Quantifying vertical fluxes of water, uranium, and metals-mixtures in the unsaturated zone at various mine life-cycle stages

Abstract:

There is a critical information gap about processes affecting movement of metals in the shallow subsurface and their responses to mining operations. This study characterizes fluxes of water and metals to identify the primary mechanisms of metals transport in shallow soils at the north rim of the Grand Canyon around four semi-arid mine sites. The stages of mine development ranged from an unmined breccia-pipe uranium deposit (EZ2), to recent production (Arizona 1), to active reclamation (Pinenut), and post-reclamation (Kanab North). Surface and depth-profile cores were collected outside the perimeter of the mine yards at each site and inside the Kanab North mine yard for analysis of bulk metals concentrations and soil physical and hydrologic properties. Dissolution experiments were conducted to characterize solubility of metals in surface soils including As, Cd, Co, Cu, Mo, Ni, Pb, Sb, Tl, U, and Zn. To distinguish transport of solid versus dissolved phases, an end member mixing analysis was applied to profiles from undisturbed locations around the mines. The majority of transport occurred in solid particles in the upper 30 cm of the soil due to biotic and abiotic soil mixing processes. A Fickian-diffusion model of soil mixing with historical inputs predicted similar rates of vertical mixing among the study sites. Mixing rates were at or above rates previously observed for radiogenic nuclides at other sites. Numerical models of soil-matrix flow using decadal weather records and calibrated to profiles of moisture content, matric pressure, and Cl concentration predicted water losses to deeper layers that were minor (~1% of precipitation) and sporadic (e.g. three deep percolation events over the course of 40 years) indicating limited potential for downward transport of uranium and other soluble metals. Implications of this study include: (1) soil mixing of airborne dust from mine operations sequesters metals in soil and retards further aeolian transport, (2) further study is needed to compare biotic and abiotic processes affecting soil mixing and migration of metals, (3) downward movement of dissolved metals in soil-matrix water is likely highly sporadic (e.g. at intervals of decades), posing challenges to characterizing long-term losses to deeper zones.