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Prognosis of the sediment quality in regional waters: present and future concentrations of heavy metals in ditches

Verkenning bodemkwaliteit regionale wateren: Huidige en toekomstige gehalten van zware metalen in slootbodems

Synopsis

Sediment dredging is a major activity in the Netherlands, not only in the main rivers but also in ditches. The dredged material must meet certain objectives if it is to be disposed of in the adjacent soil. The IRIS (Integrated Risk Instrument for ditcheS) model has been developed to evaluate whether future levels of heavy metals will meet this standard. It describes the behaviour of organic and inorganic material as well as cadmium, copper, lead and zinc. A probabilistic approach dealt with the uncertainty of process parameters and emission fluxes. For uncertain and sensitive parameters Monte Carlo simulations were performed on values taken from a range. The simulations resulted in distributions for the model output variables. The model was calibrated and validated on field data in the 1990-1995 period and results showed the model to give an accurate fit to the data. Modelresults show that, at present, background loading, mainly consisting of fluxes from adjacent land, does not cause a reasonable chance of exceedance of the standards in sand- or clayditches. However, chances are very high that in peatditches concentrations will not meet standards especially in case of copper. In the latter case, additional sources will not change the probability of exceedance very much, whereas in clay and sand this particularly is the case. Again for copper, concentrations show the highest exceedance of standards. Most likely only a minor improvement might be possible the upcoming decades, because of the historical and present loading with fertilizers and/or manure and the substantial contribution from land in newly formed sediment.
 

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