INVESTIGATION OF THE POTENTIAL FOR MICROBIAL-CONTAMINATION OF DEEP GRANITIC AQUIFERS DURING DRILLING USING 16S RIBOSOMAL-RNA GENE SEQUENCING AND CULTURING METHODS

Total number of bacteria, viable counts of aerobic and anaerobic heter otrophic bacteria and 16S rRNA gene diversity were investigated during drilling of three boreholes in the walls of the Aspo hard rock labora tory tunnel, at depths ranging from 380 to 446 m below sea level. Wate r samples were taken from the drill water source, the drilling equipme nt and from the drilled boreholes. The drill water was kept under nitr ogen atmosphere and all equipment was steam cleaned before the start o f a new drilling. Total and viable counts of bacteria in the drilled b oreholes were several orders of magnitude lower than in the samples fr om the drilling equipment, except for sulphate reducing bacteria. A to tal of 158 16S rRNA genes that were cloned from the drill water source , the drilling equipment and the drilled boreholes were partially sequ enced. The drilled boreholes generally had a 16S rRNA diversity that d iffered from what was found in samples from the drilling equipment. Se veral of the sequences obtained could be identified on genus level as one of the genera Acinetobacter, Methylophilus, Pseudomonas and Shewan ella. In conclusion, the tubing used for drill water supply constitute d a source of bacterial contamination to the rest of the drilling equi pment and the boreholes. The results show, using molecular and culturi ng methods, that although large numbers of contaminating bacteria were introduced to the boreholes during drilling, they did not establish i n the borehole groundwater at detectable levels. (C) 1997 Elsevier Sci ence B.V.