Hydrogeochemical characteristics of Kodku River of Lalitpur District, Nepal

Rivers are essential for irrigation, industries, and drinking purposes (McGrane 2016), but availability and distribution of freshwater depend on the nation's social, economic, and political aspects (Mir & Jeelani 2015; Ghimire et al. 2022). In several areas, people make use of water without undergoing any evaluation or purification for crucial needs such as drinking and irrigation. In this study region, formal institutions and procedures to evaluate and control water quality are frequently absent. Hence, there is a requirement for thorough and practical water quality assessment techniques that can be implemented effectively in such circumstances.

The ions that are formed in the atmosphere and soil during chemical weathering are transported by rivers. For understanding, Earth's climate development requires estimating chemical weathering rates and identifying the driving forces (Sun et al. 2010). Rivers carry the impact/signature of natural processes and/or anthropogenic activities. Many studies have shown the carbonate and silicate dominance in the river water chemistry in the Canadian Shield including the Bow River, southern Alberta (Grasby & Hutcheon 2000; Millot et al. 2002) and some depict the runoff as the controlling agent (Tipper et al. 2006). In some cases, temperature replaces runoff as a controlling factor (Dessert et al. 2001; Dalai et al. 2002). Erosion and rock–water interaction are also acting as major contributors (Oliva et al. 2003; Hagedorn & Cartwright 2009). Numerous rivers have been studied for the effects of anthropogenic disturbance on hydrogeochemistry (Xu 2004).

The river water quality is more vulnerable to pollution than other water sources caused by anthropogenic activities, such as industrial wastes, runoffs from agricultural land and municipal wastewater (Helena et al. 1999; Simeonov et al. 2003). These human activities directly impact the distribution, quantities, and quality (physical, chemical, and biological) of water resources (WHO 1984). The different studies have demonstrated that with increasing human intervention, anthropogenic disturbances have been detected in the study of water geochemistry (Xu 2004). River water pollution is susceptible to aquatic health as well as public health with the risk of different waterborne diseases (WHO 2011). Similarly, the use of polluted river water directly impacts the plant's health and crop production (Mir & Jeelani 2015). During the last decades, due to rapid population growth and urbanization, the demand for river water is tremendously increased in Kathmandu valley and about 82% of the surface water is daily extracted for drinking water supply (Giri et al. 2022). The encroachment of river banks has increased which results in solid waste and domestic wastewater disposal in the nearby river (Tamrakar et al. 2013).

Different methods are used for the water quality assessment. Some of the indices used for the analysis are the National Sanitation Foundation Water Quality Index (NSFWQI), Canadian Council of Ministries of the Environment Water Quality Index (CCMEWQI), Oregon Water Quality Index (OWQI), and Weighted Arithmetic Water Quality Index Method (WAWQI) (Chaturvedi & Bassin 2010). In terms of others, WAWQI is the selected index for this study as it required fewer water quality parameters for a specific purpose, considered the composite influence of various parameters, and was highly significant for the communication of such information to the stakeholder and policymakers (Balogun et al. 2012). A Piper diagram is considered as the major useful tool to assort the hydrochemical facies to their dominant ions (Amadi et al. 2012). Similarly, the Gibbs diagram is another tool for determining the controlling factor of surface water chemistry (Marandi & Shand 2018). The widely used analysis tools for assessing the water suitability for irrigation sodium ion percentage (Wilcox 1955).

Comprehensive research is needed to evaluate the hydrochemistry of surface water through different techniques which the researchers are strategically adopting (Talib et al. 2019). In order to understand the geochemistry of the water through water quality indices and assessment, the study has been conducted in most developing countries (Dehghanzadeh et al. 2015). In particular, nitrate, fluoride, and arsenic were repeatedly found as the constituent for degrading water quality (Adimalla 2021). So, from the ecological health point of view and to protect the exposed population, especially the stakeholder of Kodku River, the information regarding the hydrogeochemical characteristics of the river is vital.

The Kodku River, selected as the study area, is merging with the Manahara River (a tributary of the Bagmati River) in the southern part of the Kathmandu valley. Different research (Maharjan & Dangol 2007; Tamrakar et al. 2013, 2015; Sapkota & Tamrakar 2016; Thapa & Tamrakar 2016) has been conducted in the tributaries of the Bagmati River. But the study based on the hydrogeochemical characteristics of the Kodku River has not been done yet. The Kodku River water is used for drinking (with treatment), domestic, and agricultural purposes. With the increase in human intervention and pollution-related activities, the river water quality is declining (Xu 2004; Mir & Jeelani 2015). The pollution of the river will have a high impact on the overall urban and rural environment including human and aquatic health. For the determination of drinking and domestic water quality, qualitative analysis of water is done on the basis of its hydrogeochemical characteristics, comparison with the World Health Organization (WHO) standard and Nepal Drinking Water Quality Standard (NDWQS 2015). Different indices were used for irrigation water quality based on the chemical ions. Therefore, the main goal of this study is to evaluate water quality in order to calculate the water quality index (WQI) and determine whether it is suitable for drinking and irrigation. Also, the study attempts to identify the factors controlling the ionic composition and the chemistry of the Kodku River water.

A total of 15 water samples were collected from upstream (6 samples) to downstream (9 samples) of the Kodku River during the winter season (December 2021). For the sake of representativeness and to increase the accuracy of the sampling, composite samples were taken from the different points at each sampling point. The samples were collected using wide-mouth polyethylene bottles of 1 liter, which were rinsed with the same river water twice before the sampling. The pH, electrical conductivity (EC), and total dissolved solids (TDS) of the river water were measured using a multi-probe (HI98129, HANNA Instruments), whereas dissolved oxygen (DO) was measured using a DO meter (LLC-AI314, Apera Instruments) in-situ at each site. All the samples were kept refrigerated at 4 °C before physicochemical analysis. Similarly, samples for cation analysis were acidified (pH-2) with concentrated HNO₃, and samples were transported to the Central Department of Environmental Science, Institute of Science and Technology, Tribhuvan University, Nepal for laboratory analysis.

The chemical parameters such as total hardness (TH), biological oxygen demand (BOD), chemical oxygen demand (COD), major cations (Ca²⁺, Mg²⁺, K⁺, Na⁺, NH₄⁺), and major anions (Cl⁻, ⁠, ⁠, ⁠, ⁠) were analyzed using the standard methods prescribed by APHA (2005). The concentrations of Na⁺ and K⁺ were determined using a Microprocessor Flame Photometer (ESICO MODEL 1382). Similarly, the determination of ⁠, ⁠, ⁠, and NH₄⁺ was carried out by using a Spectrophotometer (SSI UV 2101). Sulfate content was measured by the Gravimetric method using BaCl₂ crystals, whereas the EDTA titration was used for the Ca²⁺, Mg²⁺, ⁠, TH, CaH, and MgH. Additionally, Cl⁻ was determined by the Argentometric method. Similarly, the open-air reflux method and the 5-day BOD test were performed to determine the level of COD and BOD, respectively, whereas total coliform was obtained by incubation at 45 °C (Rahman et al. 2016; Ahmed et al. 2019).

In this study, descriptive statistics were performed to evaluate and interpret the hydrogeochemical variations of the dataset. A Piper diagram (Piper 1944; Gibbs 1970) and a mixed diagram were applied to define the hydrogeochemical features using Origin software (OriginPro 2016).

All concentrations were expressed in meq/L. Besides that, the suitability of river water for irrigation purposes was made through the Wilcox diagram (Wilcox 1955).

Similarly, the calcium hardness and magnesium hardness were observed more in the downstream section with an average of 66.13 and 46.2 mg/L. The cations concentration was found in the order of Ca²⁺ > Mg²⁺ > Na⁺ > K⁺ > NH₄⁺ based on the obtained average concentration. Calcium and magnesium in river water can be derived from both carbonate and silicate weathering. Since carbonate has higher solubility, it produces more Ca²⁺ and Mg²⁺ than silicate under natural conditions (Sun et al. 2010). Therefore, the wide constitution of carbonate in the Kodku River is likely responsible for the observed higher concentrations of Ca²⁺ + Mg²⁺. The concentration of Ca²⁺ with Mg²⁺ jointly contributed 80.99% of the total cationic concentration of the Kodku River. While a low concentration of Na⁺ concentration (with respect to the WHO standard) found in Kodku depicts less silicate weathering (Sun et al. 2010). Similarly, K⁺ concentration with a mean value of 2.63 mg/L was found below the WHO value where a lower concentration could be caused by clay minerals that were immobilized during the weathering of rock and soil (Asare-Donkor et al. 2018). Although the concentration of NH₄⁺ was found between 0.06 and 2.03 mg/L (mean value: 0.72 mg/L) indicating the high anthropogenic activities such as discharge of sewage from industrial emission and agricultural runoff in the downstream section of the river (Pathak et al. 2015).

The anions concentration was found in the order of > Cl⁻ > > > based on the obtained average concentration. was the most abundant anion in the Kodku River accounting for 80.18% of the total anions followed by the Cl⁻ which accounts for 14.64%. The higher concentration of Cl⁻ is determined by the sewage discharge and agricultural runoff (Bhatt et al. 2007; Asare-Donkor et al. 2018). The concentration of ⁠, ⁠, and were found to be much lower than the WHO standard and contributed <5% of the total anions. The low concentration of is somewhat due to natural inputs which were confirmed by the high pH value in river water (Asare-Donkor et al. 2018; Pant et al. 2018, 2021). The results indicated variations in headwater and downstream sections are determined by the high extent of anthropogenic activities which was similar to the previous studies done by Pant et al. (2021), Pal et al. (2019), and Paudyal et al. (2016).

The DO concentration of the Kodku River was found in decreasing order from the headwater region to downstream with an average value of 7.83 mg/L. The concentration of BOD and COD levels (Table 2) was found in increasing order with the average value of 57.66 and 179.33 mg/L, respectively. The higher concentration of BOD and COD in the downstream regions with the increased core urban areas might be due to higher organic wastes and sewerage contamination (Mishra et al. 2017; Baniya et al. 2019; Pant et al. 2021). The biological contamination, i.e., total coliform was found in all the sampling river sites with an average concentration of 121.13 MPN/100 mL, while a higher concentration was found in the downstream section.

Prior to that, it can be deduced from the relationship between TDS and main ions that the majority of water–rock interaction is caused by the dissolution of minerals containing Ca²⁺ and (Li et al. 2018a, 2018b; Ghimire et al. 2023b). These minerals could be calcite, dolomite, or albite, whereas the and Cl⁻ could have formed through the dissolution of gypsum and halite, respectively (Wu et al. 2015; Li et al. 2018a, 2018b). However, the Cl⁻ source may be artificial since halite deposition in the Kathmandu valley has never been documented (Nakamura et al. 2014).

The presence of calcium and magnesium ions may result from the leaching of anhydrites, dolomites, gypsum, and limestone (Garrels 1976). The ratio of (Ca²⁺ + Mg²⁺) to (⁠ + ⁠) is nearly 1:1, just as it is in the aqueous environment where calcite, dolomite, and gypsum dissolutions predominate (Sonkamble et al. 2012; Chung et al. 2014; Wali et al. 2021). Accordingly, dolomite should normally dissolve if the Ca/Mg molar ratio is closer to 1 (i.e., the ratio is 1), calcite contributes more when the ratio is greater, and silicate minerals contribute more when the Ca/Mg molar ratio is higher than 2 (Singh et al. 2011; Ghimire et al. 2023a).

In addition to carbonate weathering, the silicate weathering process is the major contributor to increasing in the river water. Sodium may be liberated from halite and silicate rocks by the primary geochemical process known as silicate weathering (Zhu et al. 2019).

For the identification of the contribution of rock weathering, the ionic reactions of water samples were computed (Table 3). To identify the contribution of rock dissolution and weathering, the ionic ratios of water samples were computed (Table 3; Rajmohan & Elango 2004). In the present study, the molar ratio of /Ca²⁺ was 5.49, confirming the crucial role of carbonate weathering in the hydrochemistry of the river water (Tipper et al. 2006). Similarly, the mean ratios of (Ca²⁺ + Mg²⁺)/(Na⁺ + K⁺) and /(Na⁺ + K⁺) were found to be >1.5 in both study periods, suggesting the presence of minerals like calcite and dolomite in the river (Table 3). As the river water is composed of silicate deposits (Figure 3), the hydrochemistry of most of the river samples was controlled by carbonate weathering (Pant et al. 2021; Ghimire et al. 2023a). Since carbonate has higher solubility, it produces more Ca²⁺ and Mg²⁺ than silicate under natural conditions (Sun et al. 2010) and therefore the wide distribution of carbonate in most river water samples is likely responsible for the observed higher concentrations of Ca²⁺ + Mg²⁺. The major ion composition displayed by the Piper diagram (Figure 3) also suggests that the weathering of carbonate rocks dominates the hydrochemistry of the Kodku River.

Table 3

Ionic ratios of hydrochemical attributes of Kodku River, Nepal

Parameter .	Min .	Max .	Mean .
Ca2+/Na+	2.62	20.25	9.57
Mg2+/Na+	1.97	13.22	5.86
/Na+	3.89	13.22	7.09
/Ca2+	0.52	1.48	0.91
/(Na+ + K+)	3.18	10.22	5.49
(Ca2+ + Mg2+)/(Na+ + K+)	3.75	23.77	11.95
Ca2+/	13.96	50.54	21.48
Na+/Cl−	0.13	1.15	0.53
/(HCO3 + ⁠)	0.89	0.97	0.94
(Ca2+ + Mg2+)/Tz+	0.95	13.44	0.89
(Na+ + K+)/Tz+	0.04	0.21	0.1

Parameter .	Min .	Max .	Mean .
Ca2+/Na+	2.62	20.25	9.57
Mg2+/Na+	1.97	13.22	5.86
/Na+	3.89	13.22	7.09
/Ca2+	0.52	1.48	0.91
/(Na+ + K+)	3.18	10.22	5.49
(Ca2+ + Mg2+)/(Na+ + K+)	3.75	23.77	11.95
Ca2+/	13.96	50.54	21.48
Na+/Cl−	0.13	1.15	0.53
/(HCO3 + ⁠)	0.89	0.97	0.94
(Ca2+ + Mg2+)/Tz+	0.95	13.44	0.89
(Na+ + K+)/Tz+	0.04	0.21	0.1

Note: All ratios derived from meq, Tz⁺: the sum of total cations in meq/L.

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The WQI is a powerful tool that helps to use water quality data and change the guidelines developed by various environmental monitoring agencies (Ansari & Hemke 2013). WAWQI was used to assess river water quality for drinking purposes and other domestic purposes in the study area which is one of the most effective ways to describe the quality of water (Ogunbode & Akinola 2019). The WAWQI method of WQI has been widely used by various scientists (Tyagi et al. 2013; Olowe et al. 2016). Thus, the river water was the main source of drinking water in the headwater and midstream regions of the Kodku River and people directly used to drink river water in the headwater without any purification and filtration.

The chemical and physical characteristics of the river water are the fundamental consideration for the irrigation water quality evaluation. To evaluate the suitability of the river water for irrigation purposes, TDS (as salinity hazard) was used to describe the salts carried in the irrigation water. The Na%, SAR, PI, MAR, and KR were also calculated (Table 5).

In general, water salinity is measured by the EC or TDS (Fipps 2003). The EC value >3,000 is termed ‘Fair’ and impacts on the productivity of crops, whereas the EC value 700–3,000 refers to ‘Good’ and <700 refers to ‘Excellent’ water quality for irrigation purposes and agricultural crops (Ayers & Westcot 1985). All the samples were found to be excellent due to relatively lower EC. The SAR is also considered to be an important parameter for determining the suitability of the river water for agricultural use and can be used to indicate alkali/sodium hazards to crops (Wilcox 1955). The SAR values found to be lower than 10 for all samples showed excellence for irrigation purposes. Here, high Na% may deteriorate the soil structure, causing adverse impacts on crop growth (Bouderbala & Gharbi 2017). In this study, no water samples have higher Na% indicating no sodium hazard problem. In addition, PI values can present the permeability of the soil that is affected by Na⁺, Mg²⁺, Ca²⁺, and ⁠, and the soil type (Doneen 1964). Based on the classification, all the samples were found to be good for irrigation with class II of all water samples. Similarly, a higher amount of magnesium in water makes it more alkaline, which has an adverse impact on crop yields (Doneen 1954). Based on the MAR classification, the majority of the river water samples were suitable for having a lower amount of magnesium. Similarly, KR is an important parameter used in the evaluation of water quality for irrigation (Kelley 1963) where all the river water samples were found to be safe for irrigation due to a KR value of less than 1.

The Wilcox diagram is also known as the US Salinity diagram (Wilcox 1955) and was commonly used for the evaluation of irrigation water quality (Alavi et al. 2016; Table 6).

The assessment of the hydrogeochemical characteristics of the Kodku River revealed that the water was slightly alkaline toward the downstream. The sequence of the cation's concentration was found in the order: Ca²⁺ > Mg²⁺ > Na⁺ > K⁺ > NH₄⁺ and anion abundance of major ions was found to be in the order: > Cl⁻ > > > ⁠. The low DO with high BOD and COD section revealed the high anthropogenic activities in the downstream of Kodku River while total coliform was found in all the water samples. The WQI value of S1 (33.39), S2 (24.08), and S3 (25.83) samples was found to be good for drinking only by using some preservatives. The estimated Na%, SAR, PI, MAR, KR, and EC values showed a good category for the irrigation purposes and good for crops. Similarly, water samples falling under the C1S1 category of the Wilcox diagram also revealed that samples were effective for agricultural purposes.

The hydrogeochemical characteristics revealed that carbonate weathering dominates the water chemistry of the Kodku River with the high dominance of Ca–HCO₃ type. The Gibbs diagram revealed that samples fall under the rock weathering dominance. Similarly, the mixing diagram displayed that weathering of carbonate rocks dominates the hydrochemistry of the Kodku River. The study comes up with the insight on the hydrogeochemical properties of the river and the assessment data can be utilized to assess the suitability of the water for irrigation and drinking purposes. Although the assessment is not comprehensive over time and lacks significant information on physical and bacteriological constituents, it contributes an approach for countries at similar water resources management levels and lacking formal organizational structures, methods, and criteria to evaluate water resources for the benefit of their population.

We are grateful to the Central Department of Environmental Science, Tribhuvan University for the provision of laboratory facilities.

M.G. planned, designed the work, and collected and analyzed the river water samples. T.R. actively participated in fieldwork and chemical analysis of samples and assists in preparation of the manuscript.

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

The authors declare there is no conflict.