The fate of geogenic uranium in the environment: an example from the Swiss Plateau

Increased U concentrations in ground and drinking water have been found in the western part of Switzerland. The focus of the study was set to Mont Vully, 20 km northwest of Berne. The origin of U was investigated and has been verified to derive from an interface between the Upper Marine Molasse and the Lower Freshwater Molasse, two stratigraphic units in the Molasse Basin. During the genesis of this interface, fossils have been deposited and U was adsorbed onto them due to the reducing environment that has been present. The last glaciation formed the typically hilly area of the Swiss Plateau leading to the outcropping of the interface with the U-rich fossils. Oxygen-rich rainwater that infiltrated the water permeable Upper Marine Molasse was dammed at the Lower Freshwater Molasse which acts as an aquitard resulting in a steady leaching process of the fossils at the interface. This U-rich groundwater (350 mBq/L 238U) fed wetlands that were formed at least 8 kyrs ago on an impermeable clay layer in the adjacent land north of Mont Vully. The reducing conditions that were present in the wetland resulted in an accumulation of U that was adsorbed onto organic matter. Lasting for thousands of years, the infiltration of U-rich groundwater generated a peat horizon with up to 500 Bq/kg 238U. In the beginning of the 19th century, plenty of wetlands were drained in order to create arable land leading to a change in redox conditions underground. O-rich rainwater was then able to circulate through the peat horizon and oxidized the adsorbed U. The enabling of the leaching processes resulted in increased U concentrations in the drainage pipes with more than 600 mBq/L 238U. The investigation with airborne gamma ray spectrometry that was performed with a helicopter equipped with 4 NaI detectors revealed that further U accumulations in the area around Mont Vully could be possible. The second study site at the agricultural area around the Lyssbach was chosen for the analysis of the leaching behaviour of U. Sampling once a month the drainage pipe that drains a peat horizon with a 238U concentration of 2000 Bq/kg revealed that the leaching process depended on diverse factors such as the amount of water available and the carbonate content. By combining parameters such as the annual precipitation and the 238U concentration and flow rate of the drainage pipe revealed an overall leaching rate of 0.3% per year of the total U inventory (600 kBq/m2). This implies that leaching has to take place in the same intensity for the next several hundreds of years until the peat horizon near the Lyssbach is almost U-free. Additional laboratory experiments including a sequential extraction and leaching experiments with varying pH, time and carbonate content verified this calculation and displayed other scenarios if the leaching conditions change. The investigation of U uptake by plants was accomplished in four different study sites at the Swiss Plateau. Analyzing the soil and flooding water revealed some increased U concentrations. The following measurements of rice plants, divided into stems, leaves and panicles, as well as rice crops yielded minor transfer factors leading to the assumption that the uptake of U is too small to have an influence on increasing health risks due to the uptake of U by food