JpGU-AGU Joint Meeting 2017

Presentation information

[EE] Oral

A (Atmospheric and Hydrospheric Sciences) » A-HW Hydrology & Water Environment

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

Sun. May 21, 2017 1:45 PM - 3:15 PM 201B (International Conference Hall 2F)

convener:Noboru Okuda(Research Institute for Humanity and Nature), Shin-ichi Onodera(Graduate School of Integrated and Arts Sciences, Hiroshima University), Tohru Ikeya(Research Institute for Humanity and Nature), Adina Paytan(University of California Santa Cruz), Chairperson:Noboru Okuda(Research Institute for Humanity and Nature)

1:45 PM - 2:00 PM

[AHW32-19] Spatial Variation in Lacustrine Groundwater Discharge (LGD) as a Nutrient Source in Lake Biwa, Japan

*Shin-ichi Onodera1, Mitsuyo Saito2, Syuhei Ban3, Guangzhe Jin1, Yusuke Tomozawa1, Noboru Okuda4 (1.Graduate School of Integrated and Arts Sciences, Hiroshima University, 2.Graduate School of Environmental and Life Science, Okayama University, 3.The University of Shiga Prefecture, 4.Research Institute for Humanity and Nature)

Keywords:lacustrine groundwater discharge, phosphorus, radon

Groundwater discharge and nutrient flux into a lake has not been confirmed enough in terms of spatial variation including those in deeper zone. Biwa Lake has different characteristics between in northern and southern. In northern, the water depth varies up to 100m, groundwater discharge is also expected not only in beach sides with shallower depth but in deeper zones. We examined to indicate spatial variation of Rn-222 and to compare with the results of seepage observations by Kobayashi (1993).
Radon radioisotope (Rn-222) concentrations were measured by a RAD7 at 500 m interval along the whole shoreline of the northern lake, and surface water samples were coincidently collected. Oxygen stable isotope ratio (δ18O), Chloride anion and nutrients (nitrogen, phosphorus, and silicon) concentrations were measured in the laboratory in order to evaluate inflow of the groundwater into the lake. Those dissolved materials were also measured from the groundwater samples were collected in ca. 20 wells situated along the shore of the lake as well as those in river waters. In the eastern coast (Hikone), artesian groundwater was also collected because of aquiclude at 10m deep under the ground. Lake waters at the surface, middle and bottom layers and interstitial waters in the bottom sediments were collected for measuring Rn-222 concentrations.
At the both sites of Yasu and Takashima, high pressures of groundwater indicated flow of the water to the lake under the ground. Spatial distributions in Rn-222, Cl- and nutrient concentrations with those inδ18O along the coasts also indicated discharges of groundwater into the lake. High concentrations of dissolved phosphorus phosphate (> 0.1ppm) were detected from several wells out of 15 ones investigated. Based on the comparative results with the seepage observations, we could confirm good correlation between Rn-222 concentration and seepage observation results.
In addition, Rn-222 concentrations in lake waters were measured in the surface and bottom layers at the 4 stations with different water depths (5m, 10m and 20m) in October 2015 and July 2016. The highest Rn-222 was observed in the bottom layer at 20m-deep site in both periods. The concentration was more than 2-fold of that in the littoral site. It suggests high possibility of deep-LGD from offshore lake floor.