Health and ecological risk assessment of metals in surface water from the Himalayan tributaries of the Ganga river, India

Abstract

This study investigates the trace element concentrations in the surface waters of four north-joining Himalayan tributaries of the Ganga river (Ramganga, Ghaghara, Gandak, and Kosi), highlighting the combined effects of geogenic processes and anthropogenic activities on trace element chemistry and water quality. A knowledge gap exists in understanding the sources of trace elements in these tributaries and the contribution of trace elements from these tributaries to the Gangariver. The novelty of the study lies in its assessment of sources, human health risks, and ecological impacts. The investigation was conducted by assessing trace element concentrations and comparing them with national and international standards. Various human health and ecological risk indicators, including the Heavy Metal Pollution Index (HPI), Hazard Quotient (HQ), Health Index (HI), Chronic Daily Intake (CDI), and the Potential Ecological Risk Index (PERI), were applied. The results reveal high concentrations of copper (Cu), zinc (Zn) and lead (Pb) in the Ramganga, indicating contamination from industrial activities in the catchment. Although most trace element concentrations are within safe limits, Pb concentration in the Ramganga exceeds the limit prescribed by WHO. The Ramganga shows the highest health risks, with a HI_total of 1.876 for adults and 1.616 for children. In contrast, the Ghaghara, Gandak, and Kosi exhibit lower but moderate contamination levels. HPI values for these rivers- 93.74 for the Ghaghara, 83.95 for the Kosi, 83.13 for the Gandak, and 80.43 for the Ramganga—indicate that although contamination is below critical thresholds, targeted mitigation strategies are needed. The findings provide valuable insights into trace metal sources and their implications for human health and ecological risks, and emphasize the need for frequent monitoring and pollution control measures for maintaining sustainable water quality in these tributaries.

Introduction

Rivers are crucial agents of continental erosion, playing an important role in transporting weathered products from land to oceans. Around 90% of the materials from continents are carried to the oceans via river systems [1,2,3]. During this transport, rivers also act as conduits for various elements derived from weathering processes. Over the past few decades, geochemical studies of river waters have become increasingly important for understanding the processes that control the chemistry of water. These studies are essential for assessing water chemistry, solute fluxes, determining the weathering and erosion rates in the catchment, and evaluating broader impacts on the earth’s climate and biogeochemical cycling [4,5,6,7,8,9,10,11,12,13,14].

River water chemistry helps to understand natural geochemical cycles, with major ions and trace elements derived from rock weathering, supplemented by minor contributions from atmospheric deposition, saline/alkaline soils, and sea salts [15]. However, recent research indicates that anthropogenic activities, including industrialization and urbanization, significantly influence river chemistry, mainly through the input of heavy metals and other emerging water pollutants [16,17,18]. These anthropogenic contaminants alter the chemical composition of river waters, raising concerns regarding health and environmental risks for human populations and aquatic ecosystems.

Despite significant research on river water chemistry, knowledge gaps remain in understanding the sources of trace elements in the Himalayan tributaries of the Ganga river and their eventual contribution to its trace element load. The novelty of this study lies in its comprehensive assessment of contamination sources, human health risks, and ecological impacts, which have been inadequately addressed in previous studies. The study employs hydrogeochemical assessment of trace elements along with health and ecological risk assessments. The application of various risk indicators, including the Heavy Metal Pollution Index (HPI), Hazard Quotient (HQ), Health Index (HI), Chronic Daily Intake (CDI), and the Potential Ecological Risk Index (PERI), provides a holistic evaluation of the risks posed by potentially toxic metals in surface water [19, 20].

The methods used in this study, including trace element analysis and risk assessment indices, are particularly relevant for evaluating water quality and contamination sources in regions experiencing rapid industrialization and urbanization. These methods have been effectively applied in similar studies globally, providing a robust framework for assessing human health and ecological risks associated with trace element contamination [19,20,21,22]. The global relevance of water quality issues necessitates a comprehensive understanding of contamination sources and their implications. Therefore, this study not only contributes to regional knowledge but also provides insights applicable to other river systems worldwide.

It is noteworthy that the degradation of river water quality by high levels of trace metals poses risks to organisms through various exposure routes, such as ingestion, dermal contact, or usage for irrigation purposes [19]. These metals can have long-term health and ecological risks due to their non-biodegradable and accumulative properties. Understanding these risks is crucial for developing effective pollution control measures and ensuring sustainable water quality. This study investigates the trace element hydrogeochemistry of the Himalayan tributaries of the Ganga River, with a focus on health and ecological risk assessments.

Study area

All the tributaries (Ramganga, Ghaghara, Gandak and Kosi) in this study are fed by glacial meltwater and primarily flow through the states of Uttar Pradesh and Bihar in India. After originating in the Himalayas, these tributaries traverse various lithological units, primarily consisting of Quaternary alluvium. The Ramganga river flows through the Kumaun Himalayas and traverses the Sub-Himalaya (SH) and Lesser Himalaya (LH) [23]. LH is characterized by meta-sedimentary rocks [24], while SH consists of clastic sediments eroded from the uplifted Himalayas. After traversing 158 km, the Ramganga enters the plains at Kalagarh. In contrast, the Ghaghara, Gandak, and Kosi originate in the Tethyan Sedimentary Series (TSS), which comprises clastic sedimentary rocks and carbonates [25]. These tributaries further traverse various lithologies such as the Higher Himalayan Crystalline (HHC), LH, SH, and finally, the alluvium. The hydrogeological characteristics of these Himalayan tributaries have been documented [26], with details provided in Table 1. The geological map of the study area is shown in Fig. 1. In addition to geogenic influences, these tributaries, particularly the Ramganga, are impacted by anthropogenic activities in their catchment. The Ramganga receives significant industrial discharge while flowing through Moradabad, a major centre for brassware industries.

In general, the Ganga plains exhibit considerable climatic variation, with annual rainfall ranging from 60 cm in the west to over 160 cm in the east. Temperatures fluctuate between 5°–25 °C in winter and 20°–40 °C in summer. The region features two distinct climatic zones: Humid Subtropical Climate with Dry Winter, characterized by monsoon-driven rainfall and hot summers, and Mountain Climate, predominant in the upper reaches with lower temperatures and higher precipitation [27]. The southwest monsoon, arriving in late May or early June, plays a crucial role in sustaining river flow.

The Ganga Basin features diverse soil types shaped by variations in climate, geology, and physiography. The sampling locations on the studied tributaries lie within the floodplain of the Ganga River, predominantly consisting of alluvial soils.

Fig. 1

Geological map of the study area showing the Himalayan tributaries that join the Ganga river from the north after Azam et al. [14]

Table 1 Hydrogeological information of the Himalayan tributaries of the Ganga river

Methodology

Surface water samples were collected from the Himalayan tributaries of the Ganga River at locations upstream of their confluence with the main river. Specific sampling sites include Farrukhabad and Moradabad on the Ramganga, Manjhi near Chhapra on the Ghaghara, Hajipur on the Gandak, and Kursela on the Kosi. Sampling sites were recorded using Garmin GPSMAP-76CSX. Sampling was done across three seasons i.e. monsoon, post-monsoon, and pre-monsoon during August 2013, November 2013 and March 2014, respectively. At each locations, water samples were collected from the mid-channel of the rivers using pre-cleaned 1-liter polyethylene bottles. For the analysis of dissolved trace metals, 100 mL of each sample was filtered through a 0.45 μm syringe filter (Millipore) and acidified with HNO₃ (supra-pure). Samples were analyzed for sixteen trace elements (Ag, Al, As, Ba, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Se, Sr, V, and Zn) using ICP-MS (Perkin Elmer ELAN DRCe). Calibration and assessment of analytical precision were carried out using the multi-element standard (Merck CertiPUR). In our earlier study, the trace element data were utilized to calculate dissolved fluxes [14]. Based on the trace element dataset, the present study applies various risk indices, including Heavy Metal Pollution Index (HPI), Chronic Daily Intake (CDI), Hazard Quotient (HQ), Health Index (HI) and Potential Ecological Risk Index (PERI) to assess the potential human health and ecological risks associated with metal contamination in the surface waters of the Himalayan tributaries. These indices provide a quantitative means to assess the pollution status and their possible health and ecological impacts.

Heavy metal pollution index (HPI)

HPI is a tool for evaluating the cumulative impact of heavy metal contamination in surface water. The index developed by Mohan et al. [28] takes into account the levels of various heavy metals, evaluates each metal based on its toxicity and established regulatory limits, and provides an aggregated pollution measure for assessing water quality [29]. HPI is calculated using the following equation given by Mohan et al. [28]:

$$\:HPI=\frac{{\sum\:}_{i=1}^{n}WiQi}{{\sum\:}_{i=1}^{n}Wi}$$

Here, Wi ​represents the weight allocated to each heavy metal, calculated as inversely proportional to its permissible limit. Qi​ denotes the sub-index for each heavy metal, reflecting its contribution to the overall pollution level, while n refers to the total number of heavy metal under analysis. The sub-index for each heavy metal is calculated using the given formula:

$$\:Qi={\sum\:}_{i=1}^{n}\frac{|Ci-Ii|}{Si-Ii}\:\times\:100$$

Here, Ci​ denotes the measured concentration of each heavy metal in the water [34] with the average concentrations (μg/L) across three seasons used for Ci​. Ii represent the ideal concentration or background level of the i-th metal, whereas Si specifies the standard limit for the i-th heavy metal. According to the Bureau of Indian Standards (BIS 2012) drinking water quality guidelines [30], the maximum desirable level was considered as the ideal value (Ii), while the maximum permissible limit, applicable when no alternative source is available, was used as the standard value (Si). The modulus sign ensures that only magnitude is being considered, regardless of the algebraic sign for calculation. The sub-index scales the pollution level for each metal, ensuring that its contribution is normalized with respect to its permissible limit.

The unit weight (Wi) indicates the relative significance of each heavy metal and is determined as inversely proportional to Si [31, 32]. Heavy metals with stricter regulatory standards indicating higher toxicity are given greater significance in the overall index. Wherever there was no relaxation for permissible limits, an acceptable limit was used for the calculation of Wi [19].

$$\:Wi=\:\frac{K}{Si}$$

K serves as the constant for proportionality, and its value is set to 1. HPI value of 100 serves as the critical threshold, values below 100 indicates low heavy metal contamination, whereas values exceeding 100 signify that the water is not suitable for consumption [28, 32]. However, this study uses a revised classification method developed by [33]. The modified scale divides HPI values into three categories, values less than 15 are classified as low, values between 15 and 30 as medium, and values greater than 30 as high.

Potential ecological risk index (PERI)

PERI introduced by Hakanson [34], serves as a method to evaluate the ecological risks resulting from heavy metal contamination. Calculation of PERI is a multi-step process and involves the determination of contamination factor (C_f), comprehensive contamination factor (C_d), potential ecological risk (E_r) of individual metal, and overall potential ecological risk index (RI). The C_f assesses the pollution level of a particular heavy metal by comparing its observed concentration in the sample (C_i​) to a baseline or reference concentration (C_b). This reference concentration can be derived from natural background levels, geochemical averages, or water quality standards given by WHO, US EPA, or BIS. For this study, the acceptable limit of BIS 10500:2012 is taken for C_b.

$$\:{C}_{f}=\frac{{C}_{i}}{{C}_{b}}$$

The Comprehensive Contamination Factor (C_d) provides an overall measure of cumulative contamination by considering all metals in the study. When C_d is below 8, it is categorized as low; values between 8 and 16 fall under the moderate category; a C_d value ranging from 16 to 32 is considered high; and values greater than or equal to 32 indicate a very high level of contamination [35].

$$\:{C}_{d}=\sum\:Cf$$

Potential Ecological Risk of an individual metal (E_r) evaluates the risk posed by each metal by integrating its contamination factor (C_f) with its toxicity response coefficient (T_r​), representing the toxicity and environmental sensitivity. T_r value of Mn = Zn = 1, Cr = 2, Cu = Pb = Ni = 5, As = 10, Cd = 30 [34, 36,37,38]. The potential ecological risk (E_r) is classified into several categories based on its value. A value below 40 indicates low risk, whereas values ranging from 40 to 80 indicate moderate risk. Values within the range of 80 to 160 reflect a considerable risk. A value between 160 and 320 denotes a high ecological risk, and any value exceeding 320 indicates a very high risk.

$$\:{E}_{r}=Tr\times\:Cf$$

The RI quantifies the cumulative ecological risk posed by all metals analysed in the study and is determined by the summation of the E_r ​values for all metals.

$$\:RI=\sum\:Er$$

The calculated RI values are interpreted using established thresholds [34] to classify the ecological risk level as low (RI less than 50); moderate (RI between 50 and 100); considerable (RI from 100 to 200) and very high (RI greater than or equal to 200).

Human health risk assessment

Exposure to heavy metal primarily happens through three pathways: ingestion, inhalation and dermal absorption [19, 39–40]. These toxic metals can accumulate in the body through these routes, posing serious health risks to human. This research evaluated the potential health risks for both adults and children who might be exposed to these metals through ingestion and dermal contact, focusing on eleven elements: Mn, Cu, Pb, Zn, Ni, Co, As, Se, Cd, Cr, and Fe.

The health risks linked to potentially toxic metals were evaluated using the widely adopted US EPA method for carcinogenic as well as non-carcinogenic effect. This multi-step framework begins with the calculation of chronic daily intake (CDI) for both ingestion and dermal contact pathways. CDI is used to calculate hazard quotient (HQ) which is followed by the calculation of hazard index (HI) and the carcinogenic risk (CR). The calculations for the exposure pathways were based on equations adapted from US EPA guidelines [41].

$$\:{CDI}_{\text{i}\text{n}\text{g}-\text{n}\text{c}}=\:\frac{[C\:\times\:IR\:\times\:\:EF\:\times\:\:ED]}{[BW\times\:{AT}_{nc}]}$$

$$\:{CDI}_{\text{d}\text{e}\text{r}-\text{n}\text{c}}=\:\frac{\left[C\:\times\:\:SA\:\times\:\:{K}_{p}\times\:\:ET\:\times\:\:EF\:\times\:ED\right]\times\:{10}^{-3}}{[BW\times\:{AT}_{nc}]}$$

Here, the chronic daily intake for non-carcinogenic risks through ingestion (CDI_ing−nc) and dermal exposure (CDI_der−nc) is expressed in μg/kg/day. C is contaminant concentration in water (μg/L). The ingestion rate (IR) is the daily water consumption, set at 4.05 L/day and 1 L/day for adults and children, respectively [42]. Exposure frequency (EF) is taken as 350 days/year to factor in the travel or absence from the contaminated area [41]. Exposure duration (ED) is 6 years and 30 years for children and adults, respectively [41]. Body weight (BW) is 15 kg and 52 kg for children and adults, respectively [41,42,43,44]. Average time for non-carcinogenic effects (AT_nc) is measured in days by multiplying ED by 365 for child and adult [41]. Skin surface area exposed to water is 18,000 cm² for adults [41]. The dermal permeability coefficients (Kp) for various elements, measured in cm/h, are as follows: Pb = 0.004, As = Cr = 0.002, Mn = Cu = Cd = Fe = Ba = 0.001, Zn = 0.0006, Sr = 0.0005, Al = 0.0004, Ni = Co = Se = V = 0.0002 [19, 41]. Exposure time per day (ET) is 0.6 h/day [41].

The Hazard Quotient (HQ) and Hazard Index (HI) are key indicators used to assess non-carcinogenic health risks from metal exposure. These parameters help determine if the exposure levels of certain metals are within safe limits or could pose a threat to human health. HQ is the ratio of actual exposure to a reference dose, assessing the health risk from individual metal exposure through ingestion or dermal contact. It is computed using the following equation:

$$\:{HQ}_{ing/der}=\frac{{CDI}_{ing/der}}{{RfD}_{ing/der}}$$

Where, the reference dose (RfD_ing/der) represents the maximum safe dose of a substance through ingestion or dermal contact, below which adverse health effects are unlikely. These values are adjusted by incorporating the gastrointestinal absorption factor (GI_ABS) into the standard reference dose for each exposure pathway. Each metal has distinct RfD values for ingestion and dermal exposure. RfD_ing values for various elements are as follows: Mn (24), Cu (40), Pb (1.4), Zn (300), Ni (20), Co (20), As (0.3), Se (5), Cd (0.5), Cr (3) and Fe (700) [41]. RfD_der values for various elements are as follows: Mn (0.96), Cu (8), Pb (0.42), Zn (60), Ni (0.002), Co (20), As (0.3), Se (8), Cd (0.5), Cr (0.08), Ba (2000), V (100), and Fe (140) [19, 41].

Hazard index (HI)

HI reflects the overall risk from exposure to multiple metals. It is obtained by adding the HQ values of all the metals, as represented in the following equation:

$$\:{HI}_{ing/der}=\sum\:_{i=1}^{n}{HQ}_{ing/der}$$

$$\:{HI}_{Total}=\:{HI}_{ing}+\:{HI}_{der}$$

Here, n represents the total number of metals studied and HI_Total is a cumulative metric used to evaluate the overall non-carcinogenic health hazards from combined exposure to multiple metals through each exposure pathway. It integrates the risks from both pathways to provide a holistic evaluation of possible health impacts.

Carcinogenic risk (CR) is the likelihood of a person developing cancer over a lifetime as a result of prolonged exposure to carcinogenic substances, such as heavy metals or other environmental contaminants. This metric helps evaluate the potential health effects of low-dose, long-term exposure to harmful agents. Cr is known human carcinogenic substance (group 1) and Pb is probably carcinogenic to human (group 2A) [45]. Calculations for the exposure pathways were also based on equations adapted from USEPA guidelines [41].

$$\:{CDI}_{\text{i}\text{n}\text{g}-\text{c}\text{a}}=\:\frac{[C\:\times\:IR\:\times\:\:EF\:\times\:\:ED]}{[BW\times\:{AT}_{ca}]}$$

$$\:{CDI}_{\text{d}\text{e}\text{r}-\text{c}\text{a}}=\:\frac{\left[C\:\times\:\:SA\:\times\:\:{K}_{p}\times\:\:ET\:\times\:\:EF\:\times\:ED\right]\times\:{10}^{-3}}{[BW\times\:{AT}_{ca}]}$$

Here, CDI_ing−ca and CDI_der−ca represents the chronic daily intake of carcinogenic metals through ingestion and dermal contact, respectively, over a 70-year period (measured in μg/kg/day) over the same duration, simulating a lifetime of exposure. AT_ca. is average time of exposure to carcinogenic metals and calculated by multiplying the exposure duration (ED) of adult and child with 70 and 365 [41].

$$\:{CR}_{ing}=\:{CDI}_{\text{i}\text{n}\text{g}-\text{c}\text{a}}\:\times\:\:{SF}_{\text{i}\text{n}\text{g}}\:$$

$$\:{CR}_{der}=\:\frac{{CDI}_{\text{d}\text{e}\text{r}-\text{c}\text{a}}\:\times\:\:{SF}_{\text{d}\text{e}\text{r}}}{{GI}_{\text{A}\text{B}\text{S}}}$$

$$\:{CR}_{Total}=\:{CR}_{ing}+\:{CR}_{der}$$

The slope factors (SF_ing and SF_der) quantify the cancer risk from ingestion and dermal contact with carcinogenic metals, expressed in (μg/kg/day)⁻¹, indicating the likelihood of cancer development per unit of lifetime metal exposure. SF of Cr is 500 and Pb is 8.5 [41]. Gastrointestinal absorption factor (GI_ABS) is dimensionless quantity and represents the proportion of a metal that is absorbed via the gastrointestinal tract. GI_ABS of Cr is 0.025 and Pb is 1 [41]. Carcinogenic risk (CR_ing and CDI_der) represents the estimated cancer risk resulting from the ingestion or dermal contact of a carcinogenic metal over a lifetime. The total cancer risk (CR_Total) is the combined risk from both exposure routes.

Results and discussion

Trace elements hydrogeochemistry

Trace element concentrations were analyzed in surface waters of the four tributaries. Table 2 shows the trace element concentrations of the tributaries across three different seasons. According to the average concentrations for the three seasons (Table 3), all four tributaries had concentrations of Al, Ba, Fe, and Sr exceeding 10 μg/l. The concentrations of As, Cr, Cu, Mn, Ni, V, and Zn ranged between 10 and 1 μg/l. The concentrations of Co, Se, and Cd were below 1 μg/l (Fig. 2). It was observed that concentrations of Mn, Cu, and Zn in the Ramganga were found to exceed 10 μg/l, while the concentration of V in the Gandak was less than 1.0 μg/l. Except for Al, which was more prevalent in the Gandak, the Ramganga generally displayed higher concentrations of all trace elements than other tributaries. The Gandak and Ghaghara had high levels of Sr, Al, and Fe. The elevated concentrations of trace elements in the surface water of Ramganga are primarily attributed to anthropogenic inputs, particularly the discharge of effluents from brassware industries situated in the Moradabad region, a prominent urban center along the Ramganga riverbank.

Table 2 Analytical results of trace elements in surface waters of the Himalayan tributaries of the Ganga river

Table 3 Three seasons’ average trace element concentrations in surface waters of the Himalayan tributaries of the Ganga river

Fig. 2

Bar graph showing trace element concentration (in ppb) in surface water of the studied tributaries

The trace elements present in surface waters present insights into the hydrogeochemical characteristics of the studied tributaries. These concentrations (Table 2) highlight the role of geogenic processes as well as human-induced activities on water quality. Dilution and seasonal variability also play an important role. The high levels of Fe (635.03 ppb in the Ramganga and 498.63 ppb in the Gandak) and Mn (18.97 ppb in the Ramganga) suggest significant contributions not only from weathering of iron- and manganese-bearing minerals in the Himalayan catchment but also from the brassware industries in Moradabad, Uttar Pradesh. Elements like Sr indicate inputs from geological formations common in the Himalayas. The presence of Al (ranging from 48.17 ppb in the Ghaghara to 84.63 ppb in the Gandak) suggests leaching from aluminosilicate rocks under natural weathering conditions.

Several studies have examined groundwater chemistry in the catchments of the Ramganga, Ghaghara, Gandak, and Kosi rivers, highlighting the interactions between surface water and groundwater. Tiwari et al. (2016) have studied the groundwater quality in the Ramganga Basin, highlighting issues related to As contamination [47]. Similarly, Dwivedi et al. (2023) conducted a comprehensive review on arsenic contamination in the Ghaghara Basin, emphasizing the health risks associated with elevated As levels in groundwater [48]. In the Gandak Basin, groundwater recharge is mainly from local precipitation, with contamination from As, Fe, Mn, and U in certain areas [49]. The Kosi Basin shows strong surface water–groundwater interaction, influencing groundwater chemistry [50].

Elevated levels of Cu (25.47 ppb) and Zn (219.45 ppb) in the Ramganga point to brassware industries in Moradabad city on the banks of the Ramganga river. The high concentration of Pb (13.50 ppb) in the Ramganga is likely from metal processing or battery recycling units. Children and foetuses are the most at risk and exposure to lead can cause severe and permanent brain damage [46]. Elevated Fe concentrations in all tributaries indicate geogenic contribution in the catchment and their potential release under reducing conditions. Excessive Fe exposure can have detrimental effects on the nervous system and kidneys [46]. Pollutants from agricultural activities (fertilizers and pesticides), domestic sewage, industrial effluents, and urban runoff are also common in the catchment. Overall, the elevated concentrations of Fe, Cu, Mn, Pb, and Zn in the Ramganga indicate substantial anthropogenic influence. Moderate levels of most trace elements in Ghaghara and Gandak indicate a mix of geogenic inputs and anthropogenic disturbance. The lowest concentrations for most elements in the Kosi suggest minimal anthropogenic inputs in the river. Boral et al. (2020) have studied dissolved trace and heavy metals in the Ganga river from source to sink. The study suggests that the upstream portion of the Ganga river acquires its geochemical composition predominantly from the upstream tributaries [51]. After the Ganga river enters the floodplain, trace element concentrations continue to rise, peaking in the middle section of the downstream stretch before being diluted by the confluence of major tributaries such as the Ghaghara, Son, Gandak, and Kosi [52].

The concentrations of several trace elements show distinct differences across the tributaries. The Ramganga shows an exceptionally high Mn, Cu, Zn level, while the other tributaries exhibit much lower levels. For example, the Ramganga has the highest Mn concentration (18.97 ppb), followed by the Gandak (5.80 ppb), the Ghaghara (5.40 ppb), and the Kosi (4.10 ppb). Similarly, Ramganga has the highest Zn concentration (219.45), followed by the Gandak (3.85), the Kosi (3.75) and the Ghaghara (2.20). Cu concentration is also highest in the Ramganga (25.47 ppb), followed by the Ghaghara (2.63), the Gandak (2.43) and the Kosi (1.80). According to a study, the Ramganga, Yamuna and Gomti have a high load of dissolved trace elements and concentrations decrease when the tributaries like the Ghaghara, Son, Gandak, and Kosi join the Ganga river [52].

Trace element concentrations (Table 3) were compared with the prescribed limits for surface waters established by WHO [53], EPA [54], and BIS [30] (Table 4). Concentrations of Ni range from 1.93 ppb in the Kosi to 9.83 ppb in the Ramganga. As levels are relatively consistent across tributaries, with values ranging from 2.10 (Gandak) to 3.00 (Ramganga), below the prescribed limits by WHO and BIS. Similarly, Cobalt (Co) levels are also relatively consistent across tributaries, with value ranging from 0.10 (Kosi), 0.17 (Ghaghara), 0.20 (Gandak) and 0.30 (Ramganga). Se levels are low, with a maximum of 0.77 ppb (Ramganga), within the BIS limit of 10 ppb. Cd is below detection limits in most tributaries, indicating minimal risk.

Except for Al in the Ramganga (53.23), Ghaghara (48.17), Gandak (84.63) and Kosi (79.33), Fe in the Ramganga (635.03), Ghaghara (481.17), Gandak (498.63) and Kosi (314.17) and Pb in the Ramganga (13.50), the concentrations of all metals in all four tributaries were found to be well below the permissible limits set by BIS (2012) for drinking water. Pb concentration is significant only in the Ramganga, exceeding the WHO and BIS limit indicating potential contamination in this tributary. Despite significant variation in heavy metal concentrations across the tributaries, all tributaries remain within the safe levels for the studied trace elements when compared to the prescribed limits.

Table 4 Prescribed specification of the World Health Organization (WHO), EPA and Indian standard (BIS 2012) for trace elements

Heavy metal pollution index (HPI)

HPI was calculated for Al, As, Ba, Cd, Cr, Cu, Fe, Mn, Ni, Pb, Se, and Zn for which established drinking water quality guidelines are provided by BIS [30]. HPI values calculated for the Ramganga, Ghaghara, Gandak, and Kosi reveal concerning levels of heavy metal contamination. Among the tributaries, the Ghaghara exhibits the highest HPI value of 93.74, indicating significant pollution, followed closely by the Kosi (83.95), Gandak (83.13), and Ramganga (80.43) as given in Table 5. These HPI values, while below the critical threshold of 100 [28, 32], but signal a concerning trend of deteriorating water quality falling into the high pollution category (HPI > 30) according to the revised classification system [33]. The presence of metals such as Al, Fe and Pb at concentrations exceeding desirable levels is a primary factor contributing to the elevated indices. These contaminants predominantly originate from anthropogenic activities, including agricultural runoff, industrial discharges, and untreated sewage, all of which significantly contribute to water pollution. The higher HPI of the Ghaghara indicates that it may be more heavily impacted by anthropogenic activities particularly agricultural runoff. Tributaries with elevated levels of heavy metals require efforts to reduce their discharge to safeguard aquatic ecosystems and ensure the suitability of water quality for ecological and human needs.

Table 5 HPI of surface water samples in the studied tributaries

Potential ecological risk index (PERI)

PERI is a key indicator used for the evaluation of ecological risks posed by heavy metal contamination. Developed by Hakanson [34–35], it quantifies the ecological risk posed by heavy metals in sediments. This method has since been widely adapted to other ecosystems, including aquatic environments [19, 55,56,57,58]. It combines the concentration of metals with their toxic response factors to provide a single risk value, which helps in assessing the ecological impact of metal contamination in water bodies.

The contamination levels in the studied tributaries (Ramganga, Ghaghara, Gandak, and Kosi) reflect generally low contamination, as suggested by both Comprehensive Contamination Factor (Cd) and Risk Index (RI). The Ramganga shows the highest Cd value (2.99), signaling a low contamination status. However, potential risk of individual heavy metals (Er) for Cu and Pb stand out in this river, with Cu reaching 2.55, Pb at 6.75, Ni at 2.46 all of which may suggest localized pollution sources. The other tributaries, Ghaghara, Gandak, and Kosi, show Cd values of 0.62, 0.54, and 0.44, respectively, all indicating a low contamination status (Table 6). Er for the Ghaghara has relatively low values of metals, with Mn at 5.40E-02, Cu at 2.63E-01, and Pb at 1.00E-01, suggesting minimal pollution influence. Similarly, the Gandak and Kosi rivers show lower metal concentrations, with no significant spikes in any individual metal, which suggests cleaner water quality in these tributaries.

In terms of Risk Index (RI), the values for all four rivers remain low, with the highest being 15.39 for the Ramganga and the lowest at 3.00 for the Kosi. RI values reflect a low ecological risk, confirming that contamination, while present, does not pose significant long-term threats to aquatic life in these tributaries. However, Cu, Pb, and Ni concentrations in the Ramganga remain of high concern, as these metals tend to have higher toxicity at certain concentrations, potentially affecting local ecosystems. Mn, Zn, Cd, and Cr are found in lesser amounts, with negligible impact on the overall contamination status. Overall, the low Cd and RI values across the tributaries indicate that at these levels it does not pose a severe environmental risk at present but regular monitoring is needed particularly for metals like Cu, Pb, and Ni to assess their cumulative environmental impacts over time.

Table 6 Average degree of contamination based on Cd, potential risk based on Er and RI and the corresponding evaluation standards for potential ecological risk

Health risk assessment

This research assesses both carcinogenic and non-carcinogenic health risks linked to trace element exposure in the surface waters of the four tributaries. Table 7 presents a detailed summary of HQ and HI values for oral and dermal exposure routes in adults and children throughout the tributaries. HQ is employed to evaluate risks associated with metal contamination in water, both from ingestion and dermal exposure. The HQ_ing−nc values for water range from 3.73E-04 to 7.47E-01 for adults and from 3.20E-04 to 6.39E-01 for children. Similarly, HQ_der−nc values range from 2.85E-09 to 2.80E-03 for adults and from 9.86E-09 to 9.70E-03 for children. If HQ < 1, the risk of non-carcinogenic health effects from metal exposure is considered negligible, while HQ ≥ 1 suggests a potential for adverse non-carcinogenic effects, indicating the need for further investigation. Notably, the average HQ values for ingestion (HQ_ing−nc) and dermal (HQ_der−nc) across all metals in the studied tributaries were < 1 for both adults and children, suggesting minimal risk from heavy metal exposure.

The HI provides a comprehensive picture of overall health risks. The Ingestion Health Index (HI_ing) and Dermal Health Index (HI_der) represent the total health risk from ingestion and dermal absorption of metals, respectively. HI_ing ranges from 0.667 to 1.873 in adults and from 0.571 to 1.603 in children. Similarly, HI_der ranges from 0.001 to 0.004 in adults and 0.003 to 0.013 in children. HI value of ≤ 1 indicates no significant non-carcinogenic health risks from combined exposure to all metals, while an HI > 1 suggests a potential risk, requiring further investigation and possible remediation efforts. Only HI_ing of Ramganga is greater than 1 for both adults as well as children. All other tributaries have HI less than 1 for both the pathways. Total Health Index (HI_Total) aggregates these two indices to give a complete assessment of non-carcinogenic health risks from metal pollution. HI_Total values for Ramganga are 1.876 for adults and 1.616 for children, highlighting a high overall health risk, primarily driven by ingestion pathways. All other tributaries have HI_Total less than 1. Ghaghara and Gandak exhibited moderate health risks, with HI_Total values of 0.766 and 0.670, respectively. The Kosi had the lowest health risks (HI_Total = 0.668 for adults and 0.573 for children), but still showed moderate levels of contamination. As and Mn is biggest contributor in HI for all the tributaries while Pb and Cr are present mainly in Ramganga leading to higher HI_Total.

In the Ramganga, the HQ_ing−nc values for metals such as manganese (Mn) and arsenic (As) are 0.0590 and 0.747 for adults, and 0.0505 and 0.639 for children, respectively. However, the HQ_der−nc values are relatively low (e.g., Mn: 0.0000562 for adults, 0.000195 for children), indicating minimal risk from dermal exposure. For the Ghaghara River, the HQ_ing−nc values are lower compared to the Ramganga, with Mn at 0.0168 for adults and 0.0144 for children, and As at 0.556 for adults and 0.476 for children. The HQ_der values remain low, similar to Ramganga, indicating minimal risks. The HI_Total for Ghaghara is 0.766 for adults and 0.659 for children, indicating a moderate health risk from water contamination. In the Gandak river, the HQ_ing−nc values are further reduced, with Mn at 0.0181 for adults and 0.0155 for children, and As at 0.523 for adults and 0.447 for children. These values suggest lowerhealth risks from ingestion. The HI_Total for Gandak is 0.670 for adults and 0.573 for children, indicating moderate values, but one that is lower than that in the Ramganga and the Ghaghara. The Kosi exhibits the lowest HQ_ing−nc values for both Mn (0.0128 for adults, 0.0109 for children) and As (0.548 for adults, 0.469 for children), resulting in the lowest health risk among the tributaries. The HI_Total for Kosi is 0.668 for adults and 0.573 for children, suggesting a moderate health risk, albeit lower than that in the other tributaries. Thus, the data reveal that among the studied tributaries, the Ramganga and the Ghaghara pose the highest health risks, primarily from ingestion of metalss. While the Gandak and the Kosi show lower risks, they still exhibit moderate levels of metal contamination. These findings emphasize the necessity of targeted interventions to reduce contamination, particularly in high-risk tributaries such as the Ramganga and the Ghaghara, to protect both public health and the environment.

Carcinogenic Risk (CR) values for surface water from studied tributaries reveal varying levels of potential cancer risk, ranging from 2.7E-01 to 5.1E + 02 for adults and from 8.7E-01 to 4.4E + 02 for children. An acceptable range for carcinogenic risk is between 10^− 6 and 10^− 4 [59], indicating that risks within this range are considered tolerable, while values exceeding this threshold are deemed unacceptable and pose a significant health threat. All these tributaries have CR value greater than the threshold limit and indicate a severe carcinogenic threat in both adults as well as children. Ramganga and Ghaghara having CR greater than 1, signify a significantly high probability of cancer risk to individuals exposed to the water over a prolonged period. Lower CR values in Gandak and Kosi suggest comparatively reduced risks, though they may still warrant caution depending on exposure. These findings highlight the critical need for addressing carcinogenic pollutants in these tributaries to safeguard public health.

Table 7 The HQ, HI and CR values for ingestion and dermal contact in the studied tributaries

Conclusion

• The assessment of trace element concentrations in the surface waters of the studied Himalayan tributaries reveals a complex interplay between geogenic processes and anthropogenic influences.

• Elevated levels of metals in these tributaries indicate both geological contributions from the Himalayan catchments and significant contamination from human activities.

• The concentration of Al and Fe in all tributaries and Pb in Ramganga were higher than BIS (2012) acceptable limits.

• Ramganga exhibited the highest concentrations of various trace elements among the tributaries, highlighting substantial anthropogenic influence and posing potential threats to both human health and aquatic ecosystems.

• In contrast, the Ghaghara, Gandak, and Kosi exhibited lower contamination levels, though they still demonstrated moderate risks, especially concerning Al, As, and Fe concentrations.

• HPI and PERI suggest that all four tributaries were affected by some degree of contamination, though the Ghaghara had the highest HPI value (93.74), indicating significant pollution.

• The Ramganga, Gandak, and Kosi had slightly lower HPI values, suggesting moderate contamination but still within the range of concern for water quality.

• Health risk assessments, calculated using the Hazard Quotient and Health Index, indicated varying levels of risk across the tributaries.

• For ingestion (HIing-nc), the Ramganga poses the highest health risk, higher for adults as well as children.

• Health Index, which combines risks from both ingestion and dermal exposure, further suggests the high health risks in the Ramganga (HI_Total = 1.876 for adults and 1.616 for children).

• CR values for the tributaries demonstrate significant variations and are above the threshold limits for adults as well as children, indicating a substantial carcinogenic risk to individuals with prolonged exposure.

• The study highlights the need for focused pollution mitigation efforts, especially in the Ramganga, where industrial activities are a key source of contamination.

• Regular monitoring, as well as targeted interventions to reduce anthropogenic inputs, particularly in high-risk areas, is crucial to safeguarding the long-term health and ecological sustainability of these tributaries.

Implications and limitations of the study

The present study highlights the impact of both natural and human activities on trace element concentration in the Himalayan tributaries, with Ramganga showing the highest levels due to industrial discharges. The findings emphasize the need for better pollution control strategies to protect water quality and public health. The health and ecological risk assessments provide useful insights for policymakers to take targeted action. However, some limitations exist. Statistical source apportionment was not conducted, and earlier studies were referred to for source identification. Groundwater and rainwater data were not analysed, but relevant literature was cited to provide context. Despite these limitations, the study offers a strong foundation for future research.