Uranium Tailings Acidification and Subsurface Contaminant Migration in a Sand Aquifer

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.

Field determination of sulphide oxidation rates in mine tailings

“…Problems with water quality in areas with exposed, unsaturated fine-grained mine waste containing sulphide minerals have been recognized for a long time [Boorman and Watson, 1976;Reardon and Poscente, 1984;Dubrovsky et al, 1984; Blowes and Jambor, 1990]. The contaminated effluent typically contains high concentrations of sulphate, acid, and dissolved trace metals.…”

Field Evaluation of Sulphide Oxidation Rates

“…Often, many years of monitoring and data collection are necessary to estimate the rate of acid generation and oxidation product release (Dubrovsky et al 1984;Blowes et al 1990 and. The current method of evaluation provides an instan-taneous estimation of the relative oxidation rates assuming steady state conditions of the oxygen profile, that if used over time and space can provide a more general rate description and practical monitoring tool for tailings.…”

“…Management of this waste material has traditionally involved deposition of slurried tailings into impoundments. Numerous reports have addressed the problems of water quality associated with exposed, unsaturated tailings deposits (Boorman and Watson 1976;Reardon and Poscente 1984;Dubrovsky et al 1984;Blowes et al 1987;Blowes et al 1990 andBlowes et al 1991).…”

Principles and problems of environmental pollution of groundwater resources with case examples from developing countries.

“…Waste products in urban areas are transported away by runoff. In mining areas, gangue materials dumped about recklessly on ground surface, decay and liquid wastes are leached from them becoming components of the hydrogeologic environment (20)(21)(22). In regions of intense geomorphic degradation and masswasting, physicochemical and biological weathering disintegrate pedologic and geologic materials to produce sediments that provide great quantities and varieties of pollutants.…”