of POPs In Coastal and Marine Environment
Sources of POPs inputs in the marine environment can be divided in primary sources and secondary sources.
POPs are lipophilic substances with low solubility in water and low to medium volatility. They can travel long distances in the environment as molecules in the gaseous phase or, due to their hydrophobicity, be adsorbed on particles, suspended in aerosols and transported by the winds. The principal source of widespread pesticide contamination is from agricultural use of these compounds. Aerial transport is the main route by which they reach the sea. All organochlorine pesticides volatilise, particularly in the tropics where they are still used in large quantities and where climatic conditions favour their release to the atmosphere. The presence of water vapour may enhance this property. In Nigeria, 98% of the DDT applied to a cow pea crop volatilised within four years. (Clark et al., 1997). Further, DDT, the "drins" and toxaphene adsorb strongly on particles and are carried into the sea on wind-borne dust. Some agricultural practices particularly favour the aerial distribution of pesticides:
The most important source of contaminants in some coastal environments and in the open marine ecosystem appears to be atmospheric deposition. Through atmospheric deposition, POPs impact directly the aquatic ecosystem. Atmospheric deposition occurs through three different processes:
In the Baltic Sea, the input of PCBs through wet deposition is 7%, through dry deposition 7% and, through vapour phase deposition 63%. The atmospheric transport of POPs is higher than input from other sources in this region , and for PCBs in the Baltic Sea, for example, this input represents about 77% , compared to a river input of 23% (Larsson et al., 2000), (see example below). This would also explain how high concentrations of POPs can occur in Arctic and Antarctic regions, very far away from their sources.
The following table shows a comparison between atmospheric and river inputs of some organochlorines to the world oceans (t/yr.). (Clark, 1997).
Although the total burden of POPs carried into the sea by rivers is small compared with aerial inputs, it may be locally damaging. Floods carry very large quantities of silt into the sea, and if the silt is derived from agricultural land, it may carry a considerable burden of adsorbed POPs. Before reaching the coastal environment, POPs, mainly adsorbed onto particles, cross estuaries where sudden changes in chemical and physical condition occur. Here, processes such as flocculation of organic polymers or aggregation of organic matter trap the particles with lipophilic substances. Macroagregates are subsequently deposited in estuaries and delta areas and the future fate of POPs buried in sediments is closely related with the organic matter deposited with them. Coastal environments are the most productive areas of the sea, and POPs trapped in the sediments can be absorbed by bentic fauna or can volatilise trough the upper water column when anaerobic conditions occur in the bottom sediments.
The use of DDT and the"Drins" was phased out in western European states in the early 1970s, but elevated levels of chlorinated hydrocarbons are still recorded near the mouths of the major rivers, and over 3 tons per year of PCBs enter the North Sea from river inputs, mainly from the Rhine. This could be explained by the input from sediments still contaminated with pesticides carried in the runoff from the land, or of PCBs in drainage water from poorly maintained land disposal sites.(Clark, 1997).
Open Sea Environments
Conversely pollutants originating from river and wastewater transport are not directly available for uptake into the pelagic open sea. As a result of particle association, the pollutants sediment by gravitational settling at the river mouth and may, on a longer time scale, reach the open sea by sediment transport and focusing. Despite their lipophilic properties, part of the substances are dissolved in the water. This fraction may be transported in the water phase as a result of association to dissolved or colloidal organic matter. Still, the time to reach the pelagic environment is substantial.(Larsson et al., 2000).
Although direct inputs of chlorinated hydrocarbons to the sea have largely ceased, a large quantity of pesticides and PCBs from these sources continue to contaminate bottom sediments. Chlorinated hydrocarbons are often presents in industrial outfalls to the sea. One striking case is that of the Montrose Chemical Company in Los Angeles which was the world's major manufacturer of DDT. The Los Angeles sewerage system received the effluent from this factory and, from 1949 until 1971, this resulted in the discharge of 216 tons per year of DDT residues to the sea through the ocean outfall of this sewerage system. A survey made in 1972 suggested that 20 tons of DDT resides were trapped in the upper 30 cm of the bottom sediment over an area of 50 square kilometres around the outfall.
Sewage sludge may also contain elevated concentrations of chlorinated hydrocarbons and , if dumped at sea, represents an additional source of contamination. Sewage sludge from Glasgow dumped in the Firth of Clyde in the 1960s resulted in a contribution of 1 ton per year of PCBs to the bottom sediments until the discharge of PCBs was brought under control.(Clark, 1997).
Migratory species pick up contaminants through the food on their wintering grounds or at sites along the migration pathway. In the Arctic, for example, birds that breed in the north and overwinter in more temperate (and industrialised) latitudes may contain higher levels of contaminants in their tissues than birds that overwinter in the north. The contaminants are transported north each spring when the birds migrate back. This has significant implications for Arctic predators, including humans, for which the migrating birds provide a food source. Contaminant levels in common and king eiders, (Somateria mollissima; S. spectabilis) collected as part of a survey of contaminants in harvested avian species, illustrate this trend as do samples taken from peregrine falcons. (Muir et al., 1999).
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