『Abstract
The relation among the mean slope angle of a watershed, the stream
water quality, and the chemical composition of the plagioclase
component of granite was examined for 17 watersheds located in
the southern part of the Northern Japan Alps in central Japan.
The 17 watersheds were mainly composed of the granite. The ionic
composition of the stream water can be classified into two patterns:
the Ca-HCO3-type and the Ca・Na-HCO3-type.
In all the watersheds, Na+ and Ca2+ occur
in the groundwater that flows into the stream mainly due to the
kaolinization of plagioclase. As compared to the watersheds with
Ca-HCO3-type stream water, the Na+/Ca2+
ratio for stream water in watersheds with Ca・Na-HCO3-type
stream water was closer to that for plagioclase in granite. This
implies that the groundwater flowing in the Ca・Na-HCO3-type
watersheds is deeper than that in the Ca-HCO3-type
watersheds. The mean slope angle of Ca・Na-HCO3-type
watersheds was less acute as compared to that of Ca-HCO3-type
watersheds. The mean slope angle of the watersheds is negatively
correlated with the Na+/Ca2+ ratio for stream
water (r=-0.78, P≦0.001). It is suggested that the groundwater
quality at greater depths, which is comparable with the Na/Ca
ratio of plagioclase, contributes more to the stream water quality
in the gently sloping watersheds than in the steep watersheds.
This study clarified that there is a close relation between the
mean slope angle of watersheds and the depth of groundwater discharged
to the stream. Moreover, it is believed that the relationship
between the mean slope angle of watersheds and the Na+/Ca2+
ratio for stream water is an effective and simple index for understanding
the relative depth of groundwater.
Keywords: Stream water quality; Mean slope angle of the watershed;
Granite; Chemical composition of plagioclase』
1. Introduction
2. Methods
2.1. Site description
2.2. Sampling and analysis
3. Results
3.1. Spatial variability of stream water quality
3.2. Na/Ca ratio for plagioclase
3.3. Topography of watersheds
4. Discussion
4.1. Estimation of formation processes of groundwater quality
based on stream water quality
4.2. Relation between the stream water quality and watershed
topography
4.3. Consideration of the relationship between the water quality
and the topography of a watershed
5. Summaries
Acknowledgement
References