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