GEOCHEMICAL EVOLUTION OF WATER QUALITY INSIDE AN ACTIVE LEAD/ZINC MINE TAILINGS IMPOUNDMENT

The objective of this project is to investigate the near surface aqueous geochemistry and mineralogy of an active lead/zinc mine tailings impoundment in the Viburnum Trend, Southeast Missouri Lead District. The facility is located near Bunker, Missouri, and produces mainly lead and zinc from the Bonneterre Formation dolomites. An evaporate crust of epsomite (MgSO₄.7H₂O) has formed on the surface of the impoundment, which suggests that sulfide oxidation, dissolution, and evaporative concentration are occurring. As a result of the epsomite formation and high specific conductance levels in pore water, it is hypothesized that oxidative dissolution of sulfides, generation of sulfuric acid, and subsequent dissolution of dolomite is the driving mechanism affecting the near surface geochemistry of the active tailings impoundment. Ground water-quality data were collected through a series of monitoring wells and vacuum lysimeters within the active impoundment and within an inactive Pb-Zn tailings impoundment (last used in the 1950s), in the Old Lead Belt near Park Hills, Missouri. These data show higher specific conductance values at the active impoundment, as well as elevated concentrations of lead, zinc, and iron as compared to the data collected from the inactive impoundment. The inactive impoundment is associated with an abandoned mine that also produced primarily lead and zinc from similar ore. Surface and core samples of the tailings were taken from both impoundments at the same depths and locations as the lysimeters for chemical analysis. Rapid laboratory leach experiments were performed on tailings sample composites and the resulting leachate was analyzed in the same manner as the pore water samples for comparative purposes. This data shows higher concentrations of Ca, Mg, SO₄^2-, Zn, and other elements in the shallow samples than in the deeper samples for both the active and inactive impoundments. The same trend holds true for the pore water data from the active impoundment, but there is less chemical variation between shallow and deep pore water samples in the inactive impoundment.