Natural Attenuation Reactions at a Uranium Mill Tailings Site, Western U.S.A.

Abstract

This paper presents a modeling analysis of the geochemical evolution of a contaminated sandy aquifer at a uranium mill tailings site in the western United States. The tailings pond contains fluids having a pH of 1.5 to 3.5 and high levels of As, Be, Cd, Cr, Pb, Mo, Ni, Se, 226Ra, 228Ra, 230Th, 238U, and 234U. Seepage of tailings fluids into the aquifer has formed a low-pH ground water plume. The reclamation plan is to install a low-permeability cover on the tailings pond to stop the seepage and allow the plume to be attenuated by reactions with the aquifer matrix and flushed by uncontaminated upgradient ground water. To evaluate this reclamation scenario, ground water and sediment core samples were analyzed along one flowpath. Speciation-solubility and mass-transfer modeling revealed two sets of chemical reactions for acid seepage and flushing, respectively. The current concentrations and distribution of ground water constituents can be interpreted as being controlled by stepwise pH-buffer reactions with calcite, amorphous aluminum hydroxide, and amorphous iron hydroxides. These buffer reactions divide the aquifer into zones of near-constant pH, separated by interface zones. For the flushing stage, it is predicted that reactions with surface-bound species will dominate the reaction paths, and more pore volumes are required to neutralize the plume than predicted by models that do not consider surface reactions. Direct mineralogical and surface analysis is needed to substantiate this assertion.

Uranium mill tailings: nuclear waste and natural laboratory for geochemical and radioecological investigations

“…Amorphous materials present additional problems to XRD characterization. The limitations imposed by a lack of mineralogic data on modeling the natural attenuation of an acidic plume in the aquifer underneath a UMT pond in the western US have been highlighted by and Zhu et al ( ,2002. They advocate detailed microscopic and spectroscopic examination of mineral surfaces to establish site densities of hydrous ferric oxide (HFO), amorphous aluminum hydroxide (AAH), and other reactive phases in aquifer solids, and to elucidate the nature and form of surface acidity in contaminated zones.…”

Tailings dam seepage at the rehabilitated Mary Kathleen uranium mine, Australia

“…Michel and Hardgrove, 2001;Zhu et al, 2001Zhu et al, , 2002 and laboratory studies including kinetic tests (e.g. Landa, 1991;Goulden et al, 1998;Delaney et al, 1998;Langmuir et al, 1999).…”

A high-resolution TEM-AEM, pH titration, and modeling study of Zn2+ coprecipitation with ferrihydrite

“…For example, in acid mine drainage intrusion into groundwater, low-pH mine water can react with calcite in the aquifer. Ferric iron and a suite of trace toxic metals and radionuclides precipitate simultaneously in response to the rise of pH (Zhu et al, 2002). However, most of the previous work on metal and iron oxyhydroxide interactions focus on surface adsorption (see review by Dzombak and Morel, 1990).…”