Arsenic in geothermal systems of Tengchong, China: Potential contamination on freshwater resources

To detect arsenic geochemistry and evaluate its environmental risk, 30 geothermal water samples were collected from Rehai and Ruidian geothermal fields of Tengchong, China. There were three main water types including SO4 and Na-Cl-HCO3 types in Rehai and Na-HCO3 type in Ruidian. Arsenic concentrations ranged from 22.1 to 1150.3 μg L−1. Relative high concentrations of arsenic with an average of 495.1 μg L−1, as well as K, Na, F, Cl, Li and B, were detected in neutral or alkaline geothermal water samples, which suggested that rock leaching in geothermal reservoir was the main resource of arsenic enrichment in geothermal water. Comparatively, those acid geothermal water samples which were formed by vapor rich in H2S condensing and oxidizing in the shallow groundwater or surface water had lower arsenic concentrations with an average of 77.1 μg L−1. The ratios of arsenite to total arsenic widely ranged from 0.01 to 0.97 and were determined by mixing proportions of geothermal water and shallow groundwater or surface water. Furthermore, there was a significantly positive correlation between arsenic with HCO3 (R = 0.81, P = 0.01). These results implied that shallow groundwater or surface water in Tengchong geothermal area had been probably contaminated by arsenic due to the mixing of geothermal water.