Groundwater quality characteristics and pollution causes in typical hilly areas of North China: the case of Longan District, Anyang City, Henan Province

In order to ascertain the groundwater quality in Longan District, Anyang City, a typical hilly area in North China, the data of 24 water quality indicators from 127 monitored well samples were obtained, and the groundwater quality characteristics and pollution causes were analyzed. On the basis of hydrochemical statistics and water quality evaluation, the inverse distance weighting (IDW) spatial interpolation method was used to analyze the spatial distribution of different pollutants, and the principal component analysis was used to identify pollution sources. The results showed that HCO₃-Ca type was the typical chemical type of shallow groundwater in Longan District. The NO₃ ⁻ concentration at 93.0% of samples exceeded the class Ⅲ standard defined in Standard for Groundwater Quantity (GB/T 14848-2017), whose maximum exceeding multiples was 47.1. The single factor evaluation showed that the NO₃ ⁻ pollution index of groundwater monitoring well samples in Longan District reached a maximum value of 48.10, among which 27% were heavy pollution level, and 19% were serious to extremely heavy pollution levels. The concentration of SO₄ ²⁻ reached moderate pollution level, while Cl⁻, NH₄ ⁺, and F⁻ reached mild pollution level. Nemero comprehensive evaluation results showed that the comprehensive groundwater pollution index was between 0.01 and 34.69, and the groundwater quality was generally at good condition. Among them, 4% of the monitored well samples were at extremely poor pollution level, which were mainly distributed in the middle of Longan District. The highest concentrations of NO₃ ⁻, NH₄ ⁺, SO₄ ²⁻, Cl⁻ were mainly located in the middle, presenting the spatial distribution characteristics of higher concentration in the middle than that in the east and west. The concentration of F⁻ was higher in the east and lower in the west, and the highest was mainly distributed in the groundwater retention area. Source analysis showed that the groundwater quality was mainly affected by regional geohydrologic conditions, groundwater geochemical characteristics, and agricultural non-point source pollution.