Occurrence and mobilization of radium in fresh to saline coastal groundwater inferred from geochemical and isotopic tracers (Sr, S, O, H, Ra, Rn)

Salinization in groundwater systems can induce water-rock interaction, including the release of naturally-occurring trace elements of health significance such as radium (Ra), with possible implications for the usability of water resources in addition to the increase of dissolved solids (TDS) concentrations. In general, radium mobility is limited by chemical removal mechanisms including adsorption onto clays and/or Mn and Fe oxides, exchange processes, and coprecipitation with secondary barite. In order to examine the effect of aquifer salinity gradients on the distribution of naturally-occurring Ra in fresh to saline groundwater and the relationship to water-rock interaction and Ra removal mechanisms, two contrasting systems were investigated: the shallow unconfined coastal aquifer in Agadir (southwestern Morocco) and the confined Cretaceous (Cape Fear) and Pliocene (Yorktown) aquifers of the Atlantic Coastal Plain (North Carolina, USA). Geochemical and isotopic indicators of salinity sources (e.g. cation ratios, δ¹⁸O, δ²H, Br^-/Cl^-, δ34S-SO42-,δ18O-SO42-) were used to identify the relative contributions of seawater and other saline waters and subsequent geochemical modification by water-rock interaction. Radium activities (²²⁴Ra, ²²⁶Ra, ²²⁸Ra), radon-222, alkaline earth metal (Mg, Ca, Sr, Ba) concentrations and ratios, and ⁸⁷Sr/⁸⁶Sr ratios were analyzed to identify water-rock interaction processes affecting alkaline earth metals including Ra. The Morocco coastal aquifer is generally oxic, exhibits a range of salinity and water types (Cl^- 163-2120mg/L, median 932mg/L), and exhibits Ca/Na ratios above the seawater value, typical of monovalent-divalent cation exchange (base-exchange reactions) in coastal aquifers. In contrast, the Atlantic Coastal Plain aquifers are anoxic, sulfate-reducing, cover a wider salinity range (Cl^- 5-9890mg/L, median 800mg/L) representing a transition between Na-HCO3- and Na-Cl^- waters, and exhibit Ca/Na ratios below that of modern seawater typical of reverse base-exchange reactions. Possible salinity sources in the Morocco coastal aquifer include seawater intrusion, Mesozoic evaporites, other natural saline waters, and/or wastewater, whereas the Atlantic Coastal Plain is primarily affected by old seawater present in the aquifer system. Radium activities are generally low and vary significantly within each aquifer, for example ²²⁶Ra ranges from 1.8-27.7mBq/L in the Morocco coastal aquifer (median 8.1mBq/L) and 1.9-214mBq/L in the Atlantic Coastal Plain (median 18.9mBq/L). The highest Ra activities were observed in the most saline wells sampled in the Atlantic Coastal Plain. At total dissolved solids (TDS) concentrations above an apparent threshold of ~5000mg/L, radium activities increase in a generally linear fashion with salinity in the Atlantic Coastal Plain, broadly comparable to previous studies indicating a threshold range of ~3000-10,000mg/L. At lower TDS concentrations, water-rock interaction processes that vary with local aquifer conditions appear to control Ra distribution rather than merely salinity. In the Morocco coastal aquifer, adsorption of Ra and coprecipitation with secondary barite are apparently favorable to control Ra levels in groundwater. Radium removal in the anoxic Atlantic Coastal Plain aquifers appears to be associated with adsorption and/or exchange processes, with the additional possibility of barite precipitation in the Cape Fear aquifer indicated by barite saturation. Overall, the locally-variable factors that can control Ra sources and mobility in fresh to brackish groundwater at near-neutral pH include variation in solid-phase radioactivity, redox state affecting adsorption sites, availability of competing divalent cations, and barite saturation.