Study on Natural Background Levels of Iron and Manganese in Groundwater of Yinchuan Plain.

Groundwater environmental background value is a core parameter for distinguishing natural background fluctuations from anthropogenic pollution interference, underpinning the safe utilization of groundwater resources and effective water environment management. Accurately determining such values in high iron and manganese enrichment areas bears great theoretical and practical significance, yet traditional calculation methods are susceptible to anomalous data and cannot effectively differentiate geological-origin from human-induced anomalies. Taking the Yinchuan Plain as the study area, this research adopts two-factor method of absolute content and milliequivalent percentage (two-factor method), combined with hierarchical cluster analysis (HCA) and principal component analysis (PCA) to eliminate outliers based on 741 sets of shallow groundwater hydrochemical monitoring data. The background values are calculated from the normal distribution of filtered data and verified against traditional statistical methods. Results show the upper background limit and threshold of total iron (TFe) are both 0.30 mg/L, whereas those of manganese (Mn) are 0.09 and 0.10 mg/L. Groundwater Fe and Mn enrichment in the Yinchuan Plain is dominated by natural geological processes. Quaternary unconsolidated sediments provide the primary material source, whereas the flat topography induces slow runoff and a strongly reducing environment, facilitating the reductive dissolution of Fe/Mn minerals. Complexation with HCO