Dissolved zinc concentrations averaged 9,400 µg/L (micrograms per liter) in water from abandoned lead and zinc mines, some of which discharge at the surface. Contamination of the shallow aquifer by the highly mineralized mine water is limited to the immediate mining area. The quality of water in the deep aquifer is generally excellent and unaffected by mine water.
Dissolved zinc concentrations averaged 16,000 µg/L in runoff from tailings areas. However, during a summer storm, runoff from a 0.028 km² tailings area contained maximum dissolved zinc, lead, and cadmium concentrations of 200,000; 400; and 1,400 µg/L, respectively.
Mine-water discharges increase dissolved zinc concentrations in receiving streams from a background of about 40 µg/L to about 500 µg/L during periods of low flow. The higher concentrations are sustained during high flow by runoff from the tailings areas. Deposition of tailings on stream bottoms increases zinc concentrations in bottom material from a background of about 100 µg/g (micrograms per gram) to about 2,500 µg/g and increases lead concentrations in bottom materiel from about 20 µg/g to about 450 µg/g.
Results of this study indicate the continuing need to control metal-mining wastes after mining has ceased, as well as during active mining.
The shallow aquifer