Chemical status of Loktak Lake

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By: Dr O Geeta Devi *

Among all the water bodies, lakes are closed systems, which have their source from sea or river and occupy a unique position. As they are closed bodies they are getting polluted at a fast rate due to anthropogenic activity and also suffer eutrophication due to nutrients loading, which is causing loss of aquatic life and is becoming unfit for other uses including for drinking purposes.

In India, studies of lake have not yet gained importance though some beautiful fresh water lakes of the Himalayas for example Dal Lake of Kashmir and Nanital Lake of Uttaranchal are losing their charms and reducing its area at such an alarming rate.

The Loktak lake which is the largest fresh water lake in North-East States, second largest in India and twenty fourth rank out of thirty six world important fresh water lakes, covering the southern part of Imphal Valley, Bishnupur District, Manipur State is a fast deteriorating lentic ecosystem which is showing not only the growth of macrophyte and heavy siltation from its several catchment area but also chemically polluted to some extent as compare with other freshwater lake of India.

These should be the great concern of the people of this area particularly the researchers in this field because this lake is the life-line of the State.

The water quality of Loktak Lake has a unique-status among the other Indian fresh water lake. Iron, Calcium, Magnesium Sodium, Potassium, Chlorine, Fluorine, Iodine which are the essential elements for human beings. These elements have certain values above and below which affects the human health and even to the plants and other aquatic life, survived within the lake system.

The concentration of iron in Loktak lake water ranges from 90 to 2040 mg/1 with an average of 988-75 mg/1. Great Lake Water Quality Board (1976) has recommended 300 mg/l of iron as the maximum limit for protection of fresh water aquatic life. But, the iron concentration in Loktak lake water exceeds this prescribed limit.

The high concentration of iron in water of this lake may be due to discharge of untreated sewage direct into the lake and weathered iron oxides from Dishang shales of the hill-locks in and around the lake that is rich in iron. Indian Standards Institution, ISI (1983) recommended 1000 mg/l of iron as the maximum limit for drinking water supplies. High concentration of iron in water is harmful to aquatic life as iron salts may get precipitated in the gills of fish and affect intake of oxygen.

The average concentrations of Calcium, Magnesium, Sodium and Potassium are 4.86 mg/l, 7.11 mg/l and 3.61 mg/I respectively in Loktak lake water. These values are well within the permissible limits for drinking water prescribed by World Health Organisation (WHO 1971) and I.S.I. 1983.

But the Loktak lake water shows Mg>Na>Ca>K type whe-reas Ca>Mg>Na>K is the trend shown by the world water bodies. Among the essential anions in the lake water, the average concentrations of chloride, Fluoride and Iodide are 17.26 mg/l, 0.018 mg/l and 0.051 mg/l respectively which are comparatively low as compared with the desirable values prescribed by the Indian Standard for Drinking Water that the highest desirable limit of 25 mg/l and maximum permissible limit of 1000 mg/I for chloride and the desirable limit of fluoride in drinking water is 0.6-1.2 mg/I which can be extended up to 1.5 mg/l.

The deficiency of fluoride caused dental problems and tooth decay whereas thyroid problems like goitre are caused by Iodine deficiency in lake water. And severe deficiency of chloride is the main cause of common water borne disease in the area. Certain metallic constituents such as Copper, Cobalt, Nickel, Lead are below the detectable limit whereas Mercury, Zinc, Manganese, Cadmium, are having the average concentration of 0.3 u/l, 0.048 u/l, 0.042 and 0.030 u/l respectively.

These elements have harmful environmental effects, even when they are present in water at low concentration and are known as trace elements. Such levels may rise from natural processes or from human activities.

The role of trace metals are complicated by the fact that they are essential in small amounts for the growth of organism but if exceed a certain threshold value, it may have inhibitory effect. So trace element level is very much important to know the water quality of the lake and possible effect of toxicity to aquatic life.

However, the trace element content in the Loktak lake are below the maximum permissible limits as prescribed by I.S.I. But the Mercury content is very high as compared with other fresh water lakes of India. This may be due to the constant use of insecticides, pesticides, acricides and fungicides by the farmers in and around the lake namely Khoijuman, Toubul, Kwasiphai, Potsangbam (Awang), Upokpi and Tapokpi in the Western side and some cultivated catchment area in the Eastern side.

The amount of such chemical used in Rabi and Zaid crops is more than the kharib crops. Particularly, using the organic mercurial fungicides such as Cerean dry, Agrosan GN, Hexason, Tafssan, Agallol and Tillex which are having high concentration of Mercury.

Therefore, it is the right time to organize the related awareness programme among the intensive and progressive farmers of these areas by the concerned authority to safe Loktak lake for the betterment of the ecosystem in this heavenly valley.

The longitivity of this lake is the fate of the whole population including flora and fauna in the state.

Dr O Geeta Devi wrote this article for The Sangai Express
This article was webcasted on May 24th, 2006

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