Radium levels in dredging sludge from the Rhine estuary area, 1994-2002
If dredging sludge from the Rhine estuary area is used to raise land, the radium level must be low. The radium level measured fell in the period 1994-2002, but it has not yet reached the natural background level. It is still unclear whether the downward trend will continue.
Radium levels falling in dredging sludge from Rhine estuary area
At most monitoring sites in the Rhine estuary area, the radium level in dredging sludge fell in the period 1994-2002. The main cause of the decrease is that two companies located on the Nieuwe Maas near the Petroleum and Botlek Harbours ran down their phosphorus-processing activities and terminated them altogether in 2000. These companies discharged gypsum containing radium, that is gradually deposited on the river bed. The radium level in the Nieuwe Maas would appear to be on the increase again since 2000. At present, it is unclear why. Measurements will be required in the future to establish whether the observed increase is a real trend or a temporary increase.
Dredging sludge used in past on housing estates
The use of dredging sludge to raise land increases the amount of radium present in the soil. Without human intervention, the radium level in Dutch soil varies from 10 to 60 Bq/kg (Bq: Becquerel, see Guide to 'Radiation' section ).
The radium in the ground leads to the production of radioactive radon gas. This radon gas can enter crawling spaces in homes. The occupants are therefore exposed to ionising radiation from radon. Given a raised radium level in dredging sludge of more than 60 Bq/kg, this could result in an additional exposure as a result of radon that is higher than the maximum permitted dose for the population (0.1 mSv a year, see Guide to 'Radiation' section ).
However, no such radium level has been measured anywhere at building locations raised using dredging sludge.
Future application of dredging sludge
The options for the reuse of dredging sludge are determined to a considerable extent by the degree to which the sludge is contaminated. It is therefore important to monitor the radium level properly, as well as other pollutants in dredging sludge. The average radium level in dredging sludge for the five monitoring sites in the Rhine estuary area in 2002 was just above 60 Bq/kg, and therefore exceeds the natural background level.
Dredging sludge is now stored in specially-constructed deposits and re-used on a limited scale, for example in road construction.
