Nutrient discharge by groundwater and rivers into Lake Biwa, Japan

Nutrient discharges not only by rivers but also by groundwater have been evaluated in Lake Biwa basin, Japan. In the eastern part of the basin, the densely populated urbanized areas and many agricultural fields have brought a large amount of nutrients such as NO3-, SO42+, Cl- and Na+ into Lake Biwa. The NO3- concentration of the groundwater discharged from the lake bottom is three times larger than that of river water. A high concentration of Ca2+ in three rivers and groundwater near those rivers was also found in the areas where the main geology consists of limestone. Land use, population, geology and groundwater flow systems are keys to understanding the nutrient discharge into the lake by rivers and groundwater.