Uranium in groundwater - The importance of hydraulic regime and groundwater flow system's understanding

Uranium and other natural radionuclides are common components in groundwater, but they are not routinely measured. In drinking water their total activity is screened, but in the evaluation of the measured values usually the aquifer material is only considered. However, the occurrence of radionuclides in groundwater is strongly affected by flow systems and their geochemical characteristics. Therefore hydrogeology and flow system's evaluation is crucial to understand natural radioactivity. Areas of different hydraulic regimes - recharge, throughflow and discharge - even within the same aquifer are characterized by different geochemical environment. In the present study pressure-elevation profiles were generated based on existing basic hydraulic data of wells in order to determine the flow regimes and associated vertical groundwater flow directions. 24-753 mBq L^-1 uranium activity concentrations were found in groundwater and surface water showing great areal variability. High uranium values correspond to recharge regimes with downward flow directions. Uranium mobility is enhanced by high bicarbonate content and circumneutral pH. The study emphasize the importance of groundwater flow system's understanding in those areas, where elevated background radioactivity may exist.