3. Materials and methods
A total of 63 samples were collected from the Zuli River Catchment and these samples were divided into five types: precipitation (n=5), groundwater (n=20), river water (n=25), soil, and river suspended particulates (n=13)(Fig. 1). Local precipitation of headwaters was sampled monthly from May to September for one season (Fig. 1). TDS and pH measurements were performed in the field with a calibrated portable Multi-Parameter Analyzer (HACH HQ40D; typical EMC deviation: ±0.1 pH, ±1% of TDS). Water samples were filtered in the field at 0.45 μm for hydrochemistry analyses and acidified for cations and87Sr/86Sr analysis to pH < 2 with ultrapure HNO3. For the hydrochemistry analysis, the water samples with TDS >1000 mg/l were diluted to TDS less than 500 mg/l. Samples for Boron isotopic analysis were collected in a separate bottle.
All chemical compositions of the samples (n=50) were analyzed at the State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, China. Major cations were analyzed by inductively coupled plasma-atomic emission spectrograph (ICP-AES) (Instrumental Error <5.0%), and major anions were analyzed by ion chromatography (ICS-2000) (Instrumental Error <2.0%).
For strontium isotopes, the isotopic compositions of Sr (n=18; Table 1) were determined by TIMS (Thermal Ionization Mass Spectrometry, Triton, VG354) with nine Faraday collectors at the Institute of Geochemistry, Nanjing Nantai. The 87Sr/86Sr value for the standard NIST987 was 0.710236 ± 0.000007 (2r, n=62) during the measurement period of our samples. The total blank for Sr was less than 0.5 ng, negligible with respect to the total strontium in the analyzed precipitation samples.
Boron isotopes (n=28; Table 1) were analyzed using a Neptune Plus MC-ICP-MS (Nanjing) with an ESI PFA 50 μL/min nebulizer in a quartz cyclonic spray chamber. The isotopic standard (11B/10B) standard NIST 951 =4.04362. To reduce the memory effect, continuous washing with 3% HNO3 and 1% HNO3 and Millipore water in sequence for ~25 min was conducted. The final isotopic compositions are reported as δ11B= 1000 (Rsample/Rstandard-1). The average internal analytical precision (n=100) and the external reproducibility of the measured 11B/10B ratios of 200 ng/mL NIST 951 are ±0.02 ‰ (2 SE) and 0.11 ‰ (2σ), respectively.