SEDIMENT BIOBARRIERS FOR CHLORINATED ALIPHATIC HYDROCARBONS IN GROUNDWATER REACHING SURFACE WATER

Polluted groundwater in urban and industrial areas often represents a continuous source of diffuse contamination of surface waters. However, the fate of the infiltrating groundwater pollutants might be influenced by the sediment in eutrophic water bodies since they might possess characteristic biological and physico-chemical properties influencing pollutant degradation. In a part of the eutrophic river Zenne (Vilvoorde, Belgium) we studied the intrinsic capacity of river sediment microbial communities to degrade Chlorinated Aliphatic Hydrocarbons (CAHs). The CAH concentrations determined in groundwater, sediment and pore water samples indicate that the Zenne locally drains groundwater polluted with vinyl chloride (VC) and cis-1,2-dichloroethene (cis-DCE) The groundwater discharge spatial distribution is highly heterogeneous, mostly depending on local sediment permeability in the riverbed At some locations full dechlorination was observed, while VC and cis-DCE reach the surface water and discharge into the river at spots with a high-velocity groundwater influx. To monitor the CAH influx in function of time, Teflon pore water samplers were installed at different locations and depths in the riverbed and sampled monthly Anaerobic batch degradation tests with sediment material from the Zenne showed a rapid microbial dehalogenation of both VC and cis-DCE to non-toxic ethene (1000 mu g/L in 19 days). In congruence, chloroethene degrading Dehalo-coccoides species were present in the sediment as shown by detection of their 16S rRNA genes. Furthermore, the VC reductive dehalogenases vcrA and bvcA from respectively Dehalococcoides sp strain VS and BAV1 were detected in the river sediment by PCR analysis. Both Dehalococcoides species can grow on VC and cis-DCE as a sole energy source and are probably responsible for the observed degradation of these groundwater pollutants in the laboratory and in situ in the riverbed.