Persistent organochlorine contaminants in the Firth of Clyde in relation to sewage sludge input

The concentrations of faecal sterols, chlorobiphenyls and several organochlorine pesticides have been determined in surface sediment and sediment cores from the Firth of Clyde, Scotland. Faecal sterols and elevated organochlorine levels were present over the entire Firth. Sediment from the current sewage sludge dump ground contained the highest concentrations of organic carbon (11.9%), coprostanol (176 mg/kg), 4,4′-DDE (19 μg/kg), chlordanes (39 μg/kg) hexachlorobenzene (12 μg/kg), lindane (11 μg/kg) and pentachloro-amino-diphenylether (12 μg/kg). The concentrations of chlorobiphenyls (500 μg/kg), dieldrin (62 μg/kg), 4,4′-DDT (171 μg/kg) and 4,4′-DDD (228 μg/kg) were highest in sediment from the former sludge dump ground. Chlorobiphenyl and faecal sterols levels on the former dump ground were similar to those measured 20 years ago. Examination of (1) the spatial distribution of contaminants, (2) the relative concentrations of faecal and natural sterols in sediment, and (3) the relative proportions of organochlorine contaminants by cluster analysis suggests that the sludge disposal operation is the major source of these compounds to the entire Firth. A quantitative estimate of the degree of sludge dispersion obtained from faecal sterol measurements suggests that sludge constitutes 100% of the sediment organic input up to 2 km and 60% up to 4 km from the centre of the present dump ground. Only 10% of the reported input of chlorobiphenyls to the current dump ground is present within the designated area. The concentrations of certain compounds on the dump grounds exceed proposed ‘sediment safe levels’.