Abstract
We aimed to clarify a relation between groundwater flow condition and nitrate (NO3-) attenuation zone on the catchment scale (∼km2). Especially, the relation between hydraulic gradient and NO3--N attenuation in unconfined groundwater was examined on two coastal agricultural catchments (IKN and IKS) with the different topographic gradients in an island of the Seto Inland Sea. Results show that increases of pH and decreases of ORPSHE and NO3--N concentrations along a groundwater flow in both catchments. In addition, distributions of Cl- concentrations suggest that the denitrification contributes to the NO3--N attenuation as well as the dilution process. Relations between hydraulic gradients or topographic gradients and NO3--N attenuation ratios indicate that the attenuation process is significant in the coastal area characterized by the small gradient with less than 0.04. Additionally, the results suggest that the significant NO3--N attenuation zone locates near the coastal line in the IKS with larger gradient. Meanwhile it would locate on the inland topographic transition zone as well as the coastal area in the IKN with smaller topographic gradient.
