Abstract
It has been pointed out that groundwate r in the Nasu Fan is recharged by various processes including influent stream, irrigation at paddy field, and rainwater infiltration. We evaluated quantitatively the contribution from different sources of groundwater using isotopic tracers and a 3component mixing model, with special focus on the central part of the fan. The result showed that the Sabi River recharges great portion of groundwater (approx.60%) in its adjacent area. In contrast, less contribution from the Houki River and no contribution from the Naka River and Kuma River were found. Except for the area adjacent to the Sabi River, the principal source of groundwater was estimated to be rainwater fallen onto the fun. Unexpectedly, contribution from irrigated water at paddy field was generally less than 15% because there are few paddy fields in the fan apex area.
