CHANGYUAN TANG¹ & YASUO SAKURA²

Abstract From the hydrochemical viewpoint, a wetland functions both as an interface zone for water exchange between groundwater and surface water, and as a biochemical filter through which water quality changes greatly. In order to understand clearly how and where water quality evolution processes happen in headwater wetlands, a typical headwater in Chiba, Japan, was chosen to investigate nitrate behaviour in groundwater. Major ions, DO, Eh and δ¹⁵N of groundwater from six groups of piezometers (downslope transect, L1–L5), and a spring (L6) were measured. It was found that Ca²⁺, Mg²⁺ HCO₃^- and NO₃^- concentrations decreased from L1 to L5 and increased from L5 to L6. SO₄^2- had an opposite behaviour to NO₃^-The pe-pH diagram was also used to explain the reduction status in the wetland. As a whole, the water flow system controlled the chemical evolution procedure in the wetland, where the reduction sequence changed from nitrate (from L1 to L4) to sulfate (from L4 to L5) when they migrated in the headwater wetland.