key: cord-0074574-okpti166
authors: Bishwakarma, Kiran; Wang, Guan-xing; Zhang, Fan; Adhikari, Subash; Karki, Kabita; Ghimire, Archana
title: Hydrochemical characterization and irrigation suitability of the Ganges Brahmaputra River System: review and assessment
date: 2022-02-07
journal: J Mt Sci
DOI: 10.1007/s11629-021-6834-z
sha: c1aeaec620f93450497a9c16157c2966a9b4d13d
doc_id: 74574
cord_uid: okpti166

The hydrochemical characterization and irrigation suitability assessment of the Ganges-Brahmaputra River System (GBRS) has immense importance for the livelihoods of people and ecosystem sustainability in the region. This study aims to assess the hydrochemical characteristics and evaluate the irrigation suitability of water in the GBRS by reviewing published literature of the major tributaries. The studied rivers were categorized into two groups namely Group-1 and Group-2 considering the similarities of climatic patterns, hydrochemical attributes, and drainage characteristics. The hydrochemistry of the river water was characterized by the Piper diagram, Gibbs plot, mixing plots, and ionic ratios. Furthermore, irrigation water qualities were evaluated by electrical conductivity (EC), sodium percentage (Na%), sodium adsorption ratio (SAR), magnesium hazard (MH), and Wilcox diagram. The results indicated that the hydrochemistry of the GBRS was slightly alkaline to alkaline (7.42–8.78) in nature. The average concentrations of most of the chemical attributes showed higher in Group-1, whereas the average concentrations of K(+) and NO(3)(−) were found higher in Group-2. The average concentration of the major ions followed the dominancy order Ca(2+) > Mg(2+) > Na(+) > K(+) for cations and HCO(3)(−) >SO(4)(2−) > Cl(−) > NO(3)(−) for anions in both groups. Gibbs plot and mixing plot indicated that carbonate rock weathering dominates the hydrochemical process, which was further confirmed by the Piper diagram and the ionic ratios. From the analyses of irrigational water quality, almost all the rivers (except Gomti River in terms of MH and Rangit River in terms of Na%) in the GBRS were found to be suitable based on EC, SAR, Na%, MH, and Wilcox diagram. Finally, the majority of river systems in the GBRS were characterized by carbonate dominated lithology and irrigational water quality is mostly suitable for utilization. This study could be useful for water quality management in the glacial-fed Himalayan river under the context of global climate change. ELECTRONIC SUPPLEMENTARY MATERIAL: Supplementary material (Appendix 1) is available in the online version of this article at 10.1007/s11629-021-6834-z

mostly suitable for utilization. This study could be useful for water quality management in the glacial-fed Himalayan river under the context of global climate change.

Keywords: Ganges-Brahmaputra River System; Hydrochemical characterization; Major ions; Irrigation suitability assessment

Hydrochemical characteristics in the river water have a key role in ecological and economical perspectives. The overall concentration of dissolved constituents in river water determines its suitability for domestic purposes (Salifu et al. 2017 ). The dissolved ions in water, mainly calcium (Ca 2+ ), magnesium (Mg 2+ ), sodium (Na + ), potassium (K + ), bicarbonate (HCO3 − ), carbonate (CO3 − ), sulfate (SO4 2− ), chloride (Cl − ), and nitrate (NO3 − ) are the most abundant dissolved constituents in the river water (Meybeck and Ragu 1997) . The chemical characteristics of the river basin can reflect the natural and anthropogenic interferences in the catchment. For instance, the primary source of riverine Ca 2+ , Mg 2+ , SO4 2-, and HCO3 -is natural, whereas NH4 + and NO3 -are induced from anthropogenic activities (Huang et al. 2008; Haidary et al. 2013) .

The chemical signatures and factors controlling the river hydrochemistry of the world major rivers have been well documented from the 1980s considering the Amazon River (Stallard and Edmond, 1983) , Ganges-Brahmaputra River (Sarin et al. 1989 (Sarin et al. , 1992 , Yellow River (Zhang et al. 1995) , Nile River (Dekov et al. 1997) , Indus River (Qaisar et al. 2018) , Mississippi River (Sharif et al. 2008) , Mekong River (Huang et al. 2009 ), Tigris River (Varol et al. 2013) , Yangtze River (Huang et al. 2009; Jiang et al. 2015) , and eleven major rivers originated in the Tibetan plateau (Qu et al. 2019) . The natural hydrochemistry of surface water is primarily controlled by rock weathering, precipitation, and evaporationcrystallization (Gibbs 1970) . Moreover, groundwater discharge and anthropogenic interferences also significantly influence the hydrochemical properties of surface water.

In general, the water quality is defined based on EC, TDS, major ions, organic matters, nutrients contain, and various dissolving trace elements in water and have a significant role to determine the quality of irrigation water. However, TDS, EC, and major ions have been broadly used as primary parameters for irrigation water quality assessment (Acharya et al. 2020) . In fact, salinity and ion toxicity are notable complications in irrigation water (Zaman et al. 2018) . A sufficient amount of water with a permissible limit of salinity and ion toxicity is essential for the proper growth of plants and soil permeability (Kumari 2017) . The salinity hazard, magnesium hazard and sodium hazard are major types of salt problems in irrigation water. High salinity in irrigation water is toxic for the plants. For example, irrigation water with more than 60% sodium content may lead to a breakdown in the soil's physical properties. The excess Na% combining with carbonate may result in the formation of alkali soils (Fipps 1995) . Similarly, the excessive concentration of Ca 2+ and Mg 2+ in irrigation water can alter the salinity, increase the pH of the soil, affect the permeability properties of the soil root zone, and reduce the availability of phosphorous nutrient (Al-Shammiri et al. 2005; Joshi et al. 2009; Singh et al. 2018) .

The Ganges-Brahmaputra River System (GBRS) is one of the largest river systems in the world ranking first and fourth in terms of sediment transport and river discharge, respectively (Milliman and Meade 1983; Sarin et al. 1989) (Fig. 1) . The origins of the Ganges and the Brahmaputra are both the mountains of the Himalaya, however, they have distinctly different paths before joining at Bay of Bengal (Subramanian and Ramanathan 1996) . The mainstream of the Ganges originates from the Gangotri Glacier at Gomukh (30°36ʹ N; 79°04ʹ E) in the Garhwal Himalaya range (Vass et al. 2010) and it crosses the Great Lesser Himalaya southward before flowing down into the Indo-Gangetic Plain (IGP) (Kuehl et al. 2011) . Brahmaputra (also known as the Yarlung Tsangpo in Tibet) originates from the Angsi Glacier (at 5300 m asl) located on the northern side of the Himalaya in Tibet, travels eastward through the northern slope of the Himalayas, transverse to the southward and enters into the nanga Parbat before mixed with Ganges (Das et al. 2016) . Table 1 further elaborates the key information of the GBRS.

The river hydrochemistry of the GBRS was first introduced in the 1990s. The hydrochemistry of the GBRS was dominated by carbonate and silicate weathering, while Ca 2+ , Mg 2+ and HCO3 -accounted for more than 80% of the total ions (Sarin et al. 1989 ).

The tributaries of the Ganges like Ramganga, Gomti, Gandaki, and the Yamuna have been intensively studied (Sarin et al. 1989; Rai et al. 2010; Avtar et al. 2011; Seth et al. 2016; Pant et al. 2018 ) till date and revealed the dominancy of Ca 2+ , Mg 2+ and HCO3 -, while the concentration of Na + +K + in tributaries of the lower stretches of the GBRS was relatively higher than the Himalaya level. The river water quality of the GBRS is reducing due to the intense stresses of anthropic disturbances surrounding the drainage area. The continuous decline of surface water quality of the GBRS is now a serious issue for the sustainable management of freshwater resources.

The study of hydrochemical characterization and assessment in the GBRS water is important in deciding its quality and valuable contribution to the study of the temporal and spatial changes in water resources. One of the largest river systems of the world with a major source of freshwater and sustains the livelihoods of 650 million people in its local and surrounding areas, this review study has great implications in terms of socioeconomic aspects as well. Therefore, this review study aims to analyze the variations of hydrochemistry with its controlling factors and assess the irrigation water quality.

River hydrochemistry and water quality for the irrigation purposes of tributaries in the GBRS, which are prominent for different domestic and (Rai et al. 2010; Avtar et al. 2011; Khan et al. 2016; Banerjee and Ghosh 2016; Gupta et al. 2016; Paudyal et al. 2016; Das et al. 2016; Pant et al. 2018; Tsering et al. 2019; Liu et al. 2019) . Major 12 tributaries of the GBRS (Yarlung-Tsangpo, Teesta, Brahmaputra, Rangit, Indrawoti, Dudhkoshi, Gandaki, Garra, Ken, Betwa, Gomti, and Damodar) were considered in this study. The GBRS is a large Himalayan foreland basin formed after the collision of Eurasian and Indian plates that occurred 40-65 million years ago (Ding et al. 2016) . The basin was further developed by the loading thrust and the regional lithospheric flexure (isostasy) during the period of Late Eocene to Miocene and upliftment of the northern part and subsidence of the southern part from mid-Miocene to the present day. The Ganga-Brahmaputra basin comprises Himalayan orogen lithostratigraphy (consisting of the Tethys Himalaya, Higher Himalaya, Lesser Himalaya, Siwalik domain, and the Indo-Gangetic Plain). The southern unit can be observed only in the Ganga basin and is absent in the Brahmaputra basin. Since its formation, the basin has received an uninterrupted supply of sediments from both the rising Himalaya and the Peninsular Cratonic sources, the production and delivery of which was primarily controlled by both climate and regional tectonics. The precipitation pattern in the GBRS is distinct from north to south. The Himalaya region of the GBRS has received comparatively higher precipitation. The southern region is also received higher precipitation but not as much as the Himalaya region. The upstream region especially the leeward side of the Himalaya has received comparatively lower precipitation (Fig. 1b) . The land use/cover pattern of the GBRS is varied from Himalaya to the peninsular plain area. There exist about 29 land use/cover types with dominancy of forest land, agriculture/cultivated land, shrubland, built-up area, water bodies, grassland, etc. (Thenkabail et al. 2005) (Fig. 1c ).

The hydrochemical properties of the river stream are varied on the geology of the source and somehow contribution from the anthropogenic driver. For a better spatial comparison, only the pre-monsoonal ionic information of river hydrochemistry in the published literature had been collected and discussed. The following physicochemical parameters were taken for analyses: pH, total dissolved solids (TDS), electrical conductivity (EC), calcium (Ca² + ), magnesium (Mg² + ), sodium (Na + ), potassium (K + ), sulfate (SO4 2-), chloride (Clˉ), bicarbonate (HCO3ˉ), and nitrate (NO3 -). There was a lacking EC value in some of the river hydrochemical datasets. The relation of TDS and EC for freshwater {TDS=k×EC, k=constant factor (Walton 1989) ; and k =0.55 for freshwater (Rusydi 2018 )} had been adopted to calculate the missing value of EC.

Except for Teesta (Tsering et al. 2019) , Gandaki , and Yarlung Rivers (Liu et al. 2019 ), most of the literature only published an average value of different chemical concentrations in separate study areas without detailed information on hydrochemical attributes at the individual sampling location. Therefore, further chemical characterization and irrigation suitability analyses have been carried out for Teesta, Gandaki, and Yarlung Rivers for a deeper understanding of the spatial variation.

The Piper diagram, Gibbs plot, mixing diagram, and ionic ratios were used for the comprehensive elaboration of hydrochemistry. The Piper diagram is a trilinear shape diagram and one of the commonly used effective graphical procedures for presenting water chemistry data to help in understanding the nature and variations of the dissolved ions in water (Piper 1944) . Generally, six sub-fields of chemical types can be identified in the diamond section of the Piper diagram: 1) Ca-HCO3, 2) Na-Cl, 3) Mixed Ca-Na-HCO3, 4) Mixed Ca-Mg-Cl, 5) Ca-Cl, and 6) Na-HCO3 (Khadka and Ramanathan 2013; Pant et al. 2021) . The Gibbs plot is also an analytical method to determine key factors of controlling the hydrochemistry of surface water. From the relation of the TDS versus Na + /(Na + +Ca 2+ ) and the TDS versus Cl -/(Cl -+HCO3 -), three of the key processes including atmospheric precipitation, rock dominance, and evaporation-crystallization processes controlling surface water chemistry can be presented by the Gibbs plot (Gibbs 1970) . The main application of the mixing diagram in hydrochemistry and ionic ratios is aimed to understand the types of minerals in the river catchment mainly including carbonates, silicates, and evaporates. The ionic ratios are also another analytical technique in hydrochemistry to support the results of the Piper diagram, Gibbs plot, and Mixing diagram.

The chemical and physical characteristics of river water using for irrigation purposes are the basic consideration for the irrigation water quality evaluation. Specific properties of irrigation water have relevant relation to the quantity and quality of crops, maintenance of soil properties, and surrounding environmental balance (Alobaidy et al. 2010; Bishwakarma et al. 2019 ). The Food Agriculture Organization (FAO) has provided the permissible limit for irrigation water standards according to the physicochemical parameters of irrigation water (Table  1 ). All the examined variables have been analyzed and compared with FAO guidelines and certain physicochemical parameters are taken for the scientific analysis of irrigation water quality, such as EC, Ca² + , Mg² + , Na + , and K + in this study based on the calculation of EC, sodium percentage (Na%), sodium percentage ratio (SAR), magnesium hazard (MH) and Wilcox diagram.

There exist different methods for the evaluation of irrigation water quality. The irrigation water quality had been studied individually with the following methods from the perspectives of salinity hazard, sodium hazard, magnesium hazard and comprehensive consideration:

i. EC: It is used for an indication of the salinity hazard which is the most significant water quality guideline on crop production (Salifu et al. 2017 ). The irrigation water quality referring to EC values is defined as follows (Ayers and Westcot 1985) :

EC < 700, Excellent 700, 3000 , Good > 3000, Fair

ii. Na%: It is one of the widely used parameters indicating sodium content in irrigation water for the suitability assessment. Doneen (1954) iii. SAR: It is a useful indicator of the level to which water undergoes cation exchange reactions in the soil (Joshi et al. 2009 ). The SAR is proposed by (Richards 1954 ) and calculated as:

iv. MH: It is a method for irrigation water quality assessment. The status of magnesium and calcium should be both in equilibrium for suitable irrigation purposes. Raghunath (1987) proposed the relation to calculate MH as follows:

……………………. (7) v. Wilcox Diagram: The Wilcox diagram is also known as US Salinity diagrams proposed by Wilcox in 1948 and further improved from Torn in 1951. Nowadays, this diagram is commonly used for a comprehensive evaluation of irrigation water quality (Alavi et al. 2016 ). In the Wilcox diagram, irrigation water quality has been divided into C1, C2, C3 and C4 zones, based on the salinity hazard (EC) in the horizontal axis and S1, S2, S3, and S4 zones based on sodium hazard (SAR) in the vertical axis (Lokhande and Mujawar 2016) . Water classification for agricultural uses according to the Wilcox classification is as Table 2 .

According to their similarities and differences in hydrochemical attributes, the average elevation of the river catchment, average annual river discharge, and annual precipitation in the studied rivers (Table 3) were categorized into two groups by performing hierarchical cluster analysis with Ward's method and Squared Euclidean distance which was also verified by principal component analysis. The resulting dendrogram and component plot are presented in Fig.  2 . The rivers from the peninsular region were mostly categorized in Group-1 (Ken, Betwa, Gomti, Garra, and Damodar Rivers) and the rest of the rivers mainly originating from the Himalaya region (Yarlung-Tsangpo, Teesta, Brahmaputra, Rangit, Indrawoti, Dudhkoshi, and Gandaki) were categorized in Group-2 (Fig. 1a) . Group-1 rivers are mostly draining through a southern region having comparatively moderate precipitation (average annual rainfall: 1000 mm). In addition, the average elevation of a river catchment in the Group-1 rivers is 325 m asl. The Group-2 rivers are mostly draining through the Himalaya region having relatively higher precipitation except for the Yarlung River (average annual rainfall: 1500mm; Yarlung: 750 mm). In addition, the average elevation of a river catchment in the Group-2 Rivers is 3300 m asl. Furthermore, we have done principal component analysis (PCA) for the validation of the cluster analysis. The PCA analysis with varimax rotation explaining 95.75% of the total variance also showed the same classification of the studied river as cluster analysis (Appendix 1).

The concentrations of dissolved solutes were compared to find out the ionic distribution patterns in rivers among the two groups of rivers in the GBRS (Table 4 ). The pH of all the rivers in Group-1 and Group-2 was found slightly alkaline to alkaline nature with an average value of 8.01±0.19 and 8.02±0.55, respectively. As compared with individuals, the pH value is slightly higher in the Gandaki, and Yarlung Rivers mainly draining from the Himalaya region. The average values of EC in Group-1 and Group-2 were found to be 378±185 µS/cm and 214±140 µS/cm, respectively. The average value of EC was significantly higher in Group-1. The average value of TDS was Table 2 Wilcox diagram classification (Alavi et al. 2016 )

Water quality for agriculture C1S1

Sweet-completely effective for agriculture C1S2, C2S2, C2S1

Brackish-approximate perfect for agriculture C1S3, C2S3, C3S1, C3S2, C3S3

Passion-usable for agriculture C4S4, C4S1, C1S4, C2S4, C3S4, C4S4, C4S3

Very passion-harmful to agriculture Notes: All ionic concentrations are expressed in milli-equivalents per liter (meq/L) for the calculation of the abovementioned irrigation parameters. found 251±93 mg/L and 121±68 mg/L in Group-1 and Group-2, respectively. As compared individually, the average values of EC and TDS in Gomti (Group-1), and Gandaki (Group-2) had been observed higher than the others. Different factors were affecting the EC and TDS in river water. The intense interaction of bed-rock with flowing water possibly increased the EC and TDS values (Ansari and Ahmad, 2019) . Also, the high relief, tectonic disturbance, and large-scale anthropogenic stresses were responsible to increase TDS and EC of river water (Chakrapani 2005) . The TDS values in most of the tributaries in the GBRS were greater than the global average (120 mg/L) except for some Himalaya rivers, such as Teesta, Brahmaputra, Rangit, Dudhkoshi, and Indrawoti. The lower TDS was possibly due to very few human disturbances, originating from a glacier-fed and flowing from high elevation stretches with less bedrock interaction.

The ionic concentrations of rivers in the GBRS were categorized into two groups as described above. Generally, HCO3 -, SO4 2-, Ca 2+ , Mg 2+ , and Na + were dominant ions among the observed dissolved materials in river water (Meybeck 1987) . The average concentration of major ions together with pH and TDS had been plotted in Box plot to identify the significant difference among the two groups (Fig. 3) . The average concentrations of major dissolved solutes were found significantly different. The average concentrations of most of the physicochemical parameters showed significantly higher in Group-1, i.e., TDS, EC, Ca 2+ , Mg 2+ , Na + , Cland HCO3 -. The average concentrations of NO3 -had been found highest in Group-2 especially in the Indrawoti River (65 µeq/L). The higher concentration of NO3 -in the Indrawoti River was associated with anthropic activities such as excessive use of N-containing chemical fertilizers in the vicinity of the river channel (Paudyal et al. 2016) . The comparative abundance of major cations and anions among the two groups in the GBRS had been attempted. The same as global average order, the anionic and cationic abundancy based on the average values (µeq/L) of the two groups were in the following order: HCO3 ->SO4 2->Cl ->NO3and Ca 2+ >Mg 2+ >Na + >K + , respectively. HCO3 -was most dominant and NO3 -was the least dominant anion over the GBRS. Whereas, Ca 2+ and K + were the most and least dominant cations, respectively. The relatively higher concentrations of HCO3 -and Ca 2+ indicated the contribution of rock weathering as the main processes that determined the hydrochemistry of river catchment .

The spatial variations of general hydrochemistry of the three rivers from Group-2 as shown in Table 5 and had been analysed separately for a deeper understanding. In both upstream and downstream, the ionic dominancy of Teesta rivers followed: HCO3 ->SO4 2->Cl ->NO3 -and Ca 2+ >Na + >Mg 2+ >K + for anions and cations, respectively. And major ions together with TDS, and EC, showed a non-significant difference between upstream and downstream. The variations in upstream, midstream, and downstream of Yarlung River suggested significant spatial variations of hydrochemistry. The ionic dominancy of Yarlung rivers followed: HCO3 ->SO4 2->Cl ->NO3 -and Ca 2+ >Mg 2+ >Na + >K + for anions and cations in all three segments. However, the concentration of major ions such as Ca 2+ , K + , Cland HCO3 -together with EC and TDS were higher in the upstream whereas the concentration of Mg 2+ , Na + , and SO4 2-were higher in the midstream and the concentration of NO3 -was slightly higher in the downstream region. The general hydrochemistry of the Gandaki River showed significant variations in upstream and downstream. The anionic dominancy of Gandaki River followed: SO4 2->HCO3 ->Cl ->NO3 -and HCO3 ->SO4 2->Cl ->NO3 -in upstream and downstream, respectively. Whereas, the cationic dominancy followed the same order of Ca 2+ >Mg 2+ >Na + >K + in both upstream and downstream. All the physicochemical concentrations reported were higher in upstream than downstream except NO3 -, which indicates the anthropogenic signatures in the downstream segments of the Gandaki River Basin.

The ionic concentrations of rivers in the GBRS was plotted in the Piper diagram and further projected into the central diamond field to characterize the hydrochemistry of the GBRS. On the left ternary (cation section), the plots indicated the dominancy of Ca 2+ and Mg 2+ types i.e. the carbonate dominated lithology over river catchment. The Rangit River showed a higher percentage of Na + +K + due to the weathering of phyllite and mica schist over the river catchment (Gupta et al. 2016 ). On the right ternary (anion section), all the plots were in the location of the left corner showing the dominance of bicarbonate type (HCO3 -) which also indicated carbonate lithology. The diamond field of the Piper diagram revealed the overall characteristics of the GBRS: the dominancy of the alkaline earth elements (Ca 2+ and Mg 2+ ) over the alkaline (Na + and K + ) and the weak acids (HCO3 -) over the strong acids (Cland SO4 2-) (Fig. 4) .

The hydrochemical facies of Teesta, Yarlung, and Gandaki Rivers with spatial variations plotted in the piper diagram (Fig. 5) showed the dominancy of the Calcium-bicarbonate type of hydrochemistry except the upstream of Gandaki River. The upstream of the Gandaki River plotted into the middle of the diamond plot showed the mixed type of hydrochemistry. The reason for this mechanism in upstream of Gandaki was due to the influence of local sources of Na + and K + over river catchment ). There was a significant variation of hydrochemistry among upstream, midstream, and downstream in Yarlung river stretches. However, all three segments were plotted into calcium bicarbonate types of hydrochemistry indicating the dominancy of Ca 2+ and HCO3 -over cationic and anionic budget. Similarly, the piper diagram revealed the calcium-bicarbonate type of hydrochemistry in both the upstream and downstream of the Teesta River stretch.

The average hydrochemical attributes was explored in the Gibbs plot to find out the major natural controlling factors of the river hydrochemistry (Fig. 6) . The ionic distributions were characterized by relatively moderate TDS and low ratios of Na + /(Na + +Ca 2+ ) or Cl -/(Cl -+HCO3 -). Comparatively, the ratio of Na + /(Na + +Ca 2+ ) and Cl -/(Cl -+HCO3 -) in the Group-1 rivers is higher than the Group-2. Illustrating the dominance of geogonic factors, the Gibbs plot showed rock weathering was the major natural controlling mechanism in the GBRS.

The Gibbs plot prepared for Teesta, Yarlung, and Gandaki Rivers (Fig. 7) also suggested that the natural rock weathering was the major controlling natural factor of hydrochemistry over the river basin. However, the ratio of Na + /(Na + +Ca 2+ ) or Cl -/(Cl -+HCO3 -) in the upstream of the Gandaki River was relatively higher than the downstream also had a higher TDS. This was evidence of the minor role of evaporation-crystallization processes for controlling the hydrochemistry in the upstream region. But the downstream of the Gandaki River showed the control of the rock weathering. The low ratio of Na + /(Na + +Ca 2+ ) or Cl -/(Cl -+HCO3 -) and moderate TDS of the Teesta and Yarlung rivers indicating natural rock weathering plays a vital role to control the hydrochemistry. There were no significant differences in upstream and downstream of Teesta River and upstream, midstream, downstream of Yarlung River, despite the geological variations.

The Na + -normalized Ca 2+ vs. Mg²⁺ and Na + -normalized Ca 2+ vs. HCO3ratios were plotted to evaluate the presence of carbonates, silicates, and evaporites minerals over the river basin (Gaillardet et al. 1999) . The average values of hydrochemical attributes of different rivers of the Group-1 and Group-2 of the GBRS had been plotted into the mixing diagram (Fig. 8) . The plots of all the rivers located between the carbonates and silicates end members indicating the dominancy of carbonate and silicate minerals in the GBRS. The mixing diagram further illustrated that the rivers in Group-1 Rivers moved to silicates domain and Group-2 Rivers moved to carbonates domain exhibited the lowland and highland region of the GBRS was mainly dominated by silicate carbonate minerals, respectively.

Another mixing diagram had been plotted for Teesta, Yarlung, and Gandaki Rivers using average ionic ratio in spatial scale (Fig. 9) . The upstream of Gandaki River plotted towards silicate end member and upstream of the Yarlung River plotted towards the carbonates end member indicated the hydrochemistry of upstream of the Gandaki and upstream of the Yarlung were driven by the silicate rock weathering and carbonate rock weathering, respectively. The rest were plotted between the silicates and carbonate end members suggesting carbonate and silicate rock weathering were the keys responsible to control the hydrochemistry.

The relationships among ions and ionic ratios are used to evaluate the sources of dissolved ions and controlling mechanisms of hydrochemistry in the freshwater system (Yde et al. 2008; Feng et al. 2012; Ansari and Ahmad 2019) . The ionic ratios of the two groups were presented in Table 6 . The ratio of HCO3 -/Ca 2+ was <2 suggesting the control of carbonate rock weathering in the GBRS (Thomas et al. 2015) . Moreover, the high mean ratios of (Ca 2+ +Mg 2+ )/(Na + +K + ) (2.62: Group-1 and 4.34: Group-2) and HCO3 -/(Na + +K + ) (2.90: Group-1 and 3.97: Group-2) supported to proven the dominancy of weathering of dolomite and calcite minerals. The high ratios of (Ca 2+ +Mg 2+ )/Tz + (0.72: Group-1 and 0.81: Group-2) also suggested the domination of carbonate weathering and the comparatively low ratios of (Na + +K + )/Tz + indicated less intense evaporates over carbonate weathering in the GBRS. The HCO3 -/(HCO3 -+SO4 2-) (C-ratio) explained the relative importance of carbonation and sulfide oxidation. If the C-ratio is <0.50, the coupled chemical reactions of both carbonate dissolution and sulfide oxidation are indicated, whereas if the ratio is close to 1, exclusively carbonation reactions and dissociation of CO2 deriving protons from atmospheric inputs. The C-ratios (0.78: Group-1 and 0.77: Group-2) specified the major role of carbonate and CO2 dissolution in proton producing mechanism in the GBRS.

Irrigation suitability of the major 12 tributaries in the GBRS was evaluated using different methods and presented in Table 7 and Fig. 10 . The average ionic concentrations of River water including pH and TDS were compared with the permissible limits of FAO guidelines for irrigation water and suggested that all the Rivers were within the permissible limits. The EC values were ranges 69-754 µS/cm in the GBRS and suggested all the rivers were in the "good and excellent" categories and concluded that all the rivers were acceptable for irrigation usage. The Na% in tributaries of the GBRS ranging 10.96-47.49, and were categorized in "Excellent to permissible". This also suggested all the rivers were acceptable to use for irrigation purposes from the perspective of Na%. However, the Rangit River has higher sodium content exhibiting the permissible for the irrigation analysis. The long-term utilization of Rangit River water for irrigation might be resulted in the formation of alkali soils due to the higher Na% combining with carbonate. Similarly, SAR values of all the rivers were less than 10 (ranges 0.08-1.02) and indicated the "Excellent" category which suggested all the river's water could be used for irrigation. The high magnesium content in water would cause more alkaline which would further affect crop yields and decrease the soil permeability. So, MH content (magnesium hazard) in irrigation water less than 50 is acceptable for irrigation purposes. Based on the limited MH value, most of the tributaries in the GBRS were suitable except for the Gomti River. The concentration of Mg 2+ in the Gomti River (50.62) was high due to anthropogenic contributions. Discharge of wastewater to the Gomti River channel increased by 10%-20% of the Mg 2+ content in river water (Rai et al. 2010) . The higher Mg 2+ content in the Gomti River might alter the salinity of river water, increase the pH of the soil, affect the permeability properties of the soil root zone.

The irrigation water quality of Teesta, Yarlung and Gandaki Rivers with their spatial scale also suggested that the river water could be used for irrigation without any treatment and their ionic concentrations were within the FAO permissible limits. The details of irrigation suitability were presented in Table 8 . All the parameters that we had checked in this review study indicated that the river water from upstream, midstream, and downstream of Teesta, Yarlung and Gandaki Rivers were in a safe condition for irrigational utilization. The higher EC in upstream of the Gandaki River may cause the degradation of soil permeability.

After the interpretation of the Wilcox diagram (Fig. 11) , indicating the two classes: C1S1 and C2S1. The results of the Dudhkoshi, Rangit, Teesta, Brahmaputra, and Indrawoti plotted in C1S1 (low salinity and low sodium hazard) category indicated the best quality for irrigation purposes which were completely effective for agriculture. Likewise, Gharra, Fig. 8 Mixing diagram of Na-normalized molar ratios of (a) Ca 2+ vs. HCO3 -and (b) Ca 2+ vs. Mg 2+ of the Group-1 and Group-2 Rivers.

Mixing diagram of Na-normalized molar ratios of (a) Ca 2+ vs. HCO3 -and (b) Ca 2+ vs. Mg 2+ of the Teesta, Yarlung, and Gandaki Rivers.

Betwa, Yarlung, Damodar, Ken, Gandaki and Gomti River plotted in C2S1 (medium salinity and low sodium hazard) category suggested approximately perfect for agriculture. Overall interpretation of the Wilcox diagram also suggested the safe condition of river water in the GBRS for irrigational utilization.

This review study analyzed and expressed the hydrochemical characterization and irrigation suitability of 12 small and medium tributaries in the GBRS during the pre-monsoon season using available hydrochemical attributes collected from published literature. The hydrochemistry of the GBRS indicated a slightly alkaline to alkaline nature. The average concentrations of most of the chemical variables showed significantly higher in Group-1 except SO4 2-, K + and NO3 -. The grand mean values of the major anions and cations values in all rivers follow the order of HCO3 ->SO4 2->Cl ->NO3 -and Ca 2+ >Mg 2+ >Na + >K + , respectively, the same as the global average order. Piper diagram implied that the river water hydrochemistry belongs to the Calcium-bicarbonate (Ca-HCO3) type. The river water hydrochemistry was dominated by the carbonate and silicate rock weathering which was confirmed by the Gibbs plot and mixing diagram. Furthermore, the low ionic ratio of (Na + +K + )/TZ + , high ratio of (Ca 2+ +Mg 2+ )/TZ + , and the high C-Ratio, suggesting carbonate weathering was the dominance process. The river hydrochemistry of Teesta, Yarlung and Gandaki Rivers also indicated a slightly alkaline to alkaline nature with the dominancy of calcium-bicarbonate types except in upstream of the Gandaki River. Owing to the different geological and edaphic characters, the upstream of the Gandaki River plotted into the middle of the diamond plot showing mixed types of hydrochemistry. Gibbs diagram of upstream, midstream, and downstream of Teesta, Yarlung, and Gandaki Rivers indicated that natural rock weathering was a major natural controlling mechanism of river hydrochemistry. The carbonate and silicate rock weathering predominantly controlled the hydrochemistry of these rivers. The upstream of the Gandaki River and upstream of Yarlung River were controlled by silicate rock weathering and carbonate rock weathering, respectively. As a conclusion of hydrochemical characterization of the some selected small and medium tributaries of the GBRS, it clearly showed that the majority of the hydrochemical facies were Ca-HCO3 type with the dominancy of the natural carbonate rock weathering followed by silicate rock weathering as a controlling mechanism. The contribution of evapo-crystallization in the hydrochemical budget of the GBRS was very minimal.

The suitability of river water for irrigation purposes was assessed during the pre-monsoon season by adopting different parameters, methods, and internationally accepted standards. All the rivers included in this review study were within the FAO limit and indicated being appropriate for irrigation using purpose based on EC, SAR, and Na%. However, Table 8 Irrigation suitability evaluation table of Teesta, Yarlung, and Gandaki Rivers   ID EC Class Na% Class SAR Class MH

The Gomti River is in the unsafe category according to MH. Long-term use of irrigation water with relatively higher content of Na + and Mg 2+ in Rangit River and Gomti River might have resulted in the increasing pH of the soil, affect the permeability properties of the soil root zone and reduce the availability of phosphorous nutrients. Wilcox diagram revealed that all rivers could be used for irrigation purposes with plots on the major two sections (C1S1 and C2S1). In addition, the irrigation suitability assessment of Teesta, Yarlung, and Gandaki Rivers also showed the suitability for irrigation use. In conclusion, the river water in the GBRS was suitable for irrigation except for a few instances which need special caution for utilization. However, the responds of the hydrochemical characteristics and water quality to climate change are not clear in this region for lacking different periods studies which need persistent efforts by global scientists and local governments.

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This study was financially supported by National Natural Science Foundation of China (Grant No. 41761144078) and Kathmandu Center for Research and Education, CAS-TU, Kathmandu, 44618, Nepal. The authors are obliged to Dr. Ramesh Raj Pant from Tribhuvan University, Nepal for his valuable comments and suggestions to this review work. Also,

we would like to thank all the original research work used in this review paper. Finally, the authors would like to thank the anonymous reviewers for their constructive comments and suggestions which greatly help to polish the manuscript.