CONTENTS-
Abstract
Introduction
Study Area
Sampling and Analysis
Results and Discussion
Acknowledgements
References

The major lakes in Kumaun region are extremely popular tourist destinations situated in the scenic Himalayas. The lakes are also a source of drinking water, fisheries, irrigation etc to the local people. Ca, Mg, HCO 3 dominate water composition and are sourced from carbonate lithology. Chloride, sulphate and nitrate are added from pollution sources. The dissolved trace elements concentrations are very low and attributed to the alkaline nature of water and phytoplanktons in the lakes.

Lakes represent one of the most valuable ecosystems on earth. Like any other aquatic body, lakes are characterised by both inputs and output of materials. The different substances added show variable residence time or flushing times, which vary between 1-100 years. Hence lakes manifest any excess addition of pollutant with immediate effect in its water composition and in the form of lake eutrophication (excess phosphate loading, for example). Lakes are important from economic viewpoint as well, since most of the lakes serve as drinking water resource, fisheries, irrigation, tourism, aesthetics, etc. Hence there is greater need for monitoring of lake water quality for proper management practices.

The four lakes studied namely Nainital, Bhimtal, Sattal, and Naukuchiatal are located (Figure 1) in the Nainital massif of the Himalayas in Kumaun region. Geology, structure and tectonics of the region is discussed in detail elsewhere (Valdiya, 1980; Valdiya 1988). Briefly the lithology around Nainital lake consists of carbonate rocks, calcareous states, argillaceous limestones, ferruginous shales, algal dolomites, black shales with marlite, greywacke, siltstones etc of the Krol formation (Permo-Triassic). The catchment in Bhimtal is comprised of metabasites associated with shallow water quartzites, grits, conglomerates, phyllites and rocks of zeolite and green schist facies. The Naukuchiatal lake is surrounded by metavolcanics and quartzites whereas Sattal lake is developed in the quartzitic country. The Nainital lake is a major tourist destination with a population of around 40,000 and an additional tourist influx of 100,000 per year. The landuse in the catchment is (Nachiappan et al. 2000) forests and shrubs (42%), buildings and hotels (41%), roads (2.1%), water bodies (10.3%), playground (1.1%) and barren lands (3.9%). The Naintal lake is fed by twenty three drains and Naukuciatal by several underground springs.

Water samples were collected in pre-cleaned polypropylene laboratory bottles during 1998-99. After measuring   pH, electrical conductivity and bicarbonate   in the field, samples were filtered by 0.45 m m cellulose nitrate membrane filter papers. A portion of the samples were separated and acidified with 1N pure HNO 3 for trace element   analysis. Major cations and anions were measured by ion chromatograph Dionex 300 with a precision better than ± 5%. A very good net inorganic charge balance ( ± 3%) confirms the accuracy of the analysis. Dissolved trace elements were measured by ICP-MS after addition of an indium internal standard solution and running with SLRS-3 and SLRS-4 standards.

The average chemical composition of the water in the lakes is presented in Table 1. The lakes are alkaline in nature. Electrical conductivity (EC), which is a measure of the total dissolved solids, is higher in Nainital lake (>0.3 mS/cm) when compared to Bhimtal, Sattal and Naukuchiatal (< 0.1mS/cm). This is reflected in the pre-dominantly tall stacks plotted for chlorides, sulphates, bicarbonates and magnesium in Nainital compared to other lakes (Figure 2), although the lakes are characterised by very low ionic strengths (Nainital-8.86x10 -3 moles/l, Bhimtal 1.18x 10 -3 , Sattal 2.23 x 10 -3 and Naukuchiatal 1.9x10 -3 moles/l). The variations in major ionic concentrations in the lakes are an indication that, the chemistry of the water is determined by different rock sources. The ratio Ca+Mg/Na+K which shows if carbonates or silicate rocks are predominant source for water composition, gives average values of 4-5 for Nainital, 3-4 for Bhimtal, 7-8 for Sattal and 3-6 for Naukuchiatal pointing to the dominance of carbonate rock weathering responsible for water chemistry. Carbonate rocks weather twelve times more effectively   than silicate rocks such as granites, gneisses or mica-schists (Meybeck, 1987) and hence control water composition. The congruent weathering of carbonates releases high bicarbonates to water as compared to the incongruent silicate weathering. The ratio of HCO 3 /Ca+Mg in the lakes is ~ 1, which shows the weathering of carbonate source rocks. Weathering of silicate rocks has an important bearing on atmospheric CO 2 concentrations, since for each mole of bicarbonate ion released to water from silicate weathering, one mole of CO 2 from atmosphere is consumed whereas, one half of HCO 3 coming from carbonate rocks is derived from the rock itself. Hence, excessive silicate weathering plays an important role in global CO 2 cycle and global climate change.

Table 1

Average chemical composition of water in the lakes (Cl to Mg in m mol/l, Mn to Pb in m g/l)

Rainwater analysis of December showed pH to be 6.8 and chloride concentration of 3.0 mg/l (85 m mol /l). The average rainfall in the region is around 2300 mm/yr, the total volume of the rainfall with this composition could add upto significant Na and Cl concentrations in the lakes. The origin of chloride is mostly from rainfall and pollution sources, as rock sources for chloride is minimum unless salt deposits are of common occurrence. The lakes have significant chloride concentrations with Nainital far exceeding the other lakes, which indicates that chloride is added from pollution sources. Although there are no reported halite/evaporite deposits in the region, a strong Na-Cl source is speculated as the Bhimtal-Naukuchiatal basin underwent two phases of arid climate in the past (Kotlia et.al., 1997). From the observed Cl concentrations, Na derived from silicate rocks in the lakes is calculated to be 55-80% and 30-40% of Ca is derived from silicate rocks. The carbonate constituents Ca, Mg, HCO3 when present in excessive concentrations, cause carbonate precipitation, which is enhanced by increased pCO 2 and cold temperature conditions. The variabilities with respect to geological   settings, climate, water chemistry and biological activities in lakes limit generalisations about mechanisms of carbonate sedimentation. Freshwater chalks are a common occurrence   of numerous shallow lakes and ponds (Kelts and Hsu, 1978).

The molar Mg/Ca ratio in Bhimtal, Sattal and Naukuchiatal is <0.5 and >4.5 in Nainital, which indicates high saturation index and carbonate precipitation in Nainital lake compared to the other three lakes. The rate of sedimentation in Nainital, Bhimtal, Sattal and Naukuchiatal is 11.5mm/yr, 4.7mm/yr, 3.0mm/yr and 3.72mm/yr respectively (Das et.al., 1994). The Kumaun Himalayan region is tectonically very active, the frequency of naturally occurring and excavation induced landslides (Bartarya and Valdiya, 1989) being 0.72 landslide/km 2 . Tectonic activities are known to induce higher sedimentation rate in Lake Chapala, western Mexico (Fernex et. al., 2001). However, a high carbonate precipitation as a process cannot be ruled out for the high sedimentation rates observed for Nainital lake. The Nainital lake shows significant sulphate concentrations (1024 m mol/l) far exceeding the other three lakes (<65 m mol/l) probably due to the inputs from the open drains (effluents), efficient oxidation of sulphides in the bottom of the lake and dissolution of gypsum. NO 3 concentrations in the Nainital lake is significant (>70 m mol/l),   since there is not much agricultural activity (hence no application of fertilisers) in the catchment of Nainital, the source of NO 3 could be entirely from domestic effluents and rainfall. As the rainwater analysed showed no   appreciable presence of NO 3 , domestic effluents can be thought of as the only source of NO 3 in the lake. Bhimtal, sattal and Naukuchiatal have NO 3 concentrations of 43 m mol/l, 31 m mol/l and 13 m mol/l respectively. To compute the source of NO 3 from different sources in the absence of direct point source measurement, a model calculation is carried out with Sattal as a case study for NO 3 pollution. Since NO 3 has inputs from agriculture, anthropogenic (cultural) and atmospheric precipitation, an equation with these inputs can be written as,

(NO 3 )Tot = (NO 3 )Atm +(NO 3 )Anth + (NO 3 ) Agr

As molar ratios are preferred over concentrations to avoid dilution effects, the equation becomes,

(NO 3 /Na) Sattal lake = µ 1 (NO 3 /Na)Atm + µ 2 (NO 3 /Na) Anth. + µ 3 (NO 3 )Agr.

Where µ 1, µ 2, µ 3 are mole fractions of NO 3 from atmospheric, anthropogenic and agricultural sources respectively ( µ 1+ µ 2+ µ 3 = 1). Assuming (NO 3 /Na) Atm = 0, (NO 3 /Na)Agr = 0.07 (this value is obtained from Naukuciatal lake, where the source of NO 3 is from agriculture only) and (NO 3 /Na)anth = 0.13 (Nainital lake, several drains flow to the lake) and solving the equation 33% and 67% of NO 3 in Sattal lake is estimated to come from agriculture and anthropogenic source.

Dissolved trace element concentrations are an indication of the extent of pollution in an aquatic environment. Trace element levels in lakes are mainly governed by exchanges between atmosphere-water, process in lake catchment, in water column and process at sediment surface. In regions of high industrial activities, these concentrations reach alarming proportions unless remedial measures are in practice. The lakes in the Kumaun Himalayan region are characterized by low dissolved trace elements (Figure 3) when compared to WHO (1984) standards. The alkaline pH of the lake waters is important in precipitation of the high dissolved heavy metals. Relatively Nainital lake has higher concentrations. Dissolved Zinc in the lakes is very low (<2.4x10 -9 mol/l) and indicates there is no addition from industries. The Pb concentration could be from vehicular pollution. The phytoplankton community in the lakes is very high (>2.4x10 6 cells/l) (Sharma et.al., 1982), which probably plays a role in trace element concentrations in the lakes. The concept of potential loading is useful for various pollution inputs (Cl, SO 4 , etc) in addition to those (P, N) which cause eutrophication   (Stumm and Morgan, 1984). As the excess sulphate and chlorides in the lakes are also speculated to come from pollution sources, potential loading in the lakes which is equated to human energy consumption per unit volume of the lake was calculated. Nainital (7.2) and Naukuchiatal (5.9) have higher potential loadings compared to Sattal (3.6) and Bhimtal (2.3). At the time of higher tourist influx in summer, potential loading in lakes could reach almost 50 times.

I would like to thank “All India Council of Technical Education (AICTE)” for funding the study.

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