The seas are the ultimate receptacle for land-based pollution and it is interesting to assess the stress on coastal seas caused by this pollution.
Causes of Marine Pollution
There are some major sources of marine pollution.
1. Domestic Sewage
Domestic sewage is perhaps the major pollutant in coastal areas of India. Some 4.4 km3 of such wastes are discharged into the seas off India each year. For example, Mumbai discharges 365 million tons of sewage effluent to the sea annually and Kolkata about 396 million tons. Perhaps the major problem in India is that only a small proportion of this sewage is treated before discharge.
The situation could improve when the tertiary level treatment plants and the 3 km sewage outfalls are completed in Mumbai and the Ganga Action Plan is implemented. Nevertheless, upgrading sewage treatment facilities would be the greatest single contribution to the reduction of coastal pollution in India. As an example of the problem, Mahim Bay in Mumbai occupies 64km2 and once had a healthy ecosystem with fisheries, oyster beds, mangroves and migrating birds.
It is now one of the most polluted areas in the country. The dissolved oxygen levels in the water of the Bay sometimes fall below 1 mg/1 and hydrogen sulphide (denoting anoxia) can be detected depending on the stages of the tide. Water has high nutrient contents (denoting Eutrophication), high biological oxygen demand and high coliforms bacteria counts.
The near shore waters display severe organic pollution, especially during ebb tides, resulting in low biomass and a fauna consisting mainly of species from low trophic levels. Essentially specially then, Mahim Bay is an open sewer which may affect the health of the local communities.
In addition, the benthic fauna of Mumbai is badly depleted, resulting in the loss of fisheries. Fisheries are retreating from shore and fishermen now have to go at least 10 km offshore to get a worthwhile catch. The main health hazards for humans are gastrointestinal diseases resulting from the consumption of contaminated seafood.
In the long run, there is another threat. Because of the high productivity of the surface water of the Arabian Sea caused by upwelling of deep sea-water, the intermediate-depth waters off the west coast of India are poor in oxygen and have a renewal time of only four years.
As a result, these waters may be vulnerable to perturbation, either by an increased flux of organic carbon from pollution or an increased productivity of surface waters caused by climatic change. In either case, this could lead to these waters becoming completely anoxic and lifeless, with major implications.
2. Industrial Wastes
About 0.44 km3 of industrial waste are annually discharged into the seas around India. Many industries such as paper, textile, chemical, pharmaceutical, plastic, food, leather, jute, pesticide and oil contribute to this waste and particular attention need to be paid to heavy metals like lead, zinc, cadmium and mercury, and also to elements such as chromium used in the leather industry.
In general, the concentrations of these elements in seawater, sediments and biota in offshore areas do not pose a problem. Discharge of these elements into the rivers also tends not to cause major problems to the marine environment. For example, 55 percentages of the metals discharged into the Ganges settle out in the estuarine and river mouth region and only 15 percentages reach the Bay of Bengal. In Mahim Bay and Thane Creek, Mumbai the concentrations of heavy metals in marine organisms are quite high, indicating environmental degradation to an extent that some marine species are unfit for human consumption. An estimate has been made of heavy- metal discharge into the Ulhas River, which drains into Thane Creek in Mumbai.
The estimate was based on monitoring of 18 of the 48 major industries operating in the region. It showed that 11 tons of Cu, 400 tons of Zn, 7 tons of Hg and 0.5 tons of Cr are discharged into the river annually. This explains the massive impact on Thane Creek.
A major source of metal pollution in India is fly ash from power stations. Over 100 million tons of fly ash is produced each year and much of it is transported in the atmosphere before deposition. It is a major source of contamination of heavy metals such as cadmium in the rivers and estuarine sediments in the vicinity of such power stations.
Other sources of metal pollution include drainage water, weathering of spoil heaps and anti- fouling paints used to paint fishing boats and trawlers.
A total of 381,000 tons of pesticides and other halogenated hydrocarbons are used in India each year, of which 55,000 tons are used in agriculture. The total annual consumption of DDT and its isomers is 107,000 tons per year.
These chemicals have two principal applications, as pesticides, herbicides and fungicides in agriculture and in the control of vector-borne diseases such as malaria.
These compounds are extremely persistent in the environment and the seas are their ultimate repositories. It is believed that up to 25 percentage of the DDT used to date may have been transferred to the sea. In India, pesticide concentrations are much higher on the east coast than on the west coast and are transported there by the large rivers on the east coast.
These compounds have a demonstrable impact on marine biota and result in reproductive failures in birds and fish and inhibition of photosynthetic activity.
It should be emphasized that terms pesticides and fungicides are a misnomer. Th compounds are in fact biocides. Their indiscriminate effects on biota have been well-known over 30 years and are well documented in Rechel Carson's classic book Silent Spring. Impact of DDT on wildlife is particularly damaging and this is the main reason that it has banned in most western countries.
In India, DDT is used principally for malarial control. Howe despite a substantial initial impact and helping to bring down new malarial cases to about 200 per year, there are now over 2 million new cases per year as a result of mosquitoes become resistant to DDT. Doubt has now been cast in the efficacy of DDT in the control of malaria,
In India, huge quantities of biocides are used each year and there is a clear need to re' reliance on them, particularly DDT which should be banned outright as it is a dangerous In the west, much effort is being directed into the development of biodegradable pesticide Although these are more expensive chemical than traditional ones there is an urgent need review India's excessive reliance on biocides in the cause of environmental restoration.
One of the main tanker routes for the transport of oil is from the Gulf to the Far through the Arabian Sea. Apart from tanker accident and oil well blowouts, the main source of pollution in the marine environment is ballast discharge and bilge washing. The occurrence of slicks and, tar balls along this tanker route are well documented. The tar particles have a reside time of 30-45days before they start sinking and tar-like residues are wasted up on the west beaches of India. Particularly during the SW monsoon a peak of about 750-1000 tons of deposits was recorded on these beaches in the mid 1970s.
This type of pollution appears to leveled off or even decreased since then as a result of a decrease in the volume of oil transport: along these routes. In any case, vigilance is required to control such pollution.
Thus these are the main source of marine pollution around India. The list is by no nr complete. For instance, it does not mention the impacts of (i) soil erosion and land reclaim (ii) disposal of solid wastes (litter), (iii) mining, (iv) modification of the hydro cycles of rivers dams and (v) the sitting of nuclear power plants on the coast. Similarly, the impact of this pollution on sensitive environments such as estuaries, coral reefs and mangrove forests is not discuss It is however clear that this is a major problem.
The main area where offshore pollution occur in India have already been identified and initial work needs to concentrate is such areas.