Kate Campbell, and two co-authors, reported in 2015 that natural uranium (U) occurs as a mixture of three radioactive isotopes: 238U, 235U, and 234U. Only 235U is fissionable and makes up about 0.7% of natural U, while 238U is overwhelmingly the most abundant at greater than 99% of the total mass of U. Prior to the 1940s, U was predominantly used as a coloring agent, and U-bearing ores were mined mainly for their radium (Ra) and/or vanadium (V) content; the bulk of the U was discarded with the tailings. Once nuclear fission was discovered, the economic importance of U increased greatly. The mining and milling of U-bearing ores is the first step in the nuclear
fuel cycle, and the contact of residual waste with natural water is a potential source of contamination of U and associated elements to the environment. Uranium is mined by three basic methods: surface (open pit), underground, and solution mining (in situ leaching or in situ recovery), depending on the deposit grade, size, location, geology and economic considerations (Abdelouas, 2006). Solid wastes at U mill tailings (UMT) sites can include both standard tailings (i.e., leached ore rock residues) and solids generated on site by waste treatment processes. The latter can include sludge or ‘‘mud’’ from neutralization of acidic mine/mill effluents, containing Fe and a range of co-precipitated constituents, or barium sulfate precipitates that selectively remove Ra (e.g., Carvalho et al., 2007). In this chapter, we review the hydrometallurgical processes by which U is extracted from ore, the biogeochemical processes that can affect the fate and transport of U and associated elements in the environment, and possible remediation strategies for site closure and aquifer restoration.
The authors presented the fourth in a series of review papers from the U.S. Geological Survey (USGS) on geochemical aspects of UMT management that span more than three decades. The first paper (Landa, 1980) in this series is a primer on the nature of tailings and radionuclide mobilization from them. The second paper (Landa, 1999) includes coverage of research carried out under the U.S. Department of Energy’s Uranium Mill Tailings Remedial Action Program (UMTRA). The third paper (Landa, 2004) reflects the increased focus of researchers on biotic effects in UMT environs. This paper expands the focus to U mining, milling, and remedial actions, and includes extensive coverage of the increasingly important alkaline in situ recovery and groundwater restoration.
The authors concluded that tools such as a qualitative, conceptual geoenvironmental model (du Bray, 1995), can facilitate identification of potential geochemical indicators of contamination and sequestration agents related to a given deposit type. In this context, geoenvironmental models include a ‘‘compilation of geologic, geochemical, geophysical, hydrologic, and engineering information pertaining to the environmental behavior of geologically similar deposits prior to mining and resulting from mining, mineral processing and smelting’’ (Seal et al., 1995). Information found in a geoenvironmental model used in conjunction with knowledge regarding mineralogy mineral chemistry, hydrology, climate and transport processes can help to establish linkages among the geologic attributes of a deposit, its environmental setting and its historical and future mining techniques and environmental behavior. Such applications can be used to inform potential for contamination, waste generation, and/or environmental restoration, as well as provide insight that can be useful in mine planning, mineral exploration, permitting of proposed mines, and mine reclamation plans (Plumlee and
Nash, 1995). For example, a geoenvironmental model for the world’s largest breccia pipe-hosted U province in Northern Arizona identified the potential for acid drainage containing toxic metal(loid)s (such as Ag, As, Ba, Cd, Co, Cr, Cu, Hg, Mo, Ni, Pb, Sb, Se, Sr, U, V, Y, and Zn) in the immediate vicinity of mineralized pipes where they are dissected and exposed to flooding and catastrophic precipitation. However, it was noted that the limestone and calcareous sandstone host rocks likely would buffer acidic runoff water and downward percolating water efficiently prior to or
during their transport in aquifer systems (Wenrich et al., 1995).
Additionally, because very little water moves downward into the pipes, and because (1) ore bodies in this deposit are located, on average, 300 m below the surface, and (2) water bodies within 600 m of the plateau surface are rare, long-distance transport of these elements was predicted to be slight (Wenrich et al., 1995). A 2009 environmental assessment of the effects of 1980s mining in the Kanab Creek, Arizona area found that trace element concentrations in water samples that were thought to be associated with U mining (2–20 lg U/L) were not significantly elevated above those waters found in non-impacted areas (0.06–21 lg/L), although the data are considered preliminary (Bills et al., 2010). Likewise, concentrations of trace metals in surface soils and stream sediments from the former Hermit Mine, which was located on a flat area not susceptible to flooding averaged 4.6 mg/kg U within the reclaimed area and dropped to regional background levels of 1.7 mg/kg U within 30 m of the reclaimed area boundary.
On the other hand, the former Hack 1 Mine site, situated adjacent to the stream channel that runs through Hack Canyon, exhibited evidence of dispersion of waste rock and stream sediments containing elevated concentrations of metals, apparently the result of historical flood events (Otton et al., 2010). This variation in surficial U suggests that risk of waste release and environmental impacts from U mining may not be uniform for entire regions or deposit types, but may be due to a number of site specific attributes such as proximity to water sources, susceptibility to wind and water erosion, mining and restoration methods, elevation, and topography, to name a few. This result provides impetus to begin to examine U resources in a context of the entire life cycle including the changes that the U deposit may undergo during extraction, restoration, and stabilization.
Sustainability in U extraction is no small task .... [from the ore deposits of from the environment? If former, the uranium supply is adequate for the needs over the next 50 years, if the latter, it's removal from areas that actually represent an environmental risk should not be treated as being more difficult than typical industrial wastes] Geoscientists and engineers realize that it can only be achieved when appropriate technologies are implemented in industry and disseminated
throughout society (Anastas, 2003). As such, the future of sustainability will depend on the cooperation and collaboration of geoscientists, engineers, and stakeholders.
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