[A-HW22] Material transportation and cycling in watershed ecosystems; from headwaters to coastal areas

[AHW22-06] Estimation of phosphorus budget and surface water – groundwater interaction in a coastal freshwater lake, Hachiro-gata, using phosphorus profile in lake bottom sediments nutrient enrichment area

*Shin-ichi Onodera¹, Mitsuyo Saito², Guangzhe Jin³, Atsushi Hayakawa⁴ (1.Graduate School of Advanced Science and Engineering, , Hiroshima University, 2.Okayama University, 3.Guangdong Ocean University, 4.Akita Prefecture University)

Keywords:phosphorus, lake and groundwater interaction, sediment core

We examined to confirm the surface water-groundwater interaction in Hachiro-gata of Akita prefecture and nutrient transport with the water flow. Hachiro-gata have decreased since 1960s due to the land reclamation. The reclamation land touches mainly at the east and south side to Hachiro-gata. The height of the reclamation land is lower than the lake water level. We installed three piezometers at the bankside of the lake and reclamation land, respectively. The water levels were monitored from September to December in 2013 and water samples were collected in September and December in 2013. In addition, the lake bottom sediment cores were collected on two sites, one was on the reclamation bank side and the other was on the central of the lake. Groundwater under the reclamation land around the lake side was estimated to be the stable direction with the gradient of from 0.05 to 0.1 from the lake to the land. In addition, DOC and nutrient concentrations of groundwater were higher in the land than in the lake and lake water. The lake water has recently eutrophic condition, and so many organic matter originated from phytoplankton are deposited. The pore water in the lake bottom near the bank had the high nutrient and DOC concentrations, but the nutrient concentration in pore water and phosphorus content in sediment were constant from the bottom surface to the deeper layer. This suggests nutrient was transported from the surface to the deeper zone. These nutrient would discharge to the reclamation land and the channel in the land and be enriched. In addition, these water is drained to the Hachiro-gata. These indicated the nutrient would be enriched more in this lake.