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.