Groundwater–Surface Water Interaction: Process Understanding, Conceptualization and Modelling (Proceedings of Symposium HS1002 at IUGG2007, Perugia, July 2007). IAHS Publ. 321, 2008, 76-82.

Effect of surface and groundwater interaction on nitrate reduction processes in a small alluvial fan catchment, western Japan

MITSUYO SAITO¹, SHIN-ICHI ONODERA² & misa sawano³

1 Center for Marine Environmental Studies, Ehime University, 2-5, Bunkyo-cho, Matsuyama, Ehime 7908577, Japan

misaito@dpc.ehime-u.ac.jp

2 Graduate School of Integrated Arts and Sciences, Hiroshima University, 1-7-1, Kagamiyama, Higashi-Hiroshima, Hiroshima 7398521, Japan

3 Oyo corporation, Japan

Abstract The purpose of our study is to demonstrate the effect of surface and groundwater interaction on nitrate attenuation in the coastal aquifer of a small alluvial fan catchment in southern Japan. NO₃^–-N concentrations in the shallow groundwater decrease from the midstream area to the downstream area with groundwater flow. The inverse relationship between HCO₃^–-C and NO₃^–-N suggests biochemical denitrification process in the downstream groundwater. This is also supported by relatively low ORP (Oxidation-Reduction-Potential) in the groundwater of the downstream area. Based on the results, it is suggested that mixing with domestic wastewater with high dissolved organic carbon (DOC) leads to supply of organic carbon to the groundwater. From the seasonal variation of NO₃^–-N attenuation, it is suggested that the effect of denitrification on the downstream groundwater is smaller in the high-flow season than that in the low-flow because of large groundwater velocity. However, estimated flux of NO₃^–-N attenuation is larger in the high-flow period because of high groundwater discharge. These results revealed the important role of interaction between surface water and groundwater in the alluvial fan on nitrate reduction in the coastal area.

Key words river–groundwater interaction; nitrate reduction; alluvial fan catchment