Replacement and authigenic mineralogy of metal contaminants in stream and estuarine sediments at Newtownards, Northern Ireland

Tidal mudflats are locally enriched in heavy metals at the head of Strangford Lough in Northern Ireland, where drainage from the hinterland enters the sea-lough via a tidal canal in an urban area. To characterise the metallic contaminants and investigate their provenance, heavy particles separated from stream, canal and estuarine sediments were analysed by electron microprobe and laser Raman microspectroscopic methods. Potential metal sources are mineralization in the catchment area and industrial or domestic pollution. Anthropogenic particles include metallic grains, alloys and compounds of Pb, Zn, Cu, Fe, Cr and Sn. Alteration of metallic particles includes de-zincification of brass in freshwater sediment and replacement of copper wire by covellite in brackish to marine sediment. Mobility of Cu, Fe and S in canal sediments is indicated by the authigenic growth of framboidal iron sulphide on oxide substrates and of chalcopyrite rims on covellite. Intricate colloform and platy crystalline textures suggest a cyclical deposition of covellite and chalcopyrite under conditions of varying redox and salinity. Lead and chromium mobility in the contaminated estuarine sediment is shown by the formation on lead-rich substrates of heterogeneous Pb- and Cr-rich sulphate-phosphate compounds and of a Pb-oxychloride phase.