Heavy metals are considered as persistent water pollutants. Where do heavy metals come from, and how are these toxic substances transported and distributed? This article reviews several literature along these concerns.
The introduction of substantial chemical, physical or biological material into the coastal zone from land-based sources due to industrialization as well as natural processes such as land erosion affects the organisms living in it. This is so considering that the process involves discharge of insidious and persistent toxic pollutants such as pesticides, heavy metals and other nondegradable and bioaccumulative chemical compounds.
The potential hazard to the marine environment of pollutants depends mostly on their concentration and persistence. Persistent pollutants, such as heavy metals, can remain in the environment unchanged for years (Guzman and Jimenez, 1992). These heavy metals eventually find their way into the tissues of marine organisms as a result of ingestion.
Continuous uptake of heavy metals would lead to an increased concentration in the organisms’ tissues (bioaccumulation) until a saturation point is reached where these metals would interfere with an organism’s vital functions.
Sources of Heavy Metals
In particular, the kinds of heavy metals incorporated in sewage outputs to the aquatic ecosystem vary widely. These are determined principally by the nature of pollution sources whence these metals came.
Possible pollution sources identified by Guzman and Jimenez (1992) include:
1. point sources such as refineries, power plants, ports, dockyards, domestic and industrial sewage;
2. non-point sources such as domestic and industrial sewage, agriculture activities, soil erosion; and
3. unpredictable point sources (e. g. oil waste at sea by tankers, major oil spills.
Discharges of oil at sea by tankers, the use of antifouling and anticorrosive paints to protect vessels and structures, oil spills during shipping and terminal transfers and effluent discharges from refineries are probably among the anthropogenic sources of lead (Pb), chromium (Cr), iron (Fe), copper (Cu), zince (Zn), cadmium (Cd), and vanadium (V) (Guzman and Jimenez, 1992). Also, all metals are normal components of fertilizers, lime and pesticides (Davies, 1980; Alloway, 1990).
Transport and Distribution of Heavy Metals
Owing to the metal’s soluble and particulate nature, heavy metal pollution transcends boundaries. This suggests that a wide range of pollution sources, both natural and anthropogenic, and a very effective mechanism for disturbing metals influence heavy metal transport and distribution (Guzman and Jimenez, 1992).
Rivers appear to be the most important sources of heavy metals in the sea and they carry much larger quantities of the elements as particulates than they do as solutes (Bryan, 1976). These heavy metals which may initially be deposited in one section of a coast or bay tend to be distributed to other regions because of tidal cycles, mixing of water layers and upwelling (Eslemont, 1999), currents, and occasional strong forces in nature such as typhoons, among others. Natural processes prevailing in particular locations influence the concentration of heavy metals.
In Darwin Harbour for example, Eslemont (1999) noted that the strong tidal cycle tends to disperse pollutants. Because of these processes, even the pristine coral reefs are influenced by metal pollution, although at lower levels than the other reefs proximate to pollution sources.
However, the influence of benthic infauna especially on sediment structure, chemistry and transport could not be underestimated. The disturbance of sedimentary deposits by living organisms (bioturbation) could influence sedimentary structure and therefore has important implications for the fate of contaminants. Mazik and Elliot (2000) have demonstrated that there was an increase in bioturbation with increasing distance from the source of pollution.
Alloway, B. J., 1990. Heavy Metals in Soils. Blackie, London (in Guzman and Jimenez, 1992).
Bryan, G. W. (1976). Heavy metal contamination in the sea. In Marine Pollution (R. Johnston, ed.), pp. 185-302. Academic Press, London (in Guzman and Jimenez, 1992).
Davies, B. E., 1980. Applied Soil Trace Elements. John Wiley & Sons, New York (in Guzman and Jimenez, 1992).
Eslemont, G., 1999. Heavy metals in corals from Heron Island and Darwin Harbour, Australia. Marine Pollution Bulletin, Vol. 38, No. 11, pp. 1051-1054.
Guzman, H. M. and C. E. Jimenez, 1992. Contamination of Coral Reefs by Heavy Metals along the Caribbean Coast of Central America (Costa Rica and Panama). Marine Pollution Bulletin, Vol. 24, No. 11, pp. 554-561.
Mazik, K. and M. Elliott, 2000. The effects of chemical pollution on the bioturbation potential of estuarine intertidal mudflats. Helgoland Marine Research, Vol. 54, Issue 2/3 pp. 99-109.
© 2014 June 12 P. A. Regoniel