Monitoring and evaluation of the environmental dissipation of the marine antifoulant 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one (DCOIT) in a Danish Harbor

The concentration of marine antifoulant 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one (DCOIT; the active ingredient in Sea-Nine® 211 Antifouling Agent) leaching into a Danish Harbor from two painted ships was quantitated at varying distances from the ships. Sediment and suspended particulate matter were also analyzed for DCOIT. Water samples were concentrated on-site using C-18 solid phase extraction and subsequently analyzed by gas chromatography–tandem mass spectrometry. A strong decline in DCOIT water concentration as a function of distance from the ships was observed. The highest concentration (maximum 283 ng/l) was measured in the immediate vicinity of the ships and the concentration declined rapidly to less than the limit of detection (5 ng/l) at 400 m from the ships’ surfaces. The measured decline curve was compared to that calculated using a one-dimensional model (ECoS). The comparison indicates that the primary mechanism of dissipation of DCOIT is not dilution resulting from dispersion but degradation with a rate constant in the order of 1 h−1. Thus the field results correlate with the earlier microcosm studies demonstrating that DCOIT biodegrades rapidly in a marine environment.