Dyke‐protected floodplains: A possible chemical time bomb?

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Abstract
For more than a century contaminated sediment has been deposited on the Rhine floodplains. Freshly deposited sediment samples from the regularly flooded dyke‐protected floodplain were obtained in 1958, 1970, 1972, 1981 and 1988. Heavy metals and arsenic as well as chemically persistent organic micropollutants [polyaromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), hexachlorobenzene (HCB) and DDT derivatives] were determined in the air‐dried samples stored in a specimen bank. Pollutant levels showed a tendency to decrease; for PCBs and HCB the highest levels were present in the 1970s. Concentrations exceed natural background levels and in some instances are well above alarm levels established for the Netherlands. Although a decrease has been seen in the average annual input of pollutants onto the floodplain, accumulation in the upper layer of the floodplain sediments continues. as a consequence of the general chemical characteristics of the sediment (high organic matter, calcium carbonate and clay contents) the pollutants remain tightly bound to the sediment particles. as the pollutant levels are high, changes which reduce the organic matter and calcium carbonate contents May, cause severe environmental problems: enhanced bioavailability and leaching of pollutants May, result from the increased solubilization of pollutants.