Accidental or deliberate spills or dumping of toxic chemicals in the marine environment contribute about 10 percent of the pollution of the sea. On the other hand, land-based sources like industrial and urban sewage contribute about 70 percent of marine pollution. Many fish and molluscs are known to accumulate certain toxic chemicals in their bodies. Fish are now found with enormous concentrates of DDT in their bodies. A number of chemicals have long lives and continue to persist in the marine environment for a long time.
Estuaries are particularly sensitive to the effects of toxic chemicals.
In many countries urban and industrial waste is dumped into large land-fill sites without consideration of geography, soil texture, the water table or potential seeping into ground water which might be used for drinking. Waste from some industries, like those producing alkaline salts, affects the fertility of the soil by making it salty. Solid wastes and liquids seeping into the soil from liquid waste sources render it unsuitable for human habitation. This is causing serious problems in many countries of the world, where land sources are already very limited because of urban sprawl and increasing populations. The phosphate fertilizers used in agricultural activities also seep into the soil and clog the waterways.