Modeling a well-characterized perfluorooctane sulfonate (PFOS) source and plume using the REMChlor-MD model to account for matrix diffusion.

Two of the most important retention processes for per- and polyfluoroalkyl substances (PFAS) in groundwater likely are sorption and matrix diffusion. The objective of this study was to model concentration and mass discharge of one PFAS, perfluorooctane sulfonate (PFOS), with matrix diffusion processes incorporated using data from a highly chemically- and geologically-characterized site. When matrix diffusion is incorporated into the REMChlor-MD model for PFOS at this research site, it easily reproduces the field data for three key metrics (concentration, mass discharge, and total mass). However, the no-matrix diffusion model produced a much poorer match. Additionally, after about 40 years of groundwater transport, field data and the REMChlor-MD model both showed the majority (80%) of the measured PFOS mass that exited the source zones was located in downgradient low permeability zones due to matrix diffusion. As such, most of the PFOS mass is not available to immediately migrate downgradient via advection in the more permeable sands at this site, which has important implications for monitored natural attenuation (MNA). Plume expansion over the next 50 years is forecasted to be limited, from a 350-m plume length in 2017 to 550 m in 2070, as matrix diffusion will attenuate groundwater plumes by slowing their expansion. This phenomenon is important for constituents that do not degrade, such as PFOS, compared to those susceptible to degradation. Overall, this work shows that matrix diffusion is a relevant process in environmental PFAS persistence and slows the rate of plume expansion over time.

Evaluation of natural attenuation of 1,4-dioxane in groundwater using a 14C assay.

Monitored Natural Attenuation (MNA) is a preferred remedy for sites contaminated with 1,4-dioxane due to its low cost and limited environmental impacts compared to active remediation. Having a robust estimate of the rate at which biodegradation occurs is an essential component of assessing MNA. In this study, an assay was developed using 14C-labeled 1,4-dioxane to measure rate constants for biodegradation based on accumulation of 14C products. Purification of the 14C-1,4-dioxane stock solution lowered the level of 14C impurities to below 1% of the total 14C activity. This enabled determination of rate constants in groundwater as low as 0.0021 yr-1, equating to a half-life greater than 300 years. Of the 54 groundwater samples collected from 10 sites in the US, statistically significant rate constants were determined with the 14C assay for 24. The median rate constant was 0.0138 yr-1 (half-life = 50 yr); the maximum rate constant was 0.367 yr-1 (half-life = 1.9 yr). The results confirmed that biodegradation of 1,4-dioxane is occurring at 9 of the 10 sites sampled, albeit with considerable variability in the level of activity. The specificity of the assay was confirmed using acetylene and the absence of oxygen to inhibit monooxygenases.

Establishing the prevalence and relative rates of 1,4-dioxane biodegradation in groundwater to improve remedy evaluations.

Options for remediating 1,4-dioxane at groundwater sites are limited due to the physical-chemical properties of this compound. The relevance of natural attenuation processes for 1,4-dioxane was investigated through data from field, lab, and modeling efforts. The objectives were to use multiple lines of evidence for 1,4-dioxane biodegradation to understand the prevalence of this activity and evaluate convergence between lines of evidence. A 14C-1,4-dioxane assay confirmed 1,4-dioxane biodegradation at 9 of 10 sites (median rate constant of 0.0105 yr-1 across wells). Site-wide rate constants were established using a calibrated fate and transport model at 8 sites (median = 0.075 yr-1). The 14C assay constants are likely more conservative, and variability in rates suggested that biodegradation at sites may be localized. Stable isotope fractionation was observed at 7 of 10 sites and served as another direct line of evidence of in situ biodegradation of 1,4-dioxane. This includes sites where indirect lines of evidence, including geochemical conditions or genetic biomarkers for degradation, would not necessarily have been supportive. This highlights the importance of collecting multiple lines of evidence to document 1,4-dioxane natural attenuation, and the widespread prevalence of biodegradation suggests that this process should be part of long-term management decisions.

Perfluoroalkyl acids on suspended particles: Significant transport pathways in surface runoff, surface waters, and subsurface soils.

Eroded particles from the source zone could transport a high concentration of perfluoroalkyl acids (PFAAs) to sediments and water bodies. Yet, the contribution of suspended particles has not been systematically reviewed. Analyzing reported studies, we quantitatively demonstrate that suspended particles in surface water can contain significantly higher concentrations of PFAAs than the sediment below, indicating the source of suspended particles are not the sediment but particles eroded and carried from the source zone upstream. The affinity of PFAAs to particles depends on the particle composition, including organic carbon fraction and iron or aluminum oxide content. In soils, most PFAAs are retained within the top 5 m below the ground surface. The distribution of PFAAs in the subsurface varies based on site properties and local weather conditions. The depth corresponding to the maximum concentration of PFAA in soil decreases with an increase in soil organic carbon or rainfall amount received in the catchment areas. We attribute a greater accumulation of PFAAs near the upper layer of the subsurface to an increase in the accumulation of particles eroded from source zones upstream receiving heavy rainfall. Precursor transformation in the aerobic zone is significantly higher than in the anaerobic zone, thereby making the aerobic subsurface zone serve as a long-term source of groundwater pollution. Collectively, these results suggest that suspended particles, often an overlooked vector for PFAAs, can be a dominant pathway for the transport of PFAAs in environments.

Spatial Trends of Anionic, Zwitterionic, and Cationic PFASs at an AFFF-Impacted Site.

Soil and groundwater from an aqueous film-forming foam (AFFF)-impacted site were sampled at high resolution (n = 105 for soil, n = 58 for groundwater) and analyzed for an extensive list of anionic, zwitterionic, and cationic poly- and perfluoroalkyl substances (PFASs). Spatial trends for perfluoroalkyl acids and many precursors enabled a better understanding of PFAS composition, transport, and transformation. All PFASs without analytical standards were semi-quantified. Summed PFAS and individual PFAS concentrations were often higher at depth than near the surface in soil and groundwater. Zwitterionic and cationic compounds composed a majority of the total PFAS mass (up to 97%) in firefighter training area (FTA) soil. Composition of PFAS class, chain length, and structural isomers changed with depth and distance from the FTA, suggesting in situ transformation and differential transport. The percentage of branched perfluorooctane sulfonate increased with depth, consistent with differential isomeric transport. However, linear perfluorooctanoic acid (PFOA) was enriched, suggesting fluorotelomer precursor transformation to linear PFOA. Perfluorohexane sulfonamide, a potential transformation product of sulfonamide-based PFASs, was present at high concentrations (maximum 448 ng/g in soil, 3.4 mg/L in groundwater). Precursor compounds may create long-term sources of perfluoroalkyl acids, although many pathways remain unknown; precursor analysis is critical for PFAS fate and transport understanding.

Mass-Based, Field-Scale Demonstration of PFAS Retention within AFFF-Associated Source Areas.

Transport of poly- and perfluoroalkyl substances (PFAS) at aqueous film-forming foam (AFFF)-impacted sites is limited by various processes that can retain PFAS mass within the source area. This study used concentration data obtained via a high-resolution sampling and analytical protocol to estimate the PFAS mass distribution in source and downgradient areas of a former firefighter training area. The total PFAS mass present at the site was approximately 222 kg, with 106 kg as perfluoroalkyl acids (PFAAs) and 116 kg as polyfluorinated precursors. Zwitterionic and cationic PFAS represented 83% of the total precursor mass and were found primarily in the source and up/side-gradient areas (75%), likely due to preferential hydrophobic partitioning, electrostatic interactions, and diffusion into lower-permeability soils. Based on the release history and the high percentage of total PFAS mass represented by precursors (primarily electrochemical fluorination-derived compounds), the estimated conversion rate of precursors to PFAAs was less than 2% annually. Eighty-two percent of the total PFAS mass was encountered in lower-permeability soils, which limited the potential for advection and transformation. This contributed to a 99% decrease in the mass discharge rate at the far-downgradient plume (0.048 kg/yr compared to the near-source area (3.6 kg/yr)). The results provide field-scale evidence of the importance of these PFAS retention processes at sites where AFFF has been released.

Enhanced Extraction of AFFF-Associated PFASs from Source Zone Soils.

Poly- and perfluoroalkyl substances (PFASs) derived from aqueous film-forming foam (AFFF) are increasingly recognized as groundwater contaminants, though the composition and distribution of AFFF-derived PFASs associated with soils and subsurface sediments remain largely unknown. This is particularly true for zwitterionic and cationic PFASs, which may be incompletely extracted from subsurface solids by analytical methods developed for anionic PFASs. Therefore, a method involving sequential basic and acidic methanol extractions was developed and evaluated for recovery of anionic, cationic, and zwitterionic PFASs from field-collected, AFFF-impacted soils. The method was validated by spike-recovery experiments with equilibrated soil-water-AFFF and analytical standards. To determine the relative importance of PFASs lacking commercially available analytical standards, their concentrations were estimated by a novel semiquantitation approach. Total PFAS concentrations determined by semiquantitation were compared with concentrations determined by the total oxidizable precursor assay. Finally, the described method was applied to two soil cores from former fire-training areas in which cations and zwitterions were found to contribute up to 97% of the total PFAS mass. This result demonstrates the need for extraction and analysis methods, such as the ones presented here, that are capable of quantifying cationic and zwitterionic PFASs in AFFF-impacted source zone soils.

Effectiveness of metal oxide catalysts for the degradation of 1,4-dioxane.

1,4-dioxane, commonly used as a solvent stabilizer and industrial solvent, is an environmental contaminant and probable carcinogen. In this study, we explored the concept of using metal oxides to activate H2O2 catalytically at neutral pH in the dark for 1,4-dioxane degradation. Based on batch kinetics measurements, materials that displayed the most suitable characteristics (high 1,4-dioxane degradation activity and high H2O2 consumption efficiency) were ZrO2, WO x /ZrO2, and CuO. In contrast, materials like TiO2, WO3, and aluminosilicate zeolite Y exhibited both low 1,4-dioxane degradation and H2O2 consumption activities. Other materials (e.g., Fe2O3 and CeO2) consumed H2O2 rapidly, however 1,4-dioxane degradation was negligible. The supported metal oxide WO x /ZrO2 was the most active for 1,4-dioxane degradation and had higher H2O2 consumption efficiency compared to ZrO2. In situ acetonitrile poisoning and FTIR spectroscopy results indicate different surface acid sites for 1,4-dioxane and H2O2 adsorption and reaction. Electron paramagnetic resonance measurements indicate that H2O2 forms hydroxyl radicals (˙OH) in the presence of CuO, and unusually, forms superoxide/peroxyl radicals (˙O2 -) in the presence of WO x /ZrO2. The identified material properties suggest metal oxides/H2O2 as a potential advanced oxidation process in the treatment of 1,4-dioxane and other recalcitrant organic compounds.

Evaluation of a national data set for insights into sources, composition, and concentrations of per- and polyfluoroalkyl substances (PFASs) in U.S. drinking water.

The United States Environmental Protection Agency (USEPA) completed nationwide screening of six perfluoroalkyl substances in U.S. drinking water from 2013 to 2015 under the Third Unregulated Contaminant Monitoring Rule (UCMR3). UCMR3 efforts yielded a dataset of 36,139 samples containing analytical results from >5000 public water systems (PWSs). This study used UCMR3 data to investigate three aspects of per- and polyfluoroalkyl substances (PFASs) in drinking water: the occurrence of PFAS and co-contaminant mixtures, trends in PFAS detections relative to PWS characteristics and potential release types, and temporal trends in PFAS occurrence. This was achieved through bivariate and multivariate analyses including categorical analysis, concentration ratios, and hierarchical cluster analysis. Approximately 50% of samples with PFAS detections contained ≥2 PFASs, and 72% of detections occurred in groundwater. Large PWSs (>10,000 customers) were 5.6 times more likely than small PWSs (≤10,000 customers) to exhibit PFAS detections; however, when detected, median total PFAS concentrations were higher in small PWSs (0.12 µg/L) than in large (0.053 µg/L). Bivariate and multivariate analyses of PFAS composition suggested PWSs reflect impacts due to firefighting foam use and WWTP effluent as compared to other source types for which data were available. Mann-Kendall analysis of quarterly total PFAS detection rates indicated an increasing trend over time (p = 0.03). UCMR3 data provide a foundation for tiered design of targeted sampling and analysis plans to address remaining knowledge gaps in the sources, composition, and concentrations of PFASs in U.S. drinking water.

Associating potential 1,4-dioxane biodegradation activity with groundwater geochemical parameters at four different contaminated sites.

1,4-Dioxane (dioxane) is a groundwater contaminant of emerging concern for which bioremediation may become a practical remediation strategy. Therefore, it is important to advance our heuristic understanding of geochemical parameters that are most influential on the potential success of intrinsic bioremediation of dioxane-impacted sites. Here, Pearson's and Spearman's correlation and linear regression analyses were conducted to discern associations between 1,4-dioxane biodegradation activity measured in aerobic microcosms and groundwater geochemical parameters at four different contaminated sites. Dissolved oxygen, which is known to limit dioxane biodegradation, was excluded as a limiting factor in this analysis. Biodegradation activity was positively associated with dioxane concentrations (p < 0.01; R < 0.70) as well as the number of catabolic thmA gene copies (p < 0.01; R = 0.80) encoding dioxane monooxygenase. Thus, whereas environmental factors such as pH, temperature, and nutrients may influence dioxane biodegradation, these parameters did not exert as strong of an influence on potential biodegradation activity as the in situ concentration of substrate dioxane at the time of sampling. This analysis infers that aerobic sites with higher dioxane concentrations are more likely to select and sustain a thriving population of dioxane degraders, while sites with relatively low dioxane concentrations would be more difficult to attenuate naturally and may require alternative remediation strategies.

1,4-Dioxane drinking water occurrence data from the third unregulated contaminant monitoring rule.

This study examined data collected from U.S. public drinking water supplies in support of the recently-completed third round of the Unregulated Contaminant Monitoring Rule (UCMR3) to better understand the nature and occurrence of 1,4-dioxane and the basis for establishing drinking water standards. The purpose was to evaluate whether the occurrence data for this emerging but federally-unregulated contaminant fit with common conceptual models, including its persistence and the importance of groundwater contamination for potential exposure. 1,4-Dioxane was detected in samples from 21% of 4864 PWSs, and was in exceedance of the health-based reference concentration (0.35µg/L) at 6.9% of these systems. In both measures, it ranked second among the 28 UCMR3 contaminants. Although much of the focus on 1,4-dioxane has been its role as a groundwater contaminant, the detection frequency for 1,4-dioxane in surface water was only marginally lower than in groundwater (by a factor of 1.25; p<0.0001). However, groundwater concentrations were higher than those in surface water (p<0.0001) and contributed to a higher frequency of exceeding the reference concentration (by a factor of 1.8, p<0.0001), indicating that surface water sources tend to be more dilute. Sampling from large systems increased the likelihood that 1,4-dioxane was detected by a factor of 2.18 times relative to small systems (p<0.0001). 1,4-Dioxane detections in drinking water were highly associated with detections of other chlorinated compounds particularly 1,1-dichlorethane (odds ratio=47; p<0.0001), which is associated with the release of 1,4-dioxane as a chlorinated solvent stabilizer. Based on aggregated nationwide data, 1,4-dioxane showed evidence of a decreasing trend in concentration and detection frequency over time. These data suggest that the loading to drinking water supplies may be decreasing. However, in the interim, some water supply systems may need to consider improving their treatment capabilities in response to further regulatory review of this compound.

Implications of matrix diffusion on 1,4-dioxane persistence at contaminated groundwater sites.

Management of groundwater sites impacted by 1,4-dioxane can be challenging due to its migration potential and perceived recalcitrance. This study examined the extent to which 1,4-dioxane's persistence was subject to diffusion of mass into and out of lower-permeability zones relative to co-released chlorinated solvents. Two different release scenarios were evaluated within a two-layer aquifer system using an analytical modeling approach. The first scenario simulated a 1,4-dioxane and 1,1,1-TCA source zone where spent solvent was released. The period when 1,4-dioxane was actively loading the low-permeability layer within the source zone was estimated to be <3years due to its high effective solubility. While this was approximately an order-of-magnitude shorter than the loading period for 1,1,1-TCA, the mass of 1,4-dioxane stored within the low-permeability zone at the end of the simulation period (26kg) was larger than that predicted for 1,1,1-TCA (17kg). Even 80years after release, the aqueous 1,4-dioxane concentration was still several orders-of-magnitude higher than potentially-applicable criteria. Within the downgradient plume, diffusion contributed to higher concentrations and enhanced penetration of 1,4-dioxane into the low-permeability zones relative to 1,1,1-TCA. In the second scenario, elevated 1,4-dioxane concentrations were predicted at a site impacted by migration of a weak source from an upgradient site. Plume cutoff was beneficial because it could be implemented in time to prevent further loading of the low-permeability zone at the downgradient site. Overall, this study documented that 1,4-dioxane within transmissive portions of the source zone is quickly depleted due to characteristics that favor both diffusion-based storage and groundwater transport, leaving little mass to treat using conventional means. Furthermore, the results highlight the differences between 1,4-dioxane and chlorinated solvent source zones, suggesting that back diffusion of 1,4-dioxane mass may be serving as the dominant long-term "secondary source" at many contaminated sites that must be managed using alternative approaches.

Evidence of 1,4-dioxane attenuation at groundwater sites contaminated with chlorinated solvents and 1,4-dioxane.

There is a critical need to develop appropriate management strategies for 1,4-dioxane (dioxane) due to its widespread occurrence and perceived recalcitrance at groundwater sites where chlorinated solvents are present. A comprehensive evaluation of California state (GeoTracker) and Air Force monitoring records was used to provide significant evidence of dioxane attenuation at field sites. Temporal changes in the site-wide maximum concentrations were used to estimate source attenuation rates at the GeoTracker sites (median length of monitoring period = 6.8 years). While attenuation could not be established at all sites, statistically significant positive attenuation rates were confirmed at 22 sites. At sites where dioxane and chlorinated solvents were present, the median value of all statistically significant dioxane source attenuation rates (equivalent half-life = 31 months; n = 34) was lower than 1,1,1-trichloroethane (TCA) but similar to 1,1-dichloroethene (1,1-DCE) and trichloroethene (TCE). Dioxane attenuation rates were positively correlated with rates for 1,1-DCE and TCE but not TCA. At this set of sites, there was little evidence that chlorinated solvent remedial efforts (e.g., chemical oxidation, enhanced bioremediation) impacted dioxane attenuation. Attenuation rates based on well-specific records from the Air Force data set confirmed significant dioxane attenuation (131 out of 441 wells) at a similar frequency and extent (median equivalent half-life = 48 months) as observed at the California sites. Linear discriminant analysis established a positive correlation between dioxane attenuation and increasing concentrations of dissolved oxygen, while the same analysis found a negative correlation with metals and CVOC concentrations. The magnitude and prevalence of dioxane attenuation documented here suggest that natural attenuation may be used to manage some but not necessarily all dioxane-impacted sites.

Membrane interface probe protocol for contaminants in low-permeability zones.

Accurate characterization of contaminant mass in zones of low hydraulic conductivity (low k) is essential for site management because this difficult-to-treat mass can be a long-term secondary source. This study developed a protocol for the membrane interface probe (MIP) as a low-cost, rapid data-acquisition tool for qualitatively evaluating the location and relative distribution of mass in low-k zones. MIP operating parameters were varied systematically at high and low concentration locations at a contaminated site to evaluate the impact of the parameters on data quality relative to a detailed adjacent profile of soil concentrations. Evaluation of the relative location of maximum concentrations and the shape of the MIP vs. soil profiles led to a standard operating procedure (SOP) for the MIP to delineate contamination in low-k zones. This includes recommendations for: (1) preferred detector (ECD for low concentration zones, PID or ECD for higher concentration zones); (2) combining downlogged and uplogged data to reduce carryover; and (3) higher carrier gas flow rate in high concentration zones. Linear regression indicated scatter in all MIP-to-soil comparisons, including R(2) values using the SOP of 0.32 in the low concentration boring and 0.49 in the high concentration boring. In contrast, a control dataset with soil-to-soil correlations from borings 1-m apart exhibited an R(2) of ≥ 0.88, highlighting the uncertainty in predicting soil concentrations using MIP data. This study demonstrates that the MIP provides lower-precision contaminant distribution and heterogeneity data compared to more intensive high-resolution characterization methods. This is consistent with its use as a complementary screening tool.

Relative contribution of DNAPL dissolution and matrix diffusion to the long-term persistence of chlorinated solvent source zones.

The relative contribution of dense non-aqueous phase liquid (DNAPL) dissolution versus matrix diffusion processes to the longevity of chlorinated source zones was investigated. Matrix diffusion is being increasingly recognized as an important non-DNAPL component of source behavior over time, and understanding the persistence of contaminants that have diffused into lower permeability units can impact remedial decision-making. In this study, a hypothetical DNAPL source zone architecture consisting of several different sized pools and fingers originally developed by Anderson et al. (1992) was adapted to include defined low permeability layers. A coupled dissolution-diffusion model was developed to allow diffusion into these layers while in contact with DNAPL, followed by diffusion out of these same layers after complete DNAPL dissolution. This exercise was performed for releases of equivalent masses (675 kg) of three different compounds, including chlorinated solvents with solubilities ranging from low (tetrachloroethene (PCE)), moderate (trichloroethene (TCE)) to high (dichloromethane (DCM)). The results of this simple modeling exercise demonstrate that matrix diffusion can be a critical component of source zone longevity and may represent a longer-term contributor to source longevity (i.e., longer time maintaining concentrations above MCLs) than DNAPL dissolution alone at many sites. For the hypothetical TCE release, the simulation indicated that dissolution of DNAPL would take approximately 38 years, while the back diffusion from low permeability zones could maintain the source for an additional 83 years. This effect was even more dramatic for the higher solubility DCM (97% of longevity due to matrix diffusion), while the lower solubility PCE showed a more equal contribution from DNAPL dissolution vs. matrix diffusion. Several methods were used to describe the resulting source attenuation curves, including a first-order decay model which showed that half-life of mass discharge from the matrix-diffusion dominated phase is in the range of 13 to 29 years for TCE. Because the mass discharge rate shifts significantly over time once DNAPL dissolution is complete, a Power-Law model was shown to be useful, especially at later stages when matrix diffusion dominates. An assessment of mass distribution showed that while relatively small percentages of the initial source mass diffused into the low permeability compartment, this mass was sufficient to sustain concentrations above drinking water standards for decades. These data show that relatively typical conditions (e.g., 50-year-old release, moderate to high solubility contaminant) are consistent with late stage sources, where mass in low permeability matrices serves as the primary source, and fit the conceptual model that mass in low permeability zones is important when evaluating source longevity.

Contaminant plume classification system based on mass discharge.

Estimation of mass discharge has become an increasingly valuable analysis technique at sites with contaminated groundwater plumes. We propose a simple plume magnitude classification system based on mass discharge comprised of 10 separate magnitude categories, such as a "Mag 7 plume." This system can be a useful tool for scientists, engineers, regulators, and stakeholders to better communicate site conceptual models, prioritize sites, evaluate plumes both spatially and temporally, and determine potential impacts.

Flux and product distribution during biological treatment of tetrachloroethene dense non-aqueous-phase liquid.

Flux in non-aqueous-phase liquid (NAPL)-contaminated systems containing active microbial populations (including Dehalococcoides sp.) was investigated using a quantitative mass balance and phase distribution approach. Batch systems containing mixed NAPL with an initial tetrachloroethene (PCE) mole fraction ranging from 0.1 to 0.4 provided a means for comparing systems where mass transfer and aqueous concentration were controlled by the initial NAPL composition. Although the use of mixed NAPL with increasing PCE mole fractions introduced a mass-transfer variable on the abiotic dissolution rate, it was determined that biological systems produced flux rates that were similar to each other regardless of the initial PCE mole fraction. Thus, organisms appeared to be dechlorinating near their maximum conversion rates, and the result was the accumulation of cis-1,2-dichloroethene (cDCE) followed by slow conversion to vinyl chloride (VC). Increases in the initial PCE mole fractions in the NAPL had a negative impact on product distribution due to the presence of a larger concentration of a more favorable electron acceptor. Because the mass converted to cDCE was present largely in the dissolved phase in all systems, the production of this metabolite was a favorable outcome in terms of NAPL dissolution. The pH dropped as low as 4.9 in active systems, indicating that the amount of HCl released during the reductive dechlorination process was large enough to overwhelm the buffering capacity. This pH effect was more pronounced in systems that exhibited extensive dechlorination to VC, further suggesting that rapid dechlorination of PCE NAPL can alter chemical characteristics in source zone regions.

Inoculation of a DNAPL source zone to initiate reductive dechlorination of PCE.

The ability to inoculate a PCE-NAPL source zone with no prior dechlorinating activity was examined using a near field-scale simulated aquifer. A known mass of PCE was added to establish a source zone, and the groundwater was depleted of oxygen using acetate and lactate prior to culture addition. An active and stable dechlorinating culture was used as an inoculum, and dechlorination activity was observed within 2 weeks following culture transfer. PCE reduction to TCE and cis-DCE was observed initially, and the formation of these compounds was accelerated by the addition of a long-term source of hydrogen (Hydrogen Releasing Compound). cis-DCE was the predominant chlorinated ethene present in the effluent after 225 days of operation, and production of VC and ethene lagged the formation of TCE and cis-OCE. However, dechlorination extent continued to improve over time, and VC eventually became a major product, suggesting that reinoculation was unnecessary. The detection of Dehalococcoides species in the source culture and in the simulated aquifer postinoculation indicated that the metabolic capability to dechlorinate beyond cis-DCE (t = 86 days and t = 245 days) was present. Elevated levels of TCE and cis-DCE were present in the source zone, but neither VC nor ethene were detected in the vicinity of NAPL. The results of this research indicated that adding dechlorinating cultures may be useful in the application of source zone bioremediation but that dechlorination beyond cis-DCE may be limited to regions downgradient of the source zone.