1:45 PM - 2:00 PM
[AHW32-19] Spatial Variation in Lacustrine Groundwater Discharge (LGD) as a Nutrient Source in Lake Biwa, Japan
Keywords:lacustrine groundwater discharge, phosphorus, radon
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.