Arbejdsrapport fra Miljøstyrelsen, 23/2005 – Belastning af danske havne med antibegroningsmidler - modelberegninger af koncentrationer i vand og sedimenter

Belastning af danske havne med antibegroningsmidler - modelberegninger af koncentrationer i vand og sedimenter

Summary and conclusions

On the basis of an existing mathematical model the load of TBT, copper, Irgarol and Sea-nine on marinas and a commercial harbour was estimated. The model estimated concentrations in water and in particles in the water phase. The concentrations of TBT and copper calculated by the model were in accordance with the concentrations found in water and surface sediments in corresponding harbours.

Model calculations showed that the load of copper on harbours was generally heavy, and that the substitution of TBT-based paint with copper-based bottom paint could not solve the problems with polluted harbour sediments. In the calculated scenarios the concentration of copper exceeded the hitherto applied limit values for harbour sediments to be dredged.

In the project a newly developed mathematical model was used for the description of the concentration of active substances from bottom paint for ships in marinas and a commercial harbour. A number of conditions were assumed prior to the calculations, including the geometry of the harbours, the number of boats/ships in the harbours, the number of vessels using bottom paint with different active substances, and the speed at which the active substances were released from the paint and degraded. Moreover, it was assumed that the exchange of the water in the harbours primarily took place via the tide and the dispersion driven by the current velocity outside the harbours. The most critical assumptions pertained to the rate of release of the substances from the ships. However, the used rates are in accordance with the values accepted by the organization of European paint producers and are not likely to be overestimated.

The applied model was based on balances and therefore it was not possible to calculate how quickly changes will occur in the sediment concentrations after the introduction of new types of paint and active substances. Such a calculation will require a more advanced model access. Instead model scenarios were carried out for the year 1990 (when all pleasure boats were supposed to be painted with stanniferous paint) and for the year 2000 when the prohibition of the use of TBT on pleasure boats less than 25 metres had been in force for10 years, and when other substances had replaced TBT.

Model calculations may be a useful tool during planning and estimation of the operation of harbours, including the identification of the types of harbours where problematic concentrations in the sludge may be generated. In this manner the necessity for dredging, depositing or cleaning can be established. The calculations carried out during this project are of a general nature, and a specific assessment of each individual harbour may turn out differently owing to regional conditions. In specific cases the model calculations should be optimized with realistic values for water exchange inside the harbours and between the harbours and their outer edge, and also more precise figures for the salinity in the harbours, tide, current velocity, sediment transport and sedimentation would improve the model estimates.