Surface Water Pollution in the Middle Chinese Loess Plateau with Special Focus on Hexavalent Chromium (Cr6+): Occurrence, Sources and Health Risks

Abstract

Due to the combined influence of unique natural environment and increasing human activities, water quality in the Chinese Loess Plateau is deteriorating. Hexavalent chromium (Cr6+) contamination is a critical water environmental issue in the loess areas. In this study, surface water, spring water, Quaternary phreatic water, and soil and rock samples were collected in the middle Chinese Loess Plateau to quantify the surface water pollution and its main affecting factors. Particularly, the occurrence and potential sources of Cr6+ as well as associated health risks in surface water were thoroughly investigated. The results show that Cr6+ is widely found in Luo River, Xingzi River, Wuding River and Bali River. The upper reaches of the Luo River are the main high-Cr6+ water distribution area. For Bali River, Wuding River, Xingzi River and the downstream of the Luo River, the values of Cr6+ in surface water were below the WHO drinking water limit (0.05 mg/L). The high Cr values in the soil and rock samples indicate that the loess and mudstone are important sources of high-Cr6+ water in the area. Groundwater which is characterized by high Cr6+ concentration is also an important factor affecting Cr6+ concentration in surface water which receives recharge from groundwater. In addition to Cr6+, TDS and nitrate are also important pollutants in surface water. The hydrochemical evolution of the surface water is controlled by rock weathering and evaporation crystallization. The health risk assessment results suggest that the inhabitants in the study area face high carcinogenic risks induced by Cr6+.