[Spatial Distribution and Ecological Risk Assessment of Heavy Metals in Soil and Water in a Water Source Area of...

To investigate the issues of heavy metal pollution in soil and water， ecological risks， and pollution source identification in the water source protection area， 254 surface soil samples and 66 water samples were collected from the 1st to 3rd grade water systems in the source protection area of the Datong River， Guangxi Zhuang Autonomous Region. A systematic assessment was conducted on these samples by integrating the single pollution index， Nemerow comprehensive pollution index， potential ecological risk index， and principal component analysis. The results showed that the average contents of heavy metals in the soil of the study area followed the order： Cr>Zn>Pb>As>Cu>Ni>Cd>Hg. Among them， 16 samples exceeded the screening values（pH<5.5） specified in the Soil Environmental Quality Risk Control Standard for Soil Contamination of Agricultural Land（GB 15618-2018）. The average contents of various heavy metals in river water were much lower than the standards for Class I surface water. In the soil of the main stream of Datong River， Cu， Cd， and Hg exhibited a pattern of segmented enrichment， which was controlled by the natural weathering process of parent rocks（Members 1-4 of the Huangdongkou Formation）. A small number of samples showed significantly high concentrations of Cr， As， Pb， as well as Cu and Hg， which could be correlated with the distribution of villages， towns， and farmlands. Multiple occurrences of As pollution were detected in the Liudian gold mining area， which were attributed to mining activities. For the soil samples， the Nemerow comprehensive pollution index（P_N） ranged from 0.11 to 10.54， with samples at the warning level and mild pollution level accounting for 33.86% and 49.61%， respectively. For the river water samples， the single pollution indices were generally far below 1， and the P_N values ranged from 0.144 to 1.733， with 96.96% of the samples falling into the safe category. Comparative analysis indicated that the selection of soil background values was the fundamental cause of the aforementioned differences. Source identification analysis revealed that Cu， Pb， Zn， Ni， and Cr in the study area were mainly from natural sources； Cd and Hg were primarily from domestic-agricultural sources； and As was mainly from mining sources. In conclusion， the enrichment of heavy metals in the source protection area of the Datong River， Guangxi Zhuang Autonomous Region， is mainly controlled by the weathering of bedrock and soil-forming processes， and the overall quality of the study area is in a clean and unpolluted state.