Subsurface flow behavior in thick colluvium and fissured bedrock in Kumano-daira, Central Japan

To study the groundwater flow path and runoff generation mechanism in the watershed underlain by thick volcanic tephra and permeable bedrock, hydrometric observations, rain water, subsurface water, and stream water sampling and chemical and isotopic analysis were performed. Four catchments were monitored; K1, K2, K3 and K4, underlain by Neogene tuff (6-10 m thick) and Rhyolite, in Kumanodaira area, Gunma Prefecture, Japan. Large landslides occurred in K4 catchment without marked rainfall in 1950, killed 50 people. Discharge was monitored outlet of each cachment. In K4 catchment, six tensiometers, buried 50-650 cm, 2 groundwater wells (9 and 12 m depth) were installed. In K4 catchment, stream runoff shows the longest recession limb compared with other catchments. The groundwater level of well B (12 m) in K4, which penetrates under the bedrock interface, shows delayed peak of 5 months after the major storm event, whereas groundwater level of well A (9 m) shows similar trend to the stream hydrograph. The chemical analysis of groundwater show that the ion concentrations of grandwater in well B are significantly differs from those stream and bedrock spring water in K4. The deuterium excess value of rainwater indicates lower at large storm events by a number of typhoons compared with smaller storm events. The deuterium excess values of stable isotopic also show the seasonal trend; a low value of rainfall is indicated in summer and a high value in winter. The deuterium excess value of soil water was found to be smaller as the depth increases. These data suggest that soil water near bedrock is mainly originated from larger rainfall events. Employing the dispersive model by using deuterium excess, mean residence times of ground water in well B was estimated as 420 days. The dispersion parameter of groundwater in well B estimated as 0.047, is smaller than that of grandwater in shallow soil (well A) estimated as 0.325. From above results, the ground water flow path in the Kumanodaira catchment was hypothesized; because of the thick colluvium, long time is required to infiltrate to the soil/bedrock interface and mainly composed of larger storm water. The soil water immediately above the soil/bedrock interface would infiltrate to the large fissures of the bedrock, preserving the isotopic signal and flowing quickly to the stream.