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Groundwater remediation strategies have advanced toward more effective and economical remedial technologies. Monitored natural attenuation (MNA) has become accepted by federal regulatory agencies as a viable remediation strategy for contaminants under site-specific conditions. At chloroethene contaminated sites where MNA is used as a remediation strategy, microbially-mediated reductive dechlorination is typically the dominant pathway for natural attenuation. The efficacy of reductive dechlorination at sites with no anthropogenic carbon sources is often influenced by the availability of readily-biodegradable natural organic carbon along with favorable geochemical conditions for supporting microbial dehalogenation. Recent research studies have suggested that the pool of labile natural organic carbon, operationally defined as potentially bioavailable organic carbon (PBOC), may be a critical component related to sustaining reductive dechlorination at MNA sites. The objective of this study was to evaluate PBOC as a quantitative measure of the labile organic carbon fraction of aquifer sediments in relation to microbial reductive dechlorination of chlorinated solvents. In the first phase of this study, the variability of PBOC in aquifer sediments was examined among 15 chloroethene contaminated sites. Results showed that PBOC displayed considerable variability among the study sites, ranging over four orders of magnitude. Regression results demonstrated that a positive correlation existed between PBOC, solid phase total organic carbon (TOCs), and reductive dechlorination activity at the sites. Results supported that greater levels of PBOC and TOCs corresponded to higher reductive dechlorination activity at the sites. Composition results showed that 6-86% of PBOC consisted of proteins and amino acids. Results also suggested a positive relationship existed between PBOC, concentrations of potentially bioavailable organic compounds present in the aquifer system, expressed as hydrolyzable amino acids (HAA), and the natural attenuation capacity (NAC) at the sites. Higher PBOC levels were consistently observed at sites with greater NAC and levels of HAA. The results of this study suggested that the variability of PBOC in the aquifer sediments exhibited a reasonable correlation with TOCs, hydrolyzable amino acids, and chloroethene transformation among the selected sites. In the second phase of this study, the relationship between PBOC in aquifer sediments and site specific performance data was evaluated among 12 chloroethene contaminated sites. Results demonstrated that PBOC in aquifer sediments was directly correlated to independent field metrics associated with reductive dechlorination. Levels of PBOC demonstrated direct relationships with hydrogen (H2) and dissolved oxygen (DO) concentrations within the groundwater system at the selected study sites. Results also indicated that PBOC demonstrated positive relationships with reductive dechlorination activity and the natural attenuation capacity of the sites. The findings of this study suggested that the level of PBOC in aquifer sediments may be a key factor in sustaining conditions favorable for microbial reductive dechlorination. In the third phase of this study, the distribution of PBOC was investigated at a chloroethene contaminated site. PBOC was measured in surficial aquifer sediment samples collected at varying depths in the vicinity of a chloroethene plume. Results demonstrated that levels of PBOC were consistently higher in aquifer sediments with minimal chloroethene exposure relative to samples collected in the PCE-contaminated source zone. Regression results demonstrated that a statistically significant inverse correlation existed between PBOC levels and chloroethene concentrations for selected temporary wells in the contaminated source zone at the study site. Consistent with these findings, results also indicated a similar trend of increased PBOC in aquifer sediments outside the chloroethene plume relative to aquifer sediments inside the plume. Results from this study further suggested that differences in extracted carbon levels at the site for surficial aquifer sediment samples in the PCE-contaminated source zone could impact the extent of reductive dechlorination within the hydrographic unit. Ph. D.

Natural Attenuation Software (NAS) was developed as a screening tool to estimate remediation timeframes for monitored natural attenuation (MNA) to lower groundwater contaminant concentrations to regulatory limits, and to assist in decision-making on the level of source zone treatment in conjunction with MNA using site-specific remediation objectives. In addition, NAS facilitates the combined use of MNA with engineered remedial actions (ERAs) so that the benefits of each technology can be maximized while minimizing costs of remediation. The primary expected benefit of NAS is to increase regulatory acceptance of MNA, thereby decreasing overall remediation costs. NAS is designed for application to ground-water systems consisting of porous, relatively homogeneous, saturated media, and assumes that groundwater flow is uniform and unidirectional. NAS consists of a combination of analytical and numerical solute transport models implemented in three main interactive modules to provide estimates for: (1) target source concentration required for a plume extent to contract to regulatory limits, (2) time required for NAPL contaminants in the source area to attenuate to a predetermined target source concentration, and (3) time required for a plume extent to contract to regulatory limits after source reduction. Natural attenuation processes that NAS models include advection, dispersion, sorption, non-aqueous phase liquid (NAPL) dissolution, and biodegradation. NAS determines redox zonation, and estimates and applies varied biodegradation rates from one redox zone to the next. Recently, NAS was enhanced to include petroleum hydrocarbons, chlorinated ethenes, chlorinated ethanes, chlorinated methanes, and chlorinated benzenes, or any user-defined contaminants (e.g., heavy metals, radioisotopes), and has included the capability to model co-mingled plumes. To enable comparison of remediation timeframe estimates between MNA and specific ERAs, NAS was modified to incorporate an estimation technique for timeframes associated with pump-and-treat remediation technology for comparison to, or in conjunction with, MNA. NAS also expanded analysis tools for improved performance assessment, as well as the assessment of sustainability of natural attenuation processes over time. A Department of Defense (DoD) Environmental Security Technology Certification Program (ESTCP) demonstration was undertaken to evaluate the capability of the NAS software to provide reasonable estimates of MNA cleanup timeframes in a variety of environments and sites throughout the United States. Overall, results suggest that NAS was satisfactory in meeting performance objectives set forth in the demonstration, and that because NAS is based on sound science, it can serve as an effective tool for decision-making and data analysis at a wide range of contaminated sites and is not limited to a small subset of “simple sites” because of its simplicity. At some sites, NAS-estimated timeframes were crucial for winning regulatory acceptance of MNA, with cost-benefit analyses providing estimates of savings associated with using MNA as a final remediation strategy. Ph. D.

Reductive dechlorination is a significant natural attenuation process in chloroethene-contaminated aquifers where organic carbon combined with reducing redox conditions support active dechlorinating microorganisms. At sites where natural organic carbon (NOC) associated with the aquifer matrix provides fermentable organics, the ability to measure the NOC is needed to assess the potential for the long-term sustainability of reductive dechlorination. This study focused on developing a method to measure the potentially bioavailable organic carbon (PBOC) associated with aquifer sediment. To measure NOC and evaluate its biodegradability, liquid extraction techniques on aquifer sediment were investigated. Single extractions with different extracting solutions showed that extractable organic carbon associated with the sediment ranged from 1-38% of the total organic carbon content (TOCs). Bioassay experiments demonstrated that 30-60% of the extractable organic carbon can be utilized by a microbial consortium. Alternating between 0.1% pyrophosphate and base solutions over multiple extractions increased the rate of removal efficiency and targeted two organic carbon pools. The result of the investigation was a laboratory method to quantify organic carbon from the aquifer matrix in terms of the PBOC. In the second part, the extractable PBOC was shown to biodegrade under anaerobic conditions, to produce H2 at levels necessary to maintain reductive dechlorination, and to support reductive dechlorination in enrichment cultures. For the third part of the research, the difference in extractable organic carbon inside and outside of a chloroethene-contaminated plume was examined through the combination of PBOC laboratory data and field parameters. Supported by ground-water constituent data, the PBOC extraction and bioassay studies showed that less extractable organic carbon was present inside than outside of the chloroethene plume. The final part of the research investigated the distribution of PBOC extractions across six contaminated sites. PBOC extractions were directly correlated to the TOCs, soft carbon content, and level of reductive dechlorination activity at the sites. Based on these correlations, a range for organic carbon potentially available to subsurface microorganisms was proposed where the upper bound consisted of the soft carbon and the lower bound consisted of the PBOC. Ph. D.