Measuring in situ biotransformation in BTEX-contaminated groundwater Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/x920g032f

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  • Benzylsuccinate (BSA), methylbenzylsuccinate (methylBSA), and ethylbenzylsuccinate (ethylBSA) are unambiguous anaerobic biotransformation products from toluene, xylenes, and ethylbenzene decay, respectively, and may be used to indicate intrinsic bioremediation is occurring at hydrocarbon-contaminated sites. In order to improve upon current methods that detect and quantify anaerobic hydrocarbon metabolites in field samples, solid-phase-extraction (SPE) and direct sample injection methods coupled with liquid chromatography/tandem mass spectrometry (LC/MS/MS) were evaluated. In laboratory studies, recoveries of authentic standards of non-deuterated or deuterated benzylsuccinates and toluates ranged from 80 to 106% with relative standard errors ranging from 2 to 4%. The method detection limits for these analytes using SPE-LC/MS/MS ranged from 0.006 to 0.029 [mu]g/L whereas those for direct injection-LC/MS/MS ranged from 0.61 to 1.5 [mu]g/L. Given the increased sensitivity of using SPE coupled with LC/MS/MS, this technique was then used to analyze for the presence of putative anaerobic alkylbenzene metabolites in groundwater from a hydrocarbon-contaminated site where single-well push-pull tests were conducted using deuterated aromatic hydrocarbons. Test solutions (250 L) containing the deuterium-labeled organic substrates toluene, ethylbenzene, and o-xylene, along with nitrate as an electron acceptor and bromide as a non-reactive tracer, were injected into each of four existing wells at an alkylbenzene-contaminated field site. Samples of the test solution/groundwater blend were then extracted over a 29-day period. Volatile alkylbenzenes were analyzed and quantified by gas chromatography/mass spectrometry (GC/MS). Metabolites were determined by solid-phase extraction (SPE) followed by high performance liquid chromatography/tandem mass spectrometry (HPLC/MS/MS) analysis. Due to the rapid transport of the test solution through the aquifer, resulting in the loss of the tracer signal, no test solution was available for three wells. Of the wells tested, only the test conducted in well CR-20 gave samples with sufficient concentration of analytes to warrant full processing of the samples and data. Decreases in deuterated alkylbenzene substrates and the appearance of signature metabolites including deuterated ethylbenzylsuccinate (ethylBSA-d₅) and o-methylbenzylsuccinate (o-methylBSA-d₁₀) were observed while deuterated benzylsuccinate (BSA-d₅) was not detected. Of the deuterated secondary metabolites monitored, m-toluate-d₇ and benzoate-d₅ also were determined in well CR-20. Although analytes were detected in well CR-20, erratic changes in analyte concentrations made it infeasible to determine rates.
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