Hydrogeochemical Interactions and Evolution of Acidic Solutions in Soil

Abstract

Leachate generated from surface disposal of acidic uranium mill tailings in New Mexico and Colorado significantly alters hydrogeochemical characteristics of subjacent sediments including pH, Eh, mineralogical transformation, and acid neutralizing capacity. Experimental investigations and thermodynamic equilibrium modeling with the geochemical code PHREEQE show that the relatively oxidizing tailings pore water is in near equilibrium with jurbanite (AlOHSO4), gypsum (CaSO4·2H2O), strengite (FePO4·2H2O), and lepidocrocite (γ-FeOOH), and is oversaturated with alunite (KAl3(SO4)2(OH)6), goethite (α-FeOOH), and jarosite (KFe3(SO4)2(OH)6). Ions concentrated in tailings pore water include Mg, Na, Mn, V, Ni, Al, Fe, Ca, K, SO4 , NO3, PO4, Mo, Se, As, and U. Leach experiments on tailings material demonstrated that As, Cr, Mo, U , and V are associated with clay minerals, jarosite, and ferric oxyhydroxide coatings. The enrichment factors (clay/sand abundance) for these solutes are greater than unity, which may be the result of anion adsorption below pHzpc literature values for ferric oxyhydroxide, silica gel, and montmorillonite. The concentrations and mobilities of several species and elements follow the order SO4>NH4>Al>Mn>NO3>U>Fe>Se>PO4>Ni>As>Cd at pH 4.0. Sulfate-dominated leachate reacts with tailings subsoil calcite producing gypsum, which results in a continued decrease in SO4 concentrations. Dissolved concentrations of U, NO3, SO4 , and other major ions remain elevated above background concentrations downgradient from the tailings impoundment.