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Concentration of the herbicides in water was investigated between 2003 and 2007 at Lake Suwa. Concentration of each herbicide in Lake Suwa was lower than half effective concentration of green algae, so we concluded that these herbicides hardly affected the phytoplankton that lived in Lake Suwa. Herbicides were detected for several months in the lake. When water exchange rate was high in summer due to large amount of rainfall, concentration of herbicides in lake water was reduced.Concentration and composition of herbicides were different between center of the lake and shore of the lake in 2007. When large amount of herbicides flowed into the lake from rivers, concentration of herbicides at shore of the lake was higher than center of the lake. This result showed that herbicides at the shore of lake were affected by rivers more than center of the lake.

In the present study, we investigated the Polycyclic Aromatic Hydrocarbons (PAHs) at the representative eutrophic lake “Lake Suwa”, which has drastically eutrophicated since 1960s by inflow of domestic and industrial effluents. We also investigated PAHs in river sediments, road dusts, suspended solids and air particles of the catchment area of Lake Suwa.We showed that the concentrations of Σ17PAHs in the lake sediments and river sediments were 364±129 ng/g-dry and 157±194 ng/g-dry, respectively. On the other hand, the concentrations of Σ17PAHs in sediments were relatively high in the northeast part of Lake Suwa.One of the major PAHs in the sediment of Lake Suwa was perylene. It would be mainly produced from precursor substances (such as perylenequinone) in anoxic lake sediment. On the other hand, the major sources of other PAHs (except perylene) were considered to be both petrogenic and pyrogenic source as diesel exhaust matter and asphalt. They entered the Lake Suwa through rivers and finally precipitated to lake sediment.

In the present study, we estimated inflow and effluent load of Polycyclic Aromatic Hydrocarbons (PAHs) in Lake Suwa, Japan. Estimation of PAHs inflow and effluent load were performed by measuring changes in stream flow and concentration of suspended solids (SS) in the lake and river water. Results showed that the concentration of SS increased with stream flow. Annual (March 2004 to February 2005) inflow and effluent load of SS at the lake were calculated as 8.9×106 kg and 7.6×106 kg, respectively. The mass balance between inflow and effluent load indicated that 1.3×106 kg of SS were sedimentated to the lake. The total amount of Σ9PAHs (Phenanthrene, Pyrene, Chrysene, Benzo[e]pyrene, Benzo[b]fluoranthene, Benzo[k]fluoranthene, Benzo[a]pyrene, Dibenz[a,h]anthracene, Benzo[ghi]perylene) in lake SS varied by 230±120 g/lake (average±SD); the maximum amount exceeded 500 g/lake, which was observed in May and October 2004. Specifically, for Benzo[a]pyrene the total amount in lake SS was 14±7 g/lake. Estimation of total Σ9PAHs flowing into Lake Suwa and out through Kamaguchi Gate indicated Σ9PAHs values of 8.6 and 3.5 kg/yr, respectively. The mass balance between Σ9PAHs inflow and outflow indicated that approximately 5.1 kg/yr of Σ9PAHs were sedimentated with SS to the lake. The annual flux of PAHs in Lake Suwa calculated in the present study agreed with the deposition flux of PAHs in a previous study estimated from sediment cores (4 kg/yr) (Ikenaka et al.: Environ. Pollut., 138, 530-538 (2005)). Inflow loads of PAHs markedly increased in May 21, October 9, and October 21 when heavy rains occurred, occupying approximately 27 % of the annual inflow. These results indicated that large quantities of PAHs flowed into the lake when river flow increased by heavy rain.