Fate of Cs, Sr and Pu in soil profiles at a water recharge site in Basel, Switzerland.

An important process in the production of drinking water is the recharge of the withdrawn ground water with river water at protected recharge fields. While it is well known that undisturbed soils are efficiently filtering and adsorbing radionuclides, the goal of this study was to investigate their behaviour in an artificial recharge site that may receive rapid and additional input of radionuclides by river water (particularly when draining a catchment including nuclear power plants (NPP)). Soil profiles of recharge sites were drilled and analysed for radionuclides, specifically radiocesium (Cs), radiostrontium (Sr) and plutonium (Pu). The distribution of the analysed radionuclides were compared with an uncultivated reference soil outside the recharge site. The main activity of Cs was located in the top soil (4.5-7.5 cm) and reached down to a depth of 84 cm and 48 cm for the recharge and the reference site, respectively. The found activities of Pu originate from the global fallout after 1950. Pu appeared to be strongly adsorbed onto soil particles. The shape of the depth profile was similar to Cs, but also similar between the recharge and the reference site. In contrast, Sr showed a uniform distribution over the entire depth of the recharge and reference profiles indicating that Sr already entered the gravel zone and the ground water. Elevated inventories of the radionuclides were observed for the recharge site. The soil of the recharge field exhibited a threefold higher activity of Cs compared to the reference soil. Also for Pu higher inventories where observed for the recharge sites (40%). Sr behaved differently, showing similar inventories between reference and recharge site. We estimate that 75-89% of the total inventory of Cs in the soil at the recharge site (7.000 Bq/m) originated from the fallout of the Chernobyl accident and from emissions of Swiss NPPs. This estimate is based on the actual activity ratio of Cs/Pu of 22 for global fallout. The investigations identified radiostrontium as potential threat to the ground water.