and processing. In densely populated regions, metals
originate from a wide range of industrial, domestic
and agricultural uses, and their inputs into river
systems have increased significantly over the past
two centuries. Regulated dredging and dumping of
metal pollutants at sea, inadvertent spills, and illegal
discharge all add to the complexity of anthropogenic
inputs to the aquatic environment. Thus, our ability
to unravel natural versus anthropogenic inputs is
often complicated by the significant and uncontrolled
human perturbation of catchments and their river
systems. Only where metal compounds are entirely
of anthropogenic origin, such as tributyl tin, can the
human impact be evaluated. In the case of lead,
however, it has been possible to identify man-made
inputs via the application of inductively coupled
plasma mass spectrometry (ICP-MS) to the
determination of lead isotopic ratios (e.g.,
206 Pb to
207 Pb) which have distinct signature in leaded
gasoline.
Because a significant proportion of the marine
environment has been altered by anthropogenic activities, natural concentrations values for dissolved
and particulate metals are difficult to obtain unambiguously. Baseline values are often assumed from
analyses of samples from remote systems that are
considered to be ‘pristine’ or from metal analyses of
sediment horizons dated as being prior to the industrial revolution. Another approach to assessing
man’s impact on the global ocean is to compare the
rates of metal emission to the atmosphere from
natural and anthropogenic sources. In Table 1 the
‘interference factor’ is 41 for all metals, with relatively high values for lead and cadmium, suggesting
that there is a significant anthropogenic alteration of
their natural cycles.
Macrotidal. estuaries have strong internal cycles
and particles may be retained within the system for
years, and in large estuaries for decades. Thus, estuaries are a significant repository for metals, although no systematic inventories of the sediment
metal burden have been made. Suspended particles
advecting from estuaries into shelf seas are trapped
in the coastal margin and estimates show that
B90% of the fluvial suspended load (and associated
metals) of the Mississippi, St. Lawrence, Rho ˆ ne, and
rivers in the south east of the United States is deposited in the coastal margin. Early diagenesis of
deposited material may result in release of metals
into sediment pore waters and, since the dissolved
phase is generally considered to be more bioavailable, the composition of interstitial waters could be
more important in the overall toxicity of the sediments than is their total metal content. Accurate
quantification of interstitial water composition in
estuaries and shelf seas is hindered because of the
heterogeneous distribution of sediment texture and
because a satisfactory method has not yet been developed for application in shallow waters that are
highly dynamic.
Particulate metals deposited in the coastal margin
are slowly advected onto the continental slope by
seabed currents and wave action. Sediment diagenesis and diffusion releases dissolved metals into
the oceanic water column, where they may be involved in upwelling processes at the shelf break
(Figure 1). Comparisons of the relative magnitudes
of the combined river and atmospheric fluxes with
the upwelling flux suggests that the latter is greater
by a factor 2 for copper, of 2–7 for zinc, and of about
10 for cadmium. In contrast for inorganic mercury
the upwelling flux to shelf seas is half the magnitude
of the combined river and atmospheric input, while
for methylated mercury the main source to shelf seas
is the upwelling flux.
Distributions
The temporal and spatial distributions of dissolved
metal pollutants are highly dependent on two important processes in the coastal boundary zone.
Local hydrodynamics. Water is dispersed in estuaries and coastal waters according to the local
hydrodynamics, which can be characterized by the
flushing time. The dispersion and dilution of metal
pollutants from point and diffuse sources, and therefore their range of concentrations, will be affected by
the flushing time. A flushing time of 0.5 y for coastal
waters is typical for the North Sea and Irish Sea, a
value of 2 y is representative of the waters around
Bermuda, while a value of 5 y is representative of a
semi-enclosed sea such as the Baltic. Waters with
longer flushing times will register a slower response to
changes in metal inputs, whereas changes in metal
concentrations will be detected earlier in waters with
shorter flushing times.
Particle–water interactions. Metal partitioning
between the dissolved and particulate phases is a
crucial factor because solutes are transported in a
different way to particles. The latter experiences
gravitational settling and aggregation, as well as
advection and mixing. The concentrations of SPM
and the types of SPM play a significant role on the
fraction of metal carried in the particulate phase.
Lead has a relatively high K D , largely owing to its
tendency to complex with carboxyl and phonolic
groups that dominate the surfaces of natural particles. In contrast, the relatively low K D for cadmium
is the result of its ability to complex with chloride
METAL POLLUTION 267
originate from a wide range of industrial, domestic
and agricultural uses, and their inputs into river
systems have increased significantly over the past
two centuries. Regulated dredging and dumping of
metal pollutants at sea, inadvertent spills, and illegal
discharge all add to the complexity of anthropogenic
inputs to the aquatic environment. Thus, our ability
to unravel natural versus anthropogenic inputs is
often complicated by the significant and uncontrolled
human perturbation of catchments and their river
systems. Only where metal compounds are entirely
of anthropogenic origin, such as tributyl tin, can the
human impact be evaluated. In the case of lead,
however, it has been possible to identify man-made
inputs via the application of inductively coupled
plasma mass spectrometry (ICP-MS) to the
determination of lead isotopic ratios (e.g.,
206 Pb to
207 Pb) which have distinct signature in leaded
gasoline.
Because a significant proportion of the marine
environment has been altered by anthropogenic activities, natural concentrations values for dissolved
and particulate metals are difficult to obtain unambiguously. Baseline values are often assumed from
analyses of samples from remote systems that are
considered to be ‘pristine’ or from metal analyses of
sediment horizons dated as being prior to the industrial revolution. Another approach to assessing
man’s impact on the global ocean is to compare the
rates of metal emission to the atmosphere from
natural and anthropogenic sources. In Table 1 the
‘interference factor’ is 41 for all metals, with relatively high values for lead and cadmium, suggesting
that there is a significant anthropogenic alteration of
their natural cycles.
Macrotidal. estuaries have strong internal cycles
and particles may be retained within the system for
years, and in large estuaries for decades. Thus, estuaries are a significant repository for metals, although no systematic inventories of the sediment
metal burden have been made. Suspended particles
advecting from estuaries into shelf seas are trapped
in the coastal margin and estimates show that
B90% of the fluvial suspended load (and associated
metals) of the Mississippi, St. Lawrence, Rho ˆ ne, and
rivers in the south east of the United States is deposited in the coastal margin. Early diagenesis of
deposited material may result in release of metals
into sediment pore waters and, since the dissolved
phase is generally considered to be more bioavailable, the composition of interstitial waters could be
more important in the overall toxicity of the sediments than is their total metal content. Accurate
quantification of interstitial water composition in
estuaries and shelf seas is hindered because of the
heterogeneous distribution of sediment texture and
because a satisfactory method has not yet been developed for application in shallow waters that are
highly dynamic.
Particulate metals deposited in the coastal margin
are slowly advected onto the continental slope by
seabed currents and wave action. Sediment diagenesis and diffusion releases dissolved metals into
the oceanic water column, where they may be involved in upwelling processes at the shelf break
(Figure 1). Comparisons of the relative magnitudes
of the combined river and atmospheric fluxes with
the upwelling flux suggests that the latter is greater
by a factor 2 for copper, of 2–7 for zinc, and of about
10 for cadmium. In contrast for inorganic mercury
the upwelling flux to shelf seas is half the magnitude
of the combined river and atmospheric input, while
for methylated mercury the main source to shelf seas
is the upwelling flux.
Distributions
The temporal and spatial distributions of dissolved
metal pollutants are highly dependent on two important processes in the coastal boundary zone.
Local hydrodynamics. Water is dispersed in estuaries and coastal waters according to the local
hydrodynamics, which can be characterized by the
flushing time. The dispersion and dilution of metal
pollutants from point and diffuse sources, and therefore their range of concentrations, will be affected by
the flushing time. A flushing time of 0.5 y for coastal
waters is typical for the North Sea and Irish Sea, a
value of 2 y is representative of the waters around
Bermuda, while a value of 5 y is representative of a
semi-enclosed sea such as the Baltic. Waters with
longer flushing times will register a slower response to
changes in metal inputs, whereas changes in metal
concentrations will be detected earlier in waters with
shorter flushing times.
Particle–water interactions. Metal partitioning
between the dissolved and particulate phases is a
crucial factor because solutes are transported in a
different way to particles. The latter experiences
gravitational settling and aggregation, as well as
advection and mixing. The concentrations of SPM
and the types of SPM play a significant role on the
fraction of metal carried in the particulate phase.
Lead has a relatively high K D , largely owing to its
tendency to complex with carboxyl and phonolic
groups that dominate the surfaces of natural particles. In contrast, the relatively low K D for cadmium
is the result of its ability to complex with chloride
METAL POLLUTION 267