METAL POLLUTION FROM LATE PALEOZOIC BLACK SHALES OF THE MIDWESTERN UNITED STATES

Some black shales found in coal basins that underlie Midwestern United States contain abundant (50-5000 g/t) U, Pb, Cu, Ni, Cr, Se, Zn, V, and Mo. Such large amounts of metals are confined to extremely thin (<1 m) and organic-rich shales that are especially prevalent in the Illinois, Forest City and Cherokee basins where thick sequences of sediments containing commercial limestones, gray shales and coals were deposited during the Pennsylvanian. The amounts of metals in the shales are comparable to those of hazardous wastes and therefore raise the possibility of contamination of streams, ponds or shallow ground water aquifers. Previous studies on the mobility of metals during weathering of black shales of the Midwest have produced contradictory results. For example, Coveney et al. (Economic Geology, 1987) showed that black shale from Indiana tends to fix Ag and Mo from solutions and Angino and Wilbur (IGC Abs. 1989) found that samples of black shale from Kansas tend to retain Cd, Pb and Zn when leached with simulated rainwater. Early UMKC M.S. theses, by Andrew Ziegler (1989) Vanette Hamilton (1989) and Suzanne Bailey (1995) suggested that metals could be mobilized by weathering. John Forseh (M.S. 1997) studied two shales, each of which contains ~2000 ppm Zn throughout the Kansas City region, and found that metals were released in some exposures but not in others. To resolve these conflicting results we have used a Varian Ultramass ICPMS unit to analyze 129 samples of natural seeps from the Midwest. It is now clear that black shales can release metals at values greatly exceeding US EPA maximum contamination values for Cd, Mn, Se, Ni, Pb, U, Zn (e.g. average values, in mg/t, up to 64 Pb, 170 Cd, 490 Se, 65 U and 3900 Zn). Although the amounts of metal-bearing fluids released by such shales are unlikely to be a major source of contamination for drinking water, the extreme values for metals that can seep from the shales suggest the advisability of base-line studies prior to excavation for mining, quarries or construction projects. Remediation may be in order in extreme cases.