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GESAMP on diffuse sources of pollution

Diffuse sources result from broad-scale activities that cannot be discriminated as readily as single, site-specific discharges. The most obvious of these activities is agriculture, which results in the runoff of crop treatment residues and animal wastes. These often result in the contamination of groundwater, with associated diffuse leakage into rivers and coastal waters. Wide-scale forestry also contributes to diffuse-source transport of nutrients and soils to the marine environment. Major or widespread development activities resulting in the increased mobilization of soils would also fall into this category. Nutrients and particulate materials are the constituents of diffuse sources contributing to drainage into rivers and the marine environment that are of the greatest concern.

Atmospheric Deposition

Atmospheric deposition can be divided into two categories: substances with short atmospheric residence times and those with long ones. Short residence time materials are likely to be deposited fairly close to such sources as releases of metals from metalliferous smelting activities. Long residence time components will be widely distributed on regional, or even global, scales. More volatile substances are among those of greatest concern in relation to the atmospheric pathway to the aquatic environment. These include mercury and lead, among the inorganic chemicals, and a range of organic substances. [...]

There have been a number of studies of the atmospheric input to coastal waters, particularly in North America and Europe: in most other regions of the world it has largely been ignored. Figure 2.4 presents the percentage of the total input that has come from the atmosphere for a number of heavy metals entering Chesapeake Bay in the United States. The percentages represent deposition directly onto the Bay surface and range from 1% for manganese to 30% for lead. [...]

There has been growing concern about the input of a wide range of synthetic organic compounds to the coastal ocean. The atmospheric input of lindane (HCH) to the North Sea was compared with that from other sources, and was found to dominate it: this is typical of many synthetic organic compounds.

There have also been many investigations of the trace metal input to the North Sea, the Baltic Sea and the Medi-terranean Sea. One study of the North Sea considered not only the direct input to the surface waters, but also deposi-tion on the watershed, with subsequent riverine input, Baltic Sea inflow, Atlantic Ocean inflow and outflow, and the exchange of metals with sediments. Atmospheric input is quite important in this larger context, being approximately equal to the in-flow from the Atlantic Ocean, although still less than that entering the North Sea from dumping. It should be noted that approximately 20% of lead in the Atlantic inflow to the North Sea is also derived from the atmosphere. For artificial radionuclides, atmospheric deposition (fallout) is still a significant pathway of input to land and the ocean although it is becoming smaller as the stratospheric reservoir of fission products from atmos-pheric weapons testing is reduced by radioactive decay. Atmospheric deposition is important to the supply of some natural radionuclides, such as Beryllium-7 and Lead-210, to the Earth’s surface.

For many contaminants, a relatively small fraction of the material delivered to estuaries and the coastal zone by rivers makes its way through the near shore environment to open ocean regions. Thus, for the open ocean, atmospheric input for most contaminants is much more important than riverine input. The apparent dominance of atmospheric over riverine input for most of these substances is obvious.

Contaminants

This section deals with the classes of contaminants listed in the GPA/LBA. These are: sewage; persistent organic pollutants; heavy metals; oils (hydrocarbons); nutrients; sediment mobilization; and litter. Historically, specific contaminants, particularly chemicals, have been of foremost concern in terms of adverse effects on the marine environment. Such concerns are also reflected, but in a far more balanced context, in the GPA/LBA. In the sense that "a pollutant is a resource out of place" it should be noted that any substance -- even a regular constituent of the environment -- can cause pollution in abnormal concentrations arising from anthropogenic activities. There are some unique and surprising contaminants in some areas. One is salt (i.e., seasalt) which is discharged from seawater desalination plants at high enough volumes and concentrations in the Arabian/Persian Gulf and the Red Sea/Gulf of Aden significantly to alter the salinity of the nearshore zone, with attendant changes in community structure (Jeftic, 1998g; 1998h (Annex 2); GIPME, 1996). Similarly, although not usually given a great deal of attention by the public, heat discharges can also have significant effects, especially in small, poorly flushed, water bodies. Discharges from both power plants and desalination plants can alter temperatures and salinities in inshore areas of specific regions with potentially adverse effects. Mangrove mortality may be caused by a 3-5 o C increase in ambient water temperature in the tropics and the diversity and mass of associated fauna may diminish by 90% (Jeftic, 1998g (Annex 2)).