Uranium mobility and accumulation along the Rio Paguate, Jackpile Mine in Laguna Pueblo, NM

The mobility and accumulation of uranium (U) along the Rio Paguate, adjacent to the
Jackpile Mine, in Laguna Pueblo, New Mexico was investigated using aqueous chemistry, electron
microprobe, X-ray diffraction and spectroscopy analyses. Given that it is not common to
identify elevated concentrations of U in surface water sources, the Rio Paguate is a unique
site that concerns the Laguna Pueblo community. This study aims to better understand the solid
chemistry of abandoned mine waste sediments from the Jackpile Mine and identify key
hydrogeological and geochemical processes that affect the fate of U along the Rio Paguate.
Solid analyses using X-ray fluorescence determined that sediments located in the Jackpile Mine
contain ranges of 320 to 9200 mg kg-1 U. The presence of coffinite, a U(IV)-bearing mineral, was
identified by X-ray diffraction analyses in abandoned mine waste solids exposed to several
decades of weathering and oxidation. The dissolution of these U-bearing minerals from
abandoned mine wastes could contribute to U mobility during rain events. The U concentration
in surface waters sampled closest to mine wastes are highest during the southwestern monsoon
season. Samples collected from September 2014 to August 2016 showed higher U concentrations in
surface water adjacent to the Jackpile Mine (35.3 to 772 mg L-1) compared with those at a
wetland 4.5 kilometers downstream of the mine (5.77 to 110 mg L-1). Sediments co-located in
the stream bed and bank along the reach between the mine and wetland had low U concentrations
(range 1–5 mg kg-1) compared to concentrations in wetland sediments with higher organic matter
(14–15%) and U concentrations (2–21 mg kg-1). Approximately 10% of the total U in wetland
sediments was amenable to complexation with 1 mM sodium bicarbonate in batch experiments; a
decrease of U concentration in solution was observed over time in these experiments likely due
to re-association with sediments in the reactor. The findings from this study provide new
insights about how hydrologic events may affect the reactivity of U present in mine waste
solids exposed to surface oxidizing conditions, and the influence of organic-rich sediments on
U accumulation in the Rio Paguate. Environmental impact: Uranium mobility and accumulation in
the environment can affect surface water and sediment chemistry. Here, we investigated the
mobility of uranium in water and accumulation on sediments along the Rio Paguate and within a
wetland and reservoir 5 km south of the Jackpile Mine. Main findings of the study suggest that
ore and mine waste from surficial locations of the Jackpile Mine are the source of uranium in
the surface water. Additionally, the concentration of uranium in the Rio Paguate varies
seasonally, with higher concentrations during the southwestern monsoon season. The uranium
concentration in surface water decreases downstream of the mine and adsorption of uranium to
organic-containing sediments in a wetland 5 km downstream contributes to this decrease. These
results have important implications to better understand the mobility and accumulation of
uranium in the environment.