Importance of sediment fines in laboratory studies on the degradation of organic chemicals in aquifers

The importance of fine fractions of aquifer sediment was investigated with respect to microbial degradation capacity, as quantified in terms of number of organic compounds degraded and degradation rates. The degradation capacity was investigated for two aquifers in aerobic laboratory batch incubations with a mixture of eight aromatic hydrocarbons, benzene, toluene, o-xylene, 1,2-dichlorobenzene, 1,4-dichlorobenzene, naphthalene, biphenyl and nitrobenzene. The degradation capacity of suspensions of fines in groundwater and suspensions of complete sediment in groundwater were very similar, although not identical. Comparison of suspensions of autoclaved fines with suspensions of non-autoclaved fines showed that the degradation rates were only slightly lower in the suspension with autoclaved fines, but the lag-phases were distinctly prolonged, and thus the fines acted as an inoculum. However, an increase in bacterial numbers (enumerated by plate counting) and a substantial number of bacteria in the fines phase at the end of the experiment clearly showed that the autoclaved fines were colonized during the incubation and thus also acted as biomass support material. This indicates that a part of a bacteria able to degrade the investigated readily degradable and dissolvable compounds occurred freely suspended in the water. Diluting the suspensions of fines (five or ten times) did not influence the degradation capacity seriously, but it increased the difference between autoclaved and non-autoclaved fines, since the degradation rates decreased in the suspensions of autoclaved fines, especially for the more sorbing compounds naphthalene and biphenyl, where lag-phases were also observed. This indicates that, below a certain lower level of area of solids per litre water, the fines were unable to sustain the degrading bacterial population. The paper discusses the practical implication of the obtained results for conducting and improving laboratory degradation experiments representing aquifers (e.g. deep wells) from where solids are difficult to obtain.