ENVIRONMENTAL GEOCHEMISTRY OF METAL CONTAMINATED SEDIMENTS OF THE BIG RIVER IN SOUTHEASTERN MISSOURI

Geochemical analysis of sediments from the Big River Watershed of southeastern Missouri indicates that they contain elevated concentrations of contaminant metals such as Pb, Zn, Cu and Cd. The fate and transport of the contaminant metals in the sediment load fraction is the objective of this study. The elevated concentrations are due to natural exposures of metal enriched strata and the numerous mine tailings piles and water seeps created as a result of about 300 years of lead-zinc mining in the “Old Lead Belt”. This district produced 250 million tons of waste material with the last mines closing in 1972. The Desloge tailings pile dam was breached in 1977 and 38,000 cubic meters of tailings were introduced into the river. There is also a continuous release of tailings by wind and water erosion from the Desloge and other piles.

Inductively Coupled Plasma analysis of partially digested silt- and clay-sized fraction sediments collected three different dates at locations 2.5 and 12.5 nautical miles downstream from the Desloge tailings pile have mean Pb concentrations of ~5200 +/-1300 and 2400 +/- 400 ppm, respectively. Average Zn concentrations at 2.5 and 12.5 nautical miles downstream from the tailings are ~4600 +/-1500 and 1400 +/-200 ppm, respectively. The large standard deviations are likely due to a heterogeneous metal distribution. By comparison, background metal contents from a control site located upstream from the mines contained ~70 ppm Pb and 80 ppm Zn. The river area of primary sampling cuts through calcareous shale and dolomite of Paleozoic age which causes the Big River to have a pH of 7.2-8.3. Scanning Electron Microscopy and Energy Dispersive Spectroscopy analysis of individual metallic sediment particles collected at intervals downstream from the Desloge tailings indicate that pyrite (FeS₂) and sphalerite (ZnS) phases survive only ~2 nautical miles downstream. Galena (PbS) is more durable than the other sulfides and is found up to ~8 nautical miles downstream from the tailings. Attrition of the sulfide particles occurs by a combination of mechanical and chemical processes. No sulfide phases have been found further than ~8 nautical miles downstream. Analysis of sedimentary particles collected further downstream indicates that contaminant metals tend to adsorb onto Si, Fe and Mn oxide and/or oxyhydroxide phases.