Naturally occurring microorganisms mediate the
reductive dechlorination of the sediment-bound
chlorobenzenes. However, because of the refractory
nature of the sediment natural organic matter, as well
as strong contaminant adsorption, a low rate and
extent of biotransformation were attained. The
dechlorination rate of sediment-bound HCB was two
orders of magnitude lower than that of freshly added,
and therefore bioavailable, HCB. An increase in
contaminant hydrophobocity due to the effect of
salinity, as well as the relatively high contaminant
residence time, may have contributed to the observed
reduced contaminant bioavailability. On the other
hand, microbial reductive dechlorination of the
sediment-bound contaminants leads to the long-term
release of less chlorinated and more mobile
chlorinated benzenes. Sediment nutrients (such as
nitrogen and phosphorus) did not limit the biodegraration
of the natural organic matter. The low
electron transfer efficiency towards reductive dechlorination,
as well as the low extent of biotransformation
of the sediment contaminants, need furtherinvestigation. A better understanding of the factors
involved in the bioavailability and transformation of
the sediment-bound contaminants will lead to a more
accurate estimation of the impact of contaminant
release on the aquatic environment, as well as to the
development of means to either accelerate or control
the dechlorination process for remediation purposes.
Acknowledgements--We gratefully acknowledge the assistance
of Ms Emilise Cormier and Mr Kirk Manuel of the
Louisiana Department of Environmental Quality. This
work was supported by the US Environmental Protection
Agency/Hazardous Substance Research Center/South &
Southwest. This article has not been subjected to Agency
review; therefore, it does not necessarily reflect the views of
the Agency and no official endorsement should be inferred.