Effects of mixing processes on microbial degradation and the distribution of microorganisms in stationary and non-stationary contaminant plumes

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In the proposed project we want to study the effects of mixing processes and changing groundwater flow regimes on the microbial degradation potential in contaminated aquifers. Microbial degradation activities will be quantified by compound-specific isotope analysis (CSIA), i.e. the stable isotope fractionation of 13C/12C and D/H in the residual, not yet degraded contaminant fraction. In laboratory tank experiments artificial contaminant plumes (e.g. toluene) will be produced under stationary or transient flow conditions. The augmentation of the tank system with well defined bacterial cultures and the supply of mixtures of deuterated and non-deuterated contaminants will allow to quantify microbial degradation at a high resolution (mm-scale). Because of extremely different fractionation properties of aerobic and anaerobic strains we will be able to differentiate between the respective contributions of the two to the overall degradation processes. Furthermore, the effects of stationary and non-stationary flow conditions on the introduced microorganisms will be investigated. Here the overall question is how tolerant and/or flexible are microorganisms which are confronted with frequently changing environmental conditions (e.g. oxic-anoxic) and how long after a disturbance it might take to initiate and resuscitate a pronounced degradation. The concept will also be investigated in the field where a cross section through a contamination plume with a 50 x 50 cm resolution of the sampling spots will be sampled. CSIA will provide information on the spatial distribution of biodegradation activities in situ. By means of molecular methods the field samples will also be analysed for the presence of different groups (e.g. sulfate-reducing and iron-reducing bacteria) and individual strains of microorganisms (the ones we isolated from the actual site). Comparisons of results from laboratory and field investigations should reveal general insights into the effect of mixing and non-stationary flow conditions on natural attenuation processes in contaminated aquifers.