David T. Adamson

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.

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.

Support of Source Zone Bioremediation through Endogenous Biomass Decay and Electron Donor Recycling

ABSTRACT Enhanced bioremediation strategies employ intensive electron donor amendments that can be successful in generating high biomass concentrations within the targeted area, and this technology is increasingly being applied within source zones to address non–aqueous phase contaminants. An unintended consequence is potential electron donor recycling via the slow endogenous decay of these newly grown cells, which may persist in the source zone even after the enhanced bioremediation project is completed and the introduced electron donor is exhausted. This paper presents a conceptual model that outlines the endogenous decay process within source zones and identifies several key scenarios where it is an important contributor to long-term attenuation. A key concept is that this reservoir represented by decaying biomass is both potentially large and capable of several rounds of turnover before becoming exhausted. Thus, the slow decay of biomass and the recycling of these decay products within the source zone extended the duration of the treatment period. A recent survey on the performance of source depletion technologies strongly suggests that this process is being observed at sites where enhanced bioremediation has been implemented (McGuire et al., 2006, Ground Water Monitor. Remediat. 26:73–84). Concentrations continued to decline several years after treatment, providing a strong indication that there is an endogenous electron donor supply that is contributing to continued contaminant reduction over time. Little evidence of concentration rebound was observed relative to sites where other technologies were used, suggesting long-term benefits associated with enhanced bioremediation that appear to partially offset processes that can contribute to rebound (e.g., matrix diffusion). Because initial colonization can occur near the non-aqueous phase liquid (NAPL)-water interface when exogenous electron donor is readily available the endogenous cell decay occurs in an optimal location to continue to support reductive dechlorination. This electron donor recycling process is ideally suited to favor growth of dechlorinating organisms relative to competing populations because of the slow release rates associated with decay, and it should preferentially stimulate polychloroethylene (PCE) and trichloroethylene (TCE) source removal over metabolites. Endogenous decay can be directly employed as part of the remediation design through groundwater recirculation or by the construction of a carbon-based in situ biowall to generate large amounts of biomass. In the case of an in situ biowall, a key consideration is the placement, either as a permeable reactive barrier filled with fermentable carbon within the source zone, or as a barrier located upgradient of a source zone to ensure that groundwater is reduced before entering the targeted area. Regardless of the approach, the likely impact of electron donor recycling through endogenous decay is to extend low-level activity for up to several years, making enhanced bioremediation a promising technology in terms of initial performance and for long-term polishing.

Product distribution during transformation of multiple contaminants by a high-rate, tetrachlorethene-dechlorinating enrichment culture.

Radiolabeled tetrachloroethene (PCE) and carbon tetrachloride (CT) were added to batch systems containing a lactate-enrichment culture displaying apparent dehalorespiration abilities to analyze the influence of mixtures on product distribution. Both CT and PCE were readily dechlorinated, although significant carbon disulfide (CS2) formation was observed during CT transformation. Calculated 1,2-14C-PCE recoveries for biotic treatments were between 91 and 104%, but an inability to recover products such as CS2 led to lower recoveries of 14C-CT (55 to 62%). While the majority of activity in 14C-CT-spiked treatments was recovered in the volatile fraction, 14CO2 increased significantly over time. 1,2-14C-PCE was primarily recovered in volatile and non-strippable fractions, but a significant increase in 14CO2 relative to cell-free controls suggested that the presence of a non-specific dechlorination pathway complementing dehalorespiration. The addition of both CT and PCE inhibited the transformation of the individual compounds and reduced the percentages recovered as 14CO2. However, the magnitude of these reductions was not severe and appeared to be the result of slower overall transformation rather than a complete inhibition of mineralization pathways.

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.

Microbial responses to combined oxidation and catalysis treatment of 1,4-dioxane and co-contaminants in groundwater and soil

Post-treatment impacts of a novel combined hydrogen peroxide (H2O2) oxidation and WOx/ZrO2 catalysis used for the removal of 1,4-dioxane and chlorinated volatile organic compound (CVOC) contaminants were investigated in soil and groundwater microbial community. This treatment train removed ~90% 1,4-dioxane regardless of initial concentrations of 1,4-dioxane and CVOCs. The Illumina Miseq platform and bioinformatics were used to study the changes to microbial community structure. This approach determined that dynamic shifts of microbiomes were associated with conditions specific to treatments as well as 1,4-dioxane and CVOCs mixtures. The biodiversity was observed to decrease only after oxidation under conditions that included high levels of 1,4-dioxane and CVOCs, but increased when 1,4-dioxane was present without CVOCs. WOx/ZrO2 catalysis reduced biodiversity across all conditions. Taxonomic classification demonstrated oxidative tolerance for members of the genera Massilia and Rhodococcus, while catalyst tolerance was observed for members of the genera Sphingomonas and Devosia. Linear discriminant analysis effect size was a useful statistical tool to highlight representative microbes, while the multidimensional analysis elucidated the separation of microbiomes under the low 1,4-dioxane-only condition from all other conditions containing CVOCs, as well as the differences of microbial population among original, post-oxidation, and post-catalysis states. The results of this study enhance our understanding of microbial community responses to a promising chemical treatment train, and the metagenomic analysis will help practitioners predict the microbial community status during the post-treatment period, which may have consequences for long-term management strategies that include additional biodegradation treatment or natural attenuation.

Source Depletion at Contaminated Groundwater Sites: Is It Worth It?

In the late 1980s and early 1990s the research community and EPA began to focus on the constraints on aquifer remediation, particularly constraints related to Dense NonAqueous Phase Liquids (DNAPLs) as a secondary source of contamination at chlorinated solvent sites. By the late 1990s, a new paradigm was in general acceptance, where most chlorinated solvent sites were assumed to contain a difficultto-remove, continuing DNAPL source, even at sites where DNAPL was not directly observed. The presence of this DNAPL presents a tremendous challenge “unprecedented in the field of groundwater engineering” (Pankow and Cherry, 1996). One of the responses to this new paradigm has been to develop better technologies that can remove or destroy residual DNAPL and therefore deplete the source. A variety of methods have been tested, including thermal, chemical, oxidation, and biodegradation-based technologies. Despite the heavy financial investment in these approaches, there is an intense debate on whether these intensive source zone remediation technologies should be applied at many field sites. A U.S. EPA Expert Panel presented a qualitative decision process where the potential benefits from source depletion are evaluated using a “weight of evidence” approach (Kavanaugh et al., 2003). A list of potential benefits was presented, including: i) reduction of DNAPL mobility; ii) reduction of source longevity; iii) reduction of dissolved plume loading; iv) reduction of loading to receptors; iv) need to achieve cleanup quickly; and iv) intangible benefits. Quantitative results from a Department of Defense Strategic Environmental Research and Development Program (SERDP) project are presented to illustrate the performance of partial source depletion using thermal, chemical, oxidation, and biodegradation-based technologies compared to long-term containment. Planninglevel models of source depletion at chlorinated solvent sites are used along with two new databases (a cost and performance database of source depletion case studies from the literature and a source response database from actual field sites) to compare the environmental and economic benefits and costs of partial source depletion vs. longterm containment.