The Genesis Mechanism and Health Risk Assessment of High Boron Water in the Zhaxikang Geothermal Area, South Tibet

Abstract

The natural discharge of geothermal water containing harmful components affects the water quality of the surrounding environment and brings security risks to drinking water safety. The geothermal water in Tibet is characterized by high boron content, but the water pollution caused by the discharge of this high boron geothermal water is not clear. In this study, we collected geothermal water and surface water from the Zhaxikang geothermal system in southern Tibet to investigate the causes of high boron geothermal water and the water pollution of water quality by its discharge. The results indicate that the hydrochemical type of geothermal water was HCO3-Cl-Na, while that of cold spring water, mine water, river water, and lake water was SO4-HCO3-Ca-Mg. Hydrogen and oxygen isotopes show that the recharge source of cold groundwater was mainly snow-melting water and meteoric water, while in addition to that, there is magmatic water for hot springs. The boron content of geothermal water in the study area is as high as 42.36 mg/L, far exceeding the World Health Organization limit for drinking water (0.5 mg/L). The analysis of ion components and PHREEQC modeling indicated that the dissolution of silicate minerals and cation exchange controlled the composition of groundwater, and the boron in groundwater mainly came from the volatilization of magmatic components and the leaching of shallow sediments. The entropy weight water quality index was used to evaluate the water quality of the study area; about 42.9% of the groundwater samples are of good quality and can be used for drinking, mainly cold water that has not been mixed with geothermal water in the upstream. With the discharge of geothermal water into the river (with a mix ratio of ~20%), the downstream water quality gradually deteriorated. The health risk assessment of drinking water in the study area showed that the hazard index (HI) of drinking water in the mixed area was higher than 1 (with an average of 1.594 for children and 1.366 for adults), indicating that children are at a higher health risk than adults. Geothermal water with high boron content has been found all over the world, and the adverse effects of its natural drainage cannot be ignored.