Geographical distribution and long-term trends of cyclic volatile methyl siloxanes in sediments and fish of Tokyo Bay

Octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6) are notable cyclic volatile methyl siloxane (cVMS) compounds primarily used in the production of silicone polymers, which are extensively utilized across a wide range of industries, including construction, automotive, electronics, and more. The widespread use of cVMS, particularly D4, D5, and D6, in industrial applications has raised environmental concerns due to their persistence, bioaccumulation potential, and possible toxicity. Despite their low water solubility, cVMS are found to adsorb strongly to organic matter in sediments and soil, raising questions about their long-term environmental effect. Given Tokyo Bay's strategic economic importance and the high level of industrial and urban activities in its catchment, this study aimed to provide a comprehensive understanding of cVMS contamination in the area. The spatial and temporal distribution of cVMS concentrations in the sediments and fish of Tokyo Bay were systematically examined from 2011 to 2021. The results showed that cVMS were widely distributed across the bay's sediments. Regarding the samples collected in 2021, the concentration of D4, D5 and D6 ranged from <1.04 to 14.5 ng/g- dry weight (dw), from 6.3 to 494 ng/g-dw, and from 2.3 to 89.5 ng/g-dw, respectively. A clear decline of observed concentrations in the seaward direction underscored the influence of riverine inputs. Temporal trends indicated a general decline in cVMS concentrations in sediments at multiple sites, while concentrations in fish varied by species and trophic level. Ecological risk assessment revealed that current cVMS concentrations in the sediments do not pose a threat to benthic organisms. This study is the first to provide a long-term analysis of cVMS in Tokyo Bay, contributing valuable data for environmental monitoring efforts. The findings underscore the importance of continuous assessment of these compounds to understand persistence, bioaccumulation and risks to ecosystems.