The Boom Clay layer is presently investigated as a potential host rock for geodisposal of nuclear waste in Belgium. The HADES underground research facility (EIG Euridice c/o SCK?CEN), located at 230 m depth under the site of SCK?CEN (Mol, Belgium), provides access to this clay layer for in situ geological, geochemical and geomicrobiological testing. In order to predict how microbiology will affect the biogeochemical processes in a disposal scenario, the resident microbial communities of Boom Clay and the man-made structures within this clay are being characterised. \tIn this study, water samples were collected from Boom Clay via various existing HADES piezometers. Microbial cells from these samples were concentrated on a 0.45 ?m filter membrane, followed by DNA extraction, PCR amplification of the V1-V3 region of bacterial 16S rDNA, automated sequencing and an in-house developed bio-informatics pipeline. The aim was to assess differences or shared features of the microbial communities residing in piezometer boreholes, supplementary to the previous screening of a single, vertical piezometer [1], and to correlate variations to geochemical analyses. While the boreholes of two piezometers seem highly enriched in one family of Betaproteobacteria (Rhodocyclaceae), the other piezometers seem to balance dominance between members of the Chloroflexi, Proteobacteria, Actinobacteria, Firmicutes, Bacteroidetes and Chlorobi, although in different ratios and with variations in overall diversity. Along the five piezometers, bacterial communities of the filters within one piezometer seem more similar to each other compared to those in other piezometers, despite a variety of filter materials or Boom Clay layers sampled within one piezometer. \tThe observed variability in bacterial community composition suggests enrichment of certain members of the community according to the engineering properties of the piezometer installation. Although such locally enriched microbial community might not be representative for the total (engineered) clay environment, it shows that technical installations (such as piezometers) can introduce and promote local variations in the otherwise oligotrophic clay environment and the associated bioprocesses. \tFurther studies of other piezometers and of clay samples are needed, to pinpoint the source bacterial community underlying in situ enrichment, to unravel the mechanism that shapes such microbial community in different repository conditions and to outline the relevance of the (dominant) microbial classes in defining borehole water (and gas) chemistry. [1]\tWouters K, Moors H, Boven P & Leys N (2013) Evidence and characteristics of a diverse and metabolically active microbial community in deep subsurface clay borehole water. FEMS Microbiol Ecol. 86: 458-473.