Effect of subsurface flow on nutrient transport between a eutrophic coastal lake and agricultural reclamation land

Abstract:

We examined to confirm the effect of surface water-groundwater interaction on the nutrient transport in Hachiro-gata Lake of coastal Akita prefecture and nutrient transport with the water flow. The area of Hachiro-gata lake have decreased since 1960s due to the land reclamation. The reclamation land covered mainly at the east and south side to the Hachiro-gata Lake. The elevation of the reclamation land is about 3m lower than the lake water level. It suggests water flow in the underground between the lake and land would have the stable direction from the lake to the land. Because the eutrophication often occurs in Hachiro-gata Lake, the nutrient would be accumulated in sediment. 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 May to August in 2014. In addition, water samples were collected in September and December in 2013, May in 2014. We confirmed water flow from the lake to the land with the hydraulic gradient from 0.05 to 0.1. In addition, DOC and phosphorus concentrations of groundwater were higher in the land than in the lake water. The lake water has recently eutrophic condition, and so many organic matter originated from phytoplankton are deposited. The porewater in the lake sediment near the bank had the high nutrient and DOC concentrations. Based on this research, we can make a hypothesis of nutrient transport from the lake to the land with groundwater flow.