抄録
The concentration of dioxins in the water of Lake Shimoteganuma, in which exceeded the environmental quality standard of dioxins (annual mean value: 1 pg-TEQ · l-1, TEQ: toxic equivalency quantity) was exceeded from 2003 to 2007, was investigated. The neighboring lakes, Lake Teganuma and Lake Inbanuma, are shallow lakes like Lake Shimoteganuma. The concentrations of dioxins in the water of these lakes have not exceeded the environmental quality standard. When the result of the sediments of three lakes were compared, the proportion of fine sediment (<75 μm) of Lake Shimoteganuma was the smallest. However, the concentration of dioxins in fine sediment, which tend to float in water, was the highest. A water quality survey at defferent depths was performed. Because TEQ and iron (Fe) concentrations increased toward the bottom, it was surmised that Fe and dioxins were originated from the sediment. Only the TEQ of water near the bottom exceeded 1 pg-TEQ · l-1 in August. However, the TEQs of all the layers exceeded 1 pg-TEQ · 1-1 in October. It seemed that admixture between deffernt depths was more common in October. Analyzing surface water quality from the inlet to the outlet of the lake, the concentration of dioxins increased with the concentration of chlorophyll-a, SS (suspended solids) and Fe in October. The rising widths of TEQ were 1.6 and 2.0 pg-TEQ · l-1, respectively. By analyzing surface water quality for two years, TEQ was found to be over 1 pg-TEQ · l-1 from summer to autumn on the whole. TEQ was below 1 pg-TEQ · l-1 from winter to spring. The concentration 1,2,3,4,6,7,8-Heptachlorodibenzo-p-dioxin, which can be used as an estimate of TEQ, correlated with the concentrations of iron and chlorophyll-a in water of Lake Shimoteganuma.
