Spatiotemporal trends and ecological risk assessment of volatile methylsiloxanes in Tokyo Bay catchment basin, Japan: River water and sewage treatment plant samples

The large production volume of volatile methylsiloxanes (VMSs) and their high mobility and persistence present significant environmental-pollution concerns. Although the temporal trends of the persistent-compound concentrations of the environment have been considerably explored, those of surface water, namely surface water VMSs, have been barely investigated. Thus, we investigated the 2013–2021 spatiotemporal trends of the VMS (cyclic VMSs (cVMSs; D3–D6) and linear VMSs (L3–L6)) concentrations of river water and sewage treatment plant (STP) samples from the Tokyo Bay catchment basin, Japan. Our findings indicated the widespread concentrations of VMSs in the river water (2.3–1190 ng/L); the highest VMS levels were detected downstream of the STP discharges. However, no elevated VMS concentrations were detected downstream of silicone-manufacturing facilities, except for D3. The temporal trend analysis revealed statistically significant decreases in the D4, D5, and D6 concentrations within the monitoring period, with annual changes ranging from −7.7 % to −6.4 %, probably reflecting the impact of regulatory actions to address cVMSs. Our risk assessment of the adverse effects of surface water D4, D5, and D6 on aquatic organisms revealed that their distributions did not overlap between the 95th percentile field-water concentration and 5th percentile chronic no-observed-effect water concentration, whereas their cVMS levels were close to their predicted no-observed-effect concentrations. Furthermore, their risks of the aquatic environment to cVMS exposure displayed decreasing temporal trends during the study period. Overall, understanding temporal trends of surface water VMS is key to implementing necessary emission regulations and evaluating the effectiveness of future restrictions on these silicone materials.