Using the Forel-Ule index (FUI) to track the water quality of subsidence water bodies across the life cycle of coal mining in eastern China

The water quality and associated ecological risks in subsidence water bodies formed by underground coal mining are an increasing global concern. However, long-term water quality changes in these subsidence water bodies, especially across different spatial regions, remain poorly understood. This paper, by mapping the Forel-Ule index (FUI) a key indicator of water color, using Landsat datasets to reveal the dynamic evolution of water quality in 402 subsidence water bodies in the Huang-Huai-Hai Plain of eastern China from 1990 to 2020, covering their life cycle from formation to extinction. We identified three types of subsidence water bodies, including growing (14.4%), stable (35.1%), and shrinking (50.5%), almost all of which were found to exhibit eutrophic conditions. The findings revealed a blue-shift trend, indicative of improved water quality, was observed in nearly half (45.3%) of the water bodies. During mining, water quality was generally poor with higher average FUI values, but gradually improved at an average rate of −0.09 yr⁻¹. FUI values experienced a brief period of stability before deteriorating post-mining, with an average rate of 0.05 yr⁻¹. Our study provides valuable insights into the governance of subsidence water bodies in coal mining areas by revealing large-scale, long-term trends in water quality evolution.