Hydrogeochemistry of glacial meltwater
The physical analysis of meltwater indicates a little alkaline with pH ranging from 6.86-8.56 with an average value of 7.45 ±0.48 and 7.45 ±0.45 for 2016 and 2017, respectively (Table 2). Higher pH indicates that the process of dissolution is higher due to the more considerable contact period with rock, soil and rainwater. These would have imparted alkalinity to the meltwater (Kumar et al. 2014). Electrical conductivity indirectly measures the mineralization that explains the ionic strength of water (Kumar et al., 2019). The standard value of electrical conductivity was 86.14 ±16.96 μS/cm in 2016 and 91.25 ±16.62 μS/cm in 2017. The higher EC must result from the weathering, evaporation, and crystallization processes. Furthermore, the lesser conductivity in glacier discharge is influenced by increased precipitation making higher discharge and decreased influence of the evaporite dissolution process. The measured value of EC suggests that hydrochemistry of location is regulated through the interface of water and rock and depends on the weathering of rocks. Different rocks and their solubility influence the proportional concentration of ions in glacial meltwater (Pant et al. 2021). Table 2 displays the distribution of dissolved ionic concentrations with standard deviation in the meltwater discharge of the Shaune Garang glacier.
The results presented in Table 2 indicate that Ca2+ contributes 39.57% and 42.53 % in the total cationic budget in both the consecutive study periods 2016 and 2017. However, Ca2+ + Mg2+ contributes 82.10 % and 71.02 % of the total cationic budget in the catchment. The other two cations, Na+ and K+, contribute only 16.78 % and 17.57 %, respectively, during the study period 2016 and 2017. Bicarbonate (HCO3-) is the most dominant anion contributing 62.18 % and 54.44 %, respectively, in the total anionic budget of the ablation period of 2016 and 2017. Its average concentration was observed as 369.65 ±79.41 µeq/l, and 316.73 ±83.23 µeq/l in the consecutive study period. Sulphate (31.10% and 38.11 %) was the second most dominant anion, followed by chloride (5.58% and 6.64 %) and Nitrate (1.18% and 0.79 %) during consecutive years' observation. The dominance of bicarbonate in Shaune Garang Catchment is due to the silicate dominating geomorphology of the catchment. According to the findings, weathering of silicate minerals is less visible than carbonate minerals.
Figure 4 displays the concentration of different anions and cations and electrical conductivity for the glacier's meltwater at different parts of the Indian Himalaya. The concentration of cations and anions varies as per the morphology of rocks in the catchments and weather system. Meltwater draining from the Himalayan region shows the dominance of Ca2+ and HCO3-, whereas Bagni, Chaturangi, Gangotri and Dudu glaciers demonstrate the dominance of SO42- in their catchment. The dominating presence of silicate-bearing rocks is the important factor for the higher concentration of bicarbonate (HCO3-) in the meltwater of the Himalayan glacier. In addition, the dominancy of SO42- in the Bagni, Chaturangi, Gangotri and Dudu glaciers could be due to pyrite oxidation that enhances sulphate concentration. Cl− and SO42-' domination is influenced by halite and sulfide oxidation and weathering of soft sulphate minerals such as gypsum (Thomas et al., 2015). The chemical composition analysis reflects the dominance of bicarbonate (HCO3- ) as an anion in most of the glacial meltwater in the Himalayan region due to the dissolution of atmospheric carbon dioxide and carbonate (Sharma et al., 2013; Kumar et al., 2014; Singh et al., 2015). Concentration of cations in the meltwater of Dokariani, Bara Shigri and Gangotri glaciers follows a trend like Ca2+ > Mg2+ > K+ > Na+ while it follows a trend of Ca2+ > Mg2+ > Na+ > K+ for meltwater of Kafni and Chhota Shigri, the only alteration in the concentration of Na+ and K+ for different basins. However, the Bagni glacier meltwater showed the Potassium ion as the second most abundant. In the Bara Shigri glacial, meltwater concentration of cations varied as Ca2+ > Mg2+ > Na+ > K+ like the Chhota Shigri of its vicinity and the Kafni of Kumanyu Himalaya whereas anions concentration followed the pattern of HCO3- > SO42- > NO3-. It has been observed from the comparative analysis in Figure 4 that the central Indian Himalayan glacier's meltwater has the highest concentration of Ca2+ cation, and a similar observation is from the present study. Interestingly, the graph shows a higher concentration of anions and cations in discharge from glaciers located in the central part of the Indian Himalayan region than the glaciers in the western part.
Hydro‑geochemical process in the glacial catchment
Dissolved ions in the glacial meltwater are generally contributed through rock weathering, precipitation, anthropogenic influence, and atmospheric conditions (Jeelani 2011; Kumar et al. 2019). Generally, the chemical composition of glacial meltwater is governed by the chemical weathering between the interaction of water and bedrock beneath the glacier (Kumar et al., 2009; Kumar et al., 2014; Singh et al., 2017). The cation and anion in the glacier discharge are elucidated concerning the nature of rock and its weathering processes. Dissolved solute particles present in the glacial melt are determined by the processes involved in the glacial environment. The interrelationship between the physical and chemical parameters are presented through a scatter plot (Figure 5) of (Ca2++Mg2+) and (Na++K+) against total cation (TZ+). A positive correlation is observed between (Ca2++Mg2+) and TZ+. It further shows a ratio ranging from 0.75 to 0.85 with an average equivalent value of 0.75 ±0.05 during the study period.
The result demonstrates that the impact of Ca2++Mg2+ in the glacial meltwater is comparatively high compared to the total cation TZ+. The ratio of Ca2+ and Mg2+ determines the input source of calcium and magnesium ions in water. Ca2+/Mg2+ ≤ 1 (Table 3) indicates a process of dolomite dissolution, and a value >1 recommends the dominance of silicate weathering in water (Kumar and Singh 2015). As a result, silicate weathering could be a factor in the dominant concentration of Ca2+ and Mg2+ among cations in the Shaune Garang glacial discharge. The scatter plot (Figure 5) among Na++K+ and TZ+ displays a small contribution of Na++K+ in total dissolved ion with 0.24 ±0.04 and 0.25 ±0.08 during both ablation years. The results reveal carbonate weathering as a leading factor in the glacial meltwater ionic characteristics of the Shaune Garang catchment. The large equivalent ratios 3.23 ±0.75 and 3.28 ±1.14 for (Ca2++Mg2+)/(Na++K+) in the consecutive melting period of 2016 and 2017 further strengthen the understanding of carbonate weathering dominance in the catchment. High ratio of (Ca2++Mg2+)/(Na++K+) and (Ca2++Mg2+)/TZ+ in the glacial meltwater demonstrate that hydro-geochemistry of the meltwater of Shaune Garang catchment is mainly administrated by CO3 weathering with a minor contribution of SiO2. However, the evaporation process enhances the TDS concentration in water (Prasanna et al. 2010; Xing et al. 2013). The average ratio of Na+/Cl- was measured to be 4.77 ±2.27 and 3.83 ±1.90 in 2016 and 2017 (Table 3). The Na+/Cl- ratio indicates a minor contribution of atmospheric constituents in the chemical characterization of meltwater of the catchment.
The ion exchange process in the water is mainly defined because of (Ca2++Mg2+) versus (HCO3− +SO42−) (Srinivasamoorthy et al., 2008). Dominant dissolution process of calcite, dolomite, and gypsum ion exchange may shift the points rightward owing to excess of (HCO3− +SO42−). Further, the reverse ion exchange process turns leftward due to a surplus (Ca2++Mg2+). (Ca2++Mg2+) versus (HCO3− +SO42−) indicates carbonate and silicate weathering with ion exchange as leading geochemical processes in the catchment (Figure 6). The diagram displays that contribution of (HCO3− +SO42−) to the total ionic concentration in the glacial meltwater is greater than the (Ca2++Mg2+), indicating an excess of (HCO3−+SO42−) which is contributed by silicate weathering. The dominance of calcium and magnesium ions is calculated through Ca2+/Na+ and Mg2+/Na+ ratios, a product of weathering of CO3 and SiO2. The Ca2+/Na+ ratio was observed as 2.57 ±0.80 and 2.64 ±1.06, while Mg2+/Na+ was respectively 2.16 ±0.67 and 2.07 ±0.81 in the meltwater of Shaune Garang glacier (Table 3). This ratio shows the dominance of Ca2+ and Mg2+ over Na+. The result further confirms that hydro-geochemistry is governed by weathering of CO3 minerals in the catchment. Hydrogen ion availability is responsible for rapid CO3 weathering (Das and Kaur 2001). Na+ normalizes Ca2+ and HCO3- concentration and determines the effect of SiO2 weathering, evaporative dissolution or CO3 weathering in meltwater (Kumar et al., 2015). To understand the chemical weathering, sulphate mass fraction (SMF), and the ratio of sulphate (SO42−) to (SO42− + HCO3−) have been calculated in the study catchment. The chemical characteristics of meltwater show the importance of carbonation if the SMF value is (<0.5). The SMF value indicates the chemical attributes of meltwater affected by sulfide oxidation and the termination of CO3 (Tranter et al. 1993). In the Shaune Garang catchment, an average SMF value of 0.33 ± 0.07 and 0.41± 0.09, respectively, during the study period 2016 and 2017 indicates the dissolution of carbonate and sulfide oxidation. In Addition, C-ratio (HCO3−/ HCO3− +SO42−) has also been calculated to find the significance of proton-producing effects necessary for the chemical weathering of carbonate rocks. During 2016 and 2017, the C-ratios were 0.67 ±0.07 and 0.59 ±0.09, demonstrating the domination of the carbonate and sulphate weathering processes.
Mineral mapping
The Short Wavelength Infra-Red (SWIR) and Thermal Infrared (TIR) spectral resolution agree for mapping surface mineralogy. These spectral bands are available in (ASTER) and have been used to map the distribution of minerals on supraglacial debris. Minerals are mapped through the band indices like "SWIR indices", "TIR indices", and "TIR emissivity silica weight per cent" in the Shaune Garang catchment. The mineral measurement reflects the primary presence of quartz, feldspar, carbonate, and mica. High altitude glacier debris reflected the fact of "quartz, feldspar as calcium albite, and mica as biotite". The debris on the Shaune Garang glacier is dominated by muscovite (mica), calcium albite (feldspar) and quartz. Though in lesser quantity, the presence of calcite has also been noticed. To create thematic mineral abundance maps and quantitative estimation of minerals, "SWIR and TIR indices" have also been used (Ninomiya, 2004).
SWIR indices
Short Wavelength Infrared (SWIR) mineral indices were used to wavelength-dependent absorption patterns in estimating minerals in the catchment. The SWIR mineral indices were used to evaluate the mineral's dominance in the catchment. Equations 1, 2, 3 and 4 have been used respectively for understanding the dominance of layered silicate (LS), calcite (CA), hydroxyl-bearing (OH), and Alunite (AL).
Where,
ASTn is band number (n) related to ASTER
The varying indices are related to the variable absorption properties, which helps measure the types of minerals. A sensor, "radiance band ratios", can reduce the influence of the atmosphere and the topography of a region and the variation in illuminance (Abrams et al., 1983; Mather, 1987). The evidence also indicates the nonsignificant evidence in "single band or three-band true or false colour composite imageries". It is also helpful in having the quantitative estimation of mineral abundances. In this study, images of the 4-shortwave infrared (SWIR) mineral indices are displayed in Figure 7, reflecting the relative dominance of minerals and their presence on the surface. Alunite has been most dominant and abundant in higher altitudes up to the accumulation zone. "Layered silicates" and "hydroxyl-bearing minerals" are less productive, while "calcite and hydroxyl-bearing minerals" varies location-wise. Alunite index displays little white patches in the higher region with high abundance. Figure 7 shows kinematics and pulse flow movements of layered silicate debris, which can be understood through their variability and abundance. The evidence of Alunite at a higher altitude might be due to its formation mechanism. The formation of Alunite through the reaction of sulfuric acids with potassium-rich feldspars is called "alunitization". Layered silicates and "hydroxyl-bearing minerals" are in short supply, but "calcite and hydroxyl-bearing" minerals vary significantly within the catchment. Layered silicate consists of octahedral layers bound to the tetrahedral and primary component of soil. Its distribution within the catchment at lower altitudes implies the weathering mechanism due to meltwater and parental rock interaction. They have been the excellent water trapping mechanism held between layers. The essential minerals in layered silicates are kaolinite, nacrite and dickite.
TIR indices
To evaluate various minerals in the Shaune Garang catchment area, Thermal infrared (TIR) mineral indices of Carbonate, Quartz and Mafic were used. The thermal spectrum is instrumental in distinguishing the geology of earthy minerals, where TIR satellite spatial resolution is noticeably lesser than VNIR or SWIR (VNIR 15 m, SWIR 30 m, TIR 90 m). However, TIR is exclusive in targeting the profusion of carbonate, quartz, and silicate minerals. Band ratios derived from TIR estimate carbonate, quartz and silica bearing lithology (Figure 8). Equations 5, 6 and 7 were used for Carbonate Index (CI), Quartz Index (QI), and Mafic Index (MI), respectively.
Where,
"ASTn is band number (n) based on the properties of ASTER spectral
The CI is better used to detect primary carbonate minerals such as "calcite and dolomite". These two carbonates mineral have higher absorption features, which indicates the availability of calcite and dolomite. The absorption features of calcite are about 11.4 to 11.2 μm in the case of dolomite minerals. Minerals of carbonates with "hydrothermal origin" are very challenging to identify through a CI map due to an inadequate percentage of carbonate presence. However, calcite-bearing propylitic alteration is accredited as the "ASTER TIR" feature. The chlorite and epidote had lower emissivity between TIR bands 11 and 13 but slightly higher between 13 and 14 (Salisbury et al. 1992). Aspects of the spectrum with these characteristics resemble the mafic index minerals. Mafic and quartz index minerals found uneven distribution within the catchment, but carbonate minerals were found at lower altitudes along the riverside. The reason behind the occurrence of carbonate minerals along river channels might be due to the higher weathering across the river.
Chemometric Analysis
Principal component and factor analysis
Excel add-on XLSTAT was used for the analysis of normalized data under PCA. The sphericity test of Bartlett was performed on the data of both years. The Bartlett sphericity test shows that observed χ2 (342.85) is considerably more significant than the critical χ2 (85.96) in 2016 and χ2 (observed) = 125.25 larger than the critical value χ2 (critical) = 48.3 in 2017. The principal component analysis helped to understand problems under different measurement scales of the original variable avoided by diagonalizing the correlation matrix.
Table 4 demonstrates the PC value of more than 1, which explains 72.1 % of the total variance of four PCs. PCs 1, 2, 3 and 4 are capable of explaining 39.21%, 12.91%, 10.24% and 9.74% of variance in 2016. Similarly, in 2017 scree plot (Figure 9) shows four PCs, which explains 69.9.1% of the total variance. PC 1, 2, 3 and 4 can explain 26.62 %, 20.12 %, 12.64% and 10.52 % of variance. Table 4 further depicts the Eigenvalues, the percentage of variance calculated through varimax rotation matrix with Kaiser Normalization and rotated factor, and the percentage of variance in each PC. High SO42− and K+ loadings are observed in both the years, indicating silicate weathering dominance in the catchment. Moderately high loading values of Ca2+, Mg2+ and Na+ in both years indicate the dominance of the process, which is prevailing in factor 1.
Table 5 indicates that factor 4 shows the negative pH in both the years and the acceptable value of Cl- in the consecutive study period. The first two principal component loading are presented to understand the grouping and relationship of all chemical parameters. The PC loading has been calculated to understand correlations among variables and know the most influential variables. It could result from the minerals present in the soil (Yakubo et al., 2009). Throughout the study, higher loadings in Na+ and Mg2+ may be accredited to the ionic conversation between water through dissolution minerals containing sodium.
Statistically, the coefficient of determination (R2) indicates one variable's level of statistical agreement with another. Here, it is applied among the hydrogeochemical parameters and represented in (Table 6) during the study period. Water chemistry parameters such as EC and Na+ are highly interrelated with Ca2+, Mg2+, and HCO3−. Similarly, a decent relationship among (Ca2+- Mg2+), (Ca2+ - HCO3−) and (Mg2+ - HCO3−) has been observed. The above parameters have a good positive correlation (R2 > 0.5) and are an indicator of control by these parameters in the solute chemistry of the study region. The strong correlation between parameters such as Ca2+, Mg2+, (Ca2+ - HCO3− , (Mg2+ - HCO3−) indicates strong carbonate weathering (Singh et al., 2017). In the case of sulphate (SO42-) ion concentration, it shows a good relationship with Ca2+ and Mg2+, indicating the sulphate mineral's high dissolution in the glacier's catchment.
Hydro-geochemical facies of the glacial meltwater
Hydrogeochemical facies of the meltwater helps interpret the dominant anions and cations, which have been determined through a Piper plot (Figure 10). It is used to find similarities and dissimilarities among all water types, where the analogous water qualities fall together (Todd, 2001). In the cation plot, it can be seen that most of the water is concentrated in a trilinear pattern in the middle, indicating that it is a mixed water type. The calcium ions predominate in the glacial discharge. The hydro-geochemical cations in the bottom left triangle of Shaune Garang glacial discharge prove calcium ions' dominance. It substantiates the conclusions reached in the sections on hydro-geochemistry and hydro-geochemical processes, both included in the previous section. Slightly elevated sodium and potassium ion concentrations in several samples confirm the presence of sources at the various sampling locations of the Shaune Garang catchment. This must be contributing to the overall cation concentration (Karim and Veizer, 2000; Ravikumar, 2017). The Piper plot aids in the understanding of the fact, Ca2+, Mg2+ and HCO3- are the most prevalent ions in the Shaune Garang catchment. The average percentage value of (Ca2++ Mg2+) is about 77% and 81%, respectively in the years 2016 and 2017; however, for the (Na++ K+) it showed about 33 % and 29%, respectively, demonstrating that alkaline earth metals are prevailing over alkali metals (Singh et al., 2017). It further supports the dominance of dolomitic limestone containing gypsum and pyrite in the region (Figures 7 and 8). The piper diagram demonstrates that carbonate type weathering has been more instrumental in governing hydro-geochemistry in this catchment. The figure indicates the presence of the (Ca2+– HCO3-) type of water with little influence from (Ca2+– SO42-) type. Significant ions and TDS were found in higher concentrations in this catchment, indicating more significant interactive processes between rock materials and water having influential weathering due to moisture.
Apart from the Piper plot, Gibb's diagram (Gibbs 1970) was applied to understand how hydro-geochemical techniques such as precipitation, rock-water interface, and vaporization impact the environment's hydrogeology. According to the Gibbs diagram (Figure 11), the chemical weathering of rock minerals and a minimal extent of evaporation crystallisation are the essential variables to consider the meltwater quality in the Shaune Garang catchment. Gibb's diagram advocates the higher rock-water interaction resulting in higher ionic concentration in meltwater. Chemical weathering, carbonate dissolution and ionic exchange between water and clay indicate the rock-water interaction processes (Kumar et al., 2014). Increased evaporation, chemical weathering and anthropogenetic actions raise the total dissolved solids. Furthermore, the findings show that water contamination from poor sanitation has increased Na+ and Cl- ions and increased total dissolved solids (TDS) (Kumar et al., 2014).