Universität Tübingen
Control and limitations of microbial degradation in aromatic hydrocarbon plumes – experiments in 2-D model aquifers
Abstract
Several hundred thousand groundwater sites worldwide are contaminated with petroleum derivatives, particularly with monoaromatic hydrocarbons such as benzene, toluene, ethylbenzene, and xylenes (BTEX). As groundwater serves as a major drinking water resource, controlled removal of these widespread organic compounds from aquifers is necessary. Among the processes involved in natural attenuation of pollutants in steady state groundwater contaminant plumes, only the mineralization via microorganisms leads to a significant mass removal. General concepts hold that in porous aquifers microbial degradation activities are primarily limited by mixing processes. Based on this consideration, the plume fringe concept was raised prior to this work, i.e. the dominant biodegradation activity takes place at the fringes of steady-state contaminant plumes, governed by the dispersive mixing of electron donors from the plume core and dissolved electron acceptors from ambient groundwater. Objective of this thesis was to experimentally prove the plume fringe concept by providing evidence that the biodegradation of model pollutants in porous media is mainly mixing-controlled. Moreover, this work tackled the detection of new potential degradation-limiting factors. The degradation of toluene and ethylbenzene therefore was investigated in 2-D sediment microcosms. Aerobic (Pseudomonas putida strain F1 and strain mt-2) as well as anaerobic (Aromatoleum aromaticum strain EbN1) degradation in toluene and ethylbenzene plumes showed steep biogeochemical gradients, dominant microbial biomass and biodegradation activities at the plumes’ fringes in homogeneous porous media. These data confirmed the plume fringe concept and showed that biodegradation in the sediment microcosms is above all controlled by transverse dispersive mixing. If this holds true, increased dispersion, i. e. transverse dispersion and macrodispersion, should lead to an enhanced biodegradation. This hypothesis was proved in experiments where transverse dispersion was increased by means of sediment heterogeneity, i.e. an alternating succession of low- (middle sand) and high- (coarse sand) conductivity zones. Indeed, in developing toluene plumes as well as in steady state toluene and ethylbenzene plumes, aerobic degradation activity exhibited a significantly higher net removal of toluene and ethylbenzene (up to 100%) when compared to a homogeneous setup. However, under mixing-controlled conditions, the plume’s fringe also exhibited zones with overlapping reactants during anaerobic degradation experiments. This indicated additional limiting factors or processes besides mixing. The slower reaction kinetics of denitrification were suggested to be responsible as the faster aerobic degradation never showed overlaps during this work. Although biokinetics played a minor role when compared to transverse dispersive mixing, the data indicated that additional limiting factors may be of greater importance in the field.
Author and committee
dc:creator, dc:contributor.*- Author
-
- Bauer, Robert
Identifiers
dc:identifier.*- Identifier
- hdl:10900/49124