Ecological and human health risk assessments of cyanotoxins and heavy metals in a drinking water supply reservoir

Lakes and reservoirs play important roles in the natural water cycle; however, in recent decades, there has been significant concern regarding water contamination by various hazardous elements and other pollutants as a result of increasing population, rapid industrialization, and anthropogenic activities (Liyanage & Yamada 2017; Ho & Goethals 2019). Heavy metal contamination in surface water is one of the most serious global issues because of its wide sources, toxic effects, accumulative behaviors, and non-degradable properties in aquatic ecosystems (Tchounwou et al. 2012; Qing et al. 2015). The excessive trace element concentrations in lakes and reservoirs may lead to the degradation of water quality and the deterioration of ecological function and threaten human health (Bashir et al. 2020; Briffa et al. 2020). Numerous studies have demonstrated the toxic effects of heavy metals on human health worldwide (Fernández-Real & Manco 2014; Jaishankar et al. 2014; Rehman et al. 2018; Briffa et al. 2020). However, ecological and human health risk assessments of metal contamination in freshwater bodies, especially in Southeast Asia, have not been investigated to the same extent.

Some metals (Zn²⁺, Mn²⁺, Fe²⁺, Cu²⁺, Ni²⁺, and Co²⁺) are essential for living organisms in trace amounts; others (Pb²⁺, Hg²⁺, As³⁺, and Cd²⁺) have been considered as toxic substances, which can pose serious threats to human and environmental health, even at very low concentrations (Tchounwou et al. 2012; Briffa et al. 2020). The toxicological effects of heavy metals on humans have been extensively studied and reviewed (Tchounwou et al. 2012; Jaishankar et al. 2014; Briffa et al. 2020; Balali-Mood et al. 2021). Those effects include skin lesions, nervous system disorders, gastrointestinal, kidney dysfunction, and DNA damage that may lead to cell cycle modulation or cancer (Briffa et al. 2020; Balali-Mood et al. 2021). Because of their toxicity, the World Health Organization (WHO) has proposed limit values for heavy metals in drinking water (WHO 2017). Similarly, the Ministry of Natural Resources and Environment of Vietnam has established Vietnam's national technical regulation (QCVN 08-MT:2015/BTNMT) for the protection of water quality under heavy metal contamination (MONRE 2015).

Besides heavy metal contamination, the occurrence of cyanobacterial blooms, which are associated with toxic secondary metabolites, has posed a serious threat to human health, domestic animals, and livestock (Merel et al. 2013; Pham & Utsumi 2018). Microcystins (MCs), a group of cyclic heptapeptide hepatotoxins, are the most common toxin detected in cyanobacterial bloom events in freshwater ecosystems worldwide (Merel et al. 2013; Pham & Utsumi 2018). In 1998, the WHO proposed a provisional guideline value of 1 μg/L for total MC-LR in drinking water (Ibelings et al. 2014; WHO 2017). As in many countries, water contamination with MCs is a problem also in Vietnam's freshwaters, particularly for domestic and recreational purposes (Duong et al. 2014; Nguyen et al. 2020; Pham et al. 2020).

In eutrophic environments, the dominant cyanobacteria can co-occur with other chemical pollutants, such as heavy metals, pesticides, and pathogenic microbes (Defarge et al. 2018; El-Alfy et al. 2019; Metcalf & Codd 2020). It is well known that humans are subjected to exposure testing with a chemical mixture, imitating the natural condition context (Sexton & Hattis 2007; Silins & Högberg 2011). When exposed to a mixture of toxic substances, the adverse effects on susceptible individuals or populations are unpredictable (Metcalf & Codd 2020). The co-occurrence of cyanotoxins and heavy metals in lakes and reservoirs has been extensively studied and reviewed (Trollope & Evans 1976; Ni et al. 2019; Metcalf & Codd 2020; Kelly et al. 2021). However, previous studies have only focused on risk assessments of an individual toxic substance (Jaishankar et al. 2014; Qing et al. 2015; Rehman et al. 2018; Balali-Mood et al. 2021; Nguyen et al. 2021), and very little is known about ecological and human health risk assessments of cyanotoxins simultaneously with other environmental pollutants, such as heavy metals.

The Tri An Reservoir (TAR) is a drinking water source for millions of inhabitants in Ho Chi Minh City, and Dong Nai, Binh Duong, and Ba-Ria Vung Tau Provinces in Southern Vietnam. Other uses of this water body include industrial water supply, irrigation of plantations recreation, and hydropower operation (Nguyen et al. 2020). In recent decades, rapid industrial and economic development and agricultural activities have resulted in water quality impairment in the TAR (Pham et al. 2020). The occurrence of toxic cyanobacterial blooms and MCs has been reported from the TAR (Pham et al. 2020). The health risks associated with cyanobacterial exposure in a small community living near the TAR have been assessed (Nguyen et al. 2021). However, there is still considerable uncertainty concerning the potential health risks related to toxic cyanobacteria exposure by other residents who are living near the TAR. In addition, the ecological and health risks of heavy metals in such a drinking water supply remain unknown. Therefore, the main goal of this study is to investigate the ecological and human health risks associated with cyanotoxins and heavy metal exposures.

Field trips were conducted every 2 months throughout 2018 and 2019 at five designated stations (S1, S2, S3, S4, and S5) in the TAR (Figure 1). For quantitative analysis of cyanobacteria, 1-L subsurface water samples (∼20 cm) were collected and preserved in the field with formalin. Surface water samples for analyzing heavy metals [arsenic (As), iron (Fe), lead (Pb), zinc (Zn)] and chlorophyll-a (Chl-a) were collected by hand using 2-L acid-washed bottles and transported to the laboratory using a cool box with ice.

Quantitative analysis of cyanobacteria was done by using a Sedgewick-Rafter counting chamber according to Chorus & Welker (2021). Cyanobacterial identification was based on the standard works of Komárek & Anagnostidis (1989, 1999, 2005).

To measure Chl-a, subsamples (300 mL each) were passed through glass fiber filters (Whatman GF/C, UK); then, the filter was extracted with 10 mL of 90% acetone overnight in complete darkness. Samples were then centrifuged and measured in triplicate at 630–750 nm using a spectrophotometer (UV–VIS, Harch, 500) according to the APHA (2005) methods.

For total MC (free and cell-bound contents) measurement, field water (500 mL) was digested with acetic acid (5% final concentration) and sonication for 3 min. After centrifugation at 1,800 × g for 15 min, the supernatant was passed through an HLB cartridge (60 mg, Waters, MA, USA) that had been preconditioned with 5 mL of MeOH 100% and 5 mL of Milli-Q water; next, the cartridge was washed with 5 mL of MeOH 20% and then eluted with 3 mL of MeOH 100% into a glass tube. This eluate was dried with a flow of air at room temperature. The MC content in the tube was then re-dissolved in 500 μL of MeOH 100% and centrifuged at 4,000 × g for 5 min. Samples were then passed through a Minisart RC4 filter (Sartorius, Göttingen, Germany) into a glass vial and kept at −20 °C before analysis.

The concentration of MC was measured using a high-performance liquid chromatography (HPLC) system (Dionex UltiMate 3000, Thermo Scientific, Waltham, MA, USA) equipped with a UV–VIS detector. The samples containing MCs were carried with a mobile phase consisting of methanol:0.05 M phosphate buffer (pH 2.5; 50:50 v/v) at a flow rate of 0.65 mL/min. Three MC congeners (LR, RR, and YR) were separated at 40 °C using a silica-based reverse-phase Acclaim C18 (Waltham, MA, USA). MC congeners were identified based on both retention time and characteristic UV spectra at 238 nm. MC variants, purchased from Enzo Life Sciences (Farmingdale, NY, USA), were used as standards. The detection limit of this system is 0.12 μg/L.

To measure dissolved metals (As, Fe, Pb, and Zn), subsamples were filtered through 0.45 μm filters (Sartorius, Germany), and the filtrates were then acidified with 2% nitric acid and stored in PVC bottles at 4 °C before analysis. Heavy metals were measured using an inductively coupled plasma mass spectrometry (ICP-MS) (Agilent 7700, CA, USA) according to the APHA (2005) methods. The detection limit of the system for As, Pb, Zn, and Fe is 1.0 μg/L.

In 2003, the WHO derived a series of guideline values for recreational exposure to cyanobacteria related to the probability of acute health effects (Table 1). According to these guidelines, the relative probabilities of acute health effects (RPAHEs) were divided into three categories, based on cyanobacterial density, Chl-a concentration, and MC concentration in water (U.S. EPA 2019).

According to Håkanson (1980), the ecological risks were classified into five terminologies based on the ERF values: (1) ERF < 40, low potential ecological risk; (2) 40 ≤ ERF < 80, moderate potential ecological risk; (3) 80 ≤ ERF < 160, considerable potential ecological risk; (4) 160 ≤ ERF < 320, high potential ecological risk; and ERF ≥ 320, very high ecological risk.

To assess the health risks of heavy metals, the concentration of heavy metals was compared with Vietnam's national technical regulation on domestic water quality (QCVN 01-1:2018/BYT) (MOH 2018) and the guidelines of drinking water quality of WHO criteria (WHO 2017).

A total of 3 orders, 15 genera, and 41 cyanobacterial taxa were identified in the TAR based on morphological examination (Table 2). Dolichospermum, Microcystis, and Planktothrix were the three main genera of the cyanobacterial community in the TAR with relative percentages of 17.1, 12.2, and 19.5%, respectively. Microcystis spp. are the most common bloom-forming cyanobacteria in the TAR. The recorded cyanobacteria included potentially toxic species such as Microcystis spp., Anabaena spp., Raphidiopsis spp., and Planktothrix spp.

According to WHO guideline values for recreational exposure to cyanobacteria, the mean cyanobacterial abundance indicated that the RPAHE fell into two levels: low (in February) and moderate (in other months); the mean Chl-a showed that the RPAHE shifted from moderate to high levels, while the MC indicated the RPAHE at a low level (Figure 2(c)).

During the study period, the mean concentrations of heavy metals varied largely and decreased in the following order Fe > Pb > As > Zn (Table 3). The concentration of As ranged from the UDL to 5.8 μg/L (mean from UDL to 2.2 μg/L) (Supplementary Table S2); the concentration of Fe exhibited a wide variation and ranged from 94 to 8,195 μg/L (mean from 287 to 2,275 μg/L) (Supplementary Table S2); the concentration of Pb ranged from the UDL to 6.3 μg/L (mean from the UDL to 2.5 μg/L) (Supplementary Table S3). The concentration of Zn was the UDL during the study period (Supplementary Table S3). All metals (except Zn) showed a peak in June.

Results showed that the highest concentrations of As and Pb met the guidelines of drinking water quality with respect to WHO criteria (10 μg/L; WHO 2017) and Vietnam standard (10 μg/L; MOH 2018). The criteria for Fe are not available in the WHO criteria. Our results indicated that the mean and maximal concentrations of Fe exceeded the Vietnamese standard for drinking water (300 μg/L; MOH 2018) during the study period.

The ecological risk assessment of heavy metals in surface water of the TAR is summarized in Table 4 and Supplementary Tables S4 and S5. The mean potential ecological risk indices for individual heavy metals showed that the ERF for As, Fe, Pb, and Zn during the study period were below 40, thus indicating the low potential ecological risks. The maximal concentration of Fe posed a moderate ecological risk or a considerable potential ecological risk only in June and August. Similarly, the mean ecological risk for all factors was low during the study period.

The contribution of the four metals such as As, Fe, Pb, Zn, and MC is estimated through direct ingestion and dermal absorption to the target HQ and HI is shown in Table 5 and Supplementary Table S6. Our results indicated that MC contributed the largest proportions (from 77 to 93%) to the total health risks, and among the four metals, As and Fe contributed more to the non-carcinogenic risk. The mean values of HQ for the four heavy metals and MC were in the order of MC > Fe > As > Pb > Zn. According to the U.S. EPA (2019), the potential non-carcinogenic risks were considered if HQ or HI ≥ 1. In the present study, the HQ was less than 1 for all heavy metals during the study period. Hence, the direct ingestion and dermal absorption of these four metals, individually and collectively, through water consumption do not pose any non-carcinogenic health risks for both adults and children. In contrast, the HQ values for MC are ≥1 in most sampling events, indicating that MC contamination in the surface water poses potential non-carcinogenic risks to both adults and children through direct ingestion and dermal absorption. The HI values were higher than 1 during the study period (except in February), suggesting that the residents would be under significant health risks from the intake of MCs through consumption and dermal adsorption of the water.

The TAR is the largest man-made reservoir in Vietnam, which plays a crucial role in the lives of the local people and the socio-economic development of the region (Nguyen et al. 2020; Pham et al. 2020). Previous studies have reported that the water quality of the TAR was classified from light-eutrophic to super-eutrophic conditions (Nguyen et al. 2021). Under these conditions, cyanobacteria can reproduce at explosive rates, forming dense concentrations called cyanobacterial blooms (Pham et al. 2020). During the last two decades, the TAR has experienced cyanobacterial blooms (Ha et al. 2020; Nguyen et al. 2020; Pham et al. 2020). However, little is known about the human health risks associated with incidental exposure to cyanotoxin-contaminated water. According to WHO's guideline values for recreational exposure to cyanobacteria, the TAR is unsafe for recreational purposes in terms of Chl-a concentration for most sampling events. Consequently, the water may pose human health risks. Hence, monitoring and surveillance modes should be implemented to protect public health. The WHO's guideline values for recreational exposure to cyanobacteria are associated with three levels of probability of health effects at increasing densities of total cyanobacteria, Chl-a concentration, and MC concentration (U.S. EPA 2019) (Table 1). The findings of our study indicate that among the three parameters, the Chl-a concentration was the parameter to be most often exceeded and therefore could be used as a potential indicator for warning of toxic cyanobacteria in recreational and drinking water. This parameter can be easily measured using a spectrophotometer, therefore making it easy and cost-effective to monitor from a surveillance perspective.

In the last two decades, heavy metal contamination in lakes and reservoirs has become a major environmental problem because of their high toxicity (Deng et al. 2020; Zhou et al. 2020). Heavy metals can enter surface water bodies from both natural and anthropogenic sources (Bashir et al. 2020; Chakraborty et al. 2021). While natural sources do contribute some amount of heavy metals to aquatic ecosystems, anthropogenic activities are often the primary drivers of increased heavy metal loads in the environment (Zhou et al. 2020; Chakraborty et al. 2021). In certain cases, specifically in the Irrigation-T and Drinking-Y reservoirs, China, agricultural activities and mining have been identified as the main sources of heavy metal pollution (Deng et al. 2020). In recent years, the main sources of metal pollution in global river and lake water bodies have shifted from mining and manufacturing activities to rock weathering and waste discharge (Zhou et al. 2020). In this study, we firstly reported the concentration of As, Fe, Pb, and Zn from the surface water of the TAR. The sources of Fe and Pb in the reservoir may be discharged from anthropogenic sources in the upstream rivers such as the Dong Nai River and the La Nga River (Le et al. 2019). Further study is required to understand the full extent of other heavy metal pollution and their sources in the TAR.

In this study, which focuses on the analysis of four heavy metals including As, Fe, Zn, and Pb, it was consistently observed that the concentration of Fe exceeded the standard set by the Ministry of Health in Vietnam (MOH 2018). Fe is the second most abundant element on the earth's crust (Jaishankar et al. 2014). The dominance of Fe in aquatic ecosystems has been reported (Goher et al. 2014; Yahaya et al. 2021). Although Fe is an essential micronutrient for living organisms (Abbaspour et al. 2014), excessive Fe levels can potentially lead to cell toxicity, as it can cause oxidative stress and has a high affinity to thiol groups, which are responsible for cellular defense mechanisms (Fernández-Real & Manco 2014; Jaishankar et al. 2014). As a result, an excessive amount of Fe can lead to tissue damage and results in diverse clinical manifestations and diseases, ranging from iron deficiency anemia and possibly to neurodegenerative diseases (Abbaspour et al. 2014). However, the potential ecological and human health risks associated with Fe have not been adequately addressed in previous studies (Goher et al. 2014; Soliman et al. 2015; Tytła & Kostecki 2019; Tham et al. 2021). Furthermore, there are currently no established human health risk assessment guidelines or global standards specifically for Fe in drinking water. As a result, it is crucial to further develop novel methods for assessing the ecological and human health risks posed by Fe contamination. Additionally, there is a need for the prompt establishment of provisional guideline values for Fe in drinking water to ensure its safe use for domestic purposes. This will aid in protecting both the environment and human health from the potential adverse effects of elevated Fe levels.

In Vietnam, the government, particularly the Ministry of Health (MOH) and the Ministry of Natural Resources and Environment (MONRE), is responsible for developing guidelines and regulations related to water quality and human health risk assessment. However, the specific guideline for human health risk assessment in using lake and reservoir water is not yet available in Vietnam. It indicates that there may be a gap or a need for further development in this area (Nguyen et al. 2017). Although the HQ for As, Fe, and Pb in the TAR may not pose non-carcinogenic health risks for both adults and children, Fe-detected concentrations always exceed the Vietnamese standard (MOH 2018). Thus, it is recommended to consider health safety when using the reservoir water for domestic purposes. On the other hand, the potential non-carcinogenic risks to humans caused by MCs were often present in the TAR, which are also the main contribution to the total risks. Our findings are consistent with previous studies, indicating that children and infants are more susceptible to the health risks associated with environmental pollutants compared to adolescents and adults (Ustaoğlu et al. 2020; Dessie et al. 2021; Nguyen et al. 2021). This heightened vulnerability is attributed to factors such as their developing physiology, higher intake of water and food relative to body weight, and behavioral characteristics (Iglesia et al. 2015). Nguyen et al. (2021) focused on assessing the carcinogenic and non-carcinogenic risks associated with MCs in a specific community near the TAR. However, it is important to note that their approach may not be directly applicable to all individuals living near the TAR. The present study took a broader perspective by estimating the average risk for the entire population, including both adults and children, who have been exposed to the water from the reservoir. Our results not only assessed the risks caused by MCs but also demonstrated the additional health risks stemming from certain heavy metals present in the water. The local people residing near the reservoir may indeed be at risk of exposure to such contaminants if they rely on this water source for consumption (Sexton & Linder 2011; Evans et al. 2019; Nguyen et al. 2021). The joint risks of other toxicants, such as pesticides, antibiotics, and organic pollutants, could be present in the water and pose hidden risks to the local population. These risks require further investigation.

In this study, we conducted an assessment of the ecological and human health risks associated with the presence of cyanotoxins and heavy metals in the TAR. The findings indicated that local communities and ecological systems experience consistent exposure to diverse environmental stressors. Although the ecological risk linked to heavy metals was determined to be low, the presence of cyanobacteria and their toxins in the water poses significant risks to human health. Therefore, it is crucial to take proactive measures to address these elevated human health risks related to cyanobacterial toxins in the TAR. Furthermore, the contamination of Fe in the water also contributes to potential health risks, emphasizing the need for attention. This study highlights the potential ecological and human health risks associated with exposure to a combination of environmental stressors for the local residents in the TAR region. Further studies that integrate toxicity assessments with the identification of hidden contaminants, such as antibiotics and microplastics, are essential for effective risk management and the protection of public health.

The authors would like to thank the Dong Nai Technical Resources and Environment Center for their support during the fieldtrip.

This research was funded by the International Foundation for Science (IFS) under grant no. ‘I2-A-6054-2’.

All authors contributed to the field trip, sample analysis, and writing the manuscript. All authors already read and agreed on the content of the final manuscript.

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

The authors declare there is no conflict.