2016 Volume 46 Issue 1 Pages 39-57
In order to clarify loading processes of inorganic ions in a catchment accompanied by tephra and permeable bedrock, water sampling and electric conductivity monitoring were carried out in the forested (88.3% of catchment area) Oikamanai River catchment (62.47 km2 in area) in the Tokachi coastal region of eastern Hokkaido. Surface geology of the catchment consists mostly of Neogene marine sedimentary rocks, siltstone, sandstone and conglomerate, accompanied by currently active faults. The forest soils on catchment slope include sandy tephra (Ta-b) at 30–40 cm depth, deposited by Tarumae Volcano eruption in 1667, and thicker tephra (Spfa-1) from Shikotsu volcanic eruption (ca. 40,000 years ago) and gravelly sediment at greater depths. The layers of tephra and gravelly sediment are relatively permeable, providing water pathway for subsurface flow. In this study, a linear relationship between ionic concentration (mg/L) and electric conductivity at 25℃ (EC25; mS/m)was found out for each of five major ions (Mg2+, Ca2+, Na+, SO42-, HCO3-) in water sampled in the rainfall season of 2013. Hence, the EC25 monitoring allowed us to obtain the load time series of the five ions, using hydrographs from stage-discharge rating curves. The coupling of the tank model with power function reasonably simulated the discharge and ion load time series. As a result of runoff analysis, surface flow and intermediate flow occupied 74.2% of total discharge, and the groundwater leakage, 16.8%. Being common to the five ions, the ion load by intermediate flow prevailed, occupying 40.0–70.0% of total load. The intermediate flow could occur in the permeable layers of Shikotsu tephra Spfa-1 and gravelly sediment above the bedrock, then probably producing active dissolution of the five ions.
