IJERPH, Vol. 19, Pages 16125: Water Quality Criteria and Ecological Risk Assessment of Typical Transition Metals in South Asia

3.1. Development of Acute and Chronic SSDs of Typical Transition MetalsOverall, there were 281 species with acute toxicity data for Cd (76), Cu (66), Hg (59), Mn (12), Fe (13), and Zn (55), including plants (22), fish (114), mollusks (32), amphibians (23), crustaceans (46), insects (12), invertebrates (20), and worms (10) (Table S1). Chronic toxicity data were available to a much lower extent. We only collected a total of 88 species with chronic toxicity data for Cd (34), Cu (38), and Fe (16) (Table S2), containing plants (18), fish (30), mollusks (9), crustaceans (13), insects (7), invertebrates (4), worms (5), and amphibians (2) (Table S2). It showed that more data were available for temperate than for tropical species. Since the ranges were relatively large, the acute and chronic toxicity data were log10-transformed to reduce the relative difference for these metals, facilitating calculations.The acute and chronic NPKDE-SSDs of six and three typical transition metals, respectively, in South Asia were established (Figure 1). The Pks values of the NPKED-SSD models were all greater than 0.05, and the RMSE and SSE values were minimal (Table 1), indicating that the NPKED-SSD models developed for these metals showed a good fit, high accuracy, and suitability for use in hazard simulation. 3.2. Derivation of Acute and Chronic HC5 Values for South AsiaThe acute and chronic HC5 values were derived based on SSDs to establish the acute and chronic WQC of aquatic organisms for the studied metals (Table 1). It is found that the order of the acute HC5 values is Mn (2.56 mg/L) > Fe (81.1 μg/L) > Cd (23.6 μg/L) > Zn (17.5 μg/L) > Cu (9.8 μg/L) > Hg (7.8 μg/L) (Figure S1a), indicating that Hg has the strongest acute toxic potency among these six transition metals to aquatic organisms in South Asia. As a global pollutant, Hg is persistent, highly mobile through air transport, and can bioaccumulate in organisms and food chains [23]. Although the mechanism of Hg toxicity is still largely unclear, it might be related to the fact that Hg binds to thiol groups in proteins and thus affects the enzymatic activities of organisms [18]. In addition, the toxic potencies of Fe and Mn to aquatic organisms in South Asia were low, which is consistent with our previous results on freshwater fish [3]. However, the acute HC5 values differed from those obtained in our previous study, along with a different order (i.e., Mn > Zn > Cu > Fe > Cd > Hg) [2,16]. This can be explained by the differences in regional species and the available data. The sensitive species near 5% cumulative probability for Cd, Cu, and Hg are crustaceans, and for Mn, Fe, and Zn are plants, worms, and fish, respectively. Because of the warm climate in South Asia, metals may show different toxicity levels to tropical species. Only the HC5 values of Cu and Zn were lower than those reported in previous studies, whereas the HC5 values of the other four metals were higher. Some studies have shown that in freshwater environments, temperate species are more sensitive to most metals than tropical species, although tropical species are more sensitive to Zn [24,25]. According to a previous study, the toxic potencies of Cd, Cu, Zn, and Hg to marine species are directly proportional to the temperature [26]. This explains the need to establish regional WQC [12]. The chronic HC5 values followed the order Cu (6.9 μg/L) > Fe (2.4 μg/L) > Cd (0.413 μg/L) (Figure S1b), similar to the acute HC5 values reported in our previous research [2,16]. Of these, Cd showed the strongest chronic toxic potency to aquatic organisms in South Asia. In aquatic organisms, long-term exposure to Cd can lead to adverse effects on growth, reproduction, immune and endocrine systems, and behavior [27,28]. In addition, Fe showed a stronger toxicity than Cu, which can be explained as follows: (1) due to the lack of data, we used more model biological toxicity data in the construction of chronic SSDs. Therefore, the sensitive species near 5% cumulative probability for Fe is Oncorhynchus mykiss (1.5 μg/L), which is globally distributed, but not common in South Asia; (2) the dataset used for Fe was small, resulting in a deviation of building SSD. However, further studies comparing the chronic toxicities of Fe and Cu to temperate and tropical freshwater species are necessary since we could not adequately protect tropical species by thresholds derived from temperate species [29]. 3.4. Ecological Risk Assessment of Transition Metals in Typical South Asian RiversThe major southern and southeastern Asian river basins (i.e., Indus, Brahmaputra, and Ganges) arise from the Himalayas and benefit over 1 billion people [14]. These rivers are transboundary rivers, such as the Brahmaputra and Ganges basins, which connect Bangladesh, Bhutan, India, and Nepal. South Asian rivers including these transboundary rivers are highly polluted with various metals via the input of domestic, agricultural, and industrial wastes [10,39,40,41]. For example, the Pb concentration detected in the Karnaphuli River in Bangladesh ranges from 5.29–27.45 µg/L in water and 21.98–73.42 mg/kg in sediments [41]. However, the transboundary movement of toxic pollutants from such rivers is a matter of concern. In recent decades, the transformation of contaminants between upstream and downstream countries has sparked debates among South Asian countries [14]. The contamination of rivers with metals may have devastating effects on the ecological balance of the aquatic environment, decreasing the diversity of aquatic organisms [8,42]. In this sense, the use of unified toxicity data and WQC to assess the ecological risk of transboundary rivers may be the first step to solving this issue. However, in South Asia, there is not only no available information on WQC, but also, some countries do not even have surface water quality standards (WQSs). The governments of these countries only established national drinking WQSs to protect human health [43,44,45,46,47]. In order to investigate the transition metals pollution status in major South Asian rivers, in this study, water quality data of the major South Asian rivers (i.e., the Indus, the Ganges, the Brahmaputra, the Meghna, and the Bagmati River) with the confluence of different countries (i.e., Bangladesh, India, Pakistan, and Nepal) for the six studied metals were collected from recent publications and official reports or websites (Table 3). Then, the ecological risks of typical transition metals in these major South Asian rivers were evaluated based on NPKDE-SSDs established in the present study.The collected data for Cd, Cu, and Zn were abundant and complete, which was not the case for the Hg data. It is disconcerting that the sample size of Hg is so small, since Hg is recognized as a global pollutant. We speculated that it is possibly related to the higher complexity for determination than the other metals. The Ganges River in India was sampled most commonly, where the same river in Bangladesh had the lowest sample size. According to the ecological risk assessment via the use of the constructed acute and chronic NPKDE-SSD models of typical transition metals, the acute PAF values of all metals in almost all rivers were above 0.05, and the PAF values of the maximum detected concentrations exceeded 0.69 (Figure 2). The ecological risks of long-term exposure to Cd, Fe, and Cu are extremely high; particularly for Cu, the PAFs of the detected minimum concentrations all exceeded 0.9 (Figure 2c,d), resulting in significant threats to aquatic organisms. When comparing the average concentrations of the six transition metals detected in each river with the standards of their respective countries [43,44,45,46], only the Bagmati River (Nepal) did not exceed the standard for all metals (except for Hg and Fe due to the lack of available data). Due to the high standard value of Zn, none of the rivers exceeded the threshold. However, most of the remaining metals detected in the rivers exceeded the standard, and the rivers Ganges and Brahmaputra, passing through India and Bangladesh, showed the highest levels.The Ganges River, named Ganga in India and Padma in Bangladesh, originates from the Gangotri glacier at Gomukh in the province Uttarakhand of India [75]. The water of the Ganges River in India is subject to substantial pollution through the input of untreated domestic and industrial wastes [76,77]; the concentrations of Fe, Mn, Hg, and Cd are high. The concentrations of Cd, Hg, and Fe detected in the Ganges River in India were the highest, with average values of 0.029, 0.14, and 7.74 mg/L, respectively, exceeding the national WQSs of India by more than five times [43]. The water quality of the Ganges (Padma) River in Bangladesh is better than that in India (Figure 2). The Brahmaputra River is a large trans-Himalayan river, originating from Tibet and running through parts of China, Bhutan, India, and Bangladesh before flowing into the Bay of Bengal [67,68]. The Brahmaputra River in India is contaminated with Fe, Mn, Cu, Pb, Cd, Cr, As, and Ni, mainly via untreated wastewater, sewage, and effluents from municipalities and industries from the nearby catchment areas [48,49,50,67]. The risk of Mn pollution is highest in the Brahmaputra River of Bangladesh, with average Mn concentrations 20 times higher than those in upstream India. Previous studies have also shown that Mn is a common natural contaminant of groundwater in Bangladesh, where the maximum Mn level was 4.11 mg/L (mean, 0.53 mg/L) [78], which partly explains the high infant mortality rate in Bangladesh [79]. It also demonstrated that the current WQSs and other freshwater management measures can not protect aquatic organisms, and also human health, which should be updated and new tools adopted in time.

留言 (0)

沒有登入
gif