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[AHW26-P05] Downstream changes of dissolved ion concentrations and loads in the Kasumi River, Saitama, Japan, with an alternating losing and gaining reaches
Keywords:losing reach, gaining reach, hyporheic zone, dissolved ion concentration, dissolved ion load, nitrogen isotope
The Kasumi River, a 16-km long tributary of the Iruma River, is characterized by an alternating losing and gaining reaches along the river. It was studied on July 27, 2018 for the changes of water chemistry and dissolved ion loads as well as river discharge. Our chemical analyses suggest that the Kasumi River is strongly impacted by anthropogenic nitrogen input from the vast tea fields and urbanized areas through groundwater inflow to the river. Along the river, δ15N showed a correspondence to land use, increasing from the agricultural (tea fields) and forested areas to the urbanized areas downstream.
In the 2-4 km and 0-0.5 km reaches upstream of the confluence of the Kasumi River and the Iruma River, the measurements of discharge in the Kasumi River revealed a significant decrease in discharge from 6,800 m3/day to 3,400 m3/day and from 14,000 m3/day to 72 m3/day for the 2-4 km and 0-0.5 km reaches upstream of the confluence, respectively. It was also the case on November 4, 2021 and the river showed a decrease in discharge from 14,000 m3/day to 11,000 m3/day for the 0-0.5 km reach upstream of the confluence. The hyporheic zone is expected to develop beneath the river bed of these reaches, where a hyporheic flow path begins when river water enters the hyporheic zone and ends when it re-emerges into the river downstream after travelling a certain distance. Due to the surface and subsurface hydrologic interactions mentioned above, NO3- and PO43- concentration of the river water on July 27, 2018, for example, decreased by some 30% and 50%, respectively. Our results of the survey in the Kasumi River demonstrate that the hyporheic zone serves as a sink of the dissolved ion loads including nutrients such as nitrogen and phosphorus. Denitrification and assimilation which occurs in the hyporheic zone is likely to be responsible for these considerable decreases in both the dissolved ion concentrations and the loads. Analyses of nitrogen (δ15N) and oxygen (δ18O) isotopes of NO3- indicate nitrogen can be effectively attenuated in the hyporheic zone by denitrification rather than by assimilation.
In the 2-4 km and 0-0.5 km reaches upstream of the confluence of the Kasumi River and the Iruma River, the measurements of discharge in the Kasumi River revealed a significant decrease in discharge from 6,800 m3/day to 3,400 m3/day and from 14,000 m3/day to 72 m3/day for the 2-4 km and 0-0.5 km reaches upstream of the confluence, respectively. It was also the case on November 4, 2021 and the river showed a decrease in discharge from 14,000 m3/day to 11,000 m3/day for the 0-0.5 km reach upstream of the confluence. The hyporheic zone is expected to develop beneath the river bed of these reaches, where a hyporheic flow path begins when river water enters the hyporheic zone and ends when it re-emerges into the river downstream after travelling a certain distance. Due to the surface and subsurface hydrologic interactions mentioned above, NO3- and PO43- concentration of the river water on July 27, 2018, for example, decreased by some 30% and 50%, respectively. Our results of the survey in the Kasumi River demonstrate that the hyporheic zone serves as a sink of the dissolved ion loads including nutrients such as nitrogen and phosphorus. Denitrification and assimilation which occurs in the hyporheic zone is likely to be responsible for these considerable decreases in both the dissolved ion concentrations and the loads. Analyses of nitrogen (δ15N) and oxygen (δ18O) isotopes of NO3- indicate nitrogen can be effectively attenuated in the hyporheic zone by denitrification rather than by assimilation.