Assessment of heavy metal pollution in inner-city canal sediments

The spatial distribution of heavy metal pollution of canal sediments (City of Delft, The Netherlands) was investigated in the framework of a mass balance study. Heavy metal pollution in the accumulated dredged sediments showed marked spatial and vertical variations. In the sediment top layer of 10 to 15 cm thickness, copper levels varied from 45 to 281 mg Cu/kg, lead from 218 to 562 mg Pb/kg and zinc from 216 to 1400 mg Zn/kg. Values in the sediment bottom layer of 10 to 15 cm thickness were 20 to 50% higher, indicating a diminishing pollution load over the past years. Significant statistical correlations were found between the Cu, Pb and Zn contents, pointing at (a) common source(s) of pollution for these metals. Particle size analysis showed substantially enhanced levels of heavy metals in the fraction smaller than 106 μm. Results of preliminary mass balance calculations indicate that the quality of the surface water used for flushing the city canal system has a predominant effect on the inner-city sediment accumulation and pollution rates.