyl chloride (VC), respectively. The use of specific biomarkers (16S rRNA gene) indicated the presence of Dehalococcoides sp. DNA in 20 of the 33 evaluated samples. In parallel, the analysis of changes in the bacterial community composition in the aquifers using canonical correspondence analysis (CCA) indicated the predominant influence of the chlorinated ethene concentrations (56.3% of the variance, P=0.005). The integrated approach may open new prospects for the assessment of spatial and temporal functioning of bioattenuation in contaminated groundwater systems.     

Electron donor availability for microbial reductive processes following thermal treatment

Thermal treatment is capable of removing significant free-phase chlorinated solvent mass while potentially enhancing bioremediation effectiveness by establishing temperature gradients in the perimeter of the source zone and by increasing electron donor availability. The objectives of this study were to determine the potential for enhanced reductive dechlorination activity at the intermediate temperatures that establish in the perimeter of the heated source zone, and to evaluate the effect of electron donor competition on the performance of the microbial reductive dechlorination process. Microcosms, constructed with tetrachloroethene- (PCE-) and trichloroethene- (TCE-) impacted soils from the Great Lakes, IL, and Ft. Lewis, WA, sites were incubated at temperatures of 24, 35, 50, 70, and 95 °C for 4 months. Reductive dechlorination did not occur in microcosms incubated at temperatures above 24 °C even though mesophilic PCE-to-cis-1,2-dichloroethene dechlorinators were present in Ft. Lewis soil suggesting electron donor limitations. Five days after cooling the microcosms to 24 °C and bioaugmentation with the methanogenic, PCE-to-ethene-dechlorinating consortium OW, at least 85% of the initial PCE and TCE were dechlorinated, but dechlorination ceased prior to complete conversion to ethene. Subsequent biostimulation with hydrogen gas mitigated the dechlorination stall, and conversion to ethene resumed. The results of this study demonstrated that temperatures >35 °C inhibit reductive dechlorination activity at the Great Lakes and Ft. Lewis sites, and that the majority of reducing equivalents released from the soil matrix during heat treatment are consumed in methanogenesis rather than reductive dechlorination. These observations suggest that bioaugmenting thermal treatment sites with cultures that do not contain methanogens may allow practitioners to realize enhanced dechlorination activity, a potential benefit of coupling thermal treatment with bioremediation.     

A two and half-year-performance evaluation of a field test on treatment of source zone tetrachloroethene and its chlorinated daughter products using emulsified zero valent iron nanoparticles

A field test of emulsified zero valent iron (EZVI) nanoparticles was conducted at Parris Island, SC, USA and was monitored for two and half years to assess the treatment of subsurface-source zone chlorinated volatile organic compounds (CVOCs) dominated by tetrachloroethene (PCE) and its chlorinated daughter products. Two EZVI delivery methods were used: pneumatic injection and direct injection. In the pneumatic injection plot, 2180 L of EZVI containing 225 kg of iron (Toda RNIP-10DS), 856 kg of corn oil, and 22.5 kg of surfactant were injected to remedy an estimated 38 kg of CVOCs. In the direct injection plot, 572 L of EZVI were injected to treat an estimated 0.155 kg of CVOCs. After injection of the EZVI, significant reductions in PCE and trichloroethene (TCE) concentrations were observed in downgradient wells with corresponding increases in degradation products including significant increases in ethene. In the pneumatic injection plot, there were significant reductions in the downgradient groundwater mass flux values for PCE (>85%) and TCE (>85%) and a significant increase in the mass flux of ethene. There were significant reductions in total CVOC mass (86%); an estimated reduction of 63% in the sorbed and dissolved phases and 93% reduction in the PCE DNAPL mass. There are uncertainties in these estimates because DNAPL may have been mobilized during and after injection. Following injection, significant increases in dissolved sulfide, volatile fatty acids (VFA), and total organic carbon (TOC) were observed. In contrast, dissolved sulfate and pH decreased in many wells. The apparent effective remediation seems to have been accomplished by direct abiotic dechlorination by nanoiron followed by biological reductive dechlorination stimulated by the corn oil in the emulsion.     

Natural Attenuation-Untersuchungen an einem mit LCKW kontaminierten Altdeponiestandort

The Natural Attenuation of chlorinated hydrocarbons leaking from an abandoned landfill was the focus of an investigation conducted within the framework of the Bavarian joint research project “Nachhaltige Altlastenbewältigung unter Einbeziehung des natürlichen Reinigungsvermögens”. The research focused on two contaminant plumes, one having a diffuse TCE/PCE source and the other a cis- DCE source. The TCE/PCE plume had a maximum concentration of 52 μg/l total chlorinated hydrocarbons near the source area. Cis-DCE concentrations near the source of the second plume were up to 7,000 μg/l. In addition, degradation products (VC and Ethylene) were detected in this region, which indicate reductive dechlorination of the cis-DCE, TCE or PCE pollutants. However, other investigations indicate that the natural attenuation potential of the site is low and that the reduction in concentration is dependant more on dilution and other unknown processes. Stemming from this research, a new approach was developed using Solid Phase Micro Extraction (SPME) and GC/FID analytical techniques for microcosm testing. Furthermore, an airtight column was developed that allows the detection of sorption of highly volatile substances in low concentrations within an aquifer material.     

Effects of sulfate on anaerobic chloroethene degradation by an enriched culture under transient and steady-state hydrogen supply

Complete anaerobic dechlorination of chlorinated solvents such as trichloroethene (TCE) is essential for bioremediation of chloroethene-contaminated sites. We studied the influence of sulfate on microbial dechlorination of TCE to ethene both under transient and steady-state conditions, encompassing the range of hydrogen (H2) levels commonly found at contaminated sites. The results show that sulfate at a concentration of 2.5 mM limits microbial dechlorination by a mixed anaerobic culture by reducing the rate under steady-state hydrogen supply (a few nM H2), implying a H2 limited dechlorination. Conversely, sulfate did not affect dechlorination when rapid fermentation of lactate resulted in transient buildup of H2 to levels around two orders of magnitude higher compared to steady-state conditions. This has important implications both for optimizing culture conditions for dehalogenating microorganisms and for the efficiency of cleanup strategies. Our findings may contribute to the understanding and bioremediation of chloroethene contaminated environments containing sulfate.     

Comparison of pulsed and continuous addition of H2 gas via membranes for stimulating PCE biodegradation in soil columns

Column experiments were performed to investigate a technology for remediating aquifers contaminated with chlorinated solvents. The technology involves installation of hollow-fiber membranes in the subsurface to supply hydrogen gas (H2) to groundwater to support biological reductive dechlorination in situ. Three laboratory-scale columns [control (N2 only), continuous H2, and pulsed H2] were packed with aquifer material from a trichloroethene (TCE)-contaminated wetland in Minnesota and supplied with perchloroethene (PCE)-contaminated synthetic groundwater. The main goals of the research were: (1) evaluate the long-term performance of the H2 supply system and (2) compare the effects of pulsed (4 h on, 20 h off) versus continuous H2 supply (lumen partial pressure approximately 1.2 atm) on PCE dechlorination and production of by-products (i.e. methane and acetate). The silicone-coated fiberglass membranes employed in these experiments were robust, delivering H2 steadily over the entire 349-day experiment. Methane production decreased when H2 was added in a pulsed manner. Nevertheless, the percentage of added H2 used to support methanogenesis was similar in both H2-fed columns (92-93%). For much of the experiment, PCE dechlorination (observed end product = dichloroethene) in the continuous and pulsed H2 columns was comparable, and enhanced in comparison to the natural attenuation observed in the control column. Dechlorination began to decline in the pulsed H2 column after 210 days, however, while dechlorination in the continuous H2 column was sustained. Acetate was detected only in the continuous H2 column, at concentrations of up to 36 microM. The results of this research suggest that in situ stimulation of PCE dechlorination by direct H2 addition requires the continuous application of H2 at high partial pressures, favoring the production of bioavailable organic matter such as acetate to provide a carbon source, electron donor, or both for dechlorinators. Unfortunately, this strategy has proven to be inefficient, with the bulk of the added H2 used to support methanogenesis.     

Evaluation of polyethylene hollow-fiber membranes for hydrogen delivery to support reductive dechlorination in a soil column

Engineered systems are often needed to supply an electron donor, such as hydrogen (H(2)), to the subsurface to stimulate the biological dehalogenation of perchloroethene (PCE) to ethene. A column study was performed to evaluate the ability of gas permeable hollow-fiber membranes to supply H(2) directly to PCE-contaminated groundwater to facilitate bioremediation. Two glass columns were packed with soil obtained from a trichloroethene-contaminated site at Cape Canaveral, Florida, and were fed a minimal medium spiked with PCE (7 microM) for 391 days. The columns were operated in parallel, with one column receiving H(2) via polyethylene hollow-fiber membranes (lumen H(2) pressure of approximately 1atm) and a control column receiving no H(2). PCE was initially dechlorinated at a similar rate and to a similar extent in both columns, likely due to the presence of soil organic matter that was able to support dechlorination. After 265 days of operation, dechlorination performance declined in the control column and the benefits of membrane-supplied H(2) became evident. Although the membrane-supplied H(2) effectively stimulated PCE dechlorination at the end of the experiment (days 359-391), the system was inefficient in that only 5% of the supplied H(2) was used for dechlorination. Most of the remainder was used to support methanogenesis (94%). Despite the dominance of methanogens, nearly complete dechlorination of PCE to ethene was observed in the H(2)-fed column. In addition to the inefficient use of H(2), operational problems included excessive foulant accumulation on the outside of the membrane fibers and water condensation inside the fibers. Use of alternative membrane materials and changes to the operating approach (e.g. pulsing or supplying H(2) at low partial pressures) may help to overcome these problems so that this technology can provide effective and stable remediation of aquifers contaminated with chlorinated ethenes.     

Chitin and corncobs as electron donor sources for the reductive dechlorination of tetrachloroethene

Chitin, corncobs, and a mixture of chitin and corncobs were tested as potential electron donor sources for stimulating the reductive dechlorination of tetrachloroethene (PCE). Semi-batch, sand-packed columns were used to evaluate the donors with aerobic and anaerobic groundwaters containing varying degrees of alkalinity. In all experiments, acetate and butyrate were the dominant fatty acids produced, although propionate, valerate, formate, and succinate were also detected. From a multivariable regression analysis on the data, the presence of chitin, limestone, and dechlorinating culture inoculum were determined to be the most positive predictors of dechlorination activity. Chitin fermentation products supported the degradation of PCE to trichloroethene (TCE), cis-1,2-dichloroethene (DCE), and vinyl chloride (VC), even in columns containing PCE DNAPL, whereas dechlorination activity was not observed in any of the columns containing corncobs alone. The longevity and efficiency of chitin as an electron donor source demonstrates its potential usefulness for passive, in situ field applications.     

Optimization of electrochemical dechlorination of trichloroethylene in reducing electrolytes

Electrochemical dechlorination of trichloroethylene (TCE) in aqueous solution is investigated in a closed, liquid-recirculation system. The anodic reaction of cast iron generates ferrous species, creating a chemically reducing electrolyte (negative ORP value). The reduction of TCE on the cathode surface is enhanced under this reducing electrolyte because of the absence of electron competition. In the presence of the iron anode, the performances of different cathodes are compared in a recirculated electrolysis system. The copper foam shows superior capability for dechlorination of aqueous TCE. Electrolysis by cast iron anode and copper foam cathode is further optimized though a multivariable experimental design and analysis. The conductivity of the electrolyte is identified as an important factor for both final elimination efficiency (FEE) of TCE and specific energy consumption. The copper foam electrode exhibits high TCE elimination efficiency in a wide range of initial TCE concentration. Under coulostatic conditions, the optimal conditions to achieve the highest FEE are 9.525 mm thick copper foam electrode, 40 mA current and 0.042 mol L⁻¹ Na₂SO₄. This novel electrolysis system is proposed to remediate groundwater contaminated by chlorinated organic solvents, or as an improved iron electrocoagulation process capable of treating the wastewater co-contaminated with chlorinated compounds.     

Identifizierung und Quantifizierung natürlicher Abbauprozesse in einem mit Chlorethenen kontaminierten Grundwasserleiter

This article discusses the results of an investigation into natural attenuation processes at a former landfill which is leaching aliphatic hydrocarbons and chlorinated ethenes. The high degree of metabolisation of the chlorinated ethenes downstream of the landfill indicates the occurrence of reductive dechlorination, which was quantified by geochemical investigations and reactive transport modelling. In the vicinity of the deposit, the contaminated Buntsandsteinaquifer is divided into at least two subaquifers, the upper one of which is the most highly contaminated. Directly downstream of the landfill, the redox-environment is highly reduced, allowing reductive degradation processes to continue. The high transformation rates in this area lead to a conversion of more than 90% of the Perchloroethene (PCE) and Trichloroethene (TCE) leached from the landfill and leaves Cis-Dichloroethene (Cis-DCE) as the main contaminant found in the subsurface. Further downstream, the redox-environment becomes increasingly oxidised where (cometabolic) aerobic degradation processes will probably dominate, which will especially degrade the less chlorinated ethenes. Over the short term, however, the continuous delivery of PCE and TCE from the landfill will prevent the retreat of the contamination in the subsurface by natural attenuation alone. Nevertheless, in combination with source control measures, natural attenuation could be an alternative strategy for plume management.     

Temporal variations in natural attenuation of chlorinated aliphatic hydrocarbons in eutrophic river sediments impacted by a contaminated groundwater plume

Chlorinated aliphatic hydrocarbons (CAHs) often discharge into rivers as contaminated groundwater baseflow. Biotransformation, sorption and dilution of CAHs in the impacted river sediments have been reported to reduce discharge, but the effect of temporal variations in environmental conditions on the occurrence and extent of those processes in river sediments is largely unknown. We monitored the reduction of CAH discharge into the Zenne River during a 21-month period. Despite a relatively stable influx of CAHs from the groundwater, the total reduction in CAH discharge from 120 to 20 cm depth in the river sediments, on average 74 ± 21%, showed moderate to large temporal variations, depending on the riverbed location. High organic carbon and anaerobic conditions in the river sediments allowed microbial reductive dechlorination of both chlorinated ethenes and chlorinated ethanes. δ¹³C values of the CAHs showed that this biotransformation was remarkably stable over time, despite fluctuating pore water temperatures. Daughter products of the CAHs, however, were not detected in stoichiometric amounts and suggested the co-occurrence of a physical process reducing the concentrations of CAHs in the riverbed. This process was the main process causing temporal variations in natural attenuation of the CAHs and was most likely dilution by surface water-mixing. However, higher spatial resolution monitoring of flow transients in the riverbed is required to prove dilution contributions due to dynamic surface water-groundwater flow exchanges. δ¹³C values and a site-specific isotope enrichment factor for reductive dechlorination of the main groundwater pollutant vinyl chloride (VC) allowed assessment of changes over time in the extent of both biotransformation and dilution of VC for different scenarios in which those processes either occurred consecutively or simultaneously between 120 and 20 cm depth in the riverbed. The extent of reductive dechlorination of VC ranged from 27 to 89% and differed spatially but was remarkably stable over time, whereas the extent of VC reduction by dilution ranged from 6 to 94%, showed large temporal variations, and was often the main process contributing to the reduction of VC discharge into the river.     

Contamination of vinyl chloride in shallow urban rivers in Osaka, Japan

Vinyl chloride (VC) contamination had taken place in heavily polluted shallow rivers (Taishogawa and lower Hiranogawa Rivers) in Osaka, Japan. VC concentrations ranged from below detection limit to 55.6 micrograms l-1 (mean: 3.35 micrograms l-1, standard deviation: 5.96 micrograms l-1). Of 55 volatile organic compounds (VOCs) analyzed, concentrations of cis-1,2-dichloroethene (c-DCE), tetrachloroethene (PCE) and trichloroethene (TCE) were significantly correlated to VC concentrations in the rivers, indicating that they share common sources. The four VOCs were invariably present at approximate relative ratios of about 1:2.7:1.5:0.31 (VC: c-DCE: PCE: TCE). The similarity between sampling dates in the distribution pattern of the four VOCs concentrations were observed, but their concentrations were different between the dates. The concentrations of the four VOCs decreased with distance down the river. A sample from the upper Taishogawa River in July 1997 had 55.6 micrograms l-1 of VC, 152 micrograms l-1 of c-DCE, 86.2 micrograms l-1 of PCE and 18.4 micrograms l-1 of TCE, respectively. These values are about an order of magnitude higher than the other sites over the study period and are likely indicative of point source inputs.     

Relationship between Dehalococcoides DNA in ground water and rates of reductive dechlorination at field scale

Certain strains of Dehalococcoides bacteria can dechlorinate chlorinated ethylenes to harmless products. This study was conducted to determine if there is a valid association between the density of Dehalococcoides DNA in ground water and the observed rates of reductive dechlorination at field scale. Dehalococcoides DNA in water from monitoring wells was determined using the quantitative real time polymerase chain reaction (q-PCR) with DNA primer set specific for Dehalococcoides organisms. Dechlorination rate constants were extracted from field data using the BIOCHLOR software. Of the six conventional plumes surveyed, "generally useful" rates of dechlorination (greater than or equal to 0.3 per year) of cis-dichloroethylene (cis-DCE) and vinyl chloride (VC) along the flow path were observed at three sites where Dehalococcoides DNA was detected, and little attenuation of cis-DCE and VC occurred at two sites where Dehalococcoides DNA was not detected. At the two sites where there was no net direction of ground water flow, the relationship between the density of Dehalococcoides DNA in ground water and the trend in concentrations of chlorinated ethylenes over time in monitoring wells was not so consistent as that observed for the conventional plumes. A comparison of our study to a field study performed by Lendvay and his coworker indicated that monitoring wells did not efficiently sample the Dehalococcoides organisms in the aquifer.     

Risk assessment of exposure to volatile organic compounds in groundwater in Taiwan

The purpose of this study is to assess the risks from exposure to 14 volatile organic compounds (VOCs) in selected groundwater sites in Taiwan. The study employs the multimedia environment pollutant assessment system (MEPAS) model to calculate the specific non-cancer and cancer risks at an exposure level of 1 μg/L of each VOC for a variety of exposure pathways. The results show that the highest specific non-cancer risk is associated with water ingestion of vinyl chloride (VC) and that the highest specific cancer risk is associated with indoor breathing of VC. The three most important exposure pathways for risk assessment for both non-cancer and cancer risks are identified as water ingestion, dermal absorption when showering, and indoor breathing. Excess tetrachloroethylene (PCE), trichloroethylene (TCE), dichloroethylene (DCE), and VC are detected in the groundwater aquifers of one dump site and one factory. However, the study suggests that the pollutants in the contaminated groundwater aquifers do not travel extensively with groundwater flow and that the resulting VOC concentrations are below detectable levels for most of the sampled drinking-water treatment plants. Nevertheless, the non-cancer and cancer risks resulting from use of the contaminated groundwater are found to be hundred times higher than the general risk guidance values. To ensure safe groundwater utilisation, remediation initiatives for soil and groundwater are required. Finally, the study suggests that the current criteria for VOCs in drinking water might not be capable of ensuring public safety when groundwater is used as the primary water supply; more stringent quality criteria for drinking water are proposed for selected VOCs.