Water is an important resource for mankind, plants and animals. Surface water bodies viz., lakes, ponds and rivers, supply the required water apart from ground water sources. Human activities, disposal of storm water runoff, industrial wastewater, domestic sewage, agricultural runoff, etc pollutes lakes and reservoirs. These sources of pollution degrade the water quality and make it unfit for use. Oxygen demanding wastes are the prominent pollutants of lakes. Presence of phosphorus in toxic chemicals and domestic sewage discharge dominates the pollution of lakes. Knowledge of lake system is essential to understand the role of phosphorus in polluting the lakes. It is essential to restore and maintain the chemical, physical and biological integrity of water bodies to achieve the required water quality to ensure protection of fish, flora and fauna. The amount of phosphorus in the lake water, controls the quantity of algal growth and productivity of lakes. The paper also highlights the share of surface water bodies in irrigating arable lands in the country and also about world scenario. India's surface water utilisation is only 20 % of the available quantity, which demands proper watershed management. As a case study two lakes situated in Hosur industrial town on NH-7 was studied for the pollution levels and the observations are reflected in the degradation of lake water. Discussion and conclusions are drawn to emphasise controlling the pollution of lake water and its optimal utilisation.

General:

India has vast water wealth, though it is not adequate to meet the requirements of increasing irrigation needs and the necessities of an expanding population. The unique geographical location of the country with seas and ocean on three sides and great mountain ranges in the north contributes to high precipitation from monsoons. The surface runoff from precipitation provides huge quantity of water to fill tanks, lakes and reservoirs, generate flow of water through springs and rivers, snow fall in mountain tops, soil water and ground water. It has been estimated that the mean annual rainfall in the country is 1194 mm. This amounts to 392.5 million hectare-meter (Mha-m) of water over the total geographical area of the country - 328.7 Mha and snowfall precipitation amounting to 400 Mha-m. Out of this, 70 Mha-m is accounted for evaporation from the top thin layer of soil soaked with rainwater, 115 Mha-m as runoff water and 215 Mha-m as infiltration water [1], out of which 165 Mha-m is present as soil water and 50 Mha-m is available for ground water recharge.

World Water Wealth:

The world's total water quantity is estimated as 1400 x 10 15 m 3 of which about 96.56 % is contained in large water bodies as saline water. The volume of water on earth is estimated at 1.36 billion km 3 most of which (97.2 %) is in the oceans; about 2.15 % is frozen in glaciers, and the remaining 0.65 % constitutes all the water in streams, lakes, swamps, ground water and the atmosphere [2]. The different sources of water and their apportioning are as indicated in Table 1.

Table.1 World Estimate of Water Quantities [1]

Water available in lakes amounts to 0.013 % of the total world water potential. Though it is meagre, its share on surface water is substantial accounting for 98.48 % (0.815 x 10 5 m 3 ) of total quantity of river and lakes i.e., 0.8128 x 10 5 m 3 . At any instant the atmospheric water content is only a meagre quantity. About 70% of the fresh water resources are contained in polar ice caps and glaciers. Ground water available below a depth of 600 m can be taken as saline. The distribution of water as indicated in Table 1, maintains a balance to sustain life on earth. The global scenario of water balance between precipitation and its components on an annual basis is given in Table 2.

Table. 2 Annual Balance of Water Overland and Ocean [1]

The annual rainfall and runoff trends of different continents and the Indian subcontinent are indicated in Table 3. Amongst the various continents, the depth of maximum runoff occurs in South America where as the percentage of maximum runoff is observed in the case of Europe. The annual average runoff of India is 510 mm, amounting to 42.9 % of total precipitation. The Indian surface water potential is enormous; but its utilisation is one of the poorest as it utilises only 20 % of the available resources.

Table.3 Annual Rainfall, Runoff and Evaporation of Different Continents and India [1]

Irrigation potential of the world:

Area under irrigation in the world as per 1996 data is only 241.5 million hectares (Mha) which was only 15.98% of the total arable land and land under permanent crops in the world (FAO, 1997). The irrigated area in different countries of the world and a few major countries as per 1996 statistics is given in Table 4. The countries having vast area such as Australia, Brazil, Canada and Russian Federation have irrigated area less than 5 % of their arable land and land under permanent crops. The U.S.A and some of the African countries viz., Algeria, Sudan, South Africa have facilities to irrigate only 6 to 15 % of their arable lands. Asian countries like Japan, Korea, Kyrgystan, Pakistan have 60 to 82 % of their arable land is irrigated [1]. The irrigated area in India as per 1996 data was only 33.59 % of the total arable land and land under permanent crops. Egypt is the only country in the world, which has achieved 100 % irrigation of their arable land.

Table 4. Irrigated Area in Different Countries of the World in 1996 (1000 hectares) [1]

Lake Productivity

Lakes and large rivers can be classified on the basis of their level of primary productivity. Lake productivity is a measure of its ability to support a food web. Algae form the basis of this food web, supplying food for the higher organisms. A lake's productivity may be determined by measuring the amount of algal growth that can be supported by the available nutrients. Normally, a more productive lake will have a higher fish population; but the number of most desirable fish may be less as lake productivity is linked to reduced water quality because of undesirable changes that occur as algal growth increases.

Classification of Lakes

Lakes are classified based on their productivity as : [3]

•  Oligotrophic Lakes

•  Eutrophic Lakes

•  Mesotrophic Lakes

•  Senescent Lakes

Oligotrophic Lakes

These types of lakes have a low-level productivity due to limited supply of nutrients to support algal growth. This results in clear water so that the bottom of lake can be seen at a considerable depth. In this case, the euphotic zone often extends into the hypolimnion, which is aerobic. These lakes are favourable to support cold water game fish.

Eutrophic Lakes

These lakes have high productivity because of abundant supply of algal nutrients. Algae cause the water to be highly turbid, so the euphotic zone may extend only partially into the epilimnion. The dead algae settle to bottom and decomposed by benthic organisms. The decomposition is sufficient enough to deplete the hypolimnion of oxygen during summer stratification. As the hypolimnion is aerobic during summer eutrophic lakes support only warm-water fish. Highly eutrophic lakes may also have large mats of floating algae that typically impart unpleasant taste and odour to the water.

Mesotrophic Lakes

Lakes, which are intermediate between oligotrophic and eutrophic, are known as mesotrophic. These remain aerobic, even though substantial depletion of oxygen may have taken place in the hypolimnion that is the lowest layer of water in stratified lakes, which retain the winter temperature.

Senescent Lakes

These are very old and shallow lakes, which have thick organic sediments and rooted water plants in great quantity, which eventually become marshes.

Stratification of Lakes

The important property of water is that density is greatest at approximately 4º C and water of higher temperature floats and of lower temperature stays below this horizontal layer of water. This density differential per degree increases progressively with higher temperatures. During the year, water body cools and warms depending upon seasonal change and this has direct and indirect effects on number of ecosystem processes. During winter, temperature is relatively uniform throughout the lake and wind can mix lake water from top to bottom, which causes overturn. This process makes the lake waterbody stratified into three horizontal layers viz., 'Epilimnion', 'Thermocline' and 'Hypolimnion'[4]. During summer epilimnion layer is warm, aerobic and mixed; hypolimnion layer is cool, poorly mixed and anaerobic. In winter epilimnion is cool and more dense than the hypolimnion, which sinks, causing overturn. This overturn occurs once in a year, then it is known as 'monomictic' and if it occurs twice in a year that is known as 'Dimictic'. Thermal stratification has major effects on both oxygen concentration and nutrient supplies such as in summer - epilimnion is aerobic but deprived of nutrients from the bottom of lakes due to hypolimnion.

Water Regulation

When a water retaining structure like dam is put across a river, ecological conditions change in upstream as the aquatic system is switched from 'lotic' to 'lentic' conditions i.e., from a river to a lake [4]. The most obvious effects of dams are local upstream changes in aquatic ecosystem. The reduction in flow rate behind the dam will lead to deposition of fine sediments, destroy the population of the giant fresh water mussel for several kilometers of river course and submerged vegetation starts to decompose, releasing nutrients resulting in deoxygenation of bottom waters which is exacerbated by thermal stratification. Reduction in water velocity, sedimentation, deposition of transported organic matter, nutrients and release of nutrients from increased decomposition, all add up to produce quantum increase in primary production in the newly created lakes.

There are pros and cons of manmade tropical lakes as many species of organisms disappear, while other species appear, often in great abundance in the disturbed system. 'Water Fern', a weed grows and spreads severely impeding navigation, fishing and affecting water quality by indirect deoxygenation of water under weed cover. As a floating plant, it shades out other plant life in the water and no oxygen release by photosynthesis and no wind mixing of surface waters. These conditions also favour creation of pests and insects within a very short period. Fluctuations in water levels in the littoral areas of the artificially created lakes can cause problems to flora and fauna. Downstream of dams, the river loses its dynamic nature, temperature regimes are altered and water oxygen level is decreased if oxygen deficient waters of deeper layers in the lake are released. This water may contain suspended iron and manganese hydroxides and dissolved hydrogen sulphide (H 2 S). The excessive decomposition generates H 2 S, which can corrode the metal of turbines and concrete of dams.

Light and Lake Zonation

cover and water quality. The profundal or deeper zones lack plants, but receives a heterotrophic energy source in the form of rain detritus from above (dead organisms, faeces, etc). Decomposition of detritus by bacteria and fungi consume oxygen; thus a complex interrelationship occurs between the amount of nutrients in the water, the level of plant production, the rate and amount of decomposition and oxygen levels.

Pollution of Lake Water

Water bodies are polluted by human activities and the water quality is seriously degraded. Water quality management is concerned with the control of pollution. The water quality is affected by natural factors such as mineral contents of the watershed area, geometry of terrain and the climate of the region. It is essential to restore and maintain the chemical, physical and biological quality, which ensures protection and propagation of fish, wildlife, and plant life and provides recreation in and around water. The various pollutants discharged into the surface water are domestic sewage, industrial wastewater, and agricultural runoff and storm water runoff. Anything that can be oxidised in the receiving water with the consumption of 'dissolved molecular oxygen' is known as 'oxygen demanding material'. This material is biodegradable organic matter and also contains inorganic compounds. The consumption of dissolved oxygen (DO) poses threat to fish and other higher forms of aquatic life, which must have oxygen to live. The critical level of DO varies greatly among species [3].

Nitrogen and phosphorus are nutrients of primary importance for the growth of living organisms in water bodies and its excess leads to the growth of algae, which becomes an oxygen demanding material when they die and settle to the bottom of lakes. Some major sources of nutrients are phosphorus-based detergents, fertilisers and food processing wastes. Microorganisms found in wastewater include bacteria, viruses and protozoa. If the concentration of pathogens is sufficiently high, the water is unsafe for swimming and fishing. Organic and inorganic particles (suspended particles) carried by the wastewater become sediments, which reduce the life of a dam. High concentration of salts makes the water unfit for consumption of plants, animals, irrigation and human. Agricultural runoff contains pesticides and herbicides, which pollute the water. Urban runoff is a major source of zinc in many water bodies as zinc comes from tyre wear. Many industrial waste waters contain either toxic metals or toxic organic substances. Release of hot water by thermal power plants affects fish and also enhances the rate of oxygen depletion in areas where oxygen demanding wastes are present.

Eutrophication of Lakes

It is a natural process that the lakes gradually become shallower and more productive through the introduction and cycling of nutrients. That is how the oligotrophic lakes gradually become mesotrophic, eutrophic and senescent and are eventually filled-up almost completely. The duration for this process to occur depends on the original size of lake and on the rate of supply of sediments and nutrients. In case of some lakes the eutrophication process takes thousand of years and in some cases it may become eutrophic in a short time. 'Cultural Eutrophication' is caused due to intensive human activity, which cause sedimentation and supply nutrients to lakes.

Requirements for Growth of Algae

All algae require macronutrients such as carbon, nitrogen and phosphorus and also micronutrients viz., trace elements. For growth of algae all the nutrients must be available and lack of one nutrient would limit its population. The availability of each nutrient and its natural cycle is summarised below: [3]

Carbon

Algae obtain carbon from CO 2 dissolved in the water and CO 2 is in equilibrium with bicarbonate buffer system. Immediately available carbon could be determined by the alkalinity of water. When CO 2 is removed from water, it is replenished from the atmosphere, as CO 2 available from atmosphere is abundant and inexhaustible. When algae is either consumed by higher organisms or die and decompose, the organic carbon is oxidised back to CO 2, which returns to either water or to atmosphere to complete the carbon cycle.

Nitrogen

Nitrogen is available in lakes in the form of nitrate and comes from external sources i.e., in flowing streams or ground water. When nitrogen is consumed for algal growth, it is chemically reduced to amino-nitrogen (NH 2 ) and is incorporated into organic compounds. When dead algae undergo decomposition, the organic nitrogen is released to the water as ammonia, which is then oxidised back to nitrate by bacteria by the nitrification process. In aerobic sediments and in hypolimnion of eutrophic lakes, when algal decomposition has depleted the oxygen supply, nitrate is reduced by anaerobic bacteria to nitrogen gas (N 2 ) and lost from the system in a process known as 'denitrification' which reduces the average time nitrogen remains in the lake system. Some photosynthetic microorganisms can also fix nitrogen gas from the atmosphere by converting it to organic nitrogen. In lake the most important nitrogen-fixing microorganisms are photosynthetic bacteria known as 'cyanobacteria' (blue-green algae) which have nitrogen fixing ability compared to green algae when nitrate and ammonium concentrations are low but other nutrients are sufficiently abundant. Cyanobacteria is undesirable because it forms unsightly floating mats and imparts unpleasant odour and taste to water, and also produce toxins, which kill fish.

Phosphorus

In lakes, phosphorus originates from external sources and is taken up by algae in the inorganic from and incorporated into organic compounds. Phosphorus is returned to the organic form during algal decomposition. The release of phosphorus from the dead algal cells is so rapid that only a little of it leaves the epilimnion with settling algal cells. Phosphorus is transferred to sediments little by little as decomposed organic matter and some of it is co-precipitated with iron, aluminum and calcium and some bound to clay particles. The permanent removal of phosphorus from the overlying waters to sediments depends on the quantity of iron, aluminum, calcium and clay entering the lake along with phosphorus.

Trace Elements

The requirement of trace element for growth of algae is minimal and this quantity is available in the fresh waters, which enters the lakes.

Limiting Nutrient

Of all the nutrients, only phosphorus is not readily available from the atmosphere or from the natural water supply and hence it is known as 'limiting nutrient' in lakes. The algal growth and productivity of lakes is controlled by available quantity of phosphorus. It has been estimated that the phosphorus concentration should not exceed 0.010 to 0.015 mg/l to limit algal blooms [3].

Control of Phosphorus in Lakes

Phosphorus being a limiting nutrient, control of cultural eutrophication must be accomplished by reducing the phosphorus inputs to the lake. The slowing of eutrophication process can be done by precipitation of phosphorus with addition of aluminium (alum) or removing phosphorus rich sediments by dredging. Controlling its sources can reduce phosphorus input in lakes. Phosphorus released from the rocks, enter the water directly, and also in the form of dead plant matter. It is extremely difficult to control the natural inputs of phosphorus in lakes and if the sources are large, lakes become eutrophic. The most important sources of phosphorus are municipal and industrial wastewater, seepage from septic tanks, agricultural runoff that carries phosphorus fertilisers into the water.

Acidification of Lakes

When CO 2 dissolves in water, it forms carbonic acid (H 2 CO 3 ) and the pH of rainwater is approximately 5.6, which is slightly acidic. Fishes are very sensitive to low pH levels of water. High aluminium concentrations also kill fish, which is in soil [3]. Acidification releases highly toxic Al 3+ to the water. Most lakes are buffered by the carbonate buffer system. Calcareous soils contain large quantities of calcium carbonate and lakes formed in calcareous soils tend to be resistant to acidification. Other factors that affect the susceptibility of a lake to acidification is the permeability and depth of soil, the bed rock, the slope and size of watershed and type of vegetation. Small watersheds with steep slopes reduce the time for buffering to occur. The control of lake acidification is related to the control of atmospheric emissions of sulphur and nitrogen oxides.

Reduction of Evaporation Losses of Lakes

Evaporation of water from reservoirs is substantial in arid and semi-arid regions. Annual evaporation losses from reservoirs in India vary between 150 cm to 200 cm except the extremes in the states of Rajasthan and Jammu and Kashmir, where evaporation levels are 300 cm and 50 cm respectively. Annual evaporation loss of 170 cm can be taken as standard for Indian reservoirs. The various means of reducing the reservoir or lake evaporation are: [1]

Reduction of surface areas of lakes by increasing their depth.

Providing polyethylene covers to cover small ponds.

By spreading chemicals like 'acetyl-alcohol' or 'sterile alcohol' (actadecamol) of 0.15 micron thickness over reservoir surface can effectively reduce evaporation in the range of 20 to 50%. About 2.2 kg of acetyl alcohol is needed for one hectare of water area. However wind, oxidation and birds may disturb the layer requiring regular replenishment.

A case study has been taken up to investigate the pollution levels of two lakes in Hosur Town which is a major industrial town located about 40 km away from Bangalore on NH 7. The lake viz., Lake 1 situated near Hosur Bus-Stand on NH 7 and Lake 2 situated inside the Hosur Town near Taluk Office were considered for the investigation. Both lakes are confined ones and its water has not been used for any purpose. The surface runoff is the main source of inflow into the lakes and there is no domestic sewage and industrial wastewater being discharged into them except some pollution caused by anthropogenic activities like washing of clothes, vehicles, etc. As the stagnant water of the lakes become potential for the growth of algae, weeds and other vegetation. Often fishing is done as some aquacultural trace is found in both the lakes. The lake water samples were tested for their physical and chemical characteristics as indicated in Table 5. The results of the physico-chemical analysis of the four samples collected in Lake 1 during July to October 2002 is presented in Table 5. The permissible limits on various properties of water are also indicated in Table 5. The water samples of both the lakes indicate it is not fit for any domestic, industrial and irrigation purposes as water properties in both the cases exceed the permissible limits.

Table 5. Hosur Lake Water Properties

Water bodies viz., lakes, ponds, rivers are polluted by human activities which has degraded the water quality to the extent that these have become wastewater not fit for any use. Water quality management is concerned with the control of pollution from human activity such that the required purity is maintained for its intended uses. Water quality is affected by natural factors such as mineral discharge of watershed, geometry of terrain and the climate of the region. It is essential to maintain the chemical, physical and biological integrity of water bodies as a goal of water quality management. The results of the two lakes in Hosur Town that was investigated for physico-chemical parameters s indicates the quality of water is not fit for domestic, industrial and irrigation purposes.

•  India's utility of surface water is only 20 % and the balance quantity is let-off into oceans, which indicates the poor watershed management system.

•  The surface water sources are polluted by discharging domestic sewage, industrial wastewater, storm water runoff and agricultural runoff which pollutes and degrade the water quality, which in turn makes it unfit for use.

•  Polluted lake water serves as nutrient for the growth of algae and weeds, which is governed by the quantum of phosphorus present in the pollutants discharged in lakes.

•  Lake water is also governed by climatic factors of the region which causes stratification and overturn and sedimentation due to runoff, which makes the lake productive based on the available nutrients contributed by pollutants.

•  It is required to control all the pollutants discharged into lakes to preserve the water so as to be safe for irrigation, and other human needs.

•  Acid rain also causes degradation of lake water, which is caused by the automobile emission into atmosphere, which contains CO 2 , CO, N 2 O, CH 4 , etc.

•  The increase in water demand due to enhanced irrigation requirements, industrial and human needs demand more utilisation of surface water sources. Hence, controlling pollution levels of surface water bodies like lakes is essential to make it safe for utilisation.

•  Water quality analysis of two lakes of Hosur Town indicates that the pollution levels are beyond the permissible levels and its use for domestic, industrial and irrigation purposes is not suitable. Hence, it is emphasised that proper pollution control measures should be enforced to minimise the contamination levels of lake water such that its utility at least for irrigation should be envisaged.

Patra K.C, "Hydrology and Water Resources Engineering", 1 st Ed, Narosa Publishing House, New Delhi, 2000,pp.1-18.

James S. Monroe and Reed Wicander, "Physical Geology-Exploring the Earth", 3 rd Ed., Wadsworth Publishing Co., An International Thomson Publishing Co., 1998, pp.409-441.

Macanzie L.Davis and David A.Cornwell, "Introduction to Environmental Engineering", 3 rd Ed., WCB/McGraw Hill Publishers, 1998, pp.320-330.

Gerald Kiely, "Environmental Engineering", Int. Ed., Irwin/ McGraw Hill Publication, 1998, pp. 263-332.

Kotaiah B and Kumara Swamy N, "Environmental Engineering Manual", 1 st Ed., Charotar Publishing House, Anand, India, 1994.

Department of Civil Engineering,
Adhiyamaan College of Engineering,
Hosur, Tamil Nadu, India.
Phone: (04344) 560575.