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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).

A range of bacteriological, geochemical process-related and molecular techniques have been used to assess the microbial biodegradative potential in groundwater contaminated with phenol and other tar acids. The contaminant plume has travelled 500 m from the pollutant source over several decades. Samples were obtained from the plume using a multi-level sampler (MLS) positioned in two boreholes (boreholes 59 and 60) which vertically transected two areas of the plume. 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. Phenol degradation potential appeared to be influenced more by the concentration of the contaminants than the total bacterial cell numbers. However, in the areas of highest phenol concentration, the depression of cell numbers clearly had an effect. The types of bacteria present were assessed by culture and DNA amplification by polymerase chain reaction (PCR). Bacterial groups or processes associated with major geochemical processes, such as methanogenesis, sulphate reduction and denitrification, that have the potential to drive contaminant degradation, were detected at various borehole levels. A comparative molecular analysis of the microbial community between samples obtained from the MLS revealed the microbial community was diverse. The examination of microbial activity complemented those results obtained through chemical analysis, and when combined with hydrological data, showed that MLS samples provided a realistic profile of plume effects and could be related to the potential for natural attenuation of the site.

The distillation of acidified coal tars for up to 50 years has given rise to a phenol plume approximately 500 m long, 50 m deep and containing up to 15 g l-1 dissolved organic carbon (DOC) in the Triassic Sandstones aquifer. A conceptual biogeochemical model based on chemical and microbiological analysis of groundwater samples has been developed as a preliminary to more detailed studies of the controls on natural attenuation. While the development of redox zones and the production of methane and carbon dioxide provide evidence of natural attenuation, it appears that degradation is slow. The existence of sulphate in the plume indicates that this electron acceptor has not been depleted and that consequently methanogenesis is probably limited. Based on a simple estimate of sulphate input concentration, a half-life of about 15 years has been estimated for sulphate reduction. Geochemical modelling predicts that increased alkalinity within the plume has not led to carbonate precipitation, and thus within the limits of accuracy of the measurement, alkalinity may reflect the degree of biodegradation. This implies a loss of around 18% of the DOC over a 30-year period. Despite limited degradation, microbial studies show that there are diverse microbial communities in the aquifer with the potential for both anaerobic and aerobic biodegradation. Microbial activity was found to be greatest at the leading edge of the plume where DOC concentrations are 60 mg l-1 or less, but activity could still be observed in more contaminated samples even though cells could not be cultured. The study suggests that degradation may be limited by the high phenol concentrations within the core of the plume, but that once diluted by dispersion, natural attenuation may proceed. More detailed studies to confirm these initial findings are identified and form the basis of associated papers.

Nickel-resistant bacteria were isolated from effluent discharged from a sewage treatment outfall over an 18-month period. One of these strains, Enterobacter cloacae FBA30, was found to harbor a narrow host range conjugative plasmid, designated pFBA30, which confers nickel resistance on its host. A 10.2-kb SstI restriction fragment was cloned from pFBA30 and was shown to specify inducible nickel resistance, both in its original host and in laboratory strains of Escherichia coli. This DNA fragment, and a 1.75-kb SmaI fragment derived from it, were used as probes to examine other bacterial strains isolated during the study. Homologous nickel resistance genes were detected in enteric bacteria emerging directly from the treatment plant, but not in strains isolated from sediments downstream of the outfall, or in strains harboring well-characterized determinants such as ncc/nrc, cnr, or Klebsiella oxytoca type. Thus, this element constitutes a new nickel resistance determinant and was named nrf. Nickel-resistant strains were sorted into two groups, based on the sampling date and the size of restriction fragments homologous to probes developed from pFBA30. Group A isolates all carried a homologous 6.5-kb PvuII restriction fragment and expressed nickel resistance constitutively. Group B strains, isolated independently from the same outfall, were different from group A in that the nickel resistance was associated with a 4.7-kb PvuII fragment, and group B strains displayed a slower growth rate on nickel salts media. Each group contained a range of enteric bacterial species, including K. oxytoca, Citrobacter freundii, and Enterobacter spp. Localized genetic exchange probably occurs within wastewater treatment effluents, and, once effluents are discharged into the environment, enteric bacteria carrying pFBA30 like determinants do not persist downstream.