Evaluation of Bedrock Groundwater Movement in a Weathered Granite Hillslope Using Tracer Methods

We evaluated bedrock groundwater movements on a weathered granite hillslope based on hydrological and hydrochemical observations and tracer experiments. The bedrock groundwater level in the lower part of the hillslope responded rapidly to rainfall. Although the bedrock groundwater level in the middle part of the hillslope fluctuated much more slowly than the duration of rainfall events, the groundwater level of the bedrock groundwater zone rose sharply and reached the upper level of the weathered bedrock after a large rainfall event. This rapid rise of bedrock groundwater was induced by water stored in soil and/or by water in a shallow weathered bedrock layer moving through a cracked and/or fractured bedrock zone. There were several groundwater movement layers within the weathered bedrock in the middle part of the hillslope. Moreover, there were different groundwater movement flow paths from the middle part of the hillslope to the soil and bedrock layer at the bottom of the hillslope with hydraulic conductivities calculated to be in the order of 10 -4 to 10 -3 cm/s.

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