# Hydrogeochemistry of Fault-Related Hot Springs in the Qaidam Basin, China
## 3 Sampling Analysis: Methods and Results  
In October 2020, water samples for chemical analysis were collected from four major hot springs (the Yitunbulake, Aiken, Laomangya, and Gansen (hereafter referred to as YTBLK, AK, LMY, and GS, respectively) along the intersection of the Altun Tagh fault and the East Kunlun fault belt (Figure 1c). A few field procedures were performed during sampling, including measuring water temperature, pH, and electrical conductivity (EC) using a portable measuring device WTW Multi 3400i (WTW, Munich, Germany) with precision of 0.1 \({}^{\circ}\)C, 0.01, and 1 s/cm for the three metrics, respectively. The device was calibrated with a standard solution before measurements were taken. In addition, basic information such as temperature, air pressure, and data on the surroundings was recorded. Samples were filtered in the field using a hand-held syringe with 0.45 mm membrane filters. The filtered samples were collected in fresh 100 mL polyethylene bottles and later subjected to analysis of their conventional ion components, trace elements, and SiO\({}_{2}\) content. The unfiltered samples were stored in HDPE plastic bottles and later subjected to isotope analyses. Prior to use, these bottles were rinsed with the sampling-point spring water to minimize the chances of contamination. The samples were then transported to the laboratory with proper care to prevent any possible effects of evaporation. Anions and cations were measured via high-performance ion chromatography using Dionex ICS-900 (Dionex, Sunnyvale, CA, USA) apparatus. The trace elements were determined using an X SERTES II (Thermo, Waltham, MA, USA) inductively coupled mass spectrometer. Hydrogen and oxygen isotopes were determined using a High-Temperature Conversion/Elemental Analyzer (TC/AE) coupled to an L2130-I Isotope Ratio Analyzer (Picarro, Santa Clara, CA, USA). The SiO\({}_{2}\) was determined using an inductively coupled Optima 5300 DV (PerkinElmer, Waltham, MA, USA) plasma emission spectrometer. The \({}^{87}\)Sr/\({}^{86}\)Sr ratios were measured via thermal ionization mass-spectrometry using a Finnigan MAT-261 (ThermoFinnigan, Silicon Valley, CA, USA) spectrometer. The above experiments were conducted at the Key Laboratory of Earthquake Prediction, Institute of Earthquake Forecasting (China Earthquake Administration). All experiments were completed within 30 days of sampling. The gas samples were collected in October 2020 from the YTBLK and AK springs with strong activity at the intersection of the Altun Tagh fault and the East Kunlun fault belt. Samples were collected in fresh 500 mL glass bottles using the drainage method. The N\({}_{2}\), H\({}_{2}\), CO\({}_{2}\), O\({}_{2}\), CH\({}_{4}\), and Ar components of the hot spring gas were measured using Agilent Macro 3000 (Agilent Technologies, Palo Alto, CA, USA) gas chromatography equipment. The estimated measurement errors were as follows: when the content was 1%-100%, the RSD was less than 0.5%; when the content was 0.01%-0.1%, the RSD was less than 1%; when the content was 0.001%-0.01%, the RSD was less than 2%; and when the content was 0.0001%-0.001%, the RSD was less than 5% [38]. The \({}^{3}\)He/\({}^{4}\)He and \({}^{4}\)He/\({}^{20}\)Ne ratios were analyzed using a Noblesse noble gas mass spectrometer produced by Nu Instruments, Wrexham, United Kingdom. Carbon dioxide isotopes were determined using a GC-IRMS analytical system gas chromatography (Agilent 6890, Agilent Technologies, Palo Alto, CA, USA) stable-isotope ratio mass spectrometer (Delta Plus XP, Thermo-Fisher Scientific, Waltham, MA, USA), coupled with an online sample preprocessor. The above experiments were conducted at the Oil and Gas Research Center of the North-West Institute of Eco-Environment and Resources (Chinese Academy of Sciences). All experiments were completed within 30 days of sampling.