Persistent organic pollutants (POPs) are harmful to human health and to the environment. Their fate in the marine environment is not yet fully understood. The objective of this study is to advance the understanding about the fate of selected POPs in the marine environment as basis for higher accuracy estimates of their levels in the North Sea. An ocean model (FANTOM) has been developed to investigate the fate of selected POPs in the North Sea. The main focus of the model is on quantifying the distribution of POPs and their aquatic pathways within the North Sea. Key processes are three-dimensional transport of POPs with ocean currents, diffusive air-sea exchange, wet and dry atmospheric depositions, phase partitioning, degradation, and net sedimentation in bottom sediments. This is the first time that a spatially resolved, measurement-based ocean transport model has been used to study POP-like substances, at least on the regional scale.

The model was applied for the southern North Sea and tested by studying the behaviour of gamma-HCH, alpha-HCH and PCB 153 in sea water in the years 1995 to 2001. The model’s structure and processes are described in details. Concentrations of gamma-HCH, alpha-HCH and PCB 153 and their fluxes between upper sediment, sea and atmosphere were modelled, based on discharge and emission estimates available through various monitoring programmes.

Model results are evaluated against measurements. Modelled concentrations of the three selected POPs in sea water are in good agreement with the observations. The spatial distribution and the downward trend of the two HCHs in the entire North Sea are reproduced during the simulation period. The pathways of gamma-, alpha-HCH and PCB 153 in the North Sea, were investigated suggesting the importance of the temperature dependence of the air-sea exchange. Model results showed that for the North Sea as a whole the air-sea flux is depositional, whereas in the German Bight it can be net volatilisational. For PCB 153, for the German Bight and for the whole North Sea net volatilisational flux also occurred. Model experiments suggest that the flux direction and magnitude is altered significantly by the Henry’s law coefficient temperature dependency, which could be responsible for more than 50% of the variability in the sea water concentrations of the studied POPs. Uptake by particulate matter in sea water was the most important for PCB 153 with up to 90% of the total concentration being on particles, whereas for the two HCHs this fraction was below 2% during entire simulation period.

For the first time mass budgets of gamma-, alpha-HCH and PCB 153 in the North Sea and in the German Bight were calculated based on a modelling study. Calculated mass budgets show that gamma-HCH and PCB 153 are controlled predominantly by the local sources, whereas for alpha-HCH transport from remote sources is probably the major source for the North Sea environment.

This model study proves that transport models, such as FANTOM are capable to reproduce realistic multi-year temporal and spatial trends of selected POPs and can be used to address further scientific questions.