English

Abstract

The Elizabeth mine Superfund site offers unique opportunities to investigate the interplay between geochemical and physical processes in the geochemical evolution of an acidic pit lake. The mine exploited a steeply dipping massive sulfide ore body. Ores contained pyrrhotite and chalcopyrite and were hosted by siliciclastic sedimentary rocks and amphibolites. An open pit that accessed part of the ore body is filled by a long (380 m), narrow (< 25 m), and shallow (< 7 m) lake, which is divided into two parts by a rock slide. The southern end serves as a decant point, and discharges for most of the year. Quarterly sampling and continuous temperature monitoring of the water column in the lake document geochemical variations that depend on seasonal variations in the amount of precipitation and the presence or absence of ice. From spring to fall, parameters show limited variation with depth except for temperature, which decreases with depth. The lake experienced overturn in the fall. During winter under ice cover, the lake developed a chemocline shown by a sharp decrease in pH, a doubling of total dissolved solids (TDS), and a ten-fold increase in dissolved Fe with depth (0.4 - 55.2 mg/L). Oxia throughout most of the year causes removal of Fe, but low pH prevents significant sorption of Cu and other metals. Ca (25 - 72 mg/L), Mg (9.6 - 15.0 mg/L), K (6.0 - 8.3 mg/L), Si (6.7 - 9.6 mg/L), and SO4 -2 (210 - 280 mg/L) are the major dissolved constituents during non-stratified ice-free periods; Fe (0.2 - 0.5 mg/L), Al (1.0 - 3.1 mg/L), Cu (0.6 - 1.2 mg/L), and Zn (0.3 - 0.5 mg/L) are important minor dissolved constituents. Throughout the year, the water quality is dependent upon a variety of factors including the geometry of the pit, the short residence time of water within the pit, wind mixing and fall overturn, the oxidation of sulfides on the pit walls and in the unsaturated waste on the floor of the pit south of the haulage way, oxidation of iron within the water column, and mixing of surface waters with high TDS waters entering the bottom of the lake.