## 2 Materials and Methods
### Uranium Transport and Adsorption Experiments
#### 2.3.2 Simulation of Uranium Transport in the SPB Profile  
Column Design for Simulation  
Polymeric methyl methacrylate columns measuring 30 cm long by 5 cm in external diameter were designed to simulate the transport of U by the sediments in the profile. A threaded plug at the bottom controlled the exit of the effluent liquid from each treatment. A two \(\upmu\)m glass fiber filter disc was placed at the bottom of the columns to prevent material leakage, followed by a glass fiber wool support. The columns were packed in a top-down sequence, corresponding to the granulometric fractions of the sediments upstream to the floodplain-Laguna Cuervo previously described (Section Sierra Pena Blanca, Deserts and Streams and Figure 2) in the proportions obtained by [31]: 65% Fine sand (FSD), 17% Coarse silt + clay (CSC), and 18% Fine silt + clay (FSC). In the columns to simulate the transport of uranium ore, a uranophane horizon was placed between the FSD segment and an upper layer of coarse sand as a filter (Figure 3). Implementation of the Experiment  
A control column was implemented, into which distilled water was poured with a retention time of one month. A total of 4 columns were prepared, with two columns for each test with the variants shown in Table 1.  
Figure 3: **(A)** Scheme UN experiment; **(B)** Scheme URP experiment; yellow color represents the horizon of uraniferous minerals; **(C)** Photograph of packed columns. All the columns were packed with the granulometries described above; the arrow shows the flow of the feed solutions. Carrier U(VI) solutions with 569 mgU L\({}^{-1}\) UO\({}_{2}\)(NO\({}_{3}\))\({}_{2}\) 6H\({}_{2}\)O in deionized water were prepared. To simulate intermittent rains, carrier solutions were administered in each experiment with a peristaltic pump (Cole-Parmer Masterflex L/S model 07557-04, Vernon Hills, IL, USA) with a rate of 2.4 mL min\({}^{-1}\). The equilibrium solutions in the laboratory environment have a pH of 5, similar to rainwater. To saturate the pores of each column, they were fed with distilled water for 11 days. The experiments were fed with 100 mL of solution, and the feeding was closed after 100 h. Feeding with the carrier solution for the 6 month retention time experiments was performed sequentially for 21 day cycles during the first four months and every 14 days in the last two months. For the 12 month experiment, feeding was regular in 21 day cycles. Samples from each column were collected in one aliquot after every cycle to be analyzed by liquid scintillation spectrometry (LSC) and microwave plasma atomic emission (MP-AES). Characterization of Packed Columns  
Upon completion of fluid feeding in the four experiments, UN6, URP6, UN12, and URP12, the columns were sectioned into segments according to Figure 3 and stored at \(-\)18 \({}^{\circ}\)C until uranium isotope analysis. Uranium concentrations were determined in the sedimentary fractions of the lower segments since the objective was to estimate the transport of uranium toward the flood plain. In the case of the URP6 and URP12 experiments, before sectioning into segments, 1 cm diameter cylindrical \"cores\" were extracted along the entire length of the column to be analyzed by X-ray tomography. It should be noted that in elemental uranium, the isotopic abundance of \({}^{238}\)U is much higher than that of \({}^{234}\)U: \({}^{234}\)U--0.0054%; \({}^{235}\)U--0.7204% and \({}^{238}\)U--99.2742%; \({}^{234}\)U half-life = 2.455 \(\times\) 10\({}^{5}\) y; \({}^{235}\)U half-life = 7.04 \(\times\) 10\({}^{8}\) y and \({}^{238}\)U half-life = 4.468 \(\times\) 10\({}^{9}\) y [43].The adsorption experiments were performed with depleted uranium; therefore, the \({}^{234}\)U contents contribute practically no mass to the adsorbate. Table 2 presents the identification of each sample obtained. The CSC and FSC fractions from each experiment were separated into coarse silt, fine silt, and clay, and the procedure for calculation of pore volume (see Appendix A).