Pollution status and primary risk assessment of eight pesticides in coastal waters around Ishigaki Island, Japan

Abstract

Ishigaki Island is a remote island in Okinawa Prefecture, Japan, with highly biodiverse ecosystems. Agriculture is one of the most important industries on Ishigaki Island; however, information about pesticide pollution on the island remains limited. Herein, we selected the eight most popular pesticides used in Okinawa Prefecture and investigated their occurrence in the coastal waters of the island; testing was performed four times from May 2020 to March 2021. In addition, we estimated the predicted no-effect concentrations (PNECs) using probabilistic approaches based on the toxicity values obtained from the ECOTOX database of the United States Environmental Protection Agency. The maximum concentrations of dichlorophenoxyacetic acid (2,4-D), bromacil, dichlorvos (DDVP), diazinon, diuron, methidathion (DMTP), fipronil, and metribuzin were 54, 46, 90, 390, 18, 3, 60, and 16 ng/L, respectively. The measured pesticide concentration levels on the island correlated with those previously reported in other areas. Pesticide concentrations were high at the mouths of rivers and decreased with increasing distance in the offshore direction until they were almost undetectable at approximately 1,000 m away. Our results suggest that agricultural runoff was the likely source of pesticides, which then entered coastal areas via rivers. Contamination was limited to the shorelines of Ishigaki Island. Further, the maximum concentrations of diazinon, diuron, and fipronil were higher than the PNECs. The maximum concentrations of DDVP and diazinon were higher than the European Union water quality standards. Our results suggest that these pesticides pose an ecological risk to the coastal environment around Ishigaki Island. This study is the first to evaluate the spatiotemporal distributions and conduct a primary ecological risk assessment of multiple pesticides in the coastal waters of Okinawa Prefecture. These results contribute to both ecosystem conservation and the agricultural industry on Ishigaki Island.