Analysis of actinides in seawater from locations around Denmark: Every year we take 100-200 litres of sea water from about 10 locations around Zealand. The samples cover a salinity gradient between about 10-30 permille. This means we cover the mixing zone between the Baltic Sea and the North Sea. The ultimate goal of this time series which have been going on for about 25 years is to observe changes in the Baltic Sea inventory of radioisotopes. In the samples we analyse some 5-10 elements with their isotopes. Of my primary interest is the change in plutonium isotopes and the neptunium isotope 237 (Np-237). The data I apply to a simple model describing the water exchange between the two end members (Baltic Sea and North Sea). From the data and model I can calculate numbers of how much of the plutonium and neptunium that enters the Baltic Sea that will stay there due to sedimentation. I want to know to what magnitude the Baltic Sea acts as a trap for these elements. So why not just take a sediment core and look at how much is going to the sediments each year? Yes, I do this too but sediments cores are often complicated to use for this purpose since their ability to trap and accumulate these elements depends on factors like salinity and oxygen content: Factors that change continuously over the Baltic Sea. I would need to take a great number of cores to get the same results as I get with the analysis of in/outgoing water to the Baltic Sea. Of course both methods have weak points but if the results agree with each other, this is fine. It is by the way one of the important ‘tools’ in science to perform studies of parameters using two fundamental methods. If independent methods show the same results the phenomenon observed may even be true!