GEOCHEMICAL BEHAVIOR OF URANIUM MILL TAILINGS LEACHATE IN THE SUBSURFACE

Leachate generated from surface disposal of acidic uranium mill tailin gs at Maybell, CO has impacted groundwater quality within the underlyi ng mineralized Browns Park Formation. The extent of groundwater contam ination, however, is localized directly beneath the tailings impoundme nt. The milling process consisted of sulfuric acid extraction of urani um from the feed ore by a complex chemical leaching and precipitation process. Tailings leachate at the site contains elevated concentration s of Al, As, Cd, Mo, Ni, NO3, Se, U, and other solutes. From column le ach tests, the concentrations of contaminants within tailings pore flu id are SO4 > NH4 > NO3 > U > Se > Ni > As > Cd at pH 4.0. The carbonat e buffering capacity of the tailings subsoil has decreased because of calcite dissolution in the presence of acidic leachate. Groundwater qu ality data, mineralogical and microbiological studies, and geochemical modeling suggest that As, NO3, Se, U and other solutes are being remo ved from solution through precipitation, adsorption, and denitrificati on processes under reducing conditions. Presence of hydrogen sulfide, liquid and gaseous hydrocarbons, dissolved organic carbon, and abundan t pyrite within the Browns Park Formation have maintained reducing con ditions subjacent to the tailings impoundment. Groundwater is in close equilibrium with coffinite and uraninite, the primary U(IV) minerals extracted from the Browns Park Formation. Denitrifying bacteria (Pseud omonas and Flavobacterium) identified in this study catalyze redox rea ctions involving NO3. Subsequently, contaminant distributions of NO3 d ecrease 1000 times beneath the tailings impoundment. Applying geochemi cal and biochemical processes occurring at Maybell provides an excelle nt model for in situ aquifer restoration programs considered at other uranium tailings and heavy-metal-mixed waste contaminated sites.