Major ion chemistry and hydrochemical processes controlling water composition of Teesta River catchment, Sikkim Himalaya, India

ABSTRACT The study determines the major ion chemistry of the Teesta River catchment in the Eastern Himalayan region and evaluates hydrochemical processes controlling water composition of the catchment area. Water of the Teesta River basin was in neutral to alkaline condition. HCO3−, Ca2+, Na+, Mg2+ and SO42- were identified as the dominant ionic species in the Teesta catchment water composition and Ca-Mg-HCO3 as the dominant hydrochemical facies. Higher contribution of (Ca2++Mg2+) and HCO3− towards the TZ+ and TZ−, high ionic ratios of (Ca2++Mg2+)/(Na++K+), HCO3−/(Cl−+SO42-) and low (Na++K+)/TZ+ ratio suggest carbonate weathering as a major contributor of dissolved ions in the Teesta basin water. Under-saturation with respect to both carbonate and sulphate phase minerals suggests that water can dissolve these minerals during water-rock interaction. Concentration of the analysed water quality parameters were well below the specified drinking and irrigation water limits.


Introduction
The Himalayas are commonly referred as the 'third pole' because it is known as the largest repositories of snow and ice outside the polar region.About 33,050 km 2 geographical areas in Indian Himalayan ranges are reported to be glaciated and glacial melting annually contribute approximately 8.6 × 10 6 m 3 fresh water to the main river systems of the Indian subcontinent [1].Indian parts of the Himalayas contain 9575 glaciers and more than 20 principal Indian rivers are originated in the Himalayan range from glacial melting [2].Indus, Ganga and Brahmaputra Rivers and their tributaries like Jhelum, Ravi, Beas, Sutlej, Yamuna, Bhagirathi, Alaknanda, Kosi and Teesta are the main glacial fed Indian rivers which originate in the Himalayas.These Himalayan perennial rivers bring water and fertile soil from the glaciers, mountain slope, and lakes and play a major role in water supply to large population in north Indian States and sustaining agricultural productivity in the long alluvial track of Indo-Gangetic plains [3].Besides rivers, there are a number of lakes like Wular, Dal, Manimahesh, Mansarover, Deoria Tal, Naini Tal, Phewa, Gurudongmar, Chhangu etc. which are scattered in the Himalayan region from North-West to Eastern Himalaya and form an important source of fresh water.
The Himalayan River basins provide natural units for the understanding of weathering and geochemical processes, fluxes of dissolved and sediments loads and estimation of chemical and sediment erosion rates.Estimate showed that the Himalayan Rivers contributed as much as 20% of the global sediment loads transfer [4].The Ganges and Brahmaputra Rivers are annually contributing about a billion tons (10 12 kg) of sediment loads and 0.13 × 10 12 kg of dissolved ions loads to the oceans [5][6][7].Chemical denudation rates of the Himalayan Rivers are 2-3 times higher than the world average [7].Various natural and anthropogenic activities and global climatic changes across the Himalayan and Tibetan Plateau affects water chemistry and mass transfer rates of the glacial fed Himalayan Rivers.Dynamic changes in these high land river basins have serious implications on environmental sustainability, ecosystems and livelihoods in the downstream areas both at local and regional levels [8].Geochemistry of major Himalayan River systems, such as the Ganges, Brahmaputra, Indus, Huanghe, Changjiang etc. have been widely studied for understanding the weathering and solute acquisition processes, chemical and sediment erosion rate and fluxes of dissolved and suspended load to the downstream river/ocean [3][4][5][6][7][8][9][10][11][12][13][14].Studies have also been carried out on the sediment load and geochemistry of glacier meltwater and streams of the Garhwal and Kumaon Himalayas [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] and the rivers flowing in the Nepal and Tibetan Plateau [25][26][27][28][29][30][31].However, similar investigations for rivers flowing in the eastern part of the Indian Himalayan catchment, particularly for the Teesta River basin is very limited [32,33].Teesta is one of the important river flowing in the north-eastern part of India.It originates in Greater Himalaya and drains through Sikkim and West Bengal states of India before joining the river Brahmaputra in Bangladesh.Teesta and its tributaries greatly influence the ecology of surrounding areas and the physical and chemical characteristics of the streams while traversing through various valleys and ravines [18].Keeping in view, the present study aimed to investigate the hydro-geochemistry of the Teesta River basin in the Sikkim Himalaya to assess weathering and solute acquisition processes controlling water chemistry and quality of the catchment water for drinking and irrigation uses.

Teesta River basin
Teesta River originates from the Pauhunri Glacier, a great snow peak at 27°57′12′′ N latitude and 88°50′33′′ E longitude at an elevation of 7,128 m in the Eastern Himalayan region of Sikkim state [34].It travels 414 km long distance before joining the Brahmaputra River in Bangladesh.Out of the total Teesta catchment area of 12,540 km 2 , about 9,855 km 2 fall on Indian side and rest in Bangladesh.High mountain ranges in the Sikkim Himalaya, particularly in the north, north-western and north-eastern parts of the basin, are covered with snowfields.Teesta flows southward through gorges and rapids in the upper catchment and appeared as one of the most scenic rivers in Eastern India.It flows through Jalpaiguri and Cooch Behar of West Bengal state in downstream before meeting Brahmaputra River at Teestamukh Ghat in Rangpur district of Bangladesh.In its course, several tributaries join the Teesta River both on the left and right sides.Lachung Chhu, Chakung Chhu, Dik Chhu, Rani Khola and Rangpo Chhu streams join on the left bank of Teesta, while on the right side it is joined by Zemu Chhu, Rangyong Chhu and Rangeet streams (Figure 1).Teesta and most of its tributaries are flashy mountain rivers, characterised by high-speed turbulent water and capable of carrying boulders and delivering a considerable quantity of sediment downstream.Teesta basin receives an annual rainfall of about 2534 mm, and a major part of rainfall occurred due to south-west monsoon from June to September and July is the wettest month.Figure 2 shows the details of the geological settings and lithology of the Sikkim Himalaya.The northern, eastern and western portions of the Sikkim State are constituted of hard massive gneiss rocks, capable of resisting denudation [35].The rock type in this region belongs to Lesser and Higher Himalayas, the former being represented by low-grade metasedimentary and granite gneiss, whereas the central crystalline in the axial zone of the North Sikkim is represented by high-grade meta-sedimentary and granite intrusive [36].The main litho units exposed in the area include Daling meta-volcano sedimentaries, Lingtse sheared granite gneisses and high-grade Central Crystallines.The Daling Group of rocks comprises (schistose) metawacke, chlorite-muscovite phyllite intercalated with chlorite, quartz and their metamorphic equivalents up to garnet-staurolite-kyanite grade.The metawacke units are highly sheared and are found to be in association with Lingtse gneiss and metaporphyroid bands with bluish palescent quartz clasts.The Central Crystalline Gneiss comprises mica bearing quartz, feldspar gneiss, banded gneiss.Migmatite gneiss, garnetbearing psammitic gneiss and higher grade kyanite-sillimanite garnet gneiss are found to crop out in the eastern part of the area.Calc-silicate/marble bands occur within the highgrade gneissic sequence of Central Crystalline.Occurrence of the high-grade massive dolomite is reported in the Rangeet river valley region.Stringers and disseminations of sulphide mineralisation in the form of chalcopyrite and pyrite occur sporadically within the phyllitic rocks of Daling Group.Apart from these major litho units in the area, there are metabasite bodies and some leucogranites which show pegmatitic character at places.

Sampling and analysis
An organised sampling was carried out in the Teesta River basin during summer months of May 2011, representing a high flow regime.Thirty-two water samples were collected from Teesta main stream (11 No.), tributaries (9 No.) and waterfalls (12 No.) in 1-l prewashed high-density narrow-mouth polyethylene bottles from various parts of the river basin (Figure 1).pH and electrical conductivity (EC) were measured with the help of a handy multiparameter (Eutech (PCSTestr 35).The accuracy of the instrument for pH was ±0.01 pH, while for EC the accuracy was ±1% full scale.Water samples were filtered with the help of a vacuum pump using 0.45 µm Millipore membrane filter and preserved at 4°C for further analysis.All analysis was completed within 2 weeks after sampling.Concentration of major anions (F − , Cl − , SO 4 2-, NO 3 − ) was determined by ion chromatograph (Dionex IC 900) after calibrating with known standard solutions.Atomic absorption spectrophotometer (AAS) was used for estimating major cations (Ca 2+ , Mg 2+ , Na + , K + ) concentration in flame mode.The concentration of bicarbonate (HCO 3 − ) was determined by the acid titration method, while dissolved silica (H 4 SiO 4 ) was measured by molybdosilicate method using UV visible spectrophotometer [37].For quality control, each calibration curve was evaluated by analyses of standards solutions before, during and after the analyses of water samples.Ultrapure water with 18.2 MΩ resistivity and analytical grade chemicals and Merck aqueous standards were used in the analysis and calibration.The analytical precision was maintained within ±5% during the chemical analysis.An overall normalised inorganic charge balance (NICB) between cations (TZ + ) and anions (TZ − ) of ±10% is an added proof of the data precision (Supplemental Fig. 1).

pH, Electrical Conductivity (EC) and Total Dissolved Solids (TDS)
pH of the Teesta catchment water samples ranged from 6.9 to 7.8 and 6.6 to 8.7 respectively in the mainstream and tributaries, indicating near-neutral to alkaline conditions (Table 1).The electrical conductivity (EC) value in the analysed water samples varied from 77 to 144 µS cm −1 (average 101 µS cm −1 ) in the mainstream, while in the tributaries water samples, it ranged from 36 to 195 µS cm −1 (average 105 µS cm −1 ).TDS concentration varied from 59 to 115 mg L −1 in the river water and 32 to 148 mg L −1 in the tributaries water samples (Table 1).The average TDS of the waterfall samples (58 mg L −1 ) were relatively low as compared to main stream (79 mg L −1 ) and tributaries water (83.3 mg L −1 ) (Figure 3).The Teesta basin water samples can be categorised into fresh water category as the TDS of the analysed water samples were below 1000 mg L −1 [40].

Major ion chemistry
Anion chemistry of the Teesta River basin water was dominated by HCO 3 − with secondary contributions from SO 4 2-and Cl − .In general, waterfall samples had low ionic concentration compared to the Teesta main stream water, except for Cl − and NO 3 − (Figure 3).
Anions in general followed the abundance order of HCO 3 (Figure 4).HCO 3 − concentration in the analysed water samples ranged between 101  water is mostly derived from the dissolution of atmospheric CO 2, apart from oxidation of organic matters and weathering of carbonate and silicate minerals [41].HCO 3 − source estimation following Raymahashay method [42] showed that 66% of HCO 3 − in the Teesta catchment water originated from carbonate weathering and the remaining 34% from the silicate weathering.SO 4 2-concentration in the Teesta basin water ranged from 10 to 253 µM, contributing about 15% towards total anionic charge balance (TZ − ).HCO 3 − and SO 4 2-are together accounting for 86% of the total negative charge balance (TZ − ).
Sulphide oxidation and anthropogenic sources, including industrial and agricultural effluents, are the major contributor of SO 4 2-ions in fresh water system.Anthropogenic and atmospheric contribution to major ion budget of the Teesta catchment water may   not be significant, considering its location in the reasonably pristine and remote from leanly populated areas, especially at upper reaches.Further, a high ratio of Ca 2+ /SO 4 2-(avg.4.9) limits the possibility of gypsum dissolution contribution and suggest weathering of pyrites (FeS 2 ) associated with the pyritous-carbonaceous slates and phyllites in the geological units of the Higher Himalayas as the major source of sulphate in these waters [43,44].Krishnaswami and Singh [45] and Galy et al. [11] also demonstrated that sulphide oxidation is the major source of SO 4 2-in the Himalayan river basins in India.Cl − and NO 3 − concentrations in the Teesta River basin water ranged from 12 to 142 µM and 5 to 154 µM, respectively and both these ions are together contributing 11% to the TZ − .Cl − in water are mainly originated from the dissolution of halite salt (NaCl) or from atmospheric rain, while the common source of nitrates includes agricultural fertilisers, animal excreta and nitrification of ammonium [46].Low contribution of Cl − towards anionic charge balance, high Na + /Cl − ratio and long distance from coastal area limits the atmospheric contribution of Cl − .A positive correlation of NO 3 − with Cl − (0.82) suggests that NO 3 − and Cl − in the Teesta basin were mainly derived from local agricultural and anthropogenic sources (Table 2).Concentrations of F − in the Teesta River basin ranged from 10 to 88 µM, contributing about 3% to the total anionic charge balance and probably derived from the local lithology.Ca 2+ and Na + ions dominated in the cation chemistry of the Teesta River catchment water and followed the abundance order of Ca 2+ >Na + >Mg 2->K + (Figure 4).Concentration of Ca 2+ ranged from 53 to 987 µM, which accounted for 59% of the total cationic charge equivalence (TZ + ).Weathering of carbonate, sulphate and silicate minerals of calcium are the universal source of Ca 2+ in the water bodies.Concentrations of Mg 2+ in the analysed water varied from 18 to 703 µM and accounted for 16% of the TZ + charge balance.The concentration of Na + and K + ranged from 34 to 451 µM and 12 to 86 µM and contributed 20% and 5% to the total positive ion charge balance (TZ + ), respectively (Figure 4).Na + and K + in water are mainly derived from weathering of silicate minerals like albite, orthoclase, microcline and muscovite [43,47].
Silica concentration in the Teesta catchment ranged from 7 to 156 µM (avg.78 µM) and accounted for 10% of total dissolved solids loads.A negative correlation of silica with pH (Table 2) suggest its sorption by clay minerals [48,49].Dissolved silica (H 4 SiO 4 ) in aquatic ecosystems is believed to be derived primarily through rock weathering and point sources have generally negligible contribution [50].The major portion of the dissolved silica in the Teesta basin water is probably derived from the chemical breakdown of silicates during weathering processes by the reaction: Table 3 compared the average chemical composition of the Teesta basin water with other selected rivers of the central and eastern Himalayas.TDS concentration of the Teesta stream water is comparable with Bhagirathi and Bhilangana streams of upper Ganga Basin and Trishuli stream of Gandak River catchment, but it is much lower than other Himalayan steams.Relatively higher contribution of dissolved silica and Na + in the Teesta catchment water compared to other streams of Gandak, Alaknanda and Bhagirathi basin signify the contribution of silicate lithology in controlling the water composition.Interestingly stream water of Gandak, Ghaghra and Yamuna River basin having higher dissolved ion concentration as compared to Teesta, Brahmaputra, Alaknanda and Bhagirathi catchment water.Such spatial heterogeneity in water chemistry is mainly due to variation in basin geology, soil and vegetation cover, climatic condition and tectonic activities in the catchment area that controlled the chemical weathering rate and solute acquisition processes [51,52].Low concentrations of the principal ionic constituents in the water of the Teesta River catchment could be attributed to non-favourable basin lithology and steep slopes of the terrain.Steep slopes cause a quick outflow of the rainwater, allowing a short residence time for water to interact with the catchment rocks.
The trilinear Piper plot [54] of the Teesta basin water revealed Ca-Mg-HCO 3 as the dominant hydrochemical facies (Figure 5).Data points of the majority of the samples fell in the Ca 2+ zone in the cation facies, except for three tributary samples which fell in the no dominant zone.However, with respect to anion facies, all the water samples congregated only in the HCO 3 − zone.The plot of chemical data on diamond-shaped central field suggested the dominance of alkaline earth metals (Ca 2+ +Mg 2+ ) over the alkali metal cations (Na + +K + ) and the ascendancy of weak acids (HCO 3 − ) over strong acids (SO 4 2-+Cl − ).All water samples have eventually been plotted in zone 5, suggesting carbonate hardness (secondary alkalinity) exceeding 50%.

Weathering and hydrochemical processes
Weathering of rock-forming minerals govern the content of dissolved ions in the river water, however, atmospheric rain and anthropogenic sources may also contribute to it in a limited manner [55,56].The relative abundance of dissolved ions in the river water generally depends on the presence of source minerals in the catchment rocks and its solubility [7,22].Sources of the dissolved ions and geochemical processes controlling the river water composition may be assessed on the basis of available information about the catchment geology, the relative abundance of dissolved ions, inter-elemental ratios and their correlations.The ionic contribution from atmospheric sources to high altitude river water can be assessed by comparing the chemical composition with fresh snow or rains from nearby locations [57].An alternative method for assessing the atmospheric inputs in  the river water composition is a consideration of the ratio of elements to chloride, owing to the profuse concentration of chloride ion in the ocean water and its low content in most rocks [10,21].The variation plot of Na + vs Cl − for the Teesta basin water shows that plotted points fall much above the equiline, suggesting a higher ratio of Na + /Cl − i.e. 4.1 and limited contributions from atmospheric precipitation or halite dissolution (Figure 6(a)).A high concentration of HCO 3 − and its dominance over (SO 4 2-+Cl − ) in the studied water samples (Figure 6(b)) also suggest rock weathering as a major solute source with limited input from atmospheric and anthropogenic sources [41,58].The basic stoichiometry of carbonate weathering reactions depends on the fact that carbonate derived Ca 2+ +Mg 2+ should be balanced by the carbonate derived HCO 3 − .(Ca 2+ +Mg 2+ ) vs HCO 3 − plot of the Teesta basin water samples clearly shows that most of the points fall along the 1:1 equiline, suggesting thereby that Ca 2+ , Mg 2+ and HCO 3 − are mostly derived from the dissolution of carbonate rocks (Figure 6(c)).In a few water samples, (Ca 2+ +Mg 2+ ) content was found to be slightly in surplus of HCO 3 − suggesting non-carbonate source and demanding other anions i.e.SO 4 2-and Cl − to balance excess alkaline earth metals.The scatter plots between (Ca 2+ +Mg 2+ ) vs (HCO 3 − +SO 4 2-) and (Ca 2+ +Mg 2+ ) vs TZ + exhibit better correlation in the lower concentration range, but it deviate from equiline at higher concentration, warranting release of alkalis (Na + +K + ) to compensate the charge differences (Figure 6(d,e)).Further, the plots of (Ca 2+ +Mg 2+ ) vs TZ + and (Ca 2+ +Mg 2+ ) vs (Na + +K + ) suggest that (Ca 2 + +Mg 2+ ) were the major contributor towards the total cationic (TZ + ) charge balance, and it exceeded alkali content in water (Figure 6(e, f)).High Ca 2+ /SO 4 2-(4.9)and HCO 3 − /(HCO 3 − +SO 4

2-
) ratio (>0.5) also indicated that the carbonic acid-mediated reaction was more important in solute acquisition processes than sulphide oxidation or dissolution of anhydrite/gypsum in Teesta catchment water.
Considering the higher contribution of (Ca 2+ +Mg 2+ ) and HCO 3 − towards the TZ + and TZ − respectively, high ratios of (Ca 2+ +Mg 2+ )/(Na + +K + ) i.e. ) i.e. 0.8 and low ratio of (Na + +K + )/TZ + i.e. 0.24, it can be concluded that carbonate weathering was the major contributor of dissolved ions in the Teesta basin water.The high Ca 2+ /Mg 2+ ratio (average 4.19) in most of the water samples suggest the higher contribution of calcite dissolution than dolomite in solute acquisition.Carbonates can play a major role in solute acquisition processes due to its high solubility even at its low presence.Weathering of carbonates occurred as calcite (CaCO 3 ) fillings in microfractures and disseminated grains in silicate matrix are recognised as an important Ca contributor to aquatic water in silicate dominant catchment [59,60].The prior study established that the weathering rate of trace calcite in ambient rocks are ~350 times faster than plagioclase [61].Relatively higher contribution of dissolved silica to the TDS, high Na + /Cl − ratio, excess of (HCO 3 − +SO 4

2-
) over (Ca 2+ +Mg 2+ ) at higher concentration range and positive correlation of HCO 3 − with Ca 2+ (0.89), Na + (0.83) and Mg 2+ (0.81) suggest that Na + , K + along with some portion of Ca 2+ , Mg 2+ and HCO 3 − in the Teesta River water were derived from the weathering of aluminosilicate minerals like albite, anorthite, diopside and mica associated with crystalline rocks i.e. granite, granite gneiss and micaschist of the area.

Saturation index (SI) and CO 2 Partial pressure (PCO 2 )
Saturation indices (SI) helps to understand the equilibrium state of the water with respect to the particular mineral phase [62].It indicates whether a mineral will dissolve or precipitate in an existing physicochemical environment.The saturation indices are defined as the logarithm of the ratio of ion activity product (IAP) to the mineral equilibrium constant (Ksp) at a given temperature [40,62]: Positive SI suggests that the water is supersaturated in respect to particular mineral phase and thus incapable in dissolving the considered mineral, and subsequently, the mineral phase can be precipitated.Conversely, a negative SI indicates an undersaturation condition and thus there is a possibility for dissolution of the mineral phase.Saturation indices (SI) of calcite, dolomite, gypsum, anhydrite, fluorite, aragonite, quartz and SiO 2 were calculated using PHREQC Interactive software of USGS [63].The plot of saturation indices of calcite (SI c ) and dolomite (SI d ) demonstrates an undersaturation condition in respect to both dolomite and calcite except for one sample.This shows that catchment water may dissolve calcite and/or dolomite if it comes in contact with source rocks.Water samples are also undersaturated with respect to fluorite and sulphur bearing minerals like gypsum and anhydrite which suggests that the Teesta catchment water tends to dissolve fluorite, gypsum and anhydrite if it comes into contact with the host rocks along the flow path.(Figure 7(a)).The analysed water samples are found to be undersaturated with respect to amorphous silica but oversaturated with respect to quartz.Though quartz is one of the main constituent in the rocks but the low solubility of quartz suggests that silica in water were mainly sourced either from amorphous silica or weathering of aluminosilicates [64].
The CO 2 partial pressure (PCO 2 ) is estimated for the Teesta main stream, tributaries and waterfall samples based on the pH value and concentration of HCO 3 (Table 1).The CO 2 partial pressure in the catchment water samples varied from 10 −3.93 to 10 −2.02 and data plots show that PCO 2 values were higher than the atmospheric level of 10 −35 except for one sample (Figure 7(b)).The average PCO 2 value of waterfall samples (10 −2. 44) is slightly higher than the Teesta River (10 −2.80 ) and tributary (10 −2.82 ) water.The higher PCO 2 values of water bodies indicate an open system weathering and a relatively higher rate of solubility compared to the slow release of excess CO 2 gas.The relatively higher PCO 2 value of waterfall samples may be attributed to turbulent nature of water which enhances CO 2 solubility and decreases pH [62].Higher PCO 2 of the river water is a global trend suggesting that natural aquatic systems are commonly out of equilibrium with the atmosphere [65].

Suitability for drinking and irrigation uses
Table 4 provides the descriptive statistics of the physico-chemical parameters of the Teesta river basin water along with their prescribed limits established by the World Health Organisation (WHO) [66] and the Bureau of Indian Standards (BIS) [67] for drinking and public health.It is apparent from Table 4 that the concentration of the anions and cations and other water quality parameters were far below as compared to the WHO [66] and BIS [67] drinking water standards, indicating pristine nature of the water which could be used for the drinking purposes after filtration and disinfection.
Irrigation with poor quality water affects soil health and crop productivity.Electrical conductivity and sodium concentration are very important in classifying water for irrigation uses [68].The high salinity of water affects the growth of the plants, directly and indirectly, affects soil structure, permeability, and aeration.Various researchers proposed a number of indices to describe irrigation worthiness of water [69][70][71][72][73].The water quality of the Teesta catchment with regards to irrigation suitability was evaluated based on the electrical conductivity (EC), sodium absorption ratio (SAR), percent sodium (%Na) and residual sodium carbonate (RSC) and provided in Table 5.All the measured parameters ranked the Teesta catchment water in a good to excellent category that could be safely used for irrigation.These findings were in line with the findings of the previous studies on Himalayan Rivers [28,29,32,74].

Conclusions
The present study analysed the geochemical characteristics of the glacier-fed Teesta River basin water and evaluated the processes controlling the chemical composition of the catchment water.The results revealed that the Teesta River basin water in the Sikkim Himalaya region was neutral to alkaline.HCO 3 − , Ca 2+ , Na + , Mg 2+ and SO 4 2-were the dominant dissolved ions in the Teesta catchment water resources.Trilinear plot of the chemical data showed the dominance of alkaline earths (Ca 2+ +Mg 2+ ) over alkali cations (Na + +K + ) and weak acid (HCO 3 − ) over strong acids (SO 4 2-+Cl − ).Ca-Mg-HCO 3 was the  ) over (Ca 2 + +Mg 2+ ) at higher concentration range and positive correlation of HCO 3 − with Ca 2+ , Mg 2+ , Na + suggest that Na + , K + and some fraction of Ca 2+ , Mg 2+ and HCO 3 − were derived from silicate weathering.The Teesta basin waters were undersaturated with respect to both carbonate and sulphate phase minerals and capable of dissolving calcite, dolomite, gypsum, and anhydrite in interaction with host rocks.Higher PCO 2 value in the Teesta basin water indicated an open system weathering and a higher rate of solubility compared to release of excess CO 2 in low temperature turbulent and river environment.Water quality of the Teesta River was pristine nature and can be used safely for drinking and irrigation purposes.

Figure 1 .
Figure 1.Location map of the Teesta River showing water sampling locations (R = Teesta main river, T = Tributaries, F = Water fall).

Figure 3 .
Figure 3. Average concentration of analysed parameters of the Teesta river basin.

Figure 4 .
Figure 4. Contribution of individual ions towards the total anionic and cationic mass balance of the Teesta river basin.

Figure 5 .Figure 6 .
Figure 5. Piper's trilinear diagram, showing the relationship between dissolved ions and hydrochemical facies in the Teesta river basin.

Figure 7 .
Figure 7. Plot of (a) Total dissolved solid vs saturation indices (SI) (b) Total dissolved solid vs log PCO 2.

Table 1 .
Hydro-geochemical characteristic of the Teesta River basin water.

Table 2 .
Inter-elemental correlation of dissolved ions in the Teesta catchment water.

Table 3 .
Average chemical composition of the Teesta basin water in comparison to other selected Himalayan Rivers of the Ganga-Brahmaputra River systems.

Table 4 .
[67]istical summary of water quality parameters of the Teesta Catchment water and its comparison with the WHO[66]and Indian Standards[67]for drinking water.