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Oftenr a direct comparison is made between the level of metals in coastal waters and
the average world ocean values, and if the former level is considerably higher, Lhen it is
judged to be "very polluted". In fact, as any marine scientist knows, the natural background
levels of the two systems are vastly different, and estuarine areas must. be compared to
similar coastal environments.
Heavy metals in sediments
The study of sediments is of greal import.ance in measuring the distribution of heavy
retals, since for the majority of the elenrents and substances, sediments are the ultimate
rrsinkil or ttdeposit". Nat,ural sediments are mixtures of sands, clays and organic substances.
The relative abundance of these components varies considerably with dif f erent types of
sediment. The interaction of heavy metals with the sediments depends on their composition.
Higher concenLrations of metals are usually found in mud and silt, rather than in sand, so the
grain-size cfiaracteristies of the sediment samples must be assessed in interpreting the metal
values observed.
Heavv metals in marine oroanismg
The concern about heavy melals in food has stimulated the analysis of toxic metals in
fishery products: edible crustaceans, molluscs, and fish. Different species collected at the
same site show great differences in their metal concentrations. Consequently body contents
can rtly be compared between specimens of lhe same species, preferably of the same age or
size. Details m the use of aquatic organisms as biological indicators are given by Phillips in
his book rrGuantitative Aquatic Biological Indicators" (1980a). His proposal for monitoring
studies m the contamination of the Eest Asian Seas by trace metals and organochlorines
(Phillips, 1980b) is a very useful gnride.
TtE ability of bivalves to concentrate zubstances far above their levels in the
environnent makes them useful as sentinel organisms for indicating level of pollutants in
coastal rnarine waters. This is the basis for the Mussel Watch Programme (Goldberg et {.,
1978), now in qeration in the U.S. and some European countries for some years. Sorne
countries in the East Asian region have also joined the prognamme.
Hesvv metals in rivers
Yeats et al. (1978) have demonstrated that river monitoring gives a faster and more
reliabb warnlng-of changes dre to anthropogenic activity tfran npnitoring of coastal waters.
Thus major rivers ehould also be rnonitored for any possible change in pollutant levels.
Heavv metelg in the atmosphere
Garrels and Mackenzie (f971.) estimated that rivers account for 90% of the total
geaward transport of dissolved and suspended solids. However Windom (f981) reported that in
lome ceses, the atmosph,eric flux to the continental shelf environment is similar to, or
greater than the riverine flux, as in the latter mly the soluble fractions reach the
continental ehelf, while npat of the suryended sdiments sre trapped in the estuary. Thus he
irggests that atmoapheric flux cannot always be ignored.
Heevv metal record in coastal sediments
The pdiment aeeumulation ratee for cones from delta stations ean be determined by
geochronological nnthods with suitable isotopes like Pb-210. lt is then possibb to assign ages
to the vertieel cdiment strata. EvaluaLion of the vertical distribution of heavy melals in
these corea givee a historical record of the input of elements to that area.
lf a depoait ehowe increased concentrations of a s.rqeeted pollutant, s.rch as a rnetal,
in its r.pperrpst levels relative to the deeper sediment, the differences in concentrat.ion are
scribed to enthropogenic sources. However, special precautions must be taken before final
cmclusions are drewn (Bertine, 1978).