We studied variations in annual pesticide concentrations in two rivers in the Niigata Prefecture from April 2012 to March 2014 using GC/MS and LC/MS. Of the 83 pesticides targeted, 38 were detected in the Shin River and 25 in the Shinano River. Herbicides used mainly in paddy fields were detected more frequently, and at higher concentrations, than fungicides and insecticides. Although the pesticide concentrations in the Shin River were higher than those in the Shinano River, most pesticides were detected around the farming season for both the rivers. A small number of herbicides used only in paddy fields were detected in both rivers in winter. In this study, all pesticide concentrations were under their corresponding standard criteria.
