Assessment of pollution risks posed by trace elements in the riverbed sediment of the Ganges River in Eastern Uttar Pradesh, India Open Access

Heavy metal contamination poses a serious threat to aquatic life due to their high toxicity, long-term persistence in the environment, and accumulation in water bodies (Qadir & Malik 2011; Fu et al. 2014). Sources of heavy metal contamination in rivers include atmospheric deposition (Demirak et al. 2006), soil erosion (Kaushik et al. 2009), effluent disposal (treated or untreated) (Zheng et al. 2008), use of heavy metal-containing fertilizers and pesticides (Iqbal & Shah 2014), surface runoff (Zahra et al. 2014), and various chemicals from agricultural, urban, and industrial activities (Park & Presley 1997; Xiao et al. 2013). Sediment quality acts as an indication of river pollution levels, reflecting earlier contamination levels. Elevated levels of toxic heavy metals in sediment can endanger human health as they transfer to aquatic species and enter the food chain (Salati & Moore 2010; Varol & Sen 2012). Possible sources of various metals in the environment are mentioned in Table 1.

The Ganges River, spanning 2,525 km and catering to approximately 400 million people in India, has witnessed degradation in water quality over the past few decades (Pandey et al. 2014; Dwivedi et al. 2018). Industrial discharge stands as a primary source of pollution (CPCB 2013; Yadav & Pandey 2017), with only a fraction of sewage treated before discharge into the river. Despite governmental efforts since 1985, untreated sewage discharge into the Ganges River has increased (Dwivedi et al. 2018). Alongside urban sewage, industrial units discharge significant sewage loads directly into the river (Trivedi 2010), particularly in the middle section from Kannauj to Varanasi, primarily from tannery industries (Pandey et al. 2010).

While previous studies have assessed heavy metal contamination in Ganges River water (Aktar et al. 2010; Pandey et al. 2014) and bed sediment (Singh et al. 2003, 2013) at various locations, but not much research has been extensively done on the Ganges stretch between two important and densely populated cities of Uttar Pradesh i.e. Kanpur and Prayagraj. Jajmau, a hub of tannery and other heavy metal-intensive industries lying just downstream of Kanpur, contributes significant untreated sewage to the Ganges River, potentially contaminating downstream areas, including the holy city of Prayagraj (Ansari et al. 2000). This study focuses on the 225-km stretch between Kanpur and Prayagraj due to these concerns. Additionally, agricultural activities in this area involving pesticide and fertilizer use contribute to heavy metal contamination. Analysis in this study includes the deposition of eight heavy metals (Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn) to assess their spatial distribution in Ganges River sediments.

Various studies have investigated heavy metal contamination in rivers worldwide, including the Ganges River in India (Pandey & Singh 2017; Aggarwal et al. 2022, 2023, 2024), Indus river in Pakistan (Tariq et al. 1996), Gomti River in India (Singh et al. 2005), Hindon River in India (Suthar et al. 2009), and Nile River in Egypt (Rifaat 2005).

The primary aims of this investigation are to determine heavy metal concentrations in the Ganges riverbed sediment, identify their sources (natural or anthropogenic), risk evaluation using pollution indicators like enrichment factor (EF), contamination factor (CF), Pollution Load Index (PLI), and Geo-Accumulation Index (GAI); examine the heavy metal concentrations' spatial distribution pattern and evaluate environmental risks by comparing metal concentrations with the standard values. This research intends to quantify pollution levels in the Ganges River, provide environmental health risk assessments, and address pollution-related concerns in the study region.

Riverbed sediment sampling had been done within the pre-monsoon season in April 2021. During the summer and pre-monsoon season, i.e. during March to May, is the driest period for the river, with minimal glacial meltwater and little to no rainfall feeding the river. Water levels are at their lowest, often affecting drinking water availability, irrigation, and water quality. Pollution concentration is high, making this a challenging period for both river ecology and dependent communities. At each sampling location, three riverbed sediment samples were taken from a depth of 10 to 15 centimetres below the riverbank's surface. Riverbed sediment sampling at 10–15 cm depth is done to analyze historical contamination or long-term pollution trends. The 10–15 cm layer may contain older deposits, as sediments tend to accumulate in layers over time. This depth can help reveal past pollution levels, providing insight into the history of contaminants, such as metals or other pollutants, even if surface layer contamination levels have changed due to various parameters like pH, TDS, and various others. At 10–15 cm, sediment is generally more stable and less likely to be influenced by recent or seasonal variations, offering a more consistent record. Unlike the surface layers, which may fluctuate with water flow or recent sediment deposition, these deeper layers give a ‘snapshot’ of past environmental conditions. Heavy metals and other contaminants often bind tightly to sediment particles and remain buried over time. Sampling at 10–15 cm helps capture pollutants that may have settled and become buried, reducing their immediate bioavailability but still contributing to long-term contamination. When planning activities like dredging, understanding contaminant levels at deeper layers (like 10–15 cm) is essential. Disturbing these sediments could release buried pollutants back into the water, potentially affecting water quality and aquatic life. A lot of 30 samples were gathered from the 10 selected locations following standard protocols. Approximately 100 g of sediment were collected from each location using a grab sampler and packed into impermeable polythene bags (USEPA 2001). Subsequently, sediment samples were transported to the laboratory and air-dried for 72 h before undergoing further analysis.

As per APHA guidelines, the collected sediment samples underwent digestion and analysis to determine the concentrations of heavy metals. Initially, the sediment samples were dried in an oven at 100 °C for 1–2 h to achieve a constant mass. Subsequently, they were crumbled and ground to obtain fractions of <63 μm in size. Each sediment sample weighing one gram was digested by adding a 10 mL solution of HClO₄ and HNO₃ in a 1:2 ratio. This combination was heated on a hot plate for 60 min at a low temperature before being elevated to 120 °C, where fumes began to appear and the solution turned clear and transparent. The digested specimens were then passed via Whatman filter paper – Grade 42. To dilute the filtrate, add 0.1 N of HNO₃ until the final amount is 50 mL. Finally, the concentrations of trace elements such as Fe, Cd, Cu, Cr, Mn, Pb, Ni, and Zn were evaluated in the digested specimen using AAS equipment (Make-Shimadzu, Model-AA-6300).

[(X_x)/(X_Fe)]_s depicts a ratio of the concentration of ‘x’ metal in relation to the concentration of Fe metal in the specimen. [(X_x)/(X_Fe)]_b signifies a ratio of the background concentration of ‘x’ metal in relation to the background concentration of Fe metal. Background concentrations in riverbed sediment for Fe, Mn, Cd, Cu, Cr, Pb, Ni, and Zn were obtained from the geometric mean values provided by Singh et al. (2003) and are listed in Table 6. The determination of enrichment levels was based on corresponding EF values (Table 3).

The values of PLI obtained from the above equation are used to classify the pollution levels, as outlined in Table 4.

The values of CF obtained from this equation are used to classify the contamination levels, as outlined in Table 5.

The concentrations of heavy metals in the Ganges River sediments are determined and compared with the corresponding sediment quality guidelines (SQGs) values listed in Table 6. SQGs aid in assessing the potential impact of heavy metal concentrations in sediments on aquatic plants and animals. They are designed to interpret sediment quality and its potential effects on the ecosystem (Wenning 2005).

Two types of SQGs are developed for freshwater ecosystems (MacDonald et al. 2000): Probable Effect Concentration (PEC) represents the concentration above which adverse effects are expected to occur. Threshold effect concentration (TEC) refers to the concentration under which harmful effects are unlikely to occur.

Table 6 shows concentrations of metals in riverbed sediment samples taken from selected locations. We observed RSD of sample replicates between 1 and 4%, which falls within acceptable limits for reproducibility in this analysis. The average concentrations of heavy metals reveal a trend in which Fe exhibits the highest concentration, followed by Mn, Cr, Zn, Ni, Pb, Cu, and Cd, with Cd recording the lowest concentration among the collected sediment samples.

Pb is a toxic metal that is not essential for biological processes. It originates from both natural and anthropogenic sources. The primary sources of lead include airborne particles, forest fires, effluents from leather factories, vehicle emissions, paints, volcanic activity, waste incineration and pesticides. The Earth's crust typically contains lead concentrations ranging from 15 to 20 ppm (Abadin et al. 2007).

Cd is considered as a non-essential element and has adverse effects on plant growth. It is typically a after product of zinc and lead mining and smelting processes and exhibits higher mobility in aquatic environment compared to most other metals. Sources of cadmium release into the environment include metallurgical industries, atmospheric deposition, power plants, fertilizers, natural atmospheric processes, municipal solid waste (MSW) incineration, and the discharge of toxic effluents from industries and wastewater treatment facilities (WHO 2019).

Cu is an essential nutrient for river sediment and freshwater that supports aquatic life growth; however, excessive concentrations can be toxic. Copper is introduced into the environment through natural processes such as decaying vegetation, volcanic eruptions, sea spray, and forest fires, as well as anthropogenic activities including wastewater from industries and municipal corporations (Dorsey & Ingerman 2004). Following various natural processes, dissolved copper eventually becomes adsorbed in river sediment.

Fe concentrations were observed to be the highest among all elements in the Ganges River sediment, primarily due to natural processes such as weathering and erosion, as well as human activities including MSW, urban discharge, construction and demolition wastes, agricultural activities and industrial effluent.

Mn concentration in the Earth's crust typically ranges between 40 and 900 ppm (Coles et al. 2012). Sources of manganese in sediments include emissions from alloy production units, sewage sludge, mining waste, waste from various metal processing units, combustion of fossil fuels and municipal wastewater.

Ni is commonly used in alloys with zinc, chromium, copper and iron, and finds widespread applications across various industries including food processing, household appliances, fuel production, electroplating, ceramics, pigments, jewellery manufacturing, magnets, heat exchangers coins, batteries, medical prostheses and others.

Cr typically has an average concentration of 100 ppm in the Earth's crust (Abadin et al. 2012). Chromium compounds are utilized in various industries such as dyes, paint and leather industries. These compounds can be transported into the groundwater and soil of industrial spaces. Paints contain chromium as a base and are commonly used for refinishing automotives, contributing to Cr pollution in the Ganges River.

EF depicts the contamination levels of heavy metals and their source of contamination in a specific environment (Loska et al. 1997). EF of <2 indicates the presence of elements from crustal source, EF of >2 reflects higher levels of pollution from anthropogenic sources. The EF values of the studied heavy metals in this investigation are outlined in Table 7.

The EF values for Mn, Cr, Zn, Pb, Ni, Cu Cd fall within specific ranges: Mn (0.36–0.51), Cr (0.52–0.66), Zn (0.39–0.59), Pb (1.32–2.09), Ni (0.93–1.18), Cu (0.58–0.82), and Cd (2.04–4.62). Based on these EF values, enrichment levels are identified using Table 3.

For Cd, the EF values at all locations lies from 2 to 5, which indicate a moderate level of enrichment in the sediment. For lead (Pb), some locations show EF values close to 2, suggesting present enrichment at the mineral level which may become moderate in the future. However, for other metals, the EF values are <2, indicating enrichment at the mineral level.

Cadmium has substantial geochemical implications on the environment, allowing it to be conveyed across vast distances due to its solubility and mobility in water when compared to other heavy metals. The variation in EF values for different heavy metals across different locations may be attributed to variations in metal input or differences in the removal rate of each heavy metal from the sediments (Ghrefat et al. 2011).

The GAI class was determined using the Abrahim and Parker classification (Abrahim & Parker 2008), as reported in Table 3. This indicator is used to assess anthropogenic effect. Table 8 shows the GAI values for the eight heavy metals in riverbed sediment of each sampling location.

For Cd metal, the GAI values at locations L1, L2, L3, L4, L6, L7 and L8 fall within Class 1 (0 < GAI < 1), indicating an unpolluted to a moderate level of cadmium pollution at these locations. At locations L5, L9, and L10, the GAI values for cadmium lie within Class 0 (GAI < 0), indicating no pollution.

For all other metals studied, including Zn, Mn, Pb, Cr, Cu, Ni and Cd, the GAI values for all sampling locations fall within Class 0 (GAI < 0), indicating that the riverbed sediment are uncontaminated by these metals.

Table 9 shows the CF and PLI of metals at all locations. In all sediment samples, the CF values for iron (Fe), manganese (Mn), chromium (Cr), zinc (Zn), nickel (Ni), and copper (Cu) were observed to be lower than one, with an average of 0.60, 0.28, 0.36, 0.28, 0.63, and 0.42. These results show that the Ganges River sediment are not contaminated by these metals.

The CF values for cadmium (Cd) at all locations ranged from 1 to 3, with an average of 1.89 suggesting moderate pollution of Cd. The highest CF value for cadmium was calculated as 2.72 at location L2, followed by location L8 (2.19), L3 (2.10), L4 (2.05), L7 (1.97), L6 (1.88), L1 (1.74), L9 (1.47), L5 (1.45), and L10 (1.34). Similarly, the CF values for lead (Pb) at locations L1, L2, L3, L4, L5, L6, L7 and L8 ranged between 1 and 3, at an average of 1.06, indicating moderate contamination by lead at these locations. However, the CF values for Pb at locations L9 and L10 are <1, indicating a low level of contamination by lead at these locations.

In summary, the calculated CF values demonstrate that the concentrations of Fe, Mn, Cr, Zn, Ni, and Cu metals in the riverbed sediment are not currently in a disturbing condition. However, the riverbed sediment is moderately contaminated with Pb and Cd metals, which may rise in the future, providing a possible risk to the river environment, particularly given the fast-growing population in the vicinity of the Ganges River.

The PLI measures the total level of heavy metal contamination at a particular location. Table 9 shows the computed PLI values for the chosen locations for the studied heavy metals. the values of PLI were recorded <1 for all locations which indicate pollution at baseline levels only. Nawabganj location L6 recorded the highest PLI value of 0.574 followed by L4 – Dalmau-Fatehpur Bridge (0.569), L8 – Shringverpur (0.569), L2 – Maharajpur (0.562), L7 – Manikpur (0.561), L3 – Sarsaul (0.555), L9 – Phaphamau (0.538), L1 – Jajmau (0.534), L5 – Unchahar (0.532), whereas the lowest polluted location was L10 – Sangam (0.517).

Table 6 shows the recommended values of TEC and PEC for river sediments. The averages of concentrations of all metals remained lower than their PEC values, but the average concentrations of Fe, Mn, Cr, Ni, and Cd surpassed the TEC threshold by 21.43, 6.28, 20.65, 30.88, and 11.11%, respectively, suggesting that they may damage aquatic plants and animals. Based on the SQG readings, it may be concluded that the riverbed silt of a certain section of the Ganges River included greater amounts of Fe, Mn, Cr, Ni, and Cd, which can harm aquatic plants and animals.

Correlation analysis was conducted by obtaining correlation matrix with the help of Weka Tool and Microsoft excel to see whether there was a significant association between metals. Correlation matrix is the statistical method of determining the relationship between two variables in a set of data. The matrix is a tabular representation wherein each cell represents a correlation coefficient. A value of 1 indicates a strong relationship between variables, a value of 0 as neutral, while a value of −1 indicates a weak relationship. A value between 0.1 and 0.3 indicate weak positive correlation, 0.3–0.5 indicate moderate positive correlation whereas 0.5–1.0 indicate strong positive correlation. The observed metal concentrations which are mentioned in Table 6 are used to identify correlation between various metals in riverbed sediment of Ganga River using correlation matrix as shown in Table 10. The correlation matrix showed a perfect positive correlation between Mn-Pb only and strong correlation of Fe-Cr suggesting that they have similar geochemical nature or their input from a same source. Mn-Ni, Pb-Cd and Zn-Cu had moderate correlation with each other. Other metals were either weakly correlated or had negative correlation.

This study aimed to evaluate the concentrations, risk assessment and anthropogenic inputs of some trace elements like lead (Pb), cadmium (Cd), copper (Cu), iron (Fe), chromium (Cr), manganese (Mn), zinc (Zn), and nickel (Ni) in riverbed sediments of the Ganges River stretch between Kanpur and Prayagraj in eastern part of Uttar Pradesh, India: The occurrence of these metals in sediment differed over the examined stretch due to discharge from a variety of anthropogenic sources, including untreated wastewater from neighbouring industries such as pesticides, battery production, fertilizers, electroplating, paint and tanneries.

The GAI and EF values for the analyzed sediment samples indicated moderate contamination by Cd metal, while other heavy metals showed negligible pollution. The PLI values identified Nawabganj, as the most polluted location and Sangam as the least polluted. The CF values revealed moderate contamination by cadmium and lead. PLI values revealed that all selected locations are overall uncontaminated.

Higher concentrations and variability in heavy metals is seen downstream from industrialized and urban locations, suggesting that the increase in nearby industries and urbanization is likely contributing to metal contamination. It is essential for industries to undertake effective measures for the treatment of harmful effluent wastewater before discharging into the Ganges River. Government agencies such as the Central Pollution Control Board (CPCB) and State Pollution Control Boards (SPCBs) must enforce strict compliance with environmental standards for municipal and industrial effluents to protect the Ganges River from further metal depletion.

Remediation technologies such as in-situ and ex-situ physical/chemical treatments and thermal treatment can be employed for heavily polluted locations to mitigate the impact of metal contamination on the river ecosystem.

The authors are really grateful to the professors and staff at MITS, Gwalior in particular for supporting the technical and logistical implementation of the instruments like AAS. We also thank Dr Deepika Varshney for her continuous guidance and support during the organization of this manuscript.

M.A. contributed to writing – review & editing, writing – original draft, methodology, investigation, formal analysis, conceptualization. S.A. contributed to writing – review & editing. T.V. contributed to writing – review & editing. D.V. contributed to writing – revised draft, writing – review & editing, statistical analysis.

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

The authors declare there is no conflict.