More than three-quarters of marine pollution comes from land-based sources. Land-based sources enter into the marine habitat largely via direct discharges such effluent discharge pipelines and naturally via the hydrological cycle (rivers, precipitation, overland flow, soil and groundwater water flow), but also via atmospheric processes. The rest comes from shipping, dumping, and offshore mining and oil production. Coastal waters only account for a small proportion of total marine area, but they receive and retain the vast bulk of land-based pollution. More than 90% of all chemicals, refuse and other materials entering coastal waters remain there in sediments, wetlands, fringing reefs and other coastal ecosystems. They are the sinks for the majority of human-generated pollution. Consequently, marine environments suffer ecological damage, and in particular, many of the world's coastal zones are rapidly deteriorating. Land-based marine pollution may be reduced by protecting coastal zones from engineering, development and mining activities, and reducing run-off and improving planning of urban, industrial, agricultural and resort activities and developments. These strategies may require major changes in current practices, as well as the development of waste treatment technology and their wide use.
This strategy features in the framework of Agenda 21 as formulated at UNCED (Rio de Janeiro, 1992), now coordinated by the United Nations Commission on Sustainable Development and implemented through national and local authorities. Agenda 21 recommends that national planning and coordinating bodies should be given the capacity and authority to review all land-based activities and sources of pollution for their impact on the marine environment and to propose appropriate control measures.