Identification of inflow sources and pathways to a waste rock dump located in a former river channel in a mountainous abandoned mine of Japan: A case study 60 years post-construction

A legacy underground mine located in a mountainous region of Hokkaido, northern Japan. The concentrations of dissolved ions and stable water isotopes (δ¹⁸O and δ²H) in surface water from surrounding rivers and groundwater near a waste rock dump (WD) were analyzed. Continuous water quality monitoring was performed using data loggers. Tracer tests using NaCl were also performed in the rivers. Time-series analysis and correlation studies elucidated the origins and pathways of water inflow to the WD. Water inflow into the WD predominantly originated from groundwater in the surrounding mountainous terrain and river water, serving as the primary contributors to mining-influenced water (MIW). The inflow volumes and MIW generation increased during precipitation events and snowmelt periods. These processes could have been influenced by the degradation of impermeable structures around the WD, emphasizing the need for continuous water quality monitoring and repair of these structures. This research highlights the necessity of long-term planning for WD management, considering regional climatic and topographic characteristics, to mitigate water pollution risks. It provides new insights into reducing environmental impacts and enhancing the sustainability of mining operations in mountainous regions. Moreover, this study demonstrates that a combination of hydrological approaches—water quality analysis, isotopic studies, and tracer tests—is a valuable tool for gathering critical information on MIW in mountainous mining regions. • Mine drainage of the waste dump contained a mixture of water from different flow paths. • MIW in the WD originated from groundwater from surrounding mountains and river water. • Water inflow to the WD increased during precipitation and snowmelt events. • Impermeable structures around the WD were damaged allowing the water inflow. • A basic hydrological approach was effective for MIW surveys in mountainous mine areas.