Uranium Tailings Acidification and Subsurface Contaminant Migration in a Sand Aquifer

Abstract

Abstract Investigations of the geochemistry of inactive pyritic uranium tailings in the Elliot Lake Mining district of Ontario have focused on the Nordic tailings management area, where two impoundments are located in natural bedrock basins. The tailings are 8-12 m thick and overlie a localized deposit of glaciofluvial sands. Analyses of the solid, liquid, and gas phases in the vadose zone of the tailings show that gas-phase oxygen levels drop rapidly within 0.7 to 1.5 m of the surface, indicating rapid oxygen consumption during pyrite oxidation. Oxidation during the past 15 to 20 years has caused a marked depletion of near-surface pyrite. The oxidation of pyrite in the vadose zone imparts to infiltrating precipitation high concentrations of Fe, SO42-, various heavy metals, and a pH generally between 1.5 and 4. The acidic infiltration moves downward at a rate of 0.2 to 2.0 m/yr, displacing high-pH groundwater that originated as process water discharged from the mill. It now occupies the entire tailings thickness over a small area of the tailings. At one location a well-defined plume of high-Fe2+ tailings-derived groundwater has developed in the sand aquifer adjacent to the tailings. The plume consists of three zones: an inner core characterized by Fe > 5000 mg/L, pH < 4.8, and elevated concentrations of several heavy metals and radionuclides; an outer zone with Fe < 2500 mg/L, pH > 5.5, and relatively low concentrations of heavy metals and radionuclides; and a transition zone separating the two. Although the average linear groundwater velocity is about 440 m/yr near the dam, reactions such as mineral dissolution, precipitation and coprecipitation retard the migration of the front of the inner core, producing an observed frontal migration rate of approximately 1 m/yr. Groundwater from the outer zone of the plume flows laterally towards a small stream, where a portion of it is now discharging into the stream bed. The discharge results in the precipitation of amorphous ferric hydroxide on the stream bed. Most of the H+ produced by Fe precipitation is buffered, and only a moderate decrease in stream pH is observed. Inner zone conditions will not reach the stream unless input of low-pH groundwater from the tailings continues for several hundred years. Although the rate of pyrite oxidation in the Nordic Main tailings has been decreasing, there is sufficient pyrite in the tailings to generate high-Fe groundwater for several decades or more. Calculated groundwater migration rates indicate that in the next few decades acidic, low-pH groundwater will occupy the entire tailings thickness over most of the tailings area, causing an increase in the total flux of contaminated groundwater into the underlying aquifer. The outer zone of the plume has already arrived at a small stream, and acidification of the surface waters may increase if the Fe concentration in the groundwater seepage increases.