Spatiotemporal distribution and pollution assessment of trace metals in the Buriganga River, Bangladesh

Pollution of river environments by trace metals, which are highly toxic and persistent, is attracting global attention (Varol 2011; Yuan et al. 2011; Fu et al. 2014; Islam et al. 2015). Trace metals are a primary environmental concern because they are non-biodegradable and toxic and have a cumulative effect (Briffa et al. 2020). Trace metals deposited in the sediment can also be re-mixed into the water column via suspension, desorption, oxidation, or reduction processes, posing an increased risk to aquatic organisms, animals, and humans (Dong et al. 2012; Zhao et al. 2013). With the intensification of industrialization in many countries, numerous reports on trace metal contamination of river environments owing to industrial effluent discharge have emerged, raising concerns regarding the adverse effects of trace metal exposure on human health and ecosystems (Li & Zhang 2010; Islam et al. 2014b). The International Agency for Research on Cancer categorizes the metals As, Pb, and Ni as group 1 (‘carcinogenic to humans’), 2B (‘possibly carcinogenic to humans’), and 3 (‘not classifiable as to its carcinogenicity to humans’), respectively (World Health Organization 2023). Furthermore, adverse health effects, such as hypertension (As), diabetes (Cd), reproductive toxicity (Pb, Ni), dopamine oxidation (Mn), metabolic function effects (Cu), and respiratory problems (Zn), have been reported in humans who accidentally ingest trace metals (Uddin & Jeong 2021).

The Buriganga River is considered one of the most polluted rivers in the world. Anthropogenic activities have been reported as the primary source of trace metal contamination in this river, which flows southwest of Dhaka City, the capital of Bangladesh (Islam et al. 2014a; Nargis et al. 2021), with huge volumes of toxic waste discharged from thousands of industrial units and sewage lines being the primary pollutants (Islam et al. 2006; Kawser Ahmed et al. 2016). Hospital waste, boat and launch repairs, galvanizing steel, motor repairs, rechargeable batteries, foundry products, metal ceramics, textiles, dyeing, and leather industries, municipal sewage sludge, fertilizers, pesticides, insecticides, and oil emissions are also pollutant sources (Islam et al. 2015; Raknuzzaman et al. 2016). These deleterious effluents, including chromium, lead, sulfur, ammonium, salt, and other materials, severely pollute the Buriganga River (Hoque et al. 2016) and can seriously impact human health. The residents of Dhaka use this river daily as a water transportation route and for ablutions. Furthermore, oral exposure of humans to contaminants from ingesting crops cultivated using the river water is of particular concern, as the Buriganga River is utilized as a source of water for agricultural purposes (Islam et al. 2014a; Bhuyan et al. 2017). Therefore, a comprehensive study of the contamination levels of trace metals in this river is important for protecting the health of the residents of Dhaka.

This study aimed to assess the risk posed by trace metal contamination in the Buriganga River, Bangladesh. We collected water samples at 10 sites along the Buriganga River from August 2019 to February 2020. Trace metal concentrations, fraction ratios between dissolved and particulate phases, and single-factor pollution index values were determined to evaluate the level of metal contamination in the Buriganga River.

Standard solutions for Be, B, V, Cr, Mn, Ni, Cu, Zn, As, Se, Y, Mo, Ag, Cd, In, Sb, Pb, and Bi were obtained from FUJIFILM Wako Pure Chemical Industries (Osaka, Japan). Y, In, and Bi were used as internal standards. Inductively coupled plasma (ICP) analysis grade nitric acid, ultra-trace analysis grade HF, and poisonous grade HClO₄ were purchased from FUJIFILM Wako Pure Chemical Ind. Ultrapure water (18.2 MΩ cm) was used to dilute nitric acid.

The water samples were filtered using a quartz fiber filter to separate the suspended solids (SS) to determine the fraction of dissolved metals and metals adsorbed on SS. The filtered water samples and filters were transported from Dhaka to the University of Shizuoka, Japan, for instrumental analysis.

The samples were treated using a previously reported modification (Islam et al. 2014b; Bhuyan et al. 2017). Briefly, the filtered water samples (10 mL), nitric acid (60%, 75 μL), and internal standards (100 μL) were added to precleaned polyethylene centrifuge tubes and stored at 4 °C until the analysis. Ultrapure water was treated in the same manner as the filtered water samples to evaluate their blank levels.

A quarter of the filter was cut using Teflon scissors and weighed. The cut filter, nitric acid (60%, 2.5 mL), and HF solution (50%, 2.5 mL) were mixed in a Teflon beaker and heated at 200 °C for 30 min. Subsequently, HClO₄ (60%, 3.0 mL) was added to the Teflon beaker, and the mixture was heated at 200 °C for 30 min. The mixture was heated, and the solution was almost depleted. Nitric acid (0.1 mol L⁻¹, 5 mL) was then added to the Teflon beakers, and the contents were heated at 120 °C to dissolve the remaining residue. The nitric acid solution in the beaker was then transferred to precleaned polyethylene centrifuge tubes containing an internal standard (100 μL). The volume of the solution was fixed with nitric acid (0.1 mol L⁻¹) to 10 mL. Precleaned filters were treated in the same manner as the filter samples to evaluate the blank level.

The metal concentrations in the pretreated water and filter samples were measured using an ICP atomic emission spectrometer (AES) (5110 Agilent Technologies, Santa Clara, CA) and ICP mass spectrometer (ICP-MS) (Varian 820-MS, Agilent). Samples with concentrations lower than the ICP-AES calibration range were analyzed using ICP-MS.

No metals were detected in the blank samples. The calibration curves for each metal were linear over the entire range of standard concentrations (1, 10, 25, 50, 100, and 500 μg L⁻¹ for ICP-AES; 0.05, 0.1, 0.5, 1, 5, and 10 μg L⁻¹ for ICP-MS; coefficient of determination >0.99). The metal concentrations in the samples were determined to be in the ranges of the calibration curves.

Temporal distributions and spatial distributions of P_i values were investigated by Tukey's multiple comparison test. Prism 9 (GraphPad Software, Boston, MA) was used for the analysis.

The maximum trace metal concentrations found in this study were comparable to those reported in the previous studies, implying that the trace metal pollution status of the Buriganga River had not improved over the last decade (Table 1) (Mohiuddin et al. 2011; Bhuiyan et al. 2014; Islam et al. 2014a; Khan et al. 2014; Islam et al. 2015; Ministry of the Environment 2022). The trace metal concentrations in the Buriganga River reported in previous studies and in the present study were 100 times higher for Cr, 7 times higher for As, 9 times higher for Cd, and 13 times higher for Pb, than those reported in factory effluents in Japan, indicating a severe level of pollution (Table 1). As the trace metal concentrations measured in this study were comparable to those reported in previous studies on the Buriganga River and industrial wastewater in Japan, the risk of trace metal pollution was high, as discussed below (Table 1).

The Mn, Co, Zn, Cd, and Pb concentrations recorded in the present study were higher than those reported for other Bangladeshi rivers, such as the Korotoa, Meghna, and Karnaphuli Rivers (Table 1) (Islam et al. 2015; Ali et al. 2016; Bhuyan et al. 2017). Thus, the Buriganga River showed a relatively high level of pollution compared to that of other rivers, and the pollution status requires continuous investigation and monitoring. Conversely, large areas of Bangladesh may be contaminated by As and Cr, because the concentrations of As and Cr in the Buriganga River were similar to the concentrations of these metals in other rivers, and the concentrations observed exceeded the levels indicated in the guidelines (Islam et al. 2015; Ali et al. 2016; Bhuyan et al. 2017).

The mean concentrations (μg L⁻¹) of Be, B, V, Cr, Mn, Co, Ni, Cu, Zn, As, Se, Mo, Ag, Cd, Sb, and Pb were 14.1 ± 9.97, 33.2 ± 13.2, 14.0 ± 8.55, 27.7 ± 17.6, 238 ± 199, 15.3 ± 10.7, 22.2 ± 14.1, 26.0 ± 15.9, 65.1 ± 48.2, 16.5 ± 8.52, 12.6 ± 8.26, 13.4 ± 9.18, 11.8 ± 7.78, 12.7 ± 8.36, 13.1 ± 8.33, and 29.9 ± 51.4, respectively. The temperature ranged from 20.3 to 23.9 °C, and pH ranged from 5.4 to 9.1 (Supplementary Table S1). The observed concentrations were lower than the trace metal concentrations in the Buriganga River reported in previous studies but higher than the guideline values (Table 1), indicating that the degree of trace metal contamination in the Buriganga River is fairly unfavorable.

Cd, Ni, and Pb were classified as ‘heavily polluted’ (Figure 4 and Table 1). Cd is highly toxic; for example, Cd-contaminated mine drainage discharged into the Jinzu River, which flows through Toyama Prefecture in Japan, caused itai-itai disease in individuals who consumed rice cultivated using contaminated river water (Aoshima 2016). A previous study found that Cd adsorbs on SS migrating from the river to the paddy fields (Faroon et al. 2012). Indeed, the rice paddy soils were found to be contaminated with high concentrations of Cd (1–10 mg Cd kg⁻¹) (Faroon et al. 2012). In light of the threat posed by Cd exposure to humans, future studies should examine Cd concentration in rice cultivated using water from the Buriganga River. Humans may also be exposed to Ni and Pb through the same pathway as Cd, given their high partitioning ratios to SS (Figure 2).

This study revealed that the Buriganga River was contaminated by anthropogenic activities because the Sb pollution was ‘serious’ in the Buriganga River, and Cd, Ni, and Pb pollution were ‘heavily polluted.’ Be, Cr, Co, Ni, Cu, Zn, Se, Mo, Ag, Cd, Sb, and Pb should accumulate in the river sediments because of their high particle-water fraction ratio. Additionally, these metals may accumulate on crops cultivated using contaminated river water. The Mn concentration was diluted by rainwater during the rainy season, whereas the concentrations of other elements did not show seasonal trends. The source of the elements may be distributed among the sources of the Buriganga River.

This study was supported by the Mitsui & Co. Environment Fund No. R18-2009.

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

The authors declare there is no conflict.