The microbiology of groundwater, contaminated with phenol and other tar acids, was analysed using a range of bacteriological, biogeochemical process-related and molecular techniques and the data was used to assess the microbial biodegradative potential in a contaminated plume (Lerner et al., 2000). The contaminant plume front was shown to be 500 m from the pollutant source and moving at 10 m year^-1. Two multilevel samplers (MLS) positioned in different regions within the plume (boreholes 59 and 60) were constructed (Thornton et al., 2001). Activity of the microbial community, as represented by phenol degradation potential and ability to utilise a range of substrates, was found to be influenced by the plume contaminant concentration. The highest phenol concentrations were shown to depress bacterial numbers, in some areas to <10 cells per ml (Pickup et al., 2001). In both MLS, bacterial groups or biogeochemical processes (e.g. methanogenesis, sulphate reduction and denitrification) often associated with anaerobic degradation of contaminants were identified (Pickup et al., 2001). Microbial activity measurements, combined with chemical analyses and hydrological data, allowed an assessment of the effects of the contaminant plume on the groundwater microbiology which could, in turn, be related to the potential for natural attenuation of the site (Pickup et al., 2001).