Radium geochemical monitoring in well waters at regional and local scales: an environmental impact indicator-based approach

To assess radium (226Ra) as a potential indicator of impact in well waters, we investigated its behavior under natural conditions using a case study approach. 226Ra geochemistry was investigated in 67 private wells of southeastern New Brunswick, Canada, a region targeted for potential shale gas exploitation. Objectives were to i) establish 226Ra baseline in groundwater; ii) characterize 226Ra spatial distribution and temporal variability; iii) characterize 226Ra partitioning between dissolved phase and particulate forms in well waters; and iv) understand the mechanisms controlling 226Ra mobility under natural environmental settings. 226Ra levels were generally low (median = 0.061 pg L−1, or 2.2 mBq L−1), stable over time, and randomly distributed. A principal component analysis revealed that concentrations of 226Ra were controlled by key water geochemistry factors: the highest levels were observed in waters with high hardness, and/or high concentrations of individual alkaline earth elements (i.e. Mg, Ca, Sr, Ba), high concentrations of Mn and Fe, and low pH. As for partitioning, 226Ra was essentially observed in the dissolved phase (106 ± 19%) suggesting that the geochemical conditions of groundwater in the studied regions are prone to limit 226Ra sorption, enhancing its mobility. Overall, this study provided comprehensive knowledge on 226Ra background distribution at local and regional scales. Moreover, it provided a framework to establish 226Ra baselines and determine which geochemical conditions to monitor in well waters in order to use this radionuclide as an indicator of environmental impact caused by anthropogenic activities (e.g. unconventional shale gas exploitation, uranium mining, or nuclear generating power plants).