AN ASSESSMENT OF POTENTIAL HEALTH HAZARDS OF PENNSYLVANIAN BLACK SHALES

Certain black shales found in coal basins of the Midwestern United States contain abundant As, U, Pb, Cu, Ni, Cr, Se, Zn, V, Mo and other metals in amounts that would be unacceptable were they not natural. The metalliferous shales are characteristically extremely thin (<1 m) and rich in organic matter and 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 suggest the possibility of contamination of streams, ponds or shallow ground water aquifers. Early studies produced conflicting results on the mobility of metals during weathering of black shales, but recent analyses of samples from natural seeps clearly show that black shales of Kansas City and elsewhere in the Midwest can release metals that exceed 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). Results for arsenic are in progress. At one time domestic water wells tapped such beds, but apparently this is no longer the case. Because the volume of water released from the metalliferous shales is usually trivial compared to other sources of groundwater, natural leaching processes are unlikely to be a major source of contamination. However dissolved metal concentrations are commonly sufficiently elevated that prudence suggests the advisability of base-line studies prior to excavation of these unusual beds for construction or other purposes. Conceivably remediation may be in order in extreme cases.