Study on the characteristics of spatial distribution of nitrogen, phosphorus, iron and manganese in drinking water sources: a case study of Hefei, China

The upstream areas of the two drinking water sources are located in the expansive soil development zone of central Anhui Province. This study focuses on investigating the spatial distribution characteristics of N, P, Fe, Mn and pH, in surface soil, deep soil, as well as upstream tributaries, reservoir water bodies, and surface sediments within the watershed of two drinking water sources. The research results showed that the surface soil has been severely acidified in upstream of two drinking water sources in Hefei, while the deeper soil is slightly alkaline. The long-term, excessive application of nitrogen and phosphorus fertilizers is the primary cause of surface soil acidification. Iron and manganese in alkaline environment readily form insoluble hydroxide precipitates that solidify in deep soil layers. Iron and manganese in acid environment readily undergo redox reactions to form divalent iron ions and manganese ions. The divalent iron ions and manganese ions enter water bodies through leaching and surface runoff. Long-term, heavy application of nitrogen fertilizer and phosphate fertilizers have resulted in N and P enrichment in the tillage layer. Under conditions of rainfall runoff and agricultural drainage, this accelerates N and P migration into water bodies. The chemical index of alteration (CIA) of the surface and deep soils in the study area are higher than 65, which indicates that the surface and deep soils in the study area are moderately weathered. Illite and vermiculite in the expansive soil are iron-bearing silicate minerals. Therefore, the iron of Illite and vermiculite silicate minerals have the possibility of entering the surface water bodies. The concentration of iron in the water body fulfills the conditions for succession from green algae to cyanobacteria. The two reservoirs and their tributaries are potentially at risk of eutrophication. In addition, Fe and Mn of the expansive soil of the containing iron and manganese films and iron-manganese nodules readily enter surface water bodies during the rainy season. In summary, the soils, water bodies, and sediments within drinking water source watersheds are an interconnected system. Studying the spatial distribution characteristics of N, P, Fe, Mn and pH is of important significance for developing precise measures to safeguard water quality in source watersheds. Therefore, the ecological measures should be taken to cover bare soil, such as the banks of river slopes are converted into ecological berms to reduce soil erosion. The organic agriculture should be promoted in the water protection zones to reduce the use of chemical fertilizers. It is necessary to strengthen the study of the release law of surface sediments from Dongpu and Dafangying Reservoirs. It will provide scientific data for the waterworks to take water from two reservoirs in a stratified manner. It is strictly prohibited to cause surface and deep soil to be exposed by extensive excavation in water source protection zones.