A MULTI-ISOTOPE INVESTIGATION (RA, SR, B, O, H) OF GROUNDWATER SALINIZATION AND RADIUM MOBILIZATION IN ATLANTIC COASTAL PLAIN AQUIFERS, NORTH CAROLINA

Pumping in the Atlantic Coastal Plain of North Carolina is associated with drawdown in groundwater levels and increasing salinity, documented in examples from the confined, anoxic Cretaceous Cape Fear (CF) and Pliocene Yorktown (Y) sand aquifers. Chemical and isotopic characterization indicates that salinization is derived from upward leakage of saline groundwater, rather than lateral intrusion of seawater. Due to the well-documented ion exchange properties of these unfreshened aquifers, waters are strongly dominated by Na, primarily as Na-HCO₃^- waters mixing with saline Na-Cl^- waters (Cl^- concentrations 5-16,000 mg L^-1 in CF, 336-10,100 mg L^-1 in Y). Under these conditions, alkaline earth metal concentrations are suppressed by reverse ion exchange. Radium (Ra) activity is associated with salinity and approaches drinking water standards only in brackish waters. Also, in CF two shallower (60-70 m below surface), slightly less Na-dominated fresh waters (higher Ca/Na) exhibit ~2x the Ra of comparable pure Na-HCO₃^- fresh waters. In CF, these similar patterns between Ca/Cl, Mg/Cl, Sr/Cl, Ba/Cl, and Ra/Cl imply that ion exchange influences Ra activity; however, the association of Ra with salinity indicates that some recoiled Ra remains in brackish waters, perhaps in the form of RaCl⁺ and RaSO₄⁰ rather than Ra²⁺. Median ²²⁴Ra/²²⁸Ra near 1 implies Ra is broadly distributed on solids.

In each aquifer, ⁸⁷Sr/⁸⁶Sr occupies a narrow range, not correlated with salinity, 1/Sr, or Sr/Cl (0.70808-0.70899 for CF; 0.70900-0.70915 for Y) and apparently does not record ion exchange. While δ¹¹B in Y records a near-seawater signature at depth (+30 to +36‰), transition to terrestrial signature in the shallow aquifer, and Br/Cl identical to seawater throughout, δ²H and δ¹⁸O indicate, consistent with previous studies, that the saline water does not represent mixing with modern seawater intrusion. Instead, the data suggest that the saline component is old modified seawater. In sum, the potential for aquifer salinization in the Atlantic Coastal Plain is not closely linked to proximity to the ocean or estuaries because saline waters are ubiquitous at depth and upward gradients are widespread. This phenomenon may affect water quality or the accumulation of naturally-occurring contaminants (including As and Ra) in effluents of desalination plants.