Vertical flux of hexachlorobenzene in coastal waters of the north-west Mediterranean Sea.

In a study on the biogeochemistry of chlorinated hydrocarbons in the Mediterranean Sea, hexachlorobenzene (HCB) has been analysed in particulate matter collected in sediment traps moored 2 km off the coast of Monaco. HCB concentrations in this material, which was composed largely of biogenic debris, ranged from non-detectable to 7.3 ng/g dry weight. Time-series measurements (n = 18) made between March-September 1981 demonstrated that the vertical flux of HCB through 100 m averaged 59 (range, non-detectable to 153) pg/cm2 per year. There was no clear relationship between the HCB flux and the time of year. Nor were there any correlations between either HCB concentrations in fast-settling particles or the HCB flux and that of organic carbon. The similarity in concentration of HCB in sediment-trap particulates and freshly excreted zooplankton faecal pellets suggests that faecal pellet deposition acts to remove this contaminant from surface waters. Comparison of the observed average HCB flux through 100 m with estimates of the flux to sediments at 250 m depth (7.3 pg/cm2 per year, based on HCB concentrations in surface sediments and independently derived average sediment-deposition rates) indicates that as much as 87% of the particulate-associated HCB is dissolved or degraded before burial into sediments. These results demonstrate that HCB is removed from the euphotic zone by sedimentation processes less rapidly than polychlorinated biphenyls but on the same time-scale as the pesticide lindane. An upper limit on the residence time in surface waters was calculated to be 135 years.(ABSTRACT TRUNCATED AT 250 WORDS)