MINE WASTE PILES AS A SOURCE METAL CONTAMINATION AT THE TAR CREEK SUPERFUND SITE

The Tar Creek Superfund Site in northeastern Oklahoma is heavily impacted by decades of lead and zinc mining in the Tri-State Mining area in the early 1900s. A notable feature of this area is the presence of mine waste piles (locally called chat piles) that contain elevated concentrations of Zn, Pb and Cd. Runoff from these piles, together with acid mine drainage (AMD) originating from abandoned mine shafts, contribute to metal loading into Tar Creek. As part of a broader investigation of potential children's health impacts from exposure to metals in mining materials, we assessed metal concentrations and speciation in chat, the contribution of chat pile runoff as a source of metal loading into Tar Creek, factors affecting in-stream mobility of metals, and the potential mobilization of metals from chat piles by wind.

Variations in metal concentration and speciation as a function of particle size were assessed using XRF, XRD and sequential extraction techniques. Total concentrations of Zn, Pb and Cd were found to increase with decreasing particle size (up to 10% Zn and 2% Pb by mass and 200 ppm Cd in the <37 µm size fraction), indicating that particles most likely to be mobilized by wind are highly enriched in these metals. Sequential extraction experiments revealed that the Zn, Pb and Cd in these particles were fairly labile (40-80% solubilized in pH 7 1M MgCl₂ or pH 5 acetate buffer solutions) and thus potentially bioavailable for uptake by plants, animals and humans. Additional column leaching experiments will explore the kinetics of metal dissolution, the effect of pH and contact time on metal mobilization, and the potential re-adsorption of mobilized metals as they move through the pile.

Field measurements confirm a high degree of aqueous mobility of Zn, Pb and Cd from chat piles, with surface runoff from chat piles containing highly elevated concentrations of these metals (up to 40,000 ppb Zn, 450 ppb Cd, and 25 ppb Pb). Preliminary mass loading estimates along a 500-m stretch of Tar Creek impacted by chat piles and AMD indicate that chat piles contribute the majority of Cd and Pb to the creek and a substantial portion of Zn, while AMD contributes most significantly to Fe and Zn loading. Once in the creek, metal mobility was related to affinity for ferric hydroxides, which are abundant in the creek.