Estimation of Subsurface Groundwater Flow by the River Runoff and Water Quality

This study is designed to define the correlation between surface geology and discharge and chemical composition of the stream water in order to estimate the groundwater flow system.In the 29 catchments in the Tono area which consist of basement granite, and overlaying Mizunami Group (sandstone, mudstone, etc.) and Seto Group (unconsolidated sediments), flow rates and chemical composition of the stream water were analysed in a dry season.These values were compared with the surface geological condition of each catchment by multivariate analyses.Results are summarized as follows;(1) Height of runoff is smaller in the catchments where the Mizunami Group is dominantly exposed, and increases with average thickness of the Seto Group.These facts suggest that storage capacity of the Seto Group is greater than that of the Mizunami Group.(2) A discriminant analysis revealed that classification of stream water based on surface geology corresponds well with the classification based on chemical composition. This indicates the chemical composition of the stream water is relevant to the surface geology of the catchment.(3) The results of a principal component analysis for chemical composition of the stream water indicates that the residence time of groundwater in the Seto Group is smaller compared to that in the Mizunami Group.