RADIUM ATTENUATION, SPECIATION, AND MOBILIZATION IN RESPONSE TO SALINIZATION IN GROUNDWATER

Radium is a radioactive, carcinogenic, geogenic constituent that can be mobilized to groundwater as a result of the cascading effect of salinity on cations in some aquifers. Under typical surface and shallow groundwater conditions, Ra is relatively immobile due to its propensity to be adsorbed by Fe or Mn oxide and/or precipitate as sparingly soluble sulfate compounds such as radiobarite (Ba,Sr,Ra)SO₄. Increased salinity can induce reverse cation exchange that mobilizes Ra. For example, this process may help explain elevated Ra concentrations in a coastal plain aquifer of southern New Jersey, which is used for drinking water by millions of people and where increasing salinity resulting from road deicing salt is correlated with increases in Ra activities in groundwater. Also, low pH or anoxic conditions have been reported to decrease the ability of Ra to adsorb, which allows for more aqueous Ra to complex and move throughout the system.

This study complements prior work by providing a new simple-to-use geochemical modeling tool set that computes aqueous speciation of Ra, Ba, and Sr with changing salinity or pH conditions. The PHREEQC models we created (1) simulate the addition of increasing concentrations of chloride compounds (NaCl, CaCl₂, MgCl₂, or KCl) on Ra, Ba, and Sr speciation and their adsorption or precipitation as radiobarite, and (2) demonstrate Ra, Ba, and Sr attenuation by those mechanisms as a function of pH owing to the dissolution of caustic agent (NaOH, Ca(OH)₂, CaO, Na₂CO₃, or CaCO₃). A user interface was created for each model that facilitates adjustments to key variables such as titrants, sorbents, solutions, and surfaces. With these models, we demonstrate that increasing salinity or decreasing pH can cause an increase in the fraction of aqueous Ra. Increasing salinity also causes the formation of Ra-Cl complexes - one of the most bioavailable forms of Ra.

We conclude that interactions with saline waters can mobilize Ra from aquifer solids, creating potential water-quality problems for those that rely on the groundwater as a source for drinking water. This is of concern as there are various aquifers with low-pH or anoxic conditions that favor the mobilization of Ra – such as the Chickies Quartzite in southeastern Pennsylvania – that could be affected by salinization currently or in the future.