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. https://en.wikipedia.org/wiki/Phosphogypsum). 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.