GEOCHEMICAL AND BIOLOGICAL IMPACTS OF MODERN LEAD MINING IN THE MID-CONTINENTAL USA

The active lead mines of the Ozarks Region are the largest and economically most important source of lead in the United States. Our studies represent the first attempt to characterize the linkage between the geochemical and biological impacts of these modern mines.

Mineralogic characterizations of mill tailings indicate that the primary residence of metals are iron and mixed-metal sulfides and to a lesser extent, secondary metal-oxides. Laboratory leaching experiments of tailings show that water-leachable metal concentrations decrease as follows: Zn > Co > Ni > Pb > Cd.

In environments down-stream of mining operations stream water is circumneutral, however relative trace-element abundances mirror the results of laboratory leaching experiments. Trace-element concentrations range from indistinguishable from regional baselines at <1 to 10 ug/L, to an elevated range of 10 to 100 ug/L. Elevated Ni and Co concentrations in the range of 10 to 50 ug/L are among the highest observed in circumneutral drainage associated with any ore-deposit type in the world. Dilution appears to be the primary mechanism which attenuates dissolved metals in streams.

Analysis of stream-sediment pore waters indicate that pore-water geochemical trends are concordant with the overlying stream waters, suggesting strong geochemical and hydrologic connections between the benthos and stream.

Toxicity tests on the survival, growth, and reproduction of amphipods show that toxic effects of metals in stream-sediment pore-waters are greatest in sites closest to mining areas and generally decrease at distances up to 25km. However, both toxic effects and elevated metal concentrations in pore-waters are evident at some sites up to 15km downstream. The abundance of free-metal ions in pore waters, determined from speciation models, are consistent with observed differences in sediment toxicity to the amphipod H. azteca.