## 2 Materials and Methods
### Samples Preparation and Measurements  
Samples intended for measurements of concentrations of \({}^{236}\)U, \({}^{238}\)U, \({}^{240}\)Pu, and \({}^{239}\)Pu and respective isotopic ratios must be reduced to the form of pure oxides of the species to be analyzed, bound with iron oxides (U\({}_{x}\)O\({}_{y}\) + Fe\({}_{2}\)O\({}_{3}\) and Pu\({}_{x}\)O\({}_{y}\) + Fe\({}_{2}\)O\({}_{3}\)); therefore, the samples undergo a preparation, which consists of the following steps: 1. pre-treatment; 2. chemical extraction; 3. purification on the chromatographic column; 4. precipitation; 5. calcination; 6. pressing cathodes. Sediment samples are first dried in an oven at 70-80 \({}^{\circ}\)C for at least 2 days. Once water is removed, coarse sieving (>0.5 mm) removes any unwanted coarse debris. An aliquot of the dry and sieved sample (10-30 g each) is taken and weighed for reference in the final concentration measurements and burned in a muffle furnace at 550 \({}^{\circ}\)C through the night to produce ashes. Aqueous samples are filtered and acidified to eliminate suspended solids, to stop biological processes, and to keep the analytes in solution as soon as they are delivered to the laboratory. Subsequently, an aliquot is taken in clean containers of an almost uniform and known volume of 2.5 L, and the mass is weighed. At the end of the pre-treatment phase, known quantities of \"spike\" are added to the selected aliquots (standard of \({}^{242}\)Pu and standard of \({}^{233}\)U), to normalize the yields of the preparation and the measurement process. Blanks and standards for the qualitycheck are also produced at this stage to accompany the samples along all subsequent phases, up to the final analysis of the data. Chemical extraction of soils and sediments is essentially an acid digestion of the ashes resulting from the pre-treatment phase. The ashes are transferred into glasses of borosilicate and dissolved into adequate quantities of 8 M nitric acid (approximately 100 mL per 10 g of ash). The acid is added gradually and cold to allow the carbonates to dissolve and release CO\({}_{2}\). When the effervescence produced by the dissolution of the carbonates stops, the actual phase of acid leaching/mineralization begins, letting the solutions react at about 250 \({}^{\circ}\)C for 3 h, placing them on heated plates under the hood. Undigested inert material is removed by filtration before the samples are dried and fumed at around 80 \({}^{\circ}\)C with concentrated HNO\({}_{3}\) and H\({}_{2}\)O\({}_{2}\) (in a ratio of about 5:1 in volume) to remove the digestive acid and the chemical and organic contaminant, before transferring the samples into suitable solutions, for the chromatographic purification. For aqueous samples, the extraction procedure is co-precipitation with iron III hydroxides around pH 8-10, which is achieved by gradually adding adequate volumes of ammonia to an aqueous solution. Once a thick reddish deposit has formed, the supernatant is removed, until to 1/10 of the initial volume, using a pump. The residual part is transferred into polypropylene tubes and centrifuged in successive stages, each time removing the supernatant and accumulating the deposit in the tube. Finally, two or three rinses are carried out with Milli-Q water; each addition of water is followed by centrifugation and discharge to remove residual salts and bring the pH back to neutrality. At the end of the chemical extraction process, all types of samples are dissolved in a volume of 40/50 mL of HNO\({}_{3}\) 8 M, to obtain suitable eluent solutions. At the end of this phase, two different sets of columns with different ion exchange resins, Pu and U, are extracted and concentrated separately. First, Pu is retained and then released in columns with Dowex ion exchange resin, or Bio-Rad 1 \(\times\) 8. NaNO\({}_{2}\) is added aforehand to the samples before elution, and converts the Pu bared in the sample into Pu (IV) oxidation state to ensure complete affinity with resins to increase the retention efficiency of the Pu.