Long-Term Monitoring of Copper and Iron in the Surface Water of Lake Kasumigaura, Japan

Abstract

In order to learn about the environmental conditions of Lake Kasumigaura, National Institute for Environmental Studies has been pursued in a limnological survey of the Lake monthly from 1977. The concentrations of copper and iron in surface water of Lake Kasumigaura were monitored at five sites from 1989. The concentrations of copper were about 0.6-1 μg L−1 for St. 1, 3, 9 and 12, whereas they were 0.8-1.4 μg L−1 for St. 7. This similarity may be due to the formation of a stable soluble complex species, and also small variations of the copper concentrations in inflowed rivers. The seasonal changes in the copper concentrations became unclear in the middle 1990s. The iron concentrations in the Lake were lower than 10 μg L−1, except those at St. 1. It seems that most of the iron is supplied from rivers, and it precipitates at the river-mouth region. The concentrations of iron at St. 1 were 50 μg L−1 before the early 1990s, whereas they became 20 μg L−1 after the late 1990s. Although the reasons for the changes in the late 1990s are very complicated, the change in the surface level of the lake water, by controlling the Hitachi River Watergate, which started in 1996, may be one of the reasons. The surface level of the lake water became 30 cm higher after control of the Watergate ; this may have moved the sedimentation-active river-mouth area to the upper side of the lake.