Capacity Of Mulch Biobarriers To Support Complete Anaerobic Dechlorination Of Aerobic, Tce-Contaminated Groundwater

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The rise of Industrial Revolution in the 19th century has brought not only fast economic growth, but also the release of large amounts of anthropogenic compounds into the environment. Trichloroethene (TCE), a confirmed human carcinogen, is one of the most commonly found contaminants in groundwater, therefore the remediation of this compound has been extensively studied and practiced in the past several decades. For compounds like TCE that are highly oxidized, destruction by reduction reactions (called reductive dechlorination) is more favorable than oxidative reactions. Although a wide range of bioremediation methods have been applied, biological permeable reactive barriers, referred to as biobarriers are attractive for shallow groundwater contamination plumes because of their ability to capture groundwater contaminant plumes before they migrate offsite, and their low cost of operation and maintenance. However, little research has explored the ability of biobarriers to reach complete dechlorination of TCE in oxygenated groundwater. Additionally, the dissolved oxygen in groundwater has not been taken into consideration when estimating the longevity of a biobarrier, and the impact of dissolved oxygen to the dechlorination process in a biobarrier is unknown. For this Master's thesis, a column study was conducted to study the capacity of a mulch biobarrier to fully dechlorinate TCE to ethene. Six mulch (pine bark) filled columns (2 control, 4 experimental) were constructed to study the dechlorination process of TCE, with the inoculation of KB-1TM enrichment culture (at a 1:1000 dilution level). The 1 mg/L TCE-containing v inflow water was oxygenated, to examine the impact of dissolved oxygen on mulch column performance. The mulch columns (with a hydraulic residence time of 3.3 days) were able to reduce the dissolved oxygen concentration from 7.9 mg/L to a level anaerobic enough to allow reductive dechlorination to happen within three cm into the columns. Four days after inoculation, cis-1,2-dichloroethene appeared in the columns and 8 days after inoculation, vinyl chloride showed up in trace amounts. Until 130 days of operation, levels of vinyl chloride or ethene remained low, but the dechlorination process then accelerated. By Day 212, two of the four inoculated columns reached 73% and 99% complete dechlorination (i.e. ethene comprised over 73% of the chlorinated ethenes detected at the effluent ends of the columns), and by Day 297, the other two columns also reached 95% and 99% complete dechlorination. The longevity of the column was predicted to be 7 years, considering only the impact of dissolved oxygen on the consumption of mulch-derived electron donors. Using column parameters and results from another researcher's studies, a dissolved oxygen penetration front speed of 0.7 cm of column height per month was predicted-corresponding to 5 cm within 212 days. With time, signs of TCE penetration was observed in the four inoculated columns, suggesting oxygen intrusion further into the columns. From PCR tests on DNA extracted from column liquids, the existence of key KB1 dechlorination populations, Geobacter and Dehalococcoides, were confirmed using the biomarker genes pceA for Geobacter and vcrA/16S rRNA for Dehalococcoides. qPCR tests failed to quantify these populations, but results with 16S rRNA gene primers suggested 100 to 100,000 times more 16S rRNA gene copies (not specific to known inoculated dechlorinators) vi were associated with the mulch-attached biofilms than with planktonic phases (in column liquid). vii

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Mulch; Permeable Reactive Barriers; trichloroethene


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Richardson, Ruth E.

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Gossett, James Michael

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Civil and Environmental Engineering

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M.S., Civil and Environmental Engineering

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Master of Science

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dissertation or thesis

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