DYKE-PROTECTED FLOODPLAINS - A POSSIBLE CHEMICAL TIME BOMB

For more than a century contaminated sediment has been deposited on th e Rhine floodplains. Freshly deposited sediment samples from the regul arly flooded dyke-protected floodplain were obtained in 1958, 1970, 19 72, 1981 and 1988. Heavy metals and arsenic as well as chemically pers istent organic micropollutants [polyaromatic hydrocarbons (PAHs), poly chlorinated biphenyls (PCBs), hexachlorobenzene (HCB) and DDT derivati ves] were determined in the air-dried samples stored in a specimen ban k. Pollutant levels showed a tendency to decrease; for PCBs and HCB th e highest levels were present in the 1970s. Concentrations exceed natu ral background levels and in some instances are well above alarm level s established for the Netherlands. Although a decrease has been seen i n the average annual input of pollutants onto the floodplain, accumula tion in the upper layer of the floodplain sediments continues. As a co nsequence of the general chemical characteristics of the sediment (hig h organic matter, calcium carbonate and clay contents) the pollutants remain tightly bound to the sediment particles. As the pollutant level s are high, changes which reduce the organic matter and calcium carbon ate contents may cause severe environmental problems: enhanced bioavai lability and leaching of pollutants may result from the increased solu bilization of pollutants.