Sustainability of Reductive Dechlorination at Chlorinated Solvent Contaminated Sites: Metrics for Assessing Potentially Bioavailable Natural Organic Carbon in Aquifer Sediments
Thomas, Lashun King
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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.
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