Variability of the chemistry of streamwater and bedrock groundwater in a small catchment at a weathered granite mountain, Japan

Abstract:

Previous studies have noted that bedrock groundwater is one of the important factors influencing stream discharge and streamwater chemistry. To better understand the dynamics of bedrock groundwater, we investigated groundwater table movement and water chemistry of bedrock groundwater using dense borehole wells at a small catchment in a mountainous area.

The study was performed at the Fudoji Experimental Watershed located in the Tanakami Mountains in the southeastern part of central Japan. Precipitation was monitored and discharges were observed at eight small catchments, ranging in area from 0.1 to 2.3 ha. Seven small catchments (subcatchments) were included in the largest catchment (2.3 ha), within which we installed 61 borehole wells. Rainwater, streamwater from the small catchments and bedrock groundwater from the borehole wells were sampled, and the concentrations of major ions, SiO₂ and the water stable-isotope ratios (d¹⁸O and dD) were measured.

The results indicated that there were several fluctuating characteristics and that these characteristics of groundwater table change had locality. At the area having higher altitude in the ridge, the bedrock groundwater-table changes were gradual but the ranges of fluctuation were larger than those of the lower wells. At the lower-altitude points, although the bedrock groundwater table responded rapidly, the ranges of fluctuation of the groundwater table were small relative to those of the higher points. Based on the groundwater flux analysis, bedrock groundwater moves across the surface divide. A catchment inflowed by a neighboring catchment showed a high specific discharge.

The results of chemistries indicated that although the weathering processes were similar in the catchment, the weathering level varied among the borehole wells. The chemistries of bedrock groundwater at each catchment and of streamwater at each catchment showed large variability. The concentrations of Na⁺ and Ca²⁺ had local characteristics, but no clear characteristics were observed among other bedrock groundwater components. The chemical concentrations of bedrock groundwater were higher than those of streamwater. These results indicate that complex processes of chemical dynamics occur in the weathered bedrock and from the weathered bedrock to the stream.