Understanding future changes in the concentrations of elements such as Cd in mine drainages, which can cause severe environmental impacts, is crucial to strategically optimize the treatment and management of such drainages. In this study, on the basis of 17-year data (2003–2019) for 99 untreated drainages from legacy mines in Japan, we developed a Bayesian hierarchical log-linear model that can capture temporal changes in the concentrations of seven elements including six metals (Cd, Pb, As, Cu, Zn, Fe, and Mn) in individual mine drainages. We also projected future changes to understand the prospective trends nationwide. The modeling results showed that, during 2003–2019, although overall decreasing trends were observed for most elements across all the drainages evaluated, decreases in the concentrations of these elements were not evident in many mine drainages (5%–28% of drainages for individual elements); in addition, any rise in the number of mine drainages with element concentrations below nationwide drainage standards over the next 100 years will likely be limited (e.g., approximately 10 drainages for Zn and Fe at median estimates). These results have significant implications for future strategies to manage mine drainages: it is probably too optimistic to assume that the element concentrations of mine drainages will always decrease, or that these drainages will satisfy drainage standards (permits) in the not so distant future.
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