*3.2. Analytical Method*

The hydrochemical indexes of high-As groundwater (pH value, conductivity, dissolved oxygen and oxidation-reduction potential) were determined in the field by employing American Hach sensION2 and Hach LDO TM HQ 10 portable testers. The NO<sup>3</sup> − and SiO<sup>2</sup> of groundwater were determined by adopting the UV8500 UV-vis spectrophotometer. Cations (e.g., Ca, Mg, Na, K, Fe, Mn, Sr, B and Ba) were determined with PHS-2C ICP-AES. Fe elements were examined in Fe2+ and Fe3+. Se was determined under an XGY-1012 atomic fluorescence spectrometer. As3+ and As5+ were tested with XGY-1012 atomic fluorescence spectrometer, respectively. Other elements were analyzed by using the standard methods. To ensure the quality of the test data of groundwater samples, 5% repeated samples were added, and the error of all repeated samples was less than 5%. The samples were tested by the Key Laboratory of Groundwater Mineral Water and Environmental Supervising and Testing Center, Ministry of Land and Resources of the PRC.
