Marsh gas rising

Pollution of wetlands can increase emission of the greenhouse gas methane

Published: Sunday 30 November 1997

 Emission zones: pollution of< emission of methane from polluted wetlands exceeds that from non-polluted wetlands by as much as two or three times. According to a study conducted by scientists of the Centre for Water Resources and Ocean Management, Anna University, and the National Physical Laboratory, New Delhi, waste from urban areas and industrial units has polluted the wetland ecosystem near Chennai, leading to an increase in emission of the greenhouse gas methane. Such areas needs to be monitored and human-induced changes controlled, say the scientists (Ambio, Vol 26, No 6).

The study, conducted between July 1993 and June 1994 along the Adyar river right up to its confluence with the Bay of Bengal, observed that methane ('marsh gas') emission from the area has increased to about 15.58 milligrammes per meter square hour (mg m2ph), as compared to 6.02 mg m2ph reported for similar unpolluted wetlands. The results of tests over a large area along the Adyar estuary indicate that human interference is responsible for the change in methane emission from the wetlands.

It is generally agreed that the complex methane molecule -- consisting of one atom of carbon and three atoms of hydrogen (ch4) -- has too much internal energy to be synthesised in the atmosphere. Its origin, therefore, must be the earth's atmosphere. Methane is produced by a group of anaerobic bacteria called methanogens in marshes, swamps and wetland soils -- the largest natural source of the gas. Despite oxidation of methane in surface soils in wetlands -- open areas that are seasonally or permanently waterlogged -- methanogenesis is largely responsible for generation of the gas in these areas. Methane is a major greenhouse gas. Its concentration in the atmosphere has been increasing rapidly, and it has a great capacity to absorb and emit infrared radiation. What is more, it absorbs and emits infrared light at a frequency at which co2 and water vapour do not.

Studies of methane emission in the Amazon region suggest that a large amount of methane is produced in tropical wetlands. Oceanic sources contribute only a small fraction. Pressure of human activities, mainly discharge of domestic and industrial effluents, could substantially affect the rates of emission of the gas. Pollution accelerates change in the chemical composition of wetland soils and water. Change that would otherwise have come about in thousands of years takes place in a few years, forcing methane efflux.

The study established that methane emission rates are much higher in wetlands near Chennai due to the availability of organic matter to methanogens, the primary producers of methane. The major source of pollution is waste from factories, slaughter houses and laundries in Chennai. The researchers suggest that wetlands near urban areas should be monitored for methane emission, and pollution should be controlled to plug this additional source of the greenhouse gas.

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