Pollution assessment of the canal sediments in the city of Delft (the Netherlands)

Pollution sources for the deposited sediment material in the canals of Delft (the Netherlands) were investigated by detailed monitoring at 51 stations during the period 1991–1996, both in the inner city and the outskirts of the city. The sediment samples were all analysed for (heavy) metals, PAHs, PCBs, pesticides and mineral oil. According to the pollution classes used in the Netherlands, ca. 95% of all inner city stations in Delft were in the highly polluted classes 3 and 4, necessitating sediment dredging followed by disposal under strict conditions. In contrast, only 33% of the sediment samples in the Delft outer city fell into these pollution classes. Factor analysis showed that the inner and outer city sediments could be divided into two different clusters, each with their own characteristics. The chemical components mainly responsible for the sediment pollution were Cu, Zn, DDT, PAHs and Pb. For the latter two parameters, this points to traffic-related pollution loads whereas the main sources of copper and zinc are probably related to the corrosion of metal structures. Mass balance calculations for the Delft inner city canal system show that, of the accumulated heavy metals, around 65% of Zn and Pb and even 85% of Cu originate from outside the Delft inner city, viz. via open inlets with the main Rijn–Schie canal. In view of the decreasing contents in the sediment top layer, it is anticipated that, with the various pollution abatement measures being taken, a gradual improvement of the Delft canal sediment quality will take place.