Recovery of the Mersey Estuary from Metal Contamination

Abstract

The Mersey Estuary has received significant quantities of industrial wastes and sewage over several decades. Although contaminant 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 geochemical 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 o f sediments and possible salting out of neutral metal-organic complexes, assist in the retention and internal recycling of particles and associated metals between the bed and water column. Geochemical reactivity is suppressed in 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. Future declines in metals from bed sediments have been estimated using two methods and two independent data sets. Resulting values are not only metal-dependent but also vary with sediment location. Losses of Cd, Co, Cu, Hg, Ni and Zn are predicted to take up to 40 years, whilst removal of substantially elevated concentrations of Pb in sediments in the upper estuary could span hundreds of years.