Phosphogypsum is a by-product from the artificial fertilizer industry ( in an example, we have a factory located along the "nieuwe waterweg").  Uranium is known to be enriched in the phosphate rocks that are used to produce the fertilizer. In this case there is a direct relation between the concentration of phosphogypsum  and uranium (238U). (see e.g. In this sites, the concentrations of uranium on this site are much higher than can be expected from normal marine sediments and thereby point to the presence of anthropogenic material.