Research addressing radium in groundwater typically reports that hydrogeochemistry is an important control on radium occurrence. However, the presence of an adequate mass of source radium is not commonly assessed. In the Paleozoic bedrock aquifers of southern Minnesota, spatial and stratigraphic variations in radium occurrence are present although aquifer hydrogeochemical conditions are confined to a narrow range. This suggests that variation in radium source mass is an important control on radium occurrence.

It is likely that radium activities in groundwater decrease with ascending stratigraphic position in Paleozoic bedrock aquifers. The ²²⁸Ra/²²⁶Ra isotope ratios for all aquifers generally decrease with increasing stratigraphic level, presumably due to ²²⁸Ra decay along flow paths away from the radium source. Wells completed within a single deep aquifer have higher radium concentrations than multiple aquifer wells, and this may indicate that the lower concentrations are due to dilution. Finally, based on available information, groundwater from wells completed within lower portions of a single aquifer appear to have higher radium content than groundwater from wells completed only within upper portions of the same aquifer.

The evidence for peak radium occurrence at depth is consistent with a deep radium source.  The Paleozoic rocks of southern Minnesota are underlain by a major Precambrian structure, termed the Mid-Continent Rift Zone, which may be the ultimate source for radium in Minnesota’s Paleozoic bedrock aquifers. Evidence for this is provided by the stratigraphically lowest aquifer, where wells with the highest radium activities are on average six times closer to mapped basement faults than those meeting the radium MCL.