Runoff Characterization and Water Quality in the River Basin Near the Ningtyo-toge Mine, Okayama Prefecture

To understand water mass balance and behavior of major elements in river basin is an important issue in advancing the management of abandoned and closed mines. This paper describes the response of river runoff due to rainfall, and the relationship between water mass balance and river water quality. In the river basin around the Ningyo-toge Mine, the runoff increased when it rained more than 50 mm d–1 and continued for several days after the rain, while in case of heavy rain over 500 mm, the effect of intermediate runoff extended to the next month. The river water quality was Na– Cl type in the upstream, while it was Na–HCO3 type or Ca–HCO3 type in the middle to downstream, and contained dissolved Si in the entire river basin. Based on the measured anions and cations, it is inferred HCO3 and Ca2+ results from the dissolution of carbonate minerals, and that Na+ and Cl– are derived from sea salt. Since a correlation between the concentration of Si and runoff was observed, the Si concentration may be regulated by both groundwater flow to the river and the dissolution rate of feldspar.