Evaluating the fate of chlorinated ethenes in streambed sediments by combining stable isotope, geochemical and microbial methods

Abstract

a b s t r a c tThe occurrence of chlorinated ethene transformation in a streambed was investigated using concentration and carbon isotope data from water samples taken at different locations and depths within a 15 × 25 m study area across which a tetrachloroethene (PCE) plume discharges. Furthermore, it was evaluated how the degree of transformation is related to groundwater discharge rates, redox conditions, solid organic matter content (SOM) and microbial factors. Groundwater discharge rates were quantified based on streambed temperatures, and redox conditions using concentrations of dissolved redox-sensitive species. The degree of chlorinated ethene transformation was highly variable in space from no transformation to transformation beyond ethene. Complete reductive dechlorination to ethane and ethene occurred at locations with at least sulfate-reducing conditions and with a residence time in the samples streambed zone (80 cm depth) of at least 10 days. Among these locations, Dehalococcoides was detected using a PCR method where SOM contents were N 2% w/w and where transformation proceeded beyond ethene. However, it was not detected at locations with low SOM, which may cause an insufficient H 2 supply to sustain a detectably dense Dehalococcoides population. Additionally, it is possible that other organisms are responsible for the biodegradation. A microcosm study with streambed sediments demonstrated the potential of VC oxidation throughout the site even at locations without a pre-exposure to VC, consistent with the detection of the epoxyalkane:coenzyme M transferase (EaCoMT) gene involved in the degradation of chlorinated ethenes via epoxidation. In contrast, no aerobic transformation of cDCE in microcosms over a period of 1.5 years was observed. In summary, the study demonstrated that carbon isotope analysis is a sensitive tool to identify the degree of chlorinated ethene transformation even in hydrologically and geochemically complex streambed systems. In addition, it was observed that the degree of transformation is related to redox conditions, which in turn depend on groundwater discharge rates.

Heterogeneous hyporheic zone dechlorination of a TCE groundwater plume discharging to an urban river reach

“…This may be attributed to the limited availability or activity of specific dechlorinating bacteria such as Dehalococcoides, inadequate reducing conditions and insufficient electron donors (organic matter) or competing degradation processes (Bradley, 2000;Freedman and Gossett, 1989;Maymo-Gatell et al, 1995;Maymo-Gatell and Anguish, 1999). The lesser chlorinated hydrocarbons (LCHCs) may be susceptible to alternative anaerobic (Bradley et al, 1998) or aerobic (Abe et al, 2009) oxidation pathways. VC in particular may undergo aerobic oxidation, even at very low oxygen concentrations that may have been judged in error to be anaerobic (Gossett, 2010).…”

“…VC in particular may undergo aerobic oxidation, even at very low oxygen concentrations that may have been judged in error to be anaerobic (Gossett, 2010). Assessing the importance of mixing of aerobic surface water in the hyporheic zone may hence be important, but proving aerobic biodegradation of LCHCs at the field scale remains technically challenging (Abe et al, 2009;Cox et al, 2010). From a dechlorination perspective, hyporheic zone mixing of a CHC plume discharge with oxygenated surface water may be counterproductive to that reaction as anaerobic conditions may no longer prevail, at least for a period of time.…”

“…CHC plume studies include: detailed evaluation of hydrogeological and biogeochemical controls on the Pine River, Canada (Conant et al, 2004;Abe et al, 2009); assessment of biodegradation potential on the Zenne River, Belgium including the use of advanced compound-specific isotope and microbiological tools (Hamonts et al, , 2012Kuhn et al, 2009); attenuation in lower flow, higher carbon, wetland environments within the US (Lorah et al, 2005); and, the assessment of plume discharge and loss to atmosphere from small receiving streams upstream of a main river receptor in the US (Chapman et al, 2007). The challenge of monitoring scales and heterogeneity in plume discharges are also prominent in most of the above studies as well as the UK studies of Weatherill et al (2014) who examine the use of nested monitoring on the River Tern, and who assess city-scale impacts on the River Tame.…”

Stable carbon isotope analysis to distinguish biotic and abiotic degradation of 1,1,1-trichloroethane in groundwater sediments

“…Carbon isotope analysis has been used to demonstrate reductive dechlorination of chlorinated ethenes in groundwater (Hunkeler et al, 1999(Hunkeler et al, , 2011Sherwood Lollar et al, 2001;Imfeld et al, 2008) and streambed sediments (Abe et al, 2009). Sherwood Lollar et al (2010 studied the isotope fractionation during biodegradation of 1,1,1-TCA and 1,1-DCA by a dehalobacter-containing mixed culture in whole-cell and cell-free extract microcosms.…”

An integrated model for assessing the risk of TCE groundwater contamination to human receptors and surface water ecosystems

“…Water chemistry and redox parameters were measured in May-June 2005 using both multilevel samplers and seepage meters for the influent groundwater, as well as directly from the surface water (Table 1). Based on these results, the hyporheic zone could be characterized as being slightly aerobic and/or nitrate-reducing, thus preventing significant anaerobic dechlorination (Scheutz et al, 2008;Abe et al, 2009). No apparent increase in 1,2-cis-DCE was observed in the groundwater to the west of the stream (Figs.…”