Estuarine sediment can remain polluted by heavy metals and synthetic chemicals for decades after the contaminant source has stopped. A remediation mea sure used particularly in Japan is to dump sand on top of the contaminated material, in order to create a 0.5-1 m thick cap. This provides clean sediment as a habitat for organisms and it also decreases the re entrainment of the pollutants in the water column. If the overlying water is polluted, this new sand layer can quickly become contaminated by detritus in the water column. In the long term (years), the beneficial effect of capping is reduced by erosion of the sand cap and/or mixing across the sand cap.
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