Major ion chemistry of the Teesta River in Sikkim Himalaya, India: Chemical weathering and assessment of water quality

Journal of Hydrology: Regional Studies 24 (2019) 100612

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Major ion chemistry of the Teesta River in Sikkim Himalaya, India: Chemical weathering and assessment of water quality

T

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Tenzin Tseringa, , Mahmoud S.M. Abdel Waheda,b, Sidra Iftekhara, Mika Sillanpääa a b

Department of Green chemistry, LUT University, Sammonkatu 12, FI-50130 Mikkeli, Finland Department of Geology, Faculty of Science, Beni-Suef University, Beni-Suef, Egypt

A R T IC LE I N F O

ABS TRA CT

Keywords: Carbonate weathering Silicate weathering Eastern Himalayas Teesta River chemistry Major ions Sikkim Himalaya

Study Region: Teesta River of Eastern Himalayas, India. Study Focus: This article addresses the mechanisms of weathering in the Teesta River, for the first time based on the original data of major ions. Water samples were collected along the Teesta River in Sikkim Himalaya, India. The evaluation of the major ion and trace elements against the standard guideline values and the average chemical composition of world rivers were discussed. New Hydrological Insights: The predominance of Ca, Mg and HCO3 in all waters reflects the influence of carbonate weathering on the Teesta River. However, an increase in the Na/Ca ratio was linked to the increase of Si downstream, indicating that silicate weathering was predominant in the lowlands of Teesta drainage. The rate of silicate weathering is dependent on an overall balance of key factors including gradient, contact time, temperature and vegetation. The higher concentration of cations was balanced by the SO4 originating from the action of H2SO4 and H2CO3 on carbonates and silicates. Rock weathering (carbonate-silicate weathering) is the key mechanism that controls the major ion chemistry of the Teesta River followed by evaporite dissolution.

1. Introduction The status of natural water throughout the world has been gradually changing due to climate change and an increase in human activity (Raymond et al., 2008). According to research initiated by the World Health Organization (WHO), around 1.1 billion people do not have access to clean drinking water (WHO, 2011). The receding of Himalayan glaciers due to global climate change has a noticeable impact on the chemistry and the contamination level of toxic metals of the rivers in the Himalayas. (Huang et al., 2009; Qu et al., 2015; Zhang et al., 2015). Since the water from rivers is the main source of freshwater for human and animals, monitoring of river water availability and water quality are very important for society and ecosystems (Behmel et al., 2016). The Teesta River, an important tributary of the Brahmaputra River, is a source of fresh water for the aquatic ecosystem that is home to the activities of millions of people along its route (Khuman et al., 2018). It is located in the state of Sikkim in the Eastern Indian Himalaya and is characterised by a large difference in vertical topography over a short horizontal distance. Bhutan and Nepal lie east and west of Sikkim, respectively. The Teesta River originates from the Teesta Khangse (or Pauhunri) glaciers northwest of the state at 7,127 m above sea level (Meetei et al., 2007; Wiejaczka et al., 2014). However, at some of the headwater locations in the northern part, water also originates from the Tibetan Plateau as shown in Fig. 1. In comparison with other sub-basins of the Eastern Himalayas, the Teesta catchment of Sikkim has various landscapes in terms of its morphology and landforms (Singh et al., 2017; Krishna, 2005; Meetei

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Corresponding author. E-mail address: Tenzin.Tsering@lut.fi (T. Tsering).

https://doi.org/10.1016/j.ejrh.2019.100612 Received 31 January 2019; Received in revised form 20 June 2019; Accepted 21 June 2019 2214-5818/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).

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Fig. 1. Location map of the study area with sampling sites.

et al., 2007). It was proposed that the average chemical denudation rate of the Brahmaputra basin is twice as high as that of the Ganges and five times higher than the global average (Galy et al., 1999; Sarin et al., 1989). Currently, studies on the Sikkim Himalayas mostly focus on the formation of tectonics, land use and soil properties (Kellett et al., 2015; Jain et al., 2000; Prokop and Płoskonka, 2014; Rai et al., 1994; Singh et al., 2017; Singh and Goyal, 2016). An asynchronous trend of monsoon variation in the Eastern Himalayas has been recognised within the last few centuries (Bhattacharyya et al., 2007; Ghosh et al., 2018; Li et al., 2017). Flood and drought due to an excess and shortage of precipitation within the same year are more common in the 21st century (Goswami et al., 2018). Furthermore, there are areas that are more prone to flooding due to wet days and precipitation in the Eastern Himalayas (Goswami et al., 2018; Singh and Goyal, 2016). The Teesta River represents the overall fluvial system in the Sikkim Himalayas. However, there was a lack of scientific research on water chemistry in comparison to the other rivers of Indian Himalaya (Das et al., 2018; Devi et al., 2016; Gogoi et al., 2016; Philip et al., 2018). Data regarding the chemistry of major ions in the Teesta River and its comparison with other Himalayan rivers is not available due to lack of studies. Therefore, the primary objective of the current study is to determine the spatial variations of major ions and trace elements of the Teesta River in Sikkim. To the best of our knowledge, to date the geochemistry of major ions in the Teesta River has not been studied. The current study provides a baseline for further research work dealing with the fluvial system in the Eastern Himalayas. It aims to support water resource management, the implementation of water conservation policies and actions in the Eastern Himalayas.

2. Materials and methods 2.1. Site description and geology of the study area Our study area of Sikkim in the Eastern Himalaya lies within the coordinates 27° 04´ 46´´ to 28° 07´48´´ N and 88° 00´ 58´´ to 88° 55´ 25´´ E covering an area of 7096 km2. The drainage system of the Teesta River covers the entire state from the north by adjoining with many small tributaries fed by glaciers and precipitation. Water samples were collected from 19 sampling sites (site1–site 19) along the Teesta River in Sikkim, India (Fig. 1). The Rangit River, a major tributary of the Teesta River, was sampled at five locations (site13 to site 17) as shown in Fig. 1. The Teesta River flows from Sikkim to West Bengal on the Indian subcontinent and then on to Bangladesh, serving as a major source for irrigation and hydropower generation. Sikkim is sub-divided into types of geotectonic domains distinguished from each other by key tectonic elements (Karan, 1989; Sinha-Roy, 1982). The main exposed rock units in the study area are shown in Fig. 2. In the northern part of Sikkim, Tethyan Sedimentary Sequence (TSS) with Cambro-Ordovician to Tertiary sediments is exposed (Fig. 2). The Tethyan rocks are composed mainly of (from old to new) mica-schist, sandstone (metamorphosed and unmetamorphosed), fossil-rich limestone, calcareous arkose and siltstone, pyrite-rich black shale interbedded at all scales (Tipper et al., 2006), limestone and a massive carbonate sequence 2

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Fig. 2. A geological map of Sikkim with the sampling sites (modified after Kellett et al., 2014).

(Wolff-boenisch et al., 2009). The main central thrust (MCT) divides the Greater Himalayan Sequence (GHS) from the Lesser Himalayan Sequence (LHS) (Fig. 2). The GHS mainly consists of two units of silicate gneiss and calc-silicate metamorphic rocks, which include high-grade gneisses, quartzites, calc-silicate rocks, calcite rich-feldspar, intercalations of biotite schists (Bickle et al., 2001; Tipper, et al., 2006). The LHS is dominated by Precambrian rocks composed of quartzite, gneiss, schists, phyllite, granite and some limestone and marble (Wolff-boenisch et al., 2009; Swades Kumar Basu, 2013). South of the LHS is the sub-Himalayan Sequence, which comprises deposits of the Siwaliks (Mio-Pliocene) (Fig. 2). The “inverted Metamorphism” is best observed in Sikkim Himalaya (Carosi et al., 2018; Chakraborty et al., 2016; Mukhopadhyay et al., 2017). 2.2. Climate According to Meetei et al. (2007), the climate regime of Sikkim Himalaya can be classified into five types depending upon elevation. They are i) subtropical zone (up to 1000 m), (ii) warm temperate zone (between 1000 and 2000 m), (iii) cold temperate zone (between 2000 and 2500 m), (iv) cold zone (between 2500 and 4000 m) and (v) frigid zone (above 4000 m). The main sources of the Teesta River water are from glacial meltwater and precipitation (SC Mukhopadhyay, 1982). The Teesta basin is mainly glaciated terrain in the northern part (SC Mukhopadhyay, 1982). Sikkim Himalaya is rich in flora and fauna and has more than 315 glacial lakes. It has been largely influenced by the Bay of Bengal branch of the Indian summer monsoon and there is significant heterogeneity in the precipitation lapse rate (Azhoni and Goyal, 2018; Singh and Goyal, 2016). 2.3. Sampling and analyses Sampling from the Teesta River was performed in March 2018. Sampling sites were selected based on the elevation and the proximity to the source. The sampling plan focused on the entire mainstream of Teesta River in Sikkim Himalaya. The elevation of the sampling sites ranged from 3697 m to 203 m. Representative samples were collected from the headwaters of the river (Lachen and Lachung) up to the confluence with the Rangit River. The mid-stream of the Teesta River was inaccessible due to steep terrain. The pH was measured in situ using a Wagtech multimeter CP 1000. Prior to measurement, the meters were calibrated by standard solutions. Samples were collected at a depth of approximately 30 cm from the surface of water against the flow. All samples were filtered with a 0.45 μm polypropylene syringe filter and acidified with ultrapure nitric acid for metal analysis in a 30 ml LDPE bottle (prewashed with 10% nitric acid and ultra-pure water). Unacidified samples were collected in 30 mL pre-washed LDPE bottles, for cation and anion analysis. Unfiltered samples were collected in a 250 mL polycarbonate bottle for alkalinity measurement. All samples were stored at 4 °C until laboratory analysis. The samples were quantified using ICP-OES (Agilent 5100, Australia) for major elements and silicon. The accuracy of the method was assessed by the reference solution of the mono-component element (certified 3

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Table 1 Field parameters and TDS (in mg/L) of water samples collected from the Teesta River at Sikkim Himalaya, India. Sampling sites

Elevation (m a.s.l)

Latitude (° N)

Longitude (° E)

pH

Turbidity (NTU)

TDS (mg/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

3697 3481 3614 3317 2631 2213 1934 1686 1294 1295 347 295 635 537 468 341 301 250 216

27.86695 27.83541 27.60245 27.76068 27.69012 27.63390 27.65386 27.61348 27.54968 27.41765 27.24262 27.17383 27.29803 27.27837 27.24921 27.16976 27.13023 27.08429 27.07498

88.54497 88.54887 88.70598 88.72153 88.74551 88.70486 88.60594 88.62865 88.64413 88.63008 88.47662 88.53354 88.30326 88.27529 88.30284 88.29946 88.27935 88.42075 88.43220

7.16 7.25 7.31 7.23 7.55 7.69 7.54 7.83 7.76 7.30 7.77 7.69 7.95 7.79 7.75 8.26 7.84 7.98 8.49

5.47 5.44 4.27 3.36 3.80 1.54 6.43 6.83 4.20 0.63 24.00 20.40 3.91 2.54 4.34 7.24 9.74 7.90 11.40

78.33 57.39 36.47 42.85 43.99 55.32 69.74 84.17 79.85 46.32 77.24 67,26 77.23 72.94 75.16 81.00 59.26 75.58 77.73

reference material (CRM), Sigma Aldrich). Blank and replicate sample analysis was performed. The %RSD for all the analyses of elements was < 10%, which showed the high precision of the methods. An alkalinity measurement was carried out by titration with 0.02 N H2SO4. The anion analyses were measured using the ion chromatography (Dionex ICS-1000)-accredited method based on standard ISO 10304-1:2007 at the Finnish Environment Institute, Oulu. All blank samples were below the detection limit of the instruments. The total dissolved solids (TDS) were calculated by the sum of all the major ions (Ca2+, Mg2+, K+, Na+, SO42−, HCO3-, Cl- and Si). 3. Result and discussion 3.1. General observations The results of in situ measurements (pH and turbidity) are shown in Table 1. The Teesta River of Sikkim Himalaya was slightly alkaline with a narrow range of pH from 7.16 to 8.49. The average turbidity of the river is 6.76 except at sampling sites 11 and 12. At site 11 and 12, the turbidity values were almost three times higher than the average value, and the concentration of nitrate was 6.13 μmol/L and 4.68 μmol/L respectively. The higher concentration of NO3− and turbidity at sites 11 and 12 than the remaining sites indicates the contribution of a local pollutant from the nearby state´s largest town at sampling sites 11 and 12. It was observed that the mean TDS value is 66.20 mg/L. The concentration of cations and anions are plotted in Fig. 3 (data in Supplementary Table S1).

Fig. 3. Major cation (a) and anion (b) concentrations at the Teesta River sampling sites. 4

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Fig. 4. Distance of sampling site vs elevation.

The total cation (Tz+) and anion (Tz−) are comparable for all the samples having significant correlation at < 0.01 (one-tailed) with a Pearson coefficient value of 0.88. Good data quality is implied by a good correlation between the sum of major cations and anions (r2 = 0.74) and charge balance [(Z+-Z−)]/[(Z++Z−)] within ± 15%. The samples with a Tz+ value of less than Tz− may be due to the presence of organic ligands (Singh et al., 2005) but this is not discussed in this research. 3.2. Classification into upstream and downstream waters based on geologic terrains and altitudes As shown in Fig. 2, sampling sites 1–8 on the Teesta River lie on geologic terrain dominated by GHS while sites 9–19 lie in the zone of LHS. Classification of the Teesta River in Sikkim into upstream and downstream waters can facilitate the discussion of the geochemical evolution of water chemistry from headwaters to downstream. It was found that sites 1–8 are located at high altitudes (elevation ranges from 1686 m at site 8 to 3697 m at site 1) while sites 9–19 are situated at lower altitudes (from 1294 m at site 9 to 216 m at site 19) (Fig. 4). Based on these observations, sampling sites 1–8 are considered upstream waters while sites 9–19 represent downstream waters. The general trends of major ions concentration from headwaters to downstream does not show any particular pattern (Fig. 3), but a slight increase in Na+ from sites 9–19 is observed (Fig. 3a). 3.3. Mechanisms controlling the water chemistry of the Teesta River The three dominant mechanisms controlling global water chemistry are rock weathering, precipitation and the evaporationcrystallization process (Gibbs, 1970). According to Gibb´s plot, as shown in Fig. 5, the points are mainly towards carbonate and silicate end-members that show the main mechanism controlling the water chemistry of the Teesta River. Ternary plotting can help to understand the major ion chemistry of the present assessment (Fig. 6). For cation, on average, Ca2+ covers the highest total cationic equivalent charge of 63% followed by Na+ and K+ with 22.5%, Mg2+ covering 14.5%. For anion, HCO3− is the dominating ion contributing almost 55.3% of total anionic equivalent charge, followed by SO42- and Cl− with 25.7% (SO4 is the main contributor to this value), Si covering 19%. The trend of the highest composition of Ca2+ and HCO3− in the Teesta River indicates their derivation from carbonate weathering. Similar findings were observed at the Yamuna River and the Brahmaputra River (Dalai et al., 2002b; Qu et al., 2015; Zhang et al., 2015). The main bedrock of Eastern Himalaya is silicate rock and only about 1% carbonate rock. The Himalayan region has a high frequency of physical weathering and chemical erosion (Colin et al., 1999; Dalai et al., 2002a; Singh et al., 2005). The points in ternary plots seem to align from the Ca2+ apex towards the Na+ side accompanied by drifting from the SO42− side toward the HCO3- and Si domain. A general tendency of evolution from carbonate weathering to silicate weathering as water flows from upstream to downstream was observed and is shown by arrows in Fig. 6. However, HCO3- mainly originates from carbonate weathering, which demonstrates the great dominance of carbonates rocks as controls of water chemistry (Blum et al., 1998). The considerable SO42− in upstream waters was likely derived from TSS rocks as products of pyrite oxidation, as the pyrite-rich black shale was interbedded in TSS rocks. The Na-normalised molar ratio plot of Ca2+/Na+ versus HCO3−/Na+ shows the spatiotemporal variability in hydrogeochemistry with three end-members (carbonate-silicate-evaporites). CO2-driven weathering of these end-members can help in the understanding of the weathering processes in the watersheds with silicate-carbonate terrain (Fig. 7). The downstream points of the Teesta River are mainly scattered towards silicate-carbonate end-members, whereas the upstream 5

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Fig. 5. Gibb´s plot of the Teesta River.

Fig. 6. Ternary plots of cation and anion concentrations.

points are closer to the line connecting carbonates and evaporites (Fig. 7). Therefore, the model revealed that carbonate and silicate weathering has more influence downstream of the Teesta River, and upstream is more affected by carbonate weathering and evaporites. However, all samples slightly deviate from the three end-members’ mixing lines (Fig. 7b). This deviation indicates that, along with H2CO3, H2SO4 derived from pyrite oxidation and/or the oxidation of sulphur from organic-rich soils seems to contribute to the weathering process. H2SO4 from pyrite oxidation as a major weathering agent was also concluded by Meyer et al. (2017) during the study of the chemical weathering of the High Standing Island watershed in Taiwan that has pyrite widespread throughout the study area, as well as by Li et al. (2011) during the study of Jialing River chemistry in Southwest China. 3.4. Distinguishing between upstream waters and downstream waters based on K+ concentrations The Ca2+/Na+ ratios in the Himalayan stream waters showed great variability. For example, the average Ca2+/Na+ ratio 6

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Fig. 7. Bivariate Na-normalized molar ratios mixing diagram. The data for the three end-members (carbonates, silicates and evaporites) are obtained from Gaillardet et al. (1999).

calculated by Krishnaswami and Singh (1998) was 0.7, whereas Jacobson et al. (2002a) estimated it to be 0.18 for the Raikhot watershed within the Himalayan region. In the present study, the average ratio of Ca2+/Na+ varied from 2.46 upstream to 1.65 downstream (Table 2). The wide range of Ca2+/Na+ ratios in the Himalaya region is attributed to the variation of the lithology of Higher and Lesser Himalaya. The relative contribution of Ca2+ and Na+ by rocks is not well quantified as stated by Krisnaswami and Sunil Singh (2005). Likewise, the Teesta River is draining from GHS to LHS and these rock units can contribute Na+ and Ca2+ to the Himalayan watershed. Accordingly, as it is assumed that K+ originated exclusively from silicate weathering in the Himalayan streams (Hodson et al., 2002; Wolff-boenisch et al., 2009). A cluster analysis based on K+ concentrations in all waters is used as a tool to identify the magnitude of silicate weathering in the Teesta River watershed in Sikkim (Fig. 8). This cluster analysis was able to classify the sampling sites into two main groups with group 1 from sampling sites 1–8 and group 2 from sites 9–19 based on K+ concentrations. Likewise, the vegetation coverage of Sikkim Himalaya varies widely within the basin, with the upper region of north Sikkim mostly covered by snow and glaciers and the lower region towards the south covered by forest and hard rocks (Ghosh et al., 2018; Meetei et al., 2007). As shown in Table 2, the higher values of K+, Na+, Na+/Ca2+, K+/Ca2+, Si and HCO3− in downstream waters compared to upstream waters revealed that silicate weathering increases towards lowlands (downstream). Thus, K+ can be used as a useful tracer for silicate weathering (especially when the Ca/Na ratio varies greatly as in the cases of varying lithology in the Himalayan catchments). 3.5. Contribution of carbonate and silicate weathering to the major ions of the Teesta River 3.5.1. Carbonate weathering The geology of the studied watershed showed the distribution of massive carbonates in TSS rock units and the intercalations of carbonates with silicates in GHS and LHS. Blum et al. (1998) stated that, although the carbonates represent very small amounts in GHS (˜ 1%), they contribute 82% of the HCO3− to the rivers draining GHS rocks. Similar findings by Jacobson et al. (2002a) and Jacobson et al. (2002b) revealed that more than 90% of HCO3− and Ca2+ is derived from carbonate weathering in the glaciated Himalayas although carbonates are represented by only ˜ 1% wt.% in fresh glacial till (Liu et al., 2011). The binary plots of major ions (in μeq/L) were used (Fig. 9) to identify the main sources and processes controlling the major ion chemistry in the Teesta River. First, in the case of upstream waters (sites 1–9), the scatter plot of Mg2++ Ca2+ versus HCO3− (in μeq/l) of the upstream waters lies around the equiline with r2 equalling 0.92, which indicates a dominant source of these ions in the headwaters of the Teesta River from the weathering of carbonate rocks (Fig. 9a). The Teesta River upstream receives waters from TSS, which are dominated by limestone (Swades Kumar Basu, 2013). Moreover, these results agreed well with the results of Blum et al. Table 2 Average calculations (in μmol/L) showing the differences in major ions chemistry between upstream waters (sites 1–8) and downstream waters (sites 9–19). Site

K+

Na+

Ca2+

Ca2+/Na+

Na+/Ca2+

K+/Ca2+

Si

HCO3−

Upstream Downstream

21.26 43.06

106.13 164.46

250.54 253.62

2.46 1.65

0.51 0.68

0.09 0.18

147.68 203.31

450.70 618.31

7

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Fig. 8. Cluster analysis dendrogram of the sampling sites based on K + concentrations.

(1998) that GHS is characterised by the presence of small amounts of marble, calc-silicate, and calcareous sedimentary rocks. However, small amounts of carbonate have a major influence on the river chemistry of the Himalayan watershed. In upstream waters, some points are scattered slightly above the carbonate weathering line, which shows little excess of Ca2+ over HCO3-. The Ca2+ should be balanced by other anions such as SO42- sourced from evaporites or sulphide oxidation. The SO42- contribution form oxidation of the pyrite shows that SO42- concentration surpasses that of Ca2+ concentration for the most upstream sample. The studies of headwaters in the Naryani Basin in Nepal Himalaya by Galy et al. (1999) observed that oxidation of sulphides is the primary source of sulphate in rivers with geotectonic and lithologies similar to the Teesta River. The significant weak correlation coefficient (r2 = 0.22) between [HCO3-] and [SO42-] at upstream sites shows the influence of sulphuric acid as well as evaporite dissolution. The probable contribution of evaporite dissolution and sulphuric acid to the excess of Ca2+ in the upper reaches of the Teesta River without sulphur isotopes data is difficult to estimate. Nonetheless, a negative slope value (slope= -79.31, r2 = 0.09) of [Na++K+] and [SO42-]/ [SO4 2-+HCO3-] and weak correlation (r2 = 0.10, Slope =-696.98) between Mg2++Ca2+ and [SO4]/ [SO4+HCO3] are observed. Therefore, it is safe to consider that waters in the upper reaches of the Teesta River mainly accumulate from the contribution of carbonate weathering followed by sulphide oxidation and evaporite dissolution. Mg2++Ca2+ both upstream and downstream shows a positive correlation with HCO3−, the regression line having a slope value of 1.50 (r2 = 0.92) upstream, which decreases in the downstream with a slope value of 0.70 (r2 = 0.43), indicating the contribution of silicate weathering in addition to carbonate weathering at downstream sites. Thus, it can be concluded that the waters in the lower reaches collect contributions from both silicate and carbonate weathering, thereby decreasing the correlation coefficient in downstream samples as shown in Fig. 9b. These findings are in good agreement with other previous studies conducted on other river basins in Himalaya, which argued that the contribution of LHS carbonates to river fluxes in Himalaya is small (France-Lanord and Derry, 1999; Krishnaswami, 1999). In downstream waters, the initial few points appear below the equiline (Fig. 9b), which means an excess of HCO3− over Ca2+. This excess of HCO3− over Ca2+ in downstream waters probably derives from silicate weathering and this observation validates the earlier finding in Table 2 that silicate weathering perhaps increases from upstream to downstream. In the Teesta River, the stoichiometry between total (Ca2+ + Mg2+) and total HCO3− showed that some sites have higher values of (Ca2+ + Mg2+) than HCO3−, which requires additional anions such as SO42- for ionic balance. It can be suggested that sulphuric acid might play a part in carbonate weathering in the Teesta River. Krisnaswami and Sunil Singh (2005) demonstrated that sulphide oxidation is the leading supplier of SO42- in the Himalayan river basins in India, so sulphide oxidation appears to be the main source of SO42- throughout the Teesta River catchment in Sikkim. The absence of evaporites (gypsum or anhydrite) in the Himalayan river catchments was noted in Galy et al. (1999); Dalai et al. (2002a); Hodson et al. (2002) and Wolff-Boenish et al. (2009). These observations are similar to findings given by Galy et al. (1999) that sulphide oxidation acts as the most likely source of SO42- in the 8

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Fig. 9. Scatter plot of HCO3− vs Mg2++Ca2+ in a) upstream b) downstream c) HCO3−+SO42- vs Ca2++Mg2+ for all waters.

northern Himalayan basins. The significant positive correlation (r2 = 0.67) between (Ca2+ + Mg2+) and (HCO3− + SO42-) linked with a slope close to the equiline indicates the significant action of H2SO4 on carbonate weathering (Li and Ji, 2016; Meyer et al., 2017) (Fig. 9c). Excess HCO3− + SO42- over Ca2+ + Mg2+ indicates non-carbonate sources (i.e., interaction of H2SO4 and H2CO3 on silicates) and this excess should be balanced by Na+ and K+ that were found in higher amounts than Cl−.

3.5.2. Silicates weathering Silicate rocks represent the main bedrock in the Teesta River catchment area. Kinetically, the weathering of carbonate rocks is more rapid than silicate rocks in aquatic systems (Sarin and Krishnaswami, 1984). In this study, silicate weathering probably contributes to Na+ and K+ in the river water. According to Krisnaswami and Sunil Singh (2005), the dissolution of Na+ from silicates (Nasil) can be calculated as follows: Nasil ≈ Nar – Clr where, Nar and Clr are the concentrations of Na+ and Cl− in the river water. It was found that the average ratio of Na/Cl in all waters is ˜ 6.8. This indicates that the major source of Na+ in the Teesta River is from silicate weathering. The scatter plots of upstream and downstream water shown in Fig. 10a and b are above the equiline and closer to the Na++K+ domain, indicating the derivations of these ions from silicate ions. The positive correlation of Na+ and Cl− at all sites indicates that the contribution could likely be from rainwater, hot springs and salt dissolution. The annual rainfall differs across the state depending upon the zones, from 400 mm in temperate to subalpine zones to 1250 mm or less in the lower elevation of south Sikkim and < 500 mm in the dry plateaus of North Sikkim (Seidler, 2018; http://en.climate-data.org/region/779/). The South Sikkim district receives the maximum average annual rainfall mostly arriving during the monsoon season of June-September. According to Galy et al., 1999, the chemical composition of rain/water in Himalaya is 7.4 μM and rainwater contribution to the riverine dissolved load from the average rainwater concentration multiplied by 1.33–25% evapotranspiration in the Himalaya is estimated. The rain and 9

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Fig. 10. Scatter plot of Cl− vs Na+ + K+ in a) upstream and b) downstream for all waters.

snow contribution of the CL- could differ greatly from different streams and estimates of the contribution of the riverine chemical budget have not been extensively discussed. It is, however, safe to conclude that the increase in Cl− concentration from upstream to downstream is likely partly the result of rainwater. Likewise, a study conducted on the chemical composition of spring water in south Sikkim by Vishwakarma et al., 2019 estimated that Cl− is the second highest anion in spring water with an order of anionic dominance of HCO3− > Cl- > NO3 - > PO43- > SO42– > F–, so Cl− increases downstream could also be a result of hot springs and the dissolution of trace amounts of NaCl salt from soils in the Teesta River watershed. 3.6. Effect of gradient, vegetation cover and temperature on the rate of water-rock interaction 3.6.1. Potential effect of gradient and vegetation cover The elevation difference between sampling sites 1–8 (upstream) was 2011 m over a distance of about 34 km while the elevation difference between sampling site 9–19 (downstream) was 1059 m over about 70 km (see Fig. 4). As shown in Fig. 4, the gradient of flow of the Teesta River changes from a steep slope upstream to a gentle slope downstream. Kinetically, the rate of weathering of carbonate rocks is higher than silicate rocks. The upper region of north Sikkim is mostly covered by snow and glaciers while vegetation cover is mostly seen in the lowlands (Ghosh et al., 2018; Meetei et al., 2007). Vegetation cover enhances the rate of silicate weathering as it enables water retention and percolation into the soil, thus increasing the contact time allowed for reaction between the silicate minerals and pore water (Gabet et al., 2006; Tipper et al., 2006). It can thus be concluded that the rate of silicate weathering is dependent on an overall balance of key factors including gradient, contact time, temperature and vegetation. 3.6.2. Effect of temperature Due to the altitude differences, the local climate of Sikkim can play an important role in the weathering process. In the Teesta River catchment area in Sikkim, based on elevation differences between highlands and lowlands, the climate changes from subtropical at low altitudes (downstream) to cold-temperate or cold at high altitudes. Wolff-Boenish et al., 2009 concluded that the effect of the global climate on the Himalayan basins was causing the transportation of silicate particles from upper regions, which weathered in the lower regions. Mineral dissolution is kinetically dependent on temperature (Wolff-Boenish et al., 2009). As stated by Dalai et al. (2002b), there is a strong relationship between the rate of silicate weathering and temperature in the tributaries of the Yamuna River in the Lesser Himalaya Sequence (LHS). This proves that conditions for silicate weathering were more favourable in the lowlands (warmer than highlands) in Sikkim. 3.7. Comparison of major ions with the Brahmaputra, Yarlung Tsangpo and the global average (mg/L) A comparison of the mean value of the major ion compositions in the Teesta River with other rivers and the global average are shown in Table 3. The mean concentration of Mg2+ and Cl− in the Teesta River is lower than the global average value, whereas the concentrations of Ca2+ and Na+ are comparable to the global average. It is observed that the major ionic composition of the Teesta River in Sikkim Himalaya is less than the Yarlung Tsangpo on the Tibetan plateau but comparable to the Brahmaputra in India (Singh et al., 2005). The substantial difference in the chemical ion constituents in the Yarlung Tsangpo on the Tibet plateau and Sikkim in Indian Himalaya could be attributable to various factors such as soil type, elevation, rate of precipitation, mining and industrial activities. The high rate of precipitation in Sikkim Himalaya has caused a dilution effect on the ions concentration in the Teesta River. So, a lower level of overall ionic composition is observed than in Yarlung Tsangpo (Huang et al., 2009; Zhang et al., 2015). The South Asian summer monsoon has an effect on Eastern Himalaya (Gibling et al., 2005). The TDS of the Teesta River significantly increases 10

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Table 3 Average concentrations of ions in the Teesta river against Brahmaputra, Yarlung Tsangpo and the global average (unit: mg/L). Site

Ca2+

Mg2+

Na+

K+

Cl−

SO42−

HCO3−

TDS

Ref.

Sikkim Brahmaputra Yarlung Tsangpo River Asia average median Europe average median Africa average median North & Central America average median South America average median Global average median

9.46 14.30 27.90 16.50 41.20 4.40 13.80 5.40 14.20

1.39 3.15 5.1 4.8 7 2.4 3.2 2 4

3.16 2.49 5.7 7 9 4.2 3.5 2.9 5.4

1.3 1.66 1 1.6 1 2.3 1 1.3 1.2

0.90 9.93 2.39 5.80 16.30 2.70 3.00 3.90 5.50

10.2 8.51 37.4 9 35.8 2.3 8.9 3 8.9

32.14 55.9 74.1 65 121.1 33.8 43.2 24.5 52

66.20 87.85 117.3 104.7 220.1 27.6 85.4 40.8 96.9

This study (Singh et al., 2005) (Qu et al., 2017) (Meybeck and Ragu, 2012)

with a decrease in elevation as the vegetation coverage increases. The ionic concentration values of the Teesta River were within the range of the global average as shown in Table 3. However, the concentration of Cl− is almost four times lower than the global average. Sikkim Himalaya is situated near the southern part of the Tibetan plateau. It is less affected by anthropogenic activities and would receive a less cyclic salt contribution, which could be a reason for the low levels of Cl-. Taking that into account, it is considered that the contribution of Cl− by the cyclic salt decreases as the distance from the sea increases (Stallard and Edmond, 1981). 3.8. Comparison of major ions and trace elements For the elemental composition in the Teesta River, 14 elements were measured that are considered elements of health concern by the Bureau of Indian Standards and WHO (BIS, 2012; WHO, 2011). The metals B, Ba, Cr, Mn, Mo, Ni, Zn and Li were quantified with varying concentrations at different sampling sites. Cd, Pb, Cu, Al, As, and U were all below detection limits. The average concentrations of the measured elements and the standard guideline values of WHO and BIS are shown in Table 4. It was found that all the measured trace elements lie within the standard guidelines of WHO and BIS, except for boron. Likewise, the concentration of all the elements is just trace amounts, which indicates that the Teesta River does not have significant anthropogenic influence. Studies suggested that boron is an essential trace element for marine life and is abundantly available in oceans (Carrano et al., 2009; Mottram et al., 2014). The main central thrust zone of Sikkim Himalaya is also characterised by large concentrations of tourmaline that could be attributable to the relatively high concentration of boron in the river. Moreover, all the Table 4 Chemical Guideline values by WHO and BIS. Parameter

Units

WHOa (guideline value)

Note

BIS b (Acceptable limit)

BIS b (Permissible limit)

Note

This Studyc

pH Turbidity TDS Antimony (Sb) Barium (Ba) Boron (B) Cadmium (Cd) Calcium (Ca) Chlorine (Cl) Chromium (Cr)

– NTU mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L

– 5

5 (C) 0.05 (P)

6.5-8.5 1 500 – 0.7 0.5 0.003 75 – 0.05

– 5 2000 – No relaxation 1 No relaxation 200 – No relaxation

mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L

2 – – 0.4 (C) 0.07 0.07 – 0.01 0.01 – –

0.05 – 30 0.1 0.07 0.02 – 0.01 0.01 200 5

1.5 – 100 0.3 No relaxation No relaxation – No relaxation No relaxation 400 15

– – – – – – – – – For total Chromium – – – – – – – – – – –

7.69 7.02 66.20 Not detected 0.0048 1.01 Not detected 9.56 1.1 0.0015

Copper (Cu) Potassium (K) Magnesium (Mg) Manganese (Mn) Molybdenum (Mo) Nickel (Ni) Sodium (Na) Lead (Pb) Selenium (Se) Sulphate (SO42−) Zinc (Zn)

6.5-8 – – – – – – < 250 – For total Chromium – < 250 – – – – < 200 – – < 500 <1

0.02 0.7 0.5 (T) 0.003

Not detected 3.33 1.44 0.0036 0.0029 0.0006 3.40 Not detected Not detected 10.24 0.3144

a WHO guideline for drinking water quality by World Health Organization (4th edition) 2011. bBIS Indian standard drinking water specification (second revision) BIS standards 2012. This studyc means the average values of the measured elements. According to WHO, C is a concentration below or at the given guideline value that might affect the taste, appearance and odour of the water. P stands for provisional guideline value, because there is evidence of hazard but limited information on the effect on health. T is a provisional guideline value because the given value is below the level that can be achieved by practical treatment methods, source protection, etc.

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parameters including in situ measurements, major ions and trace elements of the river were found to be within the standard guideline values, apart from turbidity. 4. Conclusion This study provides valuable information on the water chemistry and mechanisms of weathering in the Teesta River of Sikkim Himalaya. It also contributes to the study of weathering of Himalayan regions that has always received varied explanations due to large variations in lithology and geotectonic domain. It also provides original data on the major ions and the trace elements of the Teesta River. The river chemistry is mainly dominated by natural phenomena, with upstream mainly influenced by carbonate weathering and downstream mainly governed by silicate weathering. Ca2+, Mg2+ and HCO3− were mainly provided from carbonate weathering while Na+ and K+ originated from silicate weathering throughout the entire watershed. H2CO3 and H2SO4 make significant contributions to the weathering process. The increase of Na+/Ca2+ linked with an increase in Si from upstream to downstream validates the contribution of silicate rocks in the weathering process of the Teesta River in the Sikkim lowlands. Gradient, vegetation cover and temperature have played supporting roles in silicate weathering downstream. Likewise, the contribution of hot spring and rainwater to the riverine chemical composition of downstream sites was observed. The cluster analysis (CA) based on K+ concentration could successfully differentiate between upstream and downstream waters. The concentration of major ions and elements are in the range of the concentration of the global average in rivers. The overall state of the Teesta River is pristine with minimal contribution from human activities and the river is suitable for irrigation purposes. The main limitation of this assessment is based on one sampling campaign, but it contributes to narrowing the gap in research into the Teesta River of the Indian Himalayas that has rarely been discussed. The dynamic landscape and vegetation coverage of Sikkim has an enormous impact on river chemistry; this study can support future studies on the geochemistry of Sikkim, which could also contribute to water resource management and the preservation of natural resources. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.ejrh.2019. 100612. References Azhoni, A., Goyal, M.K., 2018. Diagnosing climate change impacts and identifying adaptation strategies by involving key stakeholder organisations and farmers in Sikkim, India: challenges and opportunities. Sci. Total Environ. 626, 468–477. https://doi.org/10.1016/j.scitotenv.2018.01.112. Behmel, S., Damour, M., Ludwig, R., Rodriguez, M.J., 2016. 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