Spatial and temporal distribution and affecting factors of iron and manganese in the groundwater in the middle area of the Yangtze River Basin, China.

Iron (Fe) and manganese (Mn) are heavy metals ubiquitous in groundwater. High levels of Fe and Mn in groundwater can compromise water quality and pose a risk to human health if the groundwater is used for drinking or irrigation. In the middle region of the Yangtze River Basin, groundwater has been extensively used for domestic and agricultural purposes. However, little is known about the distribution of Fe and Mn in the groundwater in this area. It was found that the 74.4% and 48.9% of the groundwater exceed the China national guideline for Fe (i.e., 0.3 mg/L) and Mn (i.e., 0.1 mg/L), respectively. And 6.38% and 2.13% of the wells had Fe and Mn health risks, respectively. Spatial heterogeneity of Fe and Mn was observed. Notably, the concentrations of Fe and Mn in a plain region located between two major rivers (i.e., the Yangtze River and the Han River) were significantly higher than those in other regions. Modeling using PHREEQC revealed that the Fe-bearing minerals in the plain region were more saturated compared to those in the other regions. Besides, temporal change of Fe and Mn was observed in the plain region, significantly affected by rainfalls and groundwater levels. In addition, the distribution of Fe and Mn was significantly affected by various physicochemical factors. Particularly, Fe was more sensitive to redox potential compared to Mn. Under a reducing condition, organic matter concentration and water residence time also affect the release of Fe from Fe-bearing minerals. Overall, a comprehensive understanding of distribution characteristics of Fe and Mn and affecting factors in the middle area of the Yangtze River Basin can provide guidance for the distribution of industrial water, agricultural water, and drinking water in different regions of the study area. Especially in the plain area between the Yangtze River and the Han River, direct drinking of groundwater shall be reduced since the higher health risk value of Fe and Mn.