Application of short-term bioassay guided chemical analysis for water quality of agricultural land run-off

The effect of agricultural land run-off on the water quality of Lake Kojima, Japan, was investigated using a short-term bioassay-guided chemical analysis. Water samples were collected for 1 year starting from June 1995 to June 1996. Toxicity of the dissolved and adsorbed extracts in the water samples was evaluated using the Daphnia immobilization test and the concentrations of pesticides and putative toxic substance in the extracts were determined by high performance liquid chromatography. Most of the dissolved extracts caused immobilization of the test Daphnia magna at low concentrations during the period of paddy pesticide application. Some extracts were found to contain pesticides such as dymron, mefenacet and flutolanil, but their concentrations were too low to have a toxic effect on the daphnia. An unknown toxic compound, Peak C, was isolated from some river water samples, but it produced only a relatively weak toxicity to Daphnia. To better understand the impact of agricultural run-off on a receiving water body, the relationship between the observed toxicity and the concentrations of pesticides and Peak C in the water samples was studied both temporally and spatially.