Cap6300

The cation and anion concentrations were measured by inductively coupled plasma-optical emission spectrometry (CAP6300, Thermo, USA) and ion chromatography (ICS1100, Dionex, USA), respectively. As_Tot and Fe_Tot in the sediments were extracted by 1:1 aqua regia digestion method in a water bath [27 ]. Pre-separated As(III) and As(V) from hot spring waters and extracted As_Tot from sediments were determined using liquid chromatography-hydride generation -atomic fluorescence spectrometry (LC-HG-AFS, Haiguang AFS-9780, Beijing) according to Jiang et al. [25 (link)]. Extracted Fe_Tot from sediments was determined by the 1,10-Phenanthroline-based assay: 10 mL extracted solutions was mixed with 5 mL acetate-sodium acetate buffer (pH = 4.6), 2.5 mL 1% hydroxylamine hydrochloride solution and 5 mL 0.1% 1,10-phenanthroline solution in 50 mL volumetric flask. The mixtures were made up to a volume of 50 mL with deionized water and allowed to stand for 10 min. The absorbance of each solution at 510 nm was measured with a spectrophotometer (UV1750, Shimadzu, Japan). Dissolved organic carton (DOC) of water samples and total organic carbon (TOC) of sediment samples were determined using a TOC analyzer (TOC-V_CPH, Shimadzu, Japan) and a Macro elemental analyzer (Multi EA 4000, Analytik Jena, Germany), respectively.

Cation and anion concentrations were measured by inductively coupled plasma-optical emission spectrometry (CAP6300, Thermo, USA) and ion chromatography (ICS1100, Dionex, USA), respectively. Arsenic concentrations were determined using liquid chromatography-hydride generation-atomic fluorescence spectrometry (LC-HG-AFS, Haiguang AFS-9780, Beijing)33 (link). Thioarsenic species were identified using Q Exactive, a high resolution quadrupole orbitrap mass spectrometer (Thermo Scientific, Germany), by detecting the accurate mass and matching the isotope abundance. DOC of water samples were determined using a TOC analyzer (TOC-V_CPH, Shimadzu, Japan). As and Fe in the sediments were extracted by 1:1 aqua regia digestion in a water bath45 . Extracted Fe from sediments was determined by a 1,10-Phenanthroline-based assay: 10 mL extracted solutions were mixed with 5 mL acetate-sodium acetate buffer (pH = 4.6), 2.5 mL 1% hydroxylamine hydrochloride and 5 mL 0.1% 1,10-phenanthroline solution in a 50 mL volumetric flask. The mixture was brought up to a volume of 50 mL with deionized water and allowed to stand for 10 min. The absorbance of each solution at 510 nm was measured with a spectrophotometer (UV1750, Shimadzu, Japan). TOC of sediment samples was measured with a Macro elemental analyzer (Multi EA 4000, Analytik Jena, Germany) after inorganic carbon was digested using HCl.