1. Laboratory of Microbial Ecology and Technology, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, B-9000 Ghent, Belgium, Cell of Groundwater Modelling, Faculty of Science, Ghent University, Krijgslaan 281 S8, B-9000 Ghent, Belgium, and Avecom n.v., Bloemendalestraat 138, B-8730 Beernem, Belgium