Radiological Analyses of 226Ra and 238U in Surface Water and Sediments from the Jackpile Member of the Morrison Formation, Pueblo of Laguna, New Mexico.

Surface water and sediments from the Jackpile mine, St. Anthony mine, Rio Paguate, Rio Moquino, and Mesita Dam areas near Pueblo of Laguna, New Mexico, were analyzed for 226Ra and U using gamma (γ) spectroscopy and inductively coupled plasma mass spectroscopy, respectively. Activity ratios for 226Ra/238U for solid samples range from 0.34 ± 0.13 to 16 ± 2.9, which reflect uranium transport and accumulation (<1), relatively pristine material in secular equilibrium (1), and removal of uranium by weathering (>1). Concentrations ranging from 80 to 225 µg L-1 U were detected in unfiltered water samples near the Jackpile mine. Water samples upstream and downstream from the mine contained concentrations ranging from 12 to 15 µg L-1 U. Water samples collected from the North Pit standing pond in the Jackpile mine contained as much as 1560 pCi L-1 of 226Ra, and passing the water through a 0.2 µM filter did not substantially reduce the activity of 226Ra in the water. 234Th and 226Ra are in secular equilibrium in this water, while radon gas was lost from the water. The results of the current study provide insight into the distribution of U-series radionuclides in the Pueblo of Laguna area, including detection of high levels of radioactivity in water at some locations within the Jackpile mine.

Removal of Aqueous Uranyl and Arsenate Mixtures after Reaction with Limestone, PO43-, and Ca2.

The co-occurrence of uranyl and arsenate in contaminated water caused by natural processes and mining is a concern for impacted communities, including in Native American lands in the U.S. Southwest. We investigated the simultaneous removal of aqueous uranyl and arsenate after the reaction with limestone and precipitated hydroxyapatite (HAp, Ca10(PO4)6(OH)2). In benchtop experiments with an initial pH of 3.0 and initial concentrations of 1 mM U and As, uranyl and arsenate coprecipitated in the presence of 1 g L-1 limestone. However, related experiments initiated under circumneutral pH conditions showed that uranyl and arsenate remained soluble. Upon addition of 1 mM PO43- and 3 mM Ca2+ in solution (initial concentration of 0.05 mM U and As) resulted in the rapid removal of over 97% of U via Ca-U-P precipitation. In experiments with 2 mM PO43- and 10 mM Ca2+ at pH rising from 7.0 to 11.0, aqueous concentrations of As decreased (between 30 and 98%) circa pH 9. HAp precipitation in solids was confirmed by powder X-ray diffraction and scanning electron microscopy/energy dispersive X-ray. Electron microprobe analysis indicated U was coprecipitated with Ca and P, while As was mainly immobilized through HAp adsorption. The results indicate that natural materials, such as HAp and limestone, can effectively remove uranyl and arsenate mixtures.

Kinetics of Na- and K- uranyl arsenate dissolution.

We integrated aqueous chemistry analyses with geochemical modeling to determine the kinetics of the dissolution of Na and K uranyl arsenate solids (UAs(s)) at acidic pH. Improving our understanding of how UAs(s) dissolve is essential to predict transport of U and As, such as in acid mine drainage. At pH 2, Na0.48H0.52(UO2)(AsO4)(H2O)2.5(s) (NaUAs(s)) and K0.9H0.1(UO2)(AsO4)(H2O)2.5(s) (KUAs(s)) both dissolve with a rate constant of 3.2 × 10-7 mol m-2 s-1, which is faster than analogous uranyl phosphate solids. At pH 3, NaUAs(s) (6.3 × 10-8 mol m-2 s-1) and KUAs(s) (2.0 × 10-8 mol m-2 s-1) have smaller rate constants. Steady-state aqueous concentrations of U and As are similarly reached within the first several hours of reaction progress. This study provides dissolution rate constants for UAs(s), which may be integrated into reactive transport models for risk assessment and remediation of U and As contaminated waters.