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Geochemical evolution of sulphide-bearing waste rock soils at the Bingham Canyon Mine, Utah

¹ Kennecott Utah Copper Corporation, Bingham Canyon Mine, P.O. Box 232, Bingham Canyon, Utah 84006-0232, USA (e-mail: bordenr{at}kennecott.com)

The soils forming on waste rock dump surfaces at the Bingham Canyon Mine have paste pH values ranging from 2.08 to 7.91. Paste conductivity, a measure of soil salinity, varies between 22 and 8750 µS cm^–1. The primary controls on waste rock soil pH and salinity are the sulphide distribution in the waste rock, the amount of limestone present and the age of the waste rock dump surface. The average pH of recently exposed waste rock is 7.0 and the average conductivity is 1120 µS cm^–1. Within six years of placement on the waste rock dumps the average pH declines to 4.7, further decreasing to 3.7 after 50 years. The average conductivity increases to 3000 µS cm^–1 within six years but then declines to 855 µS cm^–1 after 50 years. The sharp drop in pH, and the peak in salinity shortly after the waste rock is placed on the dumps, reflects the rapid release of acidity and sulphate caused by oxidation of newly exposed pyrite. The salinity of the soils begins to decline as pyrite becomes depleted and sulphate is flushed from the soil by infiltration and runoff more rapidly than it is replenished by sulphide oxidation.