Evolution of Eutrophication Status in Coal Mining Subsidence Areas Driven by Photovoltaic Coverage and Entropy-based Water QualityIndex Risk Warning

Water resources in coal mining areas face various environmental issues, especially eutrophication. In this context, the present study aims to explore the mechanisms underlying the seasonal effects of floating photovoltaic (FPV) coverage on water eutrophication in the coal mining subsidence area of Huainan City. In addition, the comprehensive trophic index method (TLI) was employed to evaluate the eutrophication levels of water quality in the study area. The major factors influencing the eutrophication process were further identified using principal component analysis (PCA) and Pearson correlation analysis. The risk of eutrophication in the water bodies of the study area was also evaluated under different FPV coverage based on the entropy-weighted water quality index (EWQI) model. The results showed that total nitrogen (TN) and total phosphorus (TP) reached the highest concentrations in summer, reaching 2.2 and 0.35 mg/L, respectively, and thus exceeding the Class IV water quality standard. These high concentrations were due to the promoting effects of increased water temperature on N and P cycling. The FPV coverage negatively influenced the light and heat conditions of the water bodies through shading, weakening the self-purification ability of the water bodies and, consequently, promoting nitrogen (N) and phosphorus (P) accumulation. According to the TLI results, the new-FPV (N), high-FPV (H), low-FPV (L), and control-FPV (CK) had anaerobic (TLI = 29.55), mesotrophic (TLI = 30.66), mildly eutrophic (TLI = 51.66), and mesotrophic (TLI = 39.42) conditions, respectively. The PCA results demonstrated the strong triggering effect of TN on the eutrophication of the water bodies. The EWQI model further revealed that the eutrophication risk level of L reached the early warning class (EWQI = 18.1) due to the high TN concentrations, exceeding 1.25 mg/L. In contrast, the N and H zones were classified as safe and generally safe, respectively. The findings of this study demonstrated the mitigation effects of FPV coverage on eutrophication, providing a theoretical basis for the management of water environments in coal mining subsidence areas.