We analyzed the temporal changes in water quality variables (pH and concentrations of Fe and As) of mine drainages and downstream rivers at the Shojin-gawa Mine in Hokkaido, Japan, based on water quality monitoring data from 1992 to 2020. We also examined the correlations between mine drainages and river waters for these three water quality variables. With some exceptions (e.g., increasing trends in pH for mine drainages in the Shojin-gawa pit area), we found no trend of improvement in the three water quality variables over the studied period in most of the mine drainages and river waters. In addition, water quality variables at the most downstream study site, Orito-gawa A (the target site for achieving environmental water quality standards), showed higher correlations (r > 0.5) with those at the most downstream site of the Amemasu River affected by mine drainages from the Amemasu-gawa pit area than those at the most downstream site of the Shojin River affected by mine drainages from the Shojin-gawa pit area. Fe and As concentrations at the most downstream site of the Amemasu River were largely influenced by mine drainages from the Amemasu-gawa pit area (i.e., Taisei and Chugiri pits). These results highlight the importance of monitoring water quality at the most downstream site of the Amemasu River to assess the influence of mine drainages on the changes in water quality at site Orito-gawa A. Additionally, we observed relatively high correlations between electric conductivity (EC) and concentrations of several elements at the most downstream site of the Amemasu River. Therefore, in-situ measurement of EC may be beneficial for performing screening-level assessments or low-cost monitoring of water quality when considering and implementing sustainable water quality monitoring methods over the long term.
