Demonstrating Localized Intrinsic Biodegradation of Chlorinated Aliphatic Hydrocarbons Using Time Series Sampling

Abstract

AbstractLocalized intrinsic biodegradation of chlorinated aliphatic hydrocarbons (CAHs) was investigated by pumping from a monitoring well screened in a low conductivity carbonate mudstone. Two time series tests were conducted to assess the relative potential for intrinsic biodegradation in occur in the mudstone compared to the surrounding highly productive carbonate aquifer. Water samples were collected during pumping and analyzed for CAHs (tests 1 and 2). and water quality parameters (pH, specific conductance, dissolved oxygen, redox), election donors and acceptors, and biodegradation indicators (chloride, alkalinity) (test 2). During both lime series tests, concentrations of TCE breakdown products— DCE, vinyl chloride and ethane—decreased significantly after pumping 500 gallons of water from the well. During lest 2. concentrations of electron donors and acceptors and biodegradation indicators decreased, whereas dissolved oxygen and redox remained at low levels. Multiple lines of evidence demonstrated that that intrinsic bioremediation of CAHs is occurring in the mudstone, including: (1) presence of anaerobic conditions in the mudstone conducive for reductive dechlorination: (2) progression of electron acceptors and presence of methanogenic conditions; (3) available supply of electron donors in the mudstone: (4) occurrence of TCE breakdown products; and (5) increased chloride and alkalinity concentrations in the zone of active biodegradation.Time series testing indicated that during the early stages of pumping, effluent consisted mainly of ground water derived from the mudstone unit. With continued pumping, an increasing percentage of effluent water was derived from the less contaminated, permeable carbonate units which bounded the mudstone layer. Differences in permeabilities between the carbonate aquifer and mudstone layer probably accounted for the varying degree of intrinsic biodegradation observed between the two systems. Future remedial efforts should consider avoiding ground water extraction from the mudstone and disturbing the intrinsic biodegradation processes that are reducing contaminant mass in the aquifer.