In order to determine the sources of pollution of surface waters, an investigation into the occurrence of black list substances in two river basins has been made. Both rivers, “Voorste Stroom” and “Mark-Dintel”, are transboundary ones, coming from Belgium, and both rivers of discharge indirectly into the North Sea. The “Voorste Stroom” flows through a rural area with a number of natural parks. The area of the “Mark-Dintel” is more industrialized and the downstream polders are in agricultural use.
Over 75,000 samples have been taken throughout a whole year, and have been analyzed on the presence of over one hundred substances, a large number of them belonging to the black list of the European Community. Not only the surface waters and sediments of the “Voorste Stroom” and the “Mark-Dintel” have been sampled in various places but also the surface waters of upstream rivers, flowing to these two basins. The effluents of all the municipal wastewater treatment plants in the area have been analyzed. Furthermore, a number of industries discharging into these sewage systems, were taken into account.
In the whole area of the two basins, 82 out of 121 substances have been found to be present once or more in the surface water, leaving 39 that have not been found at all. Only 14 substances were frequently present, i.e. in over 50% of all the surface water samples. Another 17 substances were present in frequencies between 30 and 50%. The substances withe the highest frequencies belong predominantly tot the polycyclic aromatic hydrocarbons (PAH) and to the polychlorobiphenyls (PCB).
In spite of the large number of samples it was not possible to trace the real sources of all the substances polluting the surface water of the “Voorste Stroom” and “Mark-Dintel”. In a small number of cases the discharge from a municipal waste water treatment plant was of major importance tot the surface water quality, but within this study it was not possible to trace the discharge into the sewerage system. In all other cases the pollution seems to be from diffuse sources: groundwater, agricultural run-off, transboundary transport, and urban run-off. Nevertheless, more insight into the importance of the loads from the upstream rivers has been gained. The relative importance of the loads within these two areas to the North Sea may be calculated.
Occurrence, distribution and fluxes of benzotriazoles along the German large river basins into the North Sea