Distribution characteristics, enrichment patterns and health risk assessment of dissolved trace elements in river water in the source region of the Yangtze River

The security of water environment in the source region of the Yangtze River (SRYR) is vital to the water environment security of the whole basin. The results showed that the rivers in the SRYR were weakly alkaline and the values of total solid solubility (TDS), electrical conductivity (EC), turbidity concentration and salinity were higher than the values in the middle and lower reaches of the Yangtze River. The results showed that the dissolved trace elements detected displayed obvious regional distribution characteristics, showing a high concentration trend in the Chumar River, low in the Dangqu, and middle in the Tong River. All water quality indexes in the SRYR met the surface water environmental quality standard of class II based on GB 3838-2002 except Hg, while the average concentration of As exceeded 10 μg/L. The main enrichment elements in the SRYR were Li, Se, As and Pb, and their concentrations were far higher than the average concentration of the world rivers. Moreover, the HI and HQingrstion of children caused by As in the SRYR were greater than 1. This study could provide basic data for water environment protection and water resource management in the SRYR.


INTRODUCTION
Trace elements can be absorbed by organisms in aquatic ecosystems through physical, chemical and biological cycling processes (He & Charlet ; Kumar et al. ; Zhang et al. ). Enrichment of toxic metals in water systems makes the water unfit to drink, and the ingestion of considerable amounts of metal can lead to mental disease (Li et al. ). Trace elements are mainly derived from natural and human activities. Natural sources include bedrock weathering and erosion, soil leaching, volcanic eruptions and atmospheric precipitation, while human activities include mining, metal smelting and refining, agricultural runoff, industrial activities etc. (Singh et al. ; Li et al. ). In recent years, researchers have become increasingly concerned with the assessments of trace metals in the aqueous environment (Kavcar et  In the present research, distribution characteristics, enrichment patterns and health risk assessment of dissolved trace elements in river water in the SRYR were investigated, aiming to provide basic data for water environment protection and water resource management in the SRYR.

STUDY AREA
The Yangtze River is the longest river in China and the third longest in the world. The SRYR is located in the hinterland of the Qinghai-Tibet Plateau, known as the water tower of China, and the runoff accounts for about 1.3% of the total flow of the Yangtze river (Chen ). The SRYR, with high altitude and low temperature, is semi-

Sample collection
According to the regional characteristics of the SRYR, 34 sampling sites were set up ( Figure 1) and detailed information of all sampling sites is shown in Appendix A. Thirteen sampling sites (D1-D13) were distributed in Dangqu, three (T1-T3) in Tuouo River, five (C1-C5) in Chumar River and 13 (TT1-TT13) in Tongtian River.
Surface water samples were collected at a depth of about 10 cm, and filtered through pre-washed 0.45 μm Millipore nitrocellulose filters. The initial portion of the filtration was discarded to clean the membrane, and the following ones were acidified to pH < 2 with ultra-purified 6 M HNO 3 and then stored in plastic bottles for trace metal analyses. Cleaning of plastic bottles was carried out by soaking in 20% (v/v) HNO 3 for 24 h and then rinsing with milli-Q deionised water (∼18 MΩ/cm resistivity; TOC < 5 mg/L) from Milli-Q (Millipore, Direct 8).

Analysis methods
The pH, EC, TDS and turbidity in the water were measured by a portable water quality analyzer (Xylem, EXO2). Cu, Pb, Resources of the People's Republic of China ); the detection limit was 0.01 μg/L, and the recovery rate was 93-104%. A standard was inserted to ensure data accuracy after every ten samples. The blank sample was below the detection limit. The mean of three runs was obtained for each sample.
Data below the analytical detection limits were placed at a value of half the detection limit when constructing all plots and statistical calculations (Yang et al. ). In order to better analyze the situation of trace elements in the SRYR, some data was used from the previous research conducted by Qu et al. ().

Physicochemical properties of water
The results conducted from 2012 to 2019 are shown in Table 1. The pH value of the SRYR ranged from 7.6 to 9.1, with an average value of 8.2, indicating that the river water was weakly alkaline. The value of TDS ranged from 50 to 10,336 mg/L, which indicated that the regional geological structure was more complicated with a large variation (Noh et al. ). The average TDS concentration was 1,685 mg/L, which was ten times higher than the average TDS of the world's rivers (150 mg/L) (Gaillardet et al.

), and the average level of TDS is displayed as follows:
Chumar River > Tuotuo River > Tongtian River > Dangqu.

Evaluation of trace elements
The average concentration of 17 types of trace metals in the SRYR are presented in Table 2. The average value of EC fol- where C w is the average concentration (μg/L); IR is the ingestion rate (L/day); ABS GI is the gastrointestinal absorption factor; EF is the exposure frequency (days/year); ED is the exposure duration (years); S A is the exposed skin area(cm 2 ); K p is the dermal permeability coefficient; ET is the exposure time (h/day); BW is the body weight (kg); and AT is the average time for non-carcinogens (days). The reference values of exposure parameters are shown in Tables 3 and 4.
The risk index (HI) can assess the full potential non-carcinogenic risk of multiple pathways, and HI values more than 1 (HI > 1) indicate that the pollutant may have adverse effects on human health or require further study (Giri & Singh ):  River were all greater than 0.1, indicating that special attention should be paid to the adverse effects of As for local residents, especially for children. Comparing different rivers in the SRYR, the health risks caused by As followed the order of Tuotuo River > Chumar River > Tongtian River > Dangqu. In general, the health risks of trace elements in Dangqu were lower than that of the Tuotuo River, Chumar River, and Tongtian River.

CONCLUSIONS
The surface water of the SRYR was shown to be weakly alkaline and brackish, and the TDS and EC were high com-   c Wang et al. (2017).