Recovery of the Mersey Estuary from metal contamination
The Mersey Estuary has received significant quantities of industrial wastes and sewage over several decades. Although contarninant loads are reducing and the estuary is showing signs of recovery, the sediment reservoir remains a repository of historical contamination and still contains high concentrations of trace metals and organic compounds. A combination of hydrodynamic, sedimentary and geochernical processes are responsible for maintaining trace metal concentrations at present-day levels. The distributions of trace metals in bed sediments reflect changes in granulometry, differences in POC content and the magnitude of past inputs rather than the locations of point sources in the estuary. The association of contaminant metals with SPM varies not only with axial changes in salinity and particle concentration but also in response to the relative magnitudes of freshwater and tidal inflows and cyclic variations in water and particulate chemistry that occur on intratidal, intertidal and seasonal timescales. The most influential of these arise from axial changes in dissolved oxygen and the delivery. of organic carbon from both external and internal sources which modify the relative degree of sorptive control exerted by Fe, Mn and organic C at different locations in the estuary and at different times. These factors, combined with the efficient trapping of sediments and possible salting out of neutral metalorganic complexes, assist in the retention and internal recycling of particles and associated metals betweent he bed and water column. Geochernicalr eactivity is suppressedin Mersey SPM and metal decontamination is not predicted to occur through the loss of particulate metals to the surrounding coastal zone. Rather, it is envisaged that sediment resuspension and the desorption of metals into fresh and low salinity waters, supplemented by the release of metals from tidally stirred diagenetically modified sediments, are more likely to be important long term cleansing mechanisms, with the latter occurring particularly during the summer months when bacterial numbers and the degradation of accumulated organic detritus becomes more pronounced. Futured eclinesi n metalsf rom bed sedimentsh aveb eene stimatedu sing two methodsa nd two independendt ata sets. Resultingv aluesa re not only metal-dependenbtu t also vary with sedimentlo cation.L osseso f Cd, Co, Cu, Hg, Ni andZ n arep redictedt o takeu p to 40 years,w hilst removal of substantiallye levatedc oncentrations of Pb in sedimentsin the upper estuary could span hundreds of years.