Evaluation of water quality and identification of pollution factors in mining subsidence area with high phreatic water level

Huainan mining area is characterized by a high phreatic water level, as well as a wide range and large area of coal mining subsidence areas, with the water quality monitoring data missing and prominent water environment. Therefore, the evaluation of water quality monitoring and identification of pollution factors in large subsidence area is of great significance to regional water environmental governance. Specifically, the water quality of the subsidence area was evaluated with the Nemerow pollution index method by testing the water samples collected from 175 sampling points in the study area. Meanwhile, the nutritional state of water bodies was evaluated with the comprehensive nutritional status index method. The results show that: (1) For the open subsidence areas, the water quality level is evaluated to be excellent in terms of Cu, As and Cr, good in terms of DO, COD, NH₄-N, Hg and F-, and relatively good in terms of TN and TP. For the closed subsidence area, the water quality level is evaluated to be excellent in terms of Cu and Cr, good in terms of the NH₄-N, Hg, As and DO, relatively good in terms of COD and TN, and poor in terms of TP and F^- according to Level-III standard of surface water. However, the water quality level is evaluated to be excellent in terms of NH₄-N, Hg, As, Cu and Cr, good in terms of DO, COD, TP and TN, and relatively good in terms of F^- according to Level-V standard of surface water. (2) The comprehensive nutritional status index of the study area is ranged within 40‒90, and the average nutritional status index is sized 68.61. Therefore, the study area is in moderate eutrophication, which shows that the water body in the subsidence area is at moderate nutrition level. Among the 175 water samples, 8.57% of the samples are in light eutrophication, 51.43% in moderate eutrophication, 2.29% in medium nutrition, and 37.71% in severe eutrophication. (3) According to the Standard for Environmental Quality of Surface Water, few samples have DO and NH₄-N exceeding the level-V value, but most samples have COD, TP and TN exceeding the level-V value, and there are no excessive heavy metals in all waters of the study area, with excellent test results. Besides, a large number of samples have the inorganic anion index F^- greater than the level-V value. Generally, the water quality of open subsidence area is better than that of the closed subsidence area. (4) For the water, TP and F^- are poor for Zhangji and Panyi subsidence areas. For the closed water, the TP of Pan Yi, Pan San, Guqiao, Gubei and Zhangji subsidence areas are poor. The distribution of F^- pollution shows the geographical characteristics, which is generally poor in north of Huai River and good in south. The indexes of Cr, Cu, Hg, As, NH₄-N, TN, COD and DO in all waters are in the “excellent- relatively good” range. The F^- and TP indexes of all the open subsidence areas are also in the “excellent-relatively good” range. But the F^- of the closed water in Panji area and TP in Fengtai-Yingshang area are in the “poor” range. The regional pollution level is shown as Fengtai-Yingshang area>Panji area>old Mining area. (4) In order to protect the ecological environment of coal mining area, the pollution factors of coal mining subsidence area in Huainan mining area are determined to be COD, TP, TN and F-, with comprehensive consideration to the characteristics of two identification methods of pollution factors. The results provide important support for local ecological and environmental protection.