Heavy Metals and Related Human Health Risk Assessment for River Waters in the Issyk−Kul Basin, Kyrgyzstan, Central Asia

The water resources of Central Asia play an important role in maintaining the fragile balance of ecosystems and the sustainable development of human society. However, the lack of research on the heavy metals in river waters has a far−reaching influence on public health and the sustainable development in Central Asia. In order to reveal the possible sources of the heavy metals and to assess the associated human health risks, thirty−eight water samples were collected from the rivers of the Issyk−Kul Basin during the period with low river flow (May) and the period with high river flow (July and August), and the hydrochemical compositions and major ions of heavy metals were analyzed. No changes in hydrochemical facies were observed between the two periods and the river water type was calcium bicarbonate. Carbonate dissolution and silicate weathering controlled the variation of cations and anions in river waters from the Issyk−Kul Basin. There were some differences in the sources of heavy metals in water bodies between the two periods. During the period with low river flow, heavy metals (Cr) were closely clustered with major ions, indicating that they were mainly affected by water–rock interactions. During the period with high river flow, all heavy metals studied in this paper had different sources of major ions, and the heavy metals maybe influenced by human activities. From the human health risk assessment, the hazard quotients for all samples were less than 1, reflecting that there was no noncarcinogenic risk in the river waters of the Issyk−Kul Basin during the two sampling periods. However, the water samples with carcinogenic risk of arsenic exceeding the threshold (10−4) accounted for 21.1% of the total, indicating that there were some certain carcinogenic hazards for human health via water drinking with direct oral ingestion. The results are of certain significance for the utilization and protection of water resources in the basin as well as the protection of public health.


Introduction
The water resources of Central Asia play an important role in maintaining the fragile balance of ecosystems and the sustainable development of human society [1][2][3]. The shortage of water resources in Central Asia represented by the Aral Sea crisis has aroused concern worldwide [4,5]. At present, most studies focus on water quantity and water management under the influences of percentage [31,32] was used to evaluate the accuracy of the results for the main cations and anions, and the CBE was less than 5%. The contents of dissolved heavy metals in river waters, including zinc (Zn), copper (Cu), cadmium (Cd), lead (Pb), total chromium (Cr) and arsenic (As), were determined by inductively coupled plasma mass spectrometry with an Agilent 8800 system (Agilent Technologies, Santa Clara, CA, USA). The blank solution of 1 % nitric acid was measured for 21 consecutive times, and the concentration corresponding to 4.6 standard deviations was used as the detection limit for Zn (0.003 µg L −1 ), Cu (0.003 µg L −1 ), Cd (0.003 µg L −1 ), Pb (0.001 µg L −1 ), Cr (0.02 µg L −1 ), and As (0.006 µg L −1 ). The standard sample was measured 12 times and its precision and accuracy was calculated. Experiments show that the precision of heavy metal elements is between 0.74 % and 3.17 %. The aforementioned analyses were performed at the Research Center for Ecology and the Environment of Central Asia (Bishkek), Kyrgyzstan.

Human Health Risk Assessment
Direct oral ingestion is the main pathway of heavy metal exposure from aqueous systems. The human health risk assessment for noncarcinogens was calculated using Equations 1 and 2 [27,[33][34][35]. If the noncarcinogenic hazard quotient (HQ) < 1, no noncarcinogenic risks are suggested; otherwise,
where ADD: average daily dose of exposure through water absorption; CSF: cancer slope factor [37].

Results
For the river waters of the Issyk−Kul Basin shown in Tables 2 and 3, the concentration of Zn varied between 0.634 and 9.75 µg L −1 , with a mean of 5.43 µg L −1 , during the period with low river flow, and the concentration of Zn varied between 0.359 and 26.3 µg L −1 , with a mean of 9.60 µg L −1 , during the period with high river flow. The Cu concentration varied between 1.08 and 8.51 µg L −1 , during the period with low river flow, with a mean of 3.64 µg L −1 , and it varied between 0.30 and 12.5 µg L −1 during the period with high river flow, with a mean of 4.23. The Pb level varied between 0 and 2.19 µg L −1 , with a mean of 1.01 µg L −1 , during the period with low river flow, and from 0 to 4.06 µg L −1 , with a mean of 1.52 µg L −1 , during the period with high river flow. As varied between 0.461 and 2.08 µg L −1 , with a mean of 1.17, during the period with low river flow, and from 0.134 to 3.03µg L −1 , with a mean of 1.25, during the period with high river flow. The Cr value varied between 0.01 and 0.05 mg L −1 , with a mean of 0.03 mg L −1 , during the period with low river flow, and between 0.01 and 0.07 mg L −1 , with a mean of 0.036 mg L −1 , during the period with high river flow.  During the period with low river flow (May 2017), the pH varied from 7.55 to 8.94, with a mean of 8.20. During the period with high river flow (July and August 2017), the pH varied from 7.79 to 8.84, with a mean of 8.25. The EC ranged between 60 and 299 µS cm −1 , with a mean of 144.53 µS cm −1 , during the period with low river flow, and from 71 to 352 µS cm −1 , with a mean of 170 µS cm −1 , during the period with high river flow. The TDS ranged from 110 to 355 mg L −1 , with a mean of 204 mg L −1 , during the period with low river flow, and the TDS varied from 119 to 392 mg L −1 , with a mean of 217 mg L −1 , during the period with high river flow.
As seen from the table showing statistics on the concentration of major ions in the river waters, HCO 3 − was found in the highest concentration. No changes in hydrochemical facies were observed between the two periods. As seen in the Durov diagrams [40,41], the river water type was calcium bicarbonate (Figure 2).
during the period with low river flow, and the TDS varied from 119 to 392 mg L −1 , with a mean of 217 mg L −1 , during the period with high river flow. As seen from the table showing statistics on the concentration of major ions in the river waters, HCO3 − was found in the highest concentration. No changes in hydrochemical facies were observed between the two periods. As seen in the Durov diagrams [40,41], the river water type was calcium bicarbonate (Figure 2).

Discussion
In order to determine the detailed factors affecting water chemistry, Gibbs diagrams [42][43][44] and mixing diagrams [42,[45][46][47] were used. For the Gibbs and mixing diagrams, the concentrations of major ions were transformed to milliequivalents per liter [48] as seen in Figure 3. This suggests that river samples plotted in the dominant area are dominated by rock, which suggests that major ions in river water from the Issyk−Kul Basin are mainly controlled by the process of water-rock interactions. The river samples were located between the two end−members of carbonate and silicate ( Figure 3). Figure 3 shows that river water samples from the Issyk−Kul basin had high Ca/Na ratios, indicating that the importance of carbonate dissolution is greater than that of silicate weathering.
The statistical characteristics of hydrochemical data including major ions and heavy metals ( Table 2) cannot explain the relationship between various chemical proxies, so we used the hierarchical cluster analysis (HCA) [49] to clarify the possible influences on heavy metals in the river waters of the Issyk−Kul Basin [50][51][52][53]. The chemical compositions of the river waters were analysed at different periods using HCA. The results showed that there were some differences in the sources of heavy metals in water bodies between the two periods. During the period with low river flow, the heavy metal element Cr was closely related to calcium, bicarbonate, and potassium ions (Figure 4). This indicates that the heavy metal elements were mainly affected by the interaction between water and rock and mainly arise from the natural weathering of rock. However, the relationship between other heavy metals (Pb, Zn, Cu, and As) and the major ions was more distant, which reflects different ion sources and suggests effects from human activities (Figure 4). For the period with high river flow, heavy metals were clustered far from the major ions, reflecting different natural origins of these major ions ( Figure 5). It can be concluded that heavy metals are affected by different factors during different hydrological periods. mixing diagrams [42,[45][46][47] were used. For the Gibbs and mixing diagrams, the concentrations of major ions were transformed to milliequivalents per liter [48] as seen in Figure 3. This suggests that river samples plotted in the dominant area are dominated by rock, which suggests that major ions in river water from the Issyk−Kul Basin are mainly controlled by the process of water-rock interactions. The river samples were located between the two end−members of carbonate and silicate (Figure 3). Figure 3 shows that river water samples from the Issyk−Kul basin had high Ca/Na ratios, indicating that the importance of carbonate dissolution is greater than that of silicate weathering.  The Issyk−Kul Basin is an important international tourist area, and the number of international tourists visiting this area can be quite high. The noncarcinogenic and carcinogenic risks related to the water in this area have important practical significance in public health. From the calculated results (Table 4), the hazard quotient for noncarcinogenic risk was less than one during the two sampling periods, reflecting that no noncarcinogenic risk was posed by heavy metals in the river waters of the Issyk−Kul Basin; however, the heavy metal Cr was present in concentrations close to the threshold value, which needs more attention. From the carcinogenic risk index, the maximum value of the heavy metal As was more than 10 −4 (maximum was 1.25 × 10 −4 ) during the period with high river flow, and the carcinogenic risk exceeding the threshold account for 21.1% of the total samples, which indicates that there is a certain carcinogenic risk to human health ( Figure 6). Although it is not more than 10 −4 , during the period with low river flow, it is also close to the threshold and needs a high degree of attention. and rock and mainly arise from the natural weathering of rock. However, the relationship between other heavy metals (Pb, Zn, Cu, and As) and the major ions was more distant, which reflects different ion sources and suggests effects from human activities (Figure 4). For the period with high river flow, heavy metals were clustered far from the major ions, reflecting different natural origins of these major ions ( Figure 5). It can be concluded that heavy metals are affected by different factors during different hydrological periods.  The Issyk−Kul Basin is an important international tourist area, and the number of international tourists visiting this area can be quite high. The noncarcinogenic and carcinogenic risks related to the water in this area have important practical significance in public health. From the calculated results (Table 4), the hazard quotient for noncarcinogenic risk was less than one during the two sampling periods, reflecting that no noncarcinogenic risk was posed by heavy metals in the river waters of the Figure 5. Hierarchical clustering analysis of major ionic and dissolved heavy metal elements in the river waters from Issyk−Kul Lake Basin during the period with high river flow. Table 4. Human health risk assessment including the noncarcinogenic hazard quotients (non. HQ) and carcinogenic risks (Cancer. R) from heavy metals in the river waters of the Issyk−Kul Basin during the period with high river flow (H) and the period with low river flow (L).

Stage
Heavy  (1)) was shown in Table 2. b : The maximum value of (C h , Equation (1)) was shown in Table 2.  Arsenic is a Class−A human carcinogen [54], and the exposure to arsenic through direct drinking of arsenic−contaminated water or consumption of arsenic−contaminated edible crops is considered a worldwide life−threatening problem [55][56][57]. Through human health risk assessment, the heavy metal of Arsenic in river waters has shown some harm to human health. Based on this, the exact material source of arsenic and its migration and transformation in water bodies requires in−depth discussion. However, based on the existing data and other issues, it is not possible to elaborate on the above issues, and we need to carry out in−depth research in the next step.
Although the risks of noncarcinogenic and carcinogenic effects are low, the heavy metals in river waters may be affected by human activities, especially during wet periods with high river flow. As the Issyk Lake area is an internationally famous area for tourism and one of the main agricultural and livestock production areas in Kyrgyzstan, environmental protection of this area is very important. Thus, the treatment and discharge of sewage and the protection of the aquatic environment need to be given enough attention to avoid repeating the traditional pattern of experiencing pollution problems before solutions are implemented. Arsenic is a Class−A human carcinogen [54], and the exposure to arsenic through direct drinking of arsenic−contaminated water or consumption of arsenic−contaminated edible crops is considered a worldwide life−threatening problem [55][56][57]. Through human health risk assessment, the heavy metal of Arsenic in river waters has shown some harm to human health. Based on this, the exact material source of arsenic and its migration and transformation in water bodies requires in−depth discussion. However, based on the existing data and other issues, it is not possible to elaborate on the above issues, and we need to carry out in−depth research in the next step.

Conclusions
Although the risks of noncarcinogenic and carcinogenic effects are low, the heavy metals in river waters may be affected by human activities, especially during wet periods with high river flow. As the Issyk Lake area is an internationally famous area for tourism and one of the main agricultural and livestock production areas in Kyrgyzstan, environmental protection of this area is very important. Thus, the treatment and discharge of sewage and the protection of the aquatic environment need to be given enough attention to avoid repeating the traditional pattern of experiencing pollution problems before solutions are implemented.

Conclusions
Based on the concentrations of major ions and heavy metals in river water samples, the possible sources of heavy metals in the Issyk−Kul Basin, Kyrgyzstan, Central Asia, and related human health risks were revealed. The results were as follows: Carbonate dissolution and silicate weathering accounted for the variation in the major ions of river waters from the Issyk−Kul Basin. No changes in hydrochemical facies were observed between the two sampling periods, and all water samples belong to the type of calcium bicarbonate.
During the period with low river flow, the heavy metal Cr was mainly affected by water-rock interactions, and the sources of other heavy metals were different from the major ions. During the period with high river flow, all heavy metals studied in this paper had different sources of major ions, and the heavy metals were maybe influenced by human activities.
Based on the human health risk, the hazard quotients were less than 1, reflecting no noncarcinogenic risk for the heavy metals in the studied river waters. However, the water samples with carcinogenic risk for arsenic exceeding the threshold (10 −4 ) account for 21.1% of the total, indicating that there was a certain carcinogenic risk in river water in this area.

Conflicts of Interest:
The authors declare no conflict of interest.