RADIUM ISOTOPE RESPONSE DURING AQUIFER STORAGE AND RECOVERY: ALPHA RECOIL AND ELEMENT INTERACTIONS

Radium (Ra) differs from other naturally-occurring trace elements of health significance (e.g. As) by its behavior - a balance of radioactive and chemical processes. Here we report results of an investigation conducted during a pilot aquifer storage and recovery study in the Cambrian Mt. Simon sandstone of Minnesota (USA), an aquifer known for elevated levels of naturally-occurring Ra in groundwater. 63 × 10⁶ gal (2.4 × 10⁵ m³) of water were injected during ~4.5 months and stored for ~3 months after injection ended. Recovery of 52 × 10⁶ gal (2.0 × 10⁵ m³) over ~2 months resulted in storage time of 3-9 months.

The recovered water resembled injected water in terms of nonreactive ions such as Cl^-, implying only minor mixing of native groundwater and injected water. However, Ra in recovered water exceeded what would result from mixing of injected water (²²⁶Ra+²²⁸Ra < 2 pCi/L) with native groundwater (²²⁶Ra+²²⁸Ra > 6 pCi/L). During recovery, short-lived ²²⁴Ra increased to a stable value within hours. Long-lived ²²⁶Ra and ²²⁸Ra increased gradually during recovery (0.7-2.2 and 1.0-2.3 pCi/L, respectively).

Here, Ra is influenced mainly by alpha recoil (the release of Ra from α-decay of solid-phase Th) and/or chemical mechanisms of radium release and removal. Alpha recoil: The rapid increase of ²²⁴Ra, and ²²⁴Ra/²²⁸Ra ratios >1 after the first 2 hours of recovery (1.5-2.6), are consistent with alpha recoil. ²²⁴Ra/²²⁸Ra >1 occurs because ²²⁴Ra is replaced by α-recoil more rapidly than the longer-lived ²²⁸Ra. ²²⁸Ra/²²⁶Ra decreased during the recovery period (1.8-1.0), approaching the ratio of native groundwater and apparently approaching steady-state activities with increasing storage time. Chemical mechanisms: Concentrations of barium (Ba) were higher in injected water (0.19-0.24 mg/L) than in native groundwater (<0.1 mg/L). As Ra increased during recovery, Ba concentrations decreased (0.21-0.14 mg/L). Barium in injected water (present at 10⁸-times higher concentration than Ra) may have competed with Ra for adsorption sites. Such competition, if occurring, could affect the efficiency of Ra adsorption after its release into water by α-recoil. These results imply that the chemistry of injected water and native groundwater - and the duration of storage, even if only a few months - should be evaluated during aquifer storage.