The effects of eight years aeration and isolation from polluting discharges on sewage- and metal-contaminated sediments

The Manchester Ship Canal (MSC) has been the recipient of domestic and trade effluent since 1895, it continues to be grossly polluted. In 1985 the dock basins, now known as Salford Quays were isolated from the canal inorder to improve water quality and encourage redevelopment. Subsequent to isolation the dock basins received no effluent or drainage and Helixor pumps were installed to improve circulation, thereby preventing stratification and bottom water anoxia. Analysis of solid material, pore waters and phospholipid fatty acids (PLFA) in cores taken from Salford Quays and the MSC was carried out to assess changes in sediment characteristics that might affect water quality. Loss of carbon was apparent in the upper sediment of Salford Quays, as was a greater proportion of reducible Fe, Mn, Zn and Cd compared with the MSC. In Salford Quays a superficial peak in Fe and Zn concentration appeared to be attributable to migration of metals in the pore water and precipitation of Fe oxyhydroxide at the oxic sediment water interface. Despite these differences, NH +4SO 2−4concentration pore water profiles were similar in both sediments if the MSC sediments were considered from a point below their top layers, which appeared to be composed of freshly deposited sewage. Bacterial biomass extrapolated from PLFA concentration also suggested that the upper sediment of the MSC was largely faecal. PLFA analyses to characterize changes in the microbial community, however, did not reveal any systematic changes. That this may have been because of an absence of vertical zonation was supported by pore water analyses. It was also apparent that the lack of systematic change might be due at least in part to an artefact of vestigial PLFA signatures, resulting from deposition and burial, and the need for finer vertical resolution in the sampling procedure. Despite incomplete and some contradictory findings it appears that although metal mobilization may result from the development of an oxic sediment water interface, the extremely high original organic content of the sediment ensures that even after 10 years it exerts a high oxygen demand. Consequently, sediment management is likely to be a long‐term commitment and as remediation proceeds the importance of continuity in management will increase. Copyright © 1999 John Wiley & Sons, Ltd.