Decreases in TBT concentrations in southern Chesapeake Bay: Evidence for long-term TBT degradation and reduced imposex in the invasive gastropod, Rapana venosa.

This study assessed effectiveness of regulations reducing environmental butyltin concentrations in Southern Chesapeake Bay over the 1999-2021 period. Water column monitoring of the Elizabeth River from 1999 to 2006 demonstrated decreasing TBT from 2003 to 2006 (average >1 ng/L at most stations) to <1 ng L-1 by 2019 but with higher concentrations of degradation products DBT and MBT. TBT degrades to DBT and MBT within sediments, and releases degradation products over time. TBT induces imposex in exposed gastropods. Collections of the rapa whelk in 1999-2001 found TBT levels in female, imposex, and male whelks with median TBT concentrations of 10.5, 11.5, and 70 ng/g respectively. Collections in 2021 were below the TBT detection limit (<4.7 ng/g) and showed reduced incidence of imposex. The decrease of both TBT to below EPA water quality standard (7.4 ng/g) and observed gastropod imposex over the 1999-2021 period provides evidence that regulations limiting TBT use have been successful.