The writer states the organic thought related to the mountain body groundwater, discussing the hydrogeological characteristics of mountain body as a field of the hydrological process. Subsurface topography and buried geomorphologic features strongly affects the distribution and circulation of mountain body groundwater. Geological heterogeneity in a mountain body strongly affects to the mountain body-scale subsurface water-flow system. The generation of concentrated flow of subsurface flow is strongly related with the development of soil pipe which is controlled both by topological and geological condition. In addition, the development of soil pipe is very important as a subsurface geomorphologic process or as a cause of the degradation of the slope stability. The progressive processes related to piping phenomena are reviewed. Surface soil layer also plays important role in water supply to mountain body. On the other hand, the direct clarification of infiltration and recharge mechanisms is very difficult. Assessment of water balance, subsurface water potential and/or content and water quality supports to understand mountain-body groundwater movement. Finally the writer introduces the peculiar phenomena of the mountain body groundwater observed at Usu volcano and Mt. Takao-san.
In Japan, the groundwater flow system study in a bedrock aquifer is not so popular because of its minor storage capacity of the aquifer body as the groundwater resources. However, the development of the recent underground civil structure with utilizing groundwater characteristics such as the underground oil storage cavern and the environmental hydrological assessment of an underground tunnel construction begin to ask the more precise information about the groundwater flow system in the bedrock aquifer. This information is very useful to accumulate the knowledge about the groundwater flow system in the bedrock aquifer of a mountainous body. In the present paper, the author reports the groundwater flow system in the bedrock aquifer revealed not only by the field survey, but also by the computer simulation, mostly based on the author’s experience in bedrock groundwater survey.
The relationship between bedrock subsurface flow and delayed runoff response was studied by several published papers. Two types of basin topography were found among the watersheds showing delayed runoff responses: 1) underlain by thick colluvium at the “source area” of the basins, having mostly gentle relief, and 2) steep sedimentary rock or fractured rock basins, where bedrock fracture flow may be dominant. The other type-2 response is the watershed underlain by limestone or similar rock types where bedrock flow should be dominant. These two types can be conventionally classified by the basin relief, but bedrock spring could exist in the colluvium in most watersheds. This reviewing work suggests that the mechanism for quick or mostly identical time runoff response to rain storm, which can be revealed by analyzing delayed type runoff response.
To evaluate the roles of bedrock groundwater on runoff generation in a granitic steep headwater catchment, runoff, soil pore water pressures, soil temperatures and water chemistry were simultaneously at Fudoji headwater catchment, Tanakami, central Japan. The catchment is covered with a closed natural forest, and is underlain by granite. Stream water at Fudoji headwater catchment is delivered from two perennial springs, one from the soil matrix and the other from a crack in the bedrock.
Tensiometers suggested that a saturated zone was existed near the spring throughout the observation period. Upslope tensiometers indicated that a saturated zone was not formed at the upper hollow during the period of baseflow, and that most of storms produced a transient saturated groundwater zone above the bedrock. Hydrometric observations also indicated that downward hydraulic head gradients existed during the baseflow period within both near the spring and the upslope area. However, results of soil temperature suggested that the thermal variation near the spring might reflect the effect of upward water flow from the bedrock zone. Hydrochemical observations supported to the emerging of bedrock groundwater near the spring. From these results, it is summarized that, during the baseflow period, two flowpaths (infiltration in soil layer and water emerging from bedrock) meet in a small area near by channel head and form a perennial saturated zone. These results also indicated that the bedrock groundwater had a small impact on the formation of transient groundwater at the upslope area.
The larger SiO2 concentration was found at the lower sampling site. The highest SiO2 concentrations were found in the bedrock spring. Unlike SiO2, however, there was no significant difference in the Cl- concentration among perennial groundwater, spring water, stream water, and bedrock spring water. The pattern of SiO2 concentration indicated that the ratio of bedrock groundwater was greater than that of perennial groundwater near the spring. Moreover, the SiO2 concentration of stream water decreased with the increase of streamflow, suggesting that the ratio of bedrock groundwater discharge to streamflow decreased with the increase of streamflow. During the baseflow period, the ratio of bedrock groundwater discharge ranged from 34 to 93% and the mean value was 78%.
In order to clarify the rainfall-runoff processes in a granitic mountain catchment, we conducted hydrological observation in a soil degradation catchment of Setouchi Region. Streamwater, soilwater and rainwater were collected as well as runoff observation. In addition, electrical sounding were carried out at many plots on recharge area by four pole method. The relative resistances at the inside of a mountain changed heterogeneously. Relative resistance variations at the depth of 15m indicated groundwater recharge exists through fissures. NO3-N and SiO2 tracers showed the storm runoff is composed of other component as well as mixing water of soilwater, rainwater and shallow groundwater. These results including the observed recharge process suggest that this component is composed of deep groundwater via fissures. The SiO2 concentration in streamwater decreased with increase of the runoff amount. Counterclockwise hysteresis was found between the SiO2 concentration and the higher concentration appeared in the later events. It suggests the contribution ratio of deep groundwater increases at the latter half of each event and in the later events.