[A-HW32] [EE] Biodiversity, nutrients and other materials in ecosystems from headwaters to coasts

[AHW32-P19] Sewage water impacts on groundwater and coastal canals in Osaka city; high nutrient and flood impacts

*小野寺 真一¹、齋藤 光代²、金 広哲¹、谷口 正伸³ (1.広島大学大学院総合科学研究科、2.岡山大学大学院　環境生命科学研究科、3.和歌山大学)

キーワード：sewage, nutrient, coastal canal , urbanization, groundwater

Water pollution by domestic sewage is one of the critical environmental problems in the early stage of urbanization with significant growth of population. In case of Osaka metropolitan area in Japan, the pollution was significant until 1970s, while it has been improved by the development of sewage treatment systems. However, removal of nitrogen needs the advanced process therefore relatively large part of dissolved inorganic nitrogen (DIN) is usually discharged by treated sewage effluent. Besides, increase of sewage-derived pollutant loads through the combined sewage systems during rainfall events is recognized as a new problem in recent years. However, the impacts of sewage-derived loads on the water environment of river and coastal area have not been fully evaluated in previous studies. In the present research, we aimed to examine the dynamics of sewage-derived nutrients in highly urbanized coastal rivers. Study area is located on the coastal area of Osaka bay in Seto Inland Sea, western Japan. Treated sewage effluent is discharged from three sewage treatment plants (KH, SU and SA) to a river and channels. Water and sediment samples were collected and electric conductivity (EC), chlorophyll-a (Chl.-a) and dissolved oxygen concentration (DO) were measured from the discharging points to few kilometers offshore at 100-300 m intervals. Nutrients (nitrogen, phosphorus and silica), nitrogen and carbon contents and stable isotope ratios (δ¹⁵N and δ¹³C) of particulate organic matter (POM) and sediment, nitrogen and oxygen stable isotope ratios (δ¹⁵N and δ¹⁸O) in nitrate (NO₃^-) were measured. Nitrate-nitrogen (NO₃^--N) concentration were significantly high near the discharging point then it decreased to offshore suggesting that impact zone of sewage effluent is about 1 km from the discharging point. Significant NO₃^--N uptake by phytoplankton as well as dilution process were suggested in the area. However, the impact zone expanded more than twofold during the rainfall event (>20mm/h). Nutrient contents were significantly high both in the sediment and pore water near the discharging points and it caused relatively high diffusion flux to overlying water. It suggests nutrient regeneration process from the sediment is the secondary loading process in the study area.