Past waste disposal practices at a California coastal site resulted in a groundwater plume containing CVOCs (mainly chlorinated ethenes [CEs] and 1,1-dichloroethane) migrating offshore towards a marine harbor. CVOCs are present in the sediment porewater adjacent to the shoreline, but concentrations decreased significantly between 2000 and 2015, and NA in the marine sediments was suspected. To confirm NA, SiREM performed a laboratory treatability study for Geosyntec Consultants Inc. to evaluate biotic and abiotic transformation mechanisms for CVOCs at this critical seawater interface.
Treatability Study Design and NA Parameter Analysis
Sediment pore water samples were collected at depths of 1, 5 and 8 feet for use in sterile control and non-amended (i.e., NA) microcosms that were incubated in an anaerobic chamber for 9 months. Initial CVOC concentrations in the microcosms were approximately 2,000 parts per billion (ppb). The microcosms were regularly sampled for CVOCs, dissolved hydrocarbon gases (including ethene), acetylene, pH, volatile fatty acids (VFAs) and inorganic anions to assess and quantify potential NA pathways. Microbial characterization included Dehalococcoides (Dhc) and vinyl chloride reductase (vcrA) (Gene-Trac® Dhc/vcrA) analyses, and next generation sequencing (Gene-Trac® NGS) for comprehensive microbial community characterization. To elucidate biogeochemical processes by iron minerals microcosm sediments were characterized using magnetic susceptibility, acid volatile sulfides (AVS), QEMSCAN, SEM, X-ray diffraction, total sulfur, total metals and total organic carbon.
Confirmation of Significant Dechlorination by NA Processes
The results of the study were intriguing, complete anaerobic, intrinsic dechlorination of CEs was observed in microcosms from across the site with sulfate concentrations exceeding 2,000 milligrams per liter (mg/L). Furthermore, CE degradation half-lives were relatively rapid and ranged from 4-10 days in the four most active depth-locations. Despite robust dechlorination, the classic TCE-cDCE-VC-ethene sequence was only observed in one location (Figure 1A). Furthermore, this was also the sole study location where Dhc/vcrA was detected at an abundance where ethene detection would be expected. In microcosms from other locations, despite the clear losses of CE mass, Dhc/vcrA were not significantly detected and ethene was not quantified Without Dhc and ethene detection, reductive dechlorination seemed an unlikely degradation pathway, if so, what was removing the CE mass? This led us to investigate potential abiotic indicators using the SiREMNA™ suite of analyses.
Even in the absence of amendments, NA degradation half-lives for chlorinated ethenes were rapid, ranging from 4-10 days in many microcosms.
Figure 1 A) location demonstrated NA losses of TCE and DCE isomers with no accumulation of ethene in the absence of significant detection of vcrA genes. In contrast, ethene and vcrA were detected at the B) location suggesting reductive dechlorination by Dhc as the predominant NA mechanism. Note the decline in ethene over time indicating ethene losses possibly due to degradation.
Were Iron Minerals Responsible for the Observed Dechlorination?
Given that the classic signs of reductive dechlorination were absent in many microcosms, possible biogeochemical or abiotic pathways were examined. Magnetic susceptibility analysis performed on microcosm sediment indicated that magnetite was present in the range of 7.0E-08 to 2.2E-07 m3/kg. Using data from (ESTCP, 2015), this amount of magnetite would yield half-lives of several months versus the half-lives of days observed in the microcosms. Reactive minerals – iron sulfide and biotite were also found in the sediment material collected from microcosms at concentrations ranging from 0.1 to 0.2%wt for iron sulfides (mainly FeS2) and from 2.2 to 4.8%wt for biotite. Based on the published reactivity rates for these minerals, this material had the potential to degrade the chlorinated ethenes abiotically. However, as with the magnetite, anticipated abiotic degradation rates were too slow to account for the rapid losses observed. So, if iron minerals were not doing the heavy lifting, what could account for the rapid CE declines observed in these microcosms?
Reexamining the Microbiology
Given that the reactive iron minerals were unlikely to account for the rapid degradation observed, reductive dechlorination reemerged as a possible degradation mechanism. While Dhc were not abundant, could other dechlorinators be playing a role? Members of the genus Dehalogenimonas (Dhg) were recently reported to dechlorinate VC to ethene (Yang et al., 2016). Moreover, Gene-Trac® NGS data indicated the presence of a putative Dhg in proportions as high as 30% of the population in some locations. Perhaps dechlorination was being performed by a Dhg flying under the radar. How then to explain the missing ethene if it is being produced by reductive dechlorination? An answer may lie in findings that sulfate reducers can use ethene as an electron donor and sulfate as electron acceptor (Fullerton et al., 2013). The NGS data indicated a high abundance of sulfate reducers – combined with the lack of observed ethene this would be consistent with the activity of ethene oxidizing sulfate reducers.
Figure 2: The destruction of ethene under sulfate reducing conditions has been reported. Evidence for this at the Site includes high sulfate concentrations, high abundance of sulfate reducers and the absence of ethene. Diagram from Fullerton et al., 2013.
The combination of a high abundance dechlorination community dominated in some locations by Dhg and the degradation of ethene by sulfate reducers, possibly supplemented by slower abiotic degradation by iron minerals, provides a tangible explanation to square the relatively rapid dechlorination observed in microcosms in the absence of Dhc and ethene accumulation.
“SiREM’s expert application of next generation sequencing and mineralogy speciation revealed a plausible explanation for a vexing mystery that challenged the project team. SIREM’s work product has significantly strengthened our client’s arguments toward winning regulatory approval of a natural attenuation remedy at the site.”
Neal Durant, PhD, Senior Principal, Geosyntec Consultants, Washington, DC
This treatability study provided valuable information contributing to a better understanding of the significant rates of NA observed in the coastal sediment. The detailed analysis performed also provided an explanation for the range of degradation mechanisms and different microorganisms functioning at the site, thereby providing a more robust and nuanced understanding of dechlorination processes at this important site.