Merged with duplicate record 10026.1/596 on 27.03.2017 by CS (TIS)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.UK Environment Agency (North West Region
