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In previous papers, we examined the use of deserted and addled eggs of the Dipper Cinclus cinclus as monitors of the contamination of upland rivers by persistent organochlorines. Here, we examine and interpret differences between the organochlorine content of eggs from adjacent sub-catchments of rivers in Wales. We provide information particularly on individual polychlorinated biphenyls (PCBs), for which few data are available on wild birds generally.

There were no significant differences between catchments in egg concentrations of total PCBs, although two individual PCB chemicals (or congeners) showed significant differences in concentration or incidence of occurrence. PCB congener 118 (2,3’, 4,4’, 5 Pentachlorobiphenyl) showed a reduction in concentration on a SW-NE gradient through south and mid-Wales reflecting possible sources in the coastal industrial belt. However, significantly elevated concentrations in Irish eggs suggest the possibility of more remote origins to the west. PCB 101 (2,2,4,5,5’ Pentachlorobiphenyl) was found significantly more often in eggs from sub-catchments in mid-Wales than from south Wales. Further data on the sources, transport, persistence and ecotoxicity of individual PCB congeners are required before these results can be seen in context. There was a marked reduction in total PCB concentrations in Dipper eggs between 1988 and 1990 which continued into 1991.

Significant variation between catchments in HEOD (from the insecticides Aldrin and Dieldrin) reflected land use. Levels in eggs increased significantly where river catchments had high sheep densities. HEOD concentrations in eggs correlated with bacteriological features in river water which were indicators of high stock densities. These patterns are consistent with the former legal or latterly illicit use of Dieldrin in sheepdip, although the concentrations present in Dipper eggs declined between 1988 and 1990.

We conclude that, at least within regions, Dipper eggs can aid in the detection of local patterns in the contamination of rivers by some persistent organochlorines.