## 1 Introduction
### Sierra Pena Blanca, Deserts and Streams
Along its course, the stream crosses the Mesa, Pena Blanca, Escuadra, and Nopal formations from west to east, with Nopal and Escuadra being where the highest concentrations of U have been described [33]. The distribution of the granulometry follows a normal behavior, in which the proportion of fine particles is found to a greater extent near the flood plain. In contrast, the coarsest material is found mainly in the Sierra's high areas at the stream's beginning (Figure 2). In two wet mud samples and two dry mud samples collected in the floodplain during the rainy period of 2021, uranium concentrations were determined by liquid scintillation alpha spectrometry. At one point at the mouth of the stream, above the alluvial fan, the concentration of uranium in wet mud was 141 mg\({}_{\text{U}}\) kg\({}^{-1}\) of sediment. The sampling point favored the presence of organic matter and the generation of a reduction zone. However, at points located 5, 8, and 12 km away in the lagoon, U concentrations of 4.05, 3.32, and 3.00 mg\({}_{\text{U}}\) kg\({}^{-1}\) of sediment were obtained in the same order as that of the natural background of the area [34]. Silica-rich volcanic rocks such as those of Pena Blanca report U contents between 9 and 20 mg kg\({}^{-1}\)[35]. Another critical point is that particles with high uranium content have been found in sediments near uranium sources (mines) [36]. The coexistence of anomalous and low concentrations in the flood plain leads to the need to know how uranium is transported from uranium minerals in the Pena Blanca area to Laguna del Cuervo and whether dissolution exists. To find answers to these questions, experimental simulations were carried out to evaluate the transport and adsorption of uranium by sediments from SPB to the floodplain-Laguna del Cuervo, taking the characteristic profile of the Boca La Colorado stream. It is suggested that uranium transport may be dissolved, which supports the hypothesis of U leaching by rainwater from the environmentally accessible uranium minerals. The alternative hypothesis is that the uranium ore does not dissolve but is transported to the floodplain in particulate material form, together with the sediments. To elucidate these hypotheses and to enhance the understanding, a simulation of U transport in solution was conducted, (i) a batch experiment was used to evaluate the ideal adsorption of uranyl nitrate by fine sediments; (ii) two columns fed by a high concentration solution of uranyl nitrate (UN) ions, simulating the adsorption by different particle sizes of the Pena Blanca sediments. The transport of U in particulate material was  
Figure 2: Schematic diagram of eastern section of Boca La Colorado stream alluvial fan. simulated using (iii) two columns with an upper horizon of uranium ore (URP) of the area, simulating the weathering of uranophane, the transport by several granulometries of the sediment, and the interaction of the transported U with silts and clays. To interpret the experiments, uranium concentrations in effluent solutions and sediments were analyzed by alpha spectrometry with liquid scintillation counting (LSC) and microwave plasma atomic emission spectroscopy (MP-AES). Particle transport by the feed solution flow was assessed by X-ray tomography. XRD identified mineral phases. However, the uranium compounds in sediments in very low concentrations, as they are in adsorption on sediments, are below the detection limit for conventional techniques. A spectroscopy with a lower detection limit is XAFS, especially when measured in fluorescence mode [37]. XAFS is a synchrotron technique that provides a type of radial function of distances between a given atom of a chemical element, e.g., uranium, and neighboring atoms up to 6-8 A. XAFS is used to analyze the amplitude modulation of the X-ray absorption coefficient at energies close to and above an X-ray absorption edge of the element in the sample [38].