Hydrogeochemical and biological assessment of spring and stream water quality for its suitability for drinking in Kanglung locality, Trashigang, Bhutan

In the Himalayas, spring water has traditionally been the primary source of water for drinking and other domestic purposes for centuries. Springwater is generally considered pristine; however, it can be easily contaminated when it emerges from the earth's surface. Globally, drinking water pollution has become a significant challenge attributable to rapid urbanization and industrialization. According to World Health Organization (WHO), about 435 million people take water from unprotected wells and springs, and around 144 million people depend on untreated surface water from lakes, ponds, rivers and streams (WHO 2019).

Although Bhutan has the highest per capita water use capacity globally (i.e., 94,500 m³ per person per year), it is ironic that most rivers and streams are not accessible. The rivers and streams mostly flow at the bottom of the gorges and ravines, while settlements and farmland occupy the upper slopes and hilltops (ADB 2016). Consequently, springs or small mountain streams are the primary water sources for domestic and agricultural uses. However, climate change, population increase, and economic development are mounting pressures on these available water resources in terms of quality and quantity in the region (ADB 2016; Sharma et al. 2019). Recent studies (Tambe et al. 2012; Acharya 2021; Kulkarni et al. 2021) have demonstrated that springs and streams in the Himalayan areas, including Bhutan, are drying up or becoming seasonal, resulting in acute water shortages under changing climate conditions. The annual drinking water quality surveillance report of 2019 revealed that the availability of safe drinking water is still a challenge in Bhutan, which corroborates with past studies (Rahut et al. 2016; Wangdi & Clements 2017) carried out in some parts of Bhutan. From a total of 1951 drinking water samples collected nationwide for routine water quality surveillance, 47.4% tested for thermotolerant coliform, while some of the water samples had arsenic, lead, cadmium and iron concentrations exceeding the permissible limit. Yet, information on the water quality of the stream or spring water sources tapped for drinking is inadequate due to the limited coverage. Moreover, the routine analysis focuses mainly on the microbiological parameter. Physicochemical and biological characteristics of streams and springs are critical for the holistic understanding of water quality and determining its suitability for human consumption and other utility besides assessing its ecological health.

Physicochemical and biological characteristics of stream/spring waters mainly depend on natural factors such as underlying geological formations of the area, degree of chemical weathering of various rock types, atmospheric inputs and anthropogenic factors such as disposal of domestic and urban wastes, agricultural runoff, industrial and municipal effluents. All these factors make stream/spring waters vulnerable to microbial and mineral contaminations, including toxic trace elements, thus rendering them unfit for drinking and other purposes. Besides water quality information, spring/stream water chemistry are instrumental in inferring hydrological and hydrochemical processes in mountainous catchments. For instance, major ion chemistry is a powerful tool which is widely used for tracing source of solutes in groundwater aquifer system, the response of aquifers to hydrological events and describing the evolution of water as a result of water–rock interaction (Bhat et al. 2014). Despite its importance, this kind of data is virtually unavailable in Bhutan Himalayas.

Bhutan has a national drinking water quality standard for various physical, chemical and bacteriological parameters. While these standards are useful, they pose a particular problem in the case of reporting the water quality monitoring results to various stakeholders such as water service providers and the general public. This is due to an inherent complexity in comprehending the water quality results due to many individual variables. The water quality index (WQI), initially formulated by Horton (1965), later developed by Brown et al. (1970), and further improved by several researchers, provide an inclusive approach to interpreting water quality parameters of water sources for assessing potability (Aalipour et al. 2022). WQI aims to integrate complex water quality data and establish a simple criterion that describes overall water quality and is understandable to the general audience in deciphering water sources’ status in a particular watershed (Seth et al. 2016). This approach has been widely employed in monitoring the water quality of surface waters, including spring and stream waters (Thakur et al. 2020; Tiwari et al. 2020; Ahsan et al. 2021) to help strategize appropriate management plans to improve water quality. Therefore, this study aimed to analyze the potability of major springs and streams in the Kanglung locality using drinking water quality indices and to infer major geochemical processes in the springs and streams. Our findings will provide insights on the quality of a spring and a small stream to support making an informed decision for the site-specific springshed management plans and interventions to improve drinking water quality in the area.

Kanglung is under Trashigang District, Eastern Bhutan, at an elevation of about 1,930 m above sea level (a.s.l). The hydro-climate of the area is influenced mainly by the Indian summer monsoon typical of the eastern Himalayas. The monsoon season begins in June, peaks in July–August and ends in September, resulting in high streamflow. Annual rainfall measured at Kanglung meteorological station from 1996 to 2021 ranges from 885.6 mm to 1,487.3 mm. The maximum rainfall occurred in May and October, measuring 103.0 mm and 71.8 mm in 2020 and 2021, respectively. The average annual temperature maximum and minimum recorded at the station from 1996–2021 were 21.81 °C and 10.59 °C, respectively. Over 3000 people live in the Kanglung community comprising mainly of students, civil servants, corporate workers, entrepreneurs, and farmers (NSBO 2017). Recently, Kanglung has been identified as the regional educational hub because of its existing educational institutes, mainly Sherubtse College, Kunglung Thubten Choekhorling Shedra (a Buddhist monastery), and schools; and also due to its strategic location and favourable climatic conditions (Pokhrel 2015). Over the years, Kanglung has seen an increasing population partly owing to increased student intake at Sherubtse College and the establishment of new government, corporate and private facilities. The growing population has led to water shortages in the community mainly due to the increase in per capita water use. Moreover, the availability of clean drinking water is a recurrent issue in the community as the existing water supply is untreated (Tshering et al. 2021). Spring fed streams and springs are the primary source of drinking water and domestic application including irrigation for the Kanglung community. It is noteworthy to mention that the human encroachment at the water source areas has increased over the years due to the developmental activities like construction of roads and buildings. Moreover, some of the stream catchments in the study area have been witnessing frequent erosion due to intense surface runoff during storm events as an impact of land degradation at the upstream areas. Therefore, it is crucial to assess impact of various natural and manmade factors on the quality of these freshwater resources for the welfare of the community.

Eighty-nine grab water samples were collected weekly from October 2020 through April 2021. Both filtered and unfiltered water samples were collected for physico-chemical analyses. One-litre high-density polyethylene (HDPE) bottles were used to collect the unfiltered samples, while 125 mL HDPE bottles were used for collecting filtered samples. The sampling bottles were washed with deionized (DI) water and air-dried before sample collection. Water samples for major ions were filtered in the field using a pressure syringe filtering system through a 47-mm diameter Gelman A/E glass-fibre filter with a pore size of about 1.0 μm following the protocols discussed in Hill et al. (2018). The sampling bottles were rinsed at least five times with water samples during the sample collection. The HDPE bottle was filled with water sample and capped leaving no air space. The water samples collected were immediately transported to the Center for Science and Environmental Research (CSER) water chemistry laboratory, Sherubtse College, Royal University of Bhutan. The samples were stored at 4 °C until the analyses. Fifteen additional unfiltered samples for the thermotolerant coliforms analysis were collected in February 2021 (n=10) and December 2020 (n=5) in a sterile 125 mL HDPE bottle. The sample bottle was tightly capped, sealed with parafilm, and immediately sent to the Kanglung Basic Health Unit's Microbiological Laboratory, Trashigang, for analysis.

The CBE values of most of the samples were within the accepted range of ±10%.

The analysis of thermotolerant coliforms was carried out following membrane filtration and colony count techniques to assess faecal contamination as per the protocol of RCDC (2019). The results were expressed as colony-forming units (CFU) per 100 mL of sample.

The major ions compositions were plotted on a piper trilinear plot using AquaChem 4.0 software to identify the main water types. The Gibbs diagram was plotted to assess the dominant hydrogeochemical processes on the chemical characteristics of spring water, viz., precipitation influence, water-rock interaction and evaporation. Correlation matrix and principal component analysis (PCA) were performed using the SPSS software package to understand the solute acquisition process in the system. The physicochemical parameters of the analytical results of spring water were also compared with the standard guideline values for drinking water recommended by the World Health Organization (WHO) and the National Environmental Commission Secretariat (NECS), Bhutan.

The WQI obtained for each spring water was categorized as 0–25 excellent water quality, 26–50 good water quality, 51–75 poor water quality, 7–100 very poor water quality, and >100 unfit for drinking as per Ahsan et al. (2021) .

The HCO₃⁻ was the dominant anion contributing 54.3% of the total anions (TZ⁻), followed by Cl⁻ and SO₄²⁻ accounting 23.4% and 22.2%, respectively. In cation chemistry, Na⁺ was dominant accounting 42.9% of the total cations (TZ⁺), followed by Ca²⁺ (20.6%), Mg²⁺ (18.9%), and K⁺ (17.6%). The concentrations of weathering products were consistently higher in Sp3 except for Na⁺ and K⁺, corroborating with our assumption that this spring travels longer flow paths which is consistent with EC and TDS results (Figure 2). On the contrary, concentrations of dissolved ions, EC and TDS values were lower in Sp5 indicating minimum rock-water interaction likely owing to shorter flow path, and higher discharge resulting in dilution of dissolved ions. The possible sources for HCO₃⁻, Ca²⁺ and Mg²⁺ are carbonate rocks dissolution while silicates of magmatic rocks are likley sources for Na⁺ and K⁺. Weathering of the Na⁺ bearing minerals like plagioclase feldspar also release the soluble Na⁺ products in water. The concentrations of all the major ions are much lower than the permissible limits prescribed for drinking water.

The TH in the study area ranged from 19.8 to 79.3 mg/L which falls in soft (0–60 mg/L) to medium (60–120 mg/L) category. The concentration of TH of spring/stream water samples are also within the desirable limit recommended for drinking water. No adverse effect due to water hardness on health was documented but extreme hard water is known to cause cardiac and kidney diseases (Narsimha & Sudarshan 2017).

To understand sources of dissolved ions in springs/streams in the study area, the scatter plots of equivalent concentration (mEq/L) of different parameters were used. The scatter diagrams of alkaline earth metals (Ca²⁺+ Mg²⁺) and alkali metals (Na⁺+ K⁺) versus total cations (TZ⁺) (Figure 3(b) and 3(c)) with an average molar ratios of 0.49±0.11 and 0.51±0.11, respectively, shows that most of the data points fall below 1:1 equiline revealing the higher contribution of total cations. The scatter plots of (Ca²⁺+ Mg²⁺) and (Na⁺+ K⁺) with average molar ratios of 1.15±0.57 had data points plotting both above (43%) and below (57%) the 1:1 equiline (Figure 3(a)). These scatter diagrams (Figure 3(a)–3(c)) and molar ratios all indicated the importance of both silicate and carbonate weathering in controlling the spring/stream water chemistry (Sharma et al. 2012). The molar ratios of Na⁺/Cl⁻ of water samples range from 0.38 to 2.35. Out of 87 water samples, 56 had Na⁺/Cl⁻ molar ratios greater than one and rest with value less than one or close to one. Samples having a Na⁺/Cl⁻ ratio greater than 1 indicate excess Na⁺, which might be attributable to silicate weathering. However, with samples having Na⁺/Cl⁻ ratio less than 1, it shows excess Cl⁻ due to carbonate weathering (Srivastava & Parimal 2020). In the Ca²⁺+ Mg²⁺ versus HCO₃⁻ plot (Figure 3(d)), the samples spread mostly above and close to the 1:1 equiline with some samples poltting below the equiline. The points above and close shows that the HCO₃⁻ in the spring/stream water is controlled by the dissolution of alkaline earth metals as well as the calcite and dolomite minerals (Vishwakarma et al. 2018). The scatter plot of Ca²⁺+Mg²⁺ versus HCO₃⁻+SO₄²⁻ (Figure 3(e)) has sample points mostly plotted below or close to the 1:1 equiline line inferring the predominance of silicate weathering, and carbonate weathering as an additional process as the source of Mg²⁺ and Ca²⁺ in springs and streams (Kumar et al. 2008; Li et al. 2020). The excess of HCO₃⁻+ SO₄²⁻ over Ca²⁺+ Mg²⁺ concentrations can be attributable to the dissolution of silicate minerals (e.g., sodic plagioclases like albite) depicting silicate weathering and ion exchange as the dominant process in the study area (Vishwakarma et al. 2018; Egbueri 2019). Additionally, the scatter plot of Na⁺+ K⁺ versus SO₄²⁻+ Cl⁻ (Figure 3(f)) also had the locations of samples above, below and close to the 1:1 equiline. The location of samples below the 1:1 equiline indicate an impact of non-silicate minerals, whereas the locations above inferred contribution from the silicate minerals, confirming the influence of different sources of these dissolved ions in the springs and streams (Kumar et al. 2020).

Correlation matrix between various physicochemical parameters of the springwater is shown in Table 2. The interrelationships among dissolved ions can indicate the solute acquisition processes (Singh & Ramanathan 2017) regulating springwater chemistry. EC and TDS have a strong positive correlation with Ca²⁺, Mg²⁺, HCO₃⁻ and TH indicating that springwater chemistry was mostly regulated by them, and presence of Ca²⁺, Mg²⁺, HCO₃⁻ and TH greatly infuences TDS, alkalinity and EC of the springwater (Mahato et al. 2018). A good correlation exists between Ca²⁺ and Mg²⁺, Ca²⁺ and HCO₃⁻, Mg²⁺ and HCO₃⁻ indicating origin of these ions are from carbonate rock weathering (Singh et al. 2015) resulting in Ca-Mg-HCO₃ water type in the study area. Low positive correlation of Cl⁻ on EC, TDS, Ca²⁺, Mg²⁺, and TH was also seen. The weak and negative correlations of Na⁺ and K⁺ with other major ions suggest that sodium and potassium mostly originated from Na/K-feldspars or Na/K-bearing minerals.

R-mode factor analysis was used to identify major factors controlling the hydrochemistry of spring and stream waters in this study (Singh et al. 2012). Factor analysis of water samples generated five significant factors having eigenvalues greater then one (Table 3). These five factors explain about 80% of the total variance. Factor 1 accounts for 44.2% variance in the dataset and showed high loading of EC, TDS, Ca²⁺, Mg²⁺, HCO₃⁻ and TH, and negative loading of Na⁺ and K⁺ indicating the contribution from carbonate weathering. Factor 2 accounts for 11.1% variance in the dataset and had strong loading of Cl⁻ showing an input of atmospheric precipitation. Factors 3, 4 and 5 accounts for 8.6%, 7.9% and 7.8% variance in the dataset, respectively. Factors 3, 4 and 5 showed a high loading of SO₄²⁻, K⁺ and Na⁺, respectively, suggesting that these factors are associated with dissolution of sulphate minerals, oxidation of sulphides and contribution from weathering of silicate minerals.

The relative weights of physicochemical parameters for expressing drinking WQI are provided in Table 4. The WQI values of the spring and stream waters computed for Sp1, Sp2, Sp3, Sp4 and Sp5 were 5.75, 23.05, 7.84, 22.64 and 41.64, respectively, falling in the excellent class except for Sp5 which falls in the good class. The result indicated that Sp1, Sp2, Sp3, and Sp4 can be used for drinking, and other allied purposes; however, Sp5 might require some pretreatment prior to drinking. The source of contamination in Sp5 is suspected from surface runoff at upstream water sources that washdown particulate and dissolved organic matter during the storm events. Similarly, poor water quality of Sp5 could be attributable to aging water suppy facilities.

In this study, thermotolerant coliform ranged from 0 to 5 CFU/100 mL in spring and stream water samples collected in February and December coinciding with lean or dry winter season in the study area (Table 5). The result indicated that the spring waters Sp2, Sp3, and stream Sp4 were free of thermotolerant coliform contamination, while spring Sp1 and stream Sp5 had thermotolerant coliform exceeding the WHO drinking water standard (0 CFU/100 mL). The Sp5 had consistently higher thermotolerant coliform contamination. The occurrence of coliform in Sp5 could be attributable to entry of soil or organic matter (containing livestock and wild animal droppings) into the stream at the source area. The rangeland at the upstream catchment of the Sp5 stream is grazing area for the domestic animals which could be the source of organic matter. Similarly, the thermotolerant coliform contamination in the Sp1 could be due to faecal contamination from the leaking septic system, and domestic wastewater as its water source is located near the residential area (Figure 1(b)). The presence of thermotolerant coliform in water sources in the dry season suggested that the contamination will be more in wet seasons due to surface runoff during the storm events.

The springwater and streams are the primary water sources for various domestic applications in the Himalayas. However, these water resources are drying up and face increasing threats due to growing population and developmental activities, including climate change leading to reduced flow and poor drinking water quality. The present study assesses major springs and streams in the Kanglung locality to determine the water quality scenario and major geochemical processes controlling springwater chemistry. The order of anion and cation concentrations in water samples were HCO₃⁻>Cl⁻>SO₄²⁻ and Na⁺>Ca²⁺>Mg²⁺>K⁺, respectively. The scatter diagrams and molar ratios of major ions indicated the importance of silicate and carbonate weathering in controlling the springwater chemistry and indicated the influence of different sources of dissolved ions. The Piper triangular diagram demonstrated hydrochemical facies of spring water were predominantly Ca-Mg-Cl and Na-Cl types. Gibbs diagrams suggested that atmospheric precipitation and weathering of the rock-forming minerals are the dominant hydrogeochemical processes controlling dissolved ions in the study area. The water quality parameters measured in most water samples were within the permissible limits recommended for drinking water; however, turbidity and thermotolerant coliform exceeded the permissible limits in a few samples (Sp1 and Sp5). The WQI values of all the springs and streams samples fall under excellent class except for stream Sp5. The WQI results demonstrated that springs Sp1, Sp2, Sp3, and stream Sp4 are potable and suitable for various domestic purposes; however, Sp5 might require some pretreatment before drinking. As stream Sp5 is the main drinking water source for Sherubtse College, providing water to a major population centre in the Kanglung community, appropriate mitigation measures are necessary to improve the stream water quality in the catchment area. Our results also suggest better strategies and institutional mechanisms for strengthening the management of these water sources to reduce pollution. Overall, our study demonstrates that the WQI is an efficient tool for providing usable and understandable water quality information for the general public, policymakers and water service providers. Additionally, the study provides baseline water quality information on major streams and springs in the Kanglung area.

The authors wish to thank the faculty members of the Chemistry Programme, Physical Science Department, Sherubtse College for their valuable feedback during the study period. Authors also thank the Centre for Science and Environmental Research for the laboratory support. The authors are grateful to the National Center for Hydrology and Meteorology of Bhutan for providing the meteorological data for the Kanglung region.

TL & PL: sampling and laboratory analyses, and data analysis. TD: conceptualization, writing original draft manuscript, data analysis, review and editing, supervision, and project administration. ST: preparing the study area maps.

This research received no external funding.

This study did not contain any studies with human participants or animal experiments performed by any of the authors.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.