3. Data source and methods
The precipitation samples were collected from every precipitation event during the period from January to December in 2018 at the meteorological bureau of Gangcha (3301.5 m a.s.l.), Qinghai Province, China. During the sampling period, a total of 104 samples, including 83 of rainwater and 21 of snow or sleet, were collected, filtered and stored in 30 ml high-density polyethylene square bottles for isotopic analyses.
Groundwater samples were collected from 34 sites in September 2018, which were evenly distributed around Qinghai Lake (Fig. 1). Location information of sampling sites was acquired using a global positioning system (GPS). All samples were filtered through 0.45 μm nylon filters. Water samples were stored in 30 ml high-density polyethylene square bottles for isotopic analyses and two 100 ml bottles for chemical analyses. Samples for cation testing were acidified with ultrapure HCl. The samples for anion and isotope testing were transported with ice bags and refrigerated at approximately 4 ℃ until laboratory analysis. The pH and electrical conductivity (EC) of groundwater was measured in situ using a handheld meter with a probe. The stable isotopes were analysed using the Los Gatos Research IWA-45-EP Isotopic Water Analyzer. The isotopic values were reported using the standard δ notation relative to the V-SMOW (Venna Standard Mean Ocean Water) standard; the precisions were ±0.1 and ±0.2 ‰ for δ2H and δ18O, respectively. The hydrochemical parameters of the groundwater samples were examined, including TDS (total dissolved solids), TH (total hardness), Na+, K+, Mg2+, Ca2+, Cl-, SO42-, CO32-, HCO3-, and NO3-. The cations of the samples were determined using a Dionex-600 ion chromatograph, and the anions were measured using a Dionex-500 ion chromatograph. CO32-and HCO3- were directly titrated in situ with phenolphthalein, methyl orange and sulfuric acid.
The Piper diagram (Piper, 1944) and the boomerang envelope model developed by Gibbs (Gibbs, 1970) were used to reveal the hydrochemical characteristics and evolution of groundwater. A Piper diagram generally used the major cations and anions of the water body to categorise the hydrogeochemical type of groundwater, river water and lake water, etc. (Piper, 1944; He et al., 2019). Gibbs (1970) analysed the chemical composition of surface water worldwide, and divided the factors controlling the composition of water into three endmembers, namely rock weathering, atmospheric precipitation, and evaporation/crystallisation (Machender et al., 2014). Meanwhile, the groundwater quality was assessed according to the Chinese State Standards for drinking water quality (Chinese Ministry of Health, 2006) and Chinese State Standards for groundwater water quality (Chinese General Administration of Quality Supervision, 2017). As shown in the standards, the highest acceptable limits of pH, TH, TDS and the concentrations of Na+, Cl-, NO3- and SO42- were 8.5, 450 mg/L, 1000 mg/ L, 200 mg/L, 250 mg/L, 20 mg/L and 250 mg/L, respectively. In order to analyse the spatial characteristics of water level, isotope and hydrochemistry of groundwater around Qinghai Lake in more detail, the area around the lake was divided into four regions: east (G1-G11), south (G12-G20), west (G21-G29), and north (G30-G34) of Qinghai Lake.