Concentration and degradation of alternative biocides and an insecticide in surface waters and their major sinks in a semi-enclosed sea, Japan

•Surface water analysis of alternative biocides and an insecticide was done.•The pesticides were photodegraded and biodegraded.•Data generated and literature information were used in a mass distribution model.•Pesticide degradation in river water was significantly faster than that of seawater.•Sediments were found to be the major sink for the pesticides.

A mass distribution model was used to predict the fate of Diuron, Irgarol 1051 and Fenitrothion in Seto Inland Sea which is located in western Japan. This was done by using concentration, degradation, and literature data. Diuron and Irgarol 1051 in Seto Inland Sea are mainly derived from antifouling paints used for ships and boats. On the other hand Fenitrothion exclusively comes from land via rivers and atmospheric deposition. The total inputs/yr to Seto Inland Sea were found to be 104 tons, 7.65 tons and 5.14 tons for Diuron, Irgarol 1051 and Fenitrothion, respectively. The pesticide residence times were 0.26 yr, 0.36 yr and 0.17 yr for Diuron, Irgarol 1051 and Fenitrothion, respectively. Photodegradation was faster than biodegradation. In seawater, the half-life ranges were 37.9-57.3 d for photodegradation. In the same seawater the half-life ranges were 1650-2394 d for biodegradation. Photodegradation is effective in surface water (0-5 m depth) while biodegradation occurs throughout the entire water column. Plankton and fishes accumulate these pesticides significantly. The pesticides are deposited (sorbed and buried with) sediments (between 74 and 87% of total input amounts). The open ocean is an important sink accounting for between 8 and 17% of the total pesticide input amounts while photo- and biodegradation accounts for a small percentage.