Ocean dumping is currently used to dispose of industrial wastes, sewage sludge, garbage, construction debris, derelict vessels and dredged material. Most of these waste materials of our modern society are generated on land. Because of tidal activity, a high proportion of this waste remains within one kilometre of the coast, which contain the richest fishing grounds, tourist and other natural values. There are three principal modes of transferring these substances from the land into the sea: river discharge; atmospheric transport, followed by washout with rain; and coastal discharges through outfalls. Vessels contribute to a certain amount of pollution through discharge of sewage and garbage. Increasing quantities of material are being transported out to sea for dumping at designated dump sites; however, the largest amount of material will continue to be released into the sea through coastal outfalls.
The amount of solid materials entering the sea annually can be in the millions of tonnes. While this type of pollutant is largely cosmetic in character and affects mainly the amenities, there are other uses of the sea which are also affected. For example, large polypropylene or nylon ropes, floating just below the sea surface, can become entangled in the propellers of vessels, causing damage and possibly accidents at sea. Plastic sheets can also become entangled in ships' propellers, as well as clogging sea water intake systems for engine cooling and other purposes. Fishing gear may become fouled by netting and ropes, and on large solid objects deposited on the bottom of the ocean. From the ecological point of view, solids deposited on the bottom can adversely affect the benthic habitat. Plastic sheets can smother organisms beneath them, because of lack of oxygen replacement due to elimination or reduction of water exchange, and adversely affect the substrate for settlement of larvae. Plastic bags have been found on the heads of fur seals; plastic and rubber spherules have been found in the intestines of fish.
Of wastes dumped deliberately into the sea, as much as 80% consist of 'dredge spoils', the materials scraped from river and harbour bottoms to open channels and facilitate navigation. These materials are natural, for the most part, made up of sand, silt, clay and rock. They are usually barged to offshore areas and dumped, often grossly disrupting these undersea locations for a period of time. These 'spoils' often include a high proportion of deposited sewage sludge and accumulated industrial waste, with its potential toxicity.
Although about half of the oil transported in the world each year is carried by tankers, and tanker oil is dumped as part of routine operations, (such as illegal deballasting and tank washing, which flush oil into the sea), most of the oil in the sea comes from a combination of land-based industrial and municipal sources. As much as 350 million gallons (1.3 billion litres) of used automobile crankcase oil is dumped each year into water drainage systems that reach the sea. About 600,000 tonnes of oil run into the sea each year a result of petroleum carried into the atmosphere from poorly tuned automobile engines. Of the twenty largest metropolitan areas in the world, sixteen are coastal cities, or cities on rivers which empty relatively quickly into the ocean. About 30 billion gallons (113 billion litres) of industrial and municipal wastes are discharged into the coastal areas of the USA alone every year.
The chief danger of ocean sewage dumping lies in the spread of viral and bacterial diseases - directly to bathers, indirectly through fishery products. On the basis of the toxicity and the volume of many wastes entering the sea it seem likely that sea life could be seriously disrupted in some places more than others, but the threat is global.
The sea was the first sewage treatment plant. It has processed the organic waste and the dead husks of living things since life evolved there 3.5 billion years ago. The system thrives on this natural excrement. The animal life of the great depths would not survive without the nourishment of waste matter raining down from more populated surface waters. Gray whales contribute more faecal matter to the sea worldwide than the Los Angeles sewer system does; so do anchovies. There are also natural oil leaks in the sea. Seepage from sub-sea deposits wells up in many places around the world. Off the California coast, one natural seepage contributes about 660,000 tonnes of petroleum to the sea every year.
Because the sea and its organisms can handle this load of natural wastes quite well, the misconception has arisen that there is no limit to what the sea can dispose of in its self-purification systems. The human waste from a large city like Los Angeles is greatly concentrated in one location, not dispersed widely, and it is usually spiked with toxic substances that the sea may be unable to recycle naturally. There is natural seepage of petroleum into the sea from ocean floor deposits, but human-caused oil pollution is now about ten times that rising naturally from the ocean bottom. Perhaps the most pernicious misconception about the sanitation system of the ocean, however, is that coastal dumping is a negligible contribution to an environment so vast and deep, covering nearly three quarters of the planet's surface. The fact is that most of the sea's life is concentrated along the world's coastlines, where nutrient-rich currents pass across convenient hiding and attachment surfaces shallow enough to be touched by sunlight. Of the 140 million square miles (363 million square kilometres) of ocean surface, it is only the 14 million square miles (36 million square kilometres) near shore that contain the sea's most important living habitats. About 90% of the world's food fish spawn, mature, and are caught, in these coastal areas. Unfortunately, the land adjacent to these productive waters is and where human populations gather, where many industries locate, where the greatest mass of destructive waste matter is dumped.
Dumping was at sea is preferable to expanding disposal sites on land. Land dumping is more expensive, more difficult to gain approval of local authorities and more likely to meet with political opposition.