Natural radionuclides as tracers of the dispersal of dredge spoil dumped at sea

Monitoring large (underwater) surfaces, with rapidly varying composition, requires a sampling density which exceeds the capabilities of standard techniques. These techniques involve sample collection and a number of treatments and measurements in the laboratory; both steps are laborious, tedious and costly. This paper presents an in situ method in which a detector system is trailed over the surface and measures continuously the gamma rays emitted by the natural radionuclides in the sediment. Since each sediment component has its own characteristic set of activity–concentration values (radiometric fingerprint), the composition of the sediment can be deduced quantitatively. This paper shows the application of this technique for monitoring the dispersal of dredge spoil from Rotterdam harbour, dumped in the North Sea. In addition to a qualitative picture of dredge spoil dispersal, a mass–balance equation has been used to quantitatively assess the dredge spoil transport with time.