GEOCHEMICAL MODEL FOR THE ORIGIN OF ARSENIC IN GROUNDWATER IN A METALLIC ORE AREA, HANGJINHOUQI, HETAO PLAIN, CHINA

High arsenic (As) groundwater is widely distributed in Hetao Plain of Inner Mongolia, China, where Hangjinhouqi County is the greatest arsenic poisoning area. In this study, a polymetallic ore area (mainly including of pyrite, pyrrhotite, chalcopyrite, and sphalerite) in Hangjinhouqi was selected as the study area. Groundwater samples (n=38) were collected and analyzed for As, anions and cations. Results showed that the pH value ranged from 7.00 to 7.89, and As in groundwater was at the range of 76-1,093 µg/L, which was dominated by As (III) (85%) with the remaining (15%) as As (V). The major anions were Cl^- and HCO₃^-, and the major cation was Na⁺. Correlation analysis of As and other chemical components showed that As only had a significant correlation with S (r=0.33), indicating that sulfide minerals may play an important role on As accumulation in groundwater. According to the hydrogeochemical characteristics of high As groundwater, an aqueous system of “Fe-C-O-H” at 25^oC was modeled to calculate As adsorption on Fe-bearing mineral and desorption from sulfide mineral. Modeling results revealed that the weakly alkaline environment led to the migration of adsorbed As into groundwater and the original source of As in groundwater was mainly from dissolution of As-bearing sulfide mineral. In addition, accumulation processes of As into groundwater are as follows: dissolution of arsenic pyrite, adsorption of the soluble As on Fe-bearing mineral or migration of the adsorbed As, and release of the adsorbed As from adsorbents.