Occurrence and origin of polonium-210 in a sandstone aquifer in Germany

Polonium-210 (Po-210) is a naturally occurring high-energy alpha emitter with a high dose coefficient compared to other natural radionuclides. In groundwater, Po-210 concentrations usually are low due to its high surface reactivity and tendency to readily adsorb onto mineral surfaces. The nationwide median in drinking water in Germany is 1.4 mBq/L as shown by investigations of the Federal Office for Radiation Protection (BfS). However, some sites in southern Sachsen-Anhalt in Germany were revealed to have Po‑210 activity concentrations far above 100 mBq/L. This is of particular relevance, since the EU Council Directive 2013/51/Euratom and its national implementation in the German Drinking Water Ordinance specifies requirements for drinking water quality with regard to artificial and natural radioactive substances. These high values of Po-210 would require reduction measures according to legislation.  

In order to gain a better understanding of the occurrence and origin of Po-210 in groundwater, further groundwater wells and water supply facilities in southern Sachsen-Anhalt were selected for analysis, taking into account the regional geology. The investigated aquifers pertain to the rock sequences of the Lower, Middle and Upper Buntsandstein (Lower Triassic), the Lower Muschelkalk (Middle Triassic) and Cenozoic stratigraphic units. Unfiltered water samples from 50 sites in total were analyzed for their content of natural radionuclides, major and trace elements as well as their oxygen and hydrogen isotope composition and supplemented by the examination of drill core material from these rock sequences.

The data indicate that the elevated Po-210 activity concentrations are limited to the aquifer of the Middle Buntsandstein. The simultaneous absence of the parent nuclides lead-210 (Pb-210) and radon-222 in the water samples indicate a local release mechanism in the rock. In the Middle Buntsandstein the amount of Pb-210 would be sufficient to provide the amount of Po-210 found in solution and the available clay minerals like illite, chlorite, hematite and muscovite could serve as potential hosts for Pb-210 and Po-210. Redox-indicators and a relatively high content of dissolved organic material (DOC >1 mg/L) found in the water samples of the Middle Buntsandstein aquifer could be interpreted as an indication of microbially induced reduction processes.

Understanding the geochemical processes responsible for the mobilization of Po-210 could provide necessary information helping to minimize the human exposure caused by drinking water consumption. Therefore, further investigations will focus on the size distribution of Po-210-bearing particles, the presence of the short-lived mother nuclide bismuth-210 and the sulfate isotope distribution at selected Po-210-rich groundwater sites in this study area.