IMPORTANCE OF SEDIMENT FINES IN LABORATORY STUDIES ON THE DEGRADATIONOF 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 de gradation capacity was investigated for two aquifers in aerobic labora tory batch incubations with a mixture of eight aromatic hydrocarbons, benzene, toluene, o-xylene, 1,2-dichlorobenzene, 1,4-dichlorobenzene n aphthalene, biphenyl and nitrobenzene. The degradation capacity of sus pensions of fines in groundwater and suspensions of complete sediment in groundwater were very similar, although not identical. Comparison o f suspensions of autoclaved fines with suspensions of non-autoclaved f ines 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 increa se in bacterial numbers (enumerated by plate counting) and a substanti al number of bacteria in the fines phase at the end of the experiment clearly showed that the autoclaved fines were colonized during the inc ubation and thus also acted as biomass support material. This indicate s that a part of a bacteria able to degrade the investigated readily d egradable and dissolvable compounds occurred freely suspended in the w ater. Diluting the suspensions of fines (five or ten times) did not in fluence the degradation capacity seriously, but it increased the diffe rence between autoclaved and non-autoclaved fines, since the degradati on rates decreased in the suspensions of autoclaved fines, especially for the more sorbing compounds naphthalene and biphenyl, where lag-pha ses were also observed. This indicates that, below a certain lower lev el 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 labo ratory degradation experiments representing aquifers (e.g. deep wells) from where solids are difficult to obtain. (C) 1997 Elsevier Science Ltd.