Chief routes of entry of marine pollutants

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Marine pollutants find their way into the sea not only through deliberate routes such as sewage discharges and dumping (legal and illegal) but also by a variety of other, not always obvious, routes outlined below.

Drainage

Many pollutants reach the sea either through direct drainage from coastal towns and industries or indirectly via rivers. Dilute industrial effluents, treated sewage and cooling water are often discharged into rivers and estuaries. Fertilizers, pesticides and animal wastes may drain into rivers from agricultural land. Huge amounts of silt resulting from rainforest clearance are carried down to the sea by tropical rivers. Rainwater runoff from cities and towns carries oil, heavy metals and other material into rivers. A surprising area of sea-bed around domestic sewage outfall pipes is often contaminated with oil (Dipper, pers. obs.).

Dumping

Coastal towns and cities discharge raw or treated sewage into coastal waters. Tipping at sea is used to dispose of sewage sludge, industrial wastes, dredged materials, ocean incineration wastes, oil platform wastes and rubbish (the latter particularly in undeveloped countries). Ships dispose of many of their day-to-day wastes (including oil tanker washings) by dumping, often illegally. The recent (1995) appearance of dangerous phosphorus bombs on Scottish beaches is thought to be the result of poorly controlled military dumping after World War II.

Airborne pollution

Many airborne pollutants are dissolved by rain which may then fall over the sea. Others are carried into the sea as dust particles or by solution of volatile materials

Table 10.1 Approximate quantities of wastes dumped into the North Sea in 1985 (data from QSR, 1987).

Type of material

Amount dumped annually

Dredged spoil

5 million tonnes

Liquid and solid industrial waste

1.9 million tonnes

Sewage sludge

5 million tonnes

from the atmosphere. For example, huge amounts of smoke containing PAHs (polycyclic aromatic hydrocarbons) were created by burning oil wells in the 1991 Gulf War which drifted for many miles over land and sea.

Accidents

A great variety of objects and substances finds its way into the sea through shipwrecks and lost cargoes, from plastic ducks and sneakers to potentially lethal (to humans and marine organisms) chemicals. Nearly 6 tonnes of Lindane, a highly toxic pesticide, still remain on the seabed in the English Channel after being washed overboard from a ship during a storm in 1989. Oil spills are another obvious concern.

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