Heavy-metal pollution of the river Rhine and Meuse floodplains in the Netherlands

The embanked floodplains of the lower Rhine river in the Netherlands contai n large amounts of heavy metals, which is a result of many years of deposit ion of contaminated overbank sediments. The metal pollution varies greatly between the various floodplain sections as well as in vertical direction wi thin the floodplain soil profiles. The present contribution describes the k ey processes producing the spatial variability of the metal pollution in fl oodplain soils: (1) spatial patterns of the concentrations and deposition o f Cd, Cu, Pb and Zn during a single flood, which have been determined from samples collected after a high-magnitude flood event; (2) the pollution tre nds of the lower Rhine over the past 150 years, which were reconstructed on the basis of metal concentrations in sediments from small ponds within the floodplain area. During the flood the largest metal depositions (0.03 g/m( 2) Cd, 0.7 g/m(2) Cu, 1.1 g/m(2) Pb and 5.0 g/m(2) Zn for the Rhine) occurr ed along the natural levees, decreasing to about one third of these values at larger distance from the river. Deposition of heavy metals occurred sinc e the end of the nineteenth century. Periods of maximum pollution occurred in the 1930s and 1960s, when Cu, Pb and Zn concentrations were about 6-10 t imes as high as background values. The resulting metal distribution in the floodplain soil profiles is illustr ated by means of typical examples. Maximum metal concentrations in floodpla in soils vary from 30 to 130 mg/kg for Cu, from 70 to 490 mg/kg for Pb, and from 170 to 1450 mg/kg for Zn. The lowest metal pollution is found in the distal parts of floodplain sections with low flooding frequencies, where av erage sedimentation rates have been less than about 5 mm/a. The largest met al accumulations occur in low-lying floodplain sections where average sedim entation rates have been more than 10 mm/a.