ROCK-WATER INTERACTIONS CONTROLLING ZINC, CADMIUM, AND LEAD CONCENTRATIONS IN SURFACE WATERS AND SEDIMENTS, US TRISTATE MINING DISTRICT - 2- GEOCHEMICAL INTERPRETATION

We have studied principle rock-water interactions that control trace m etal concentrations in a complex geochemical environment containing mu ltiple contaminants and multiple solid phases by combining kinetic and thermodynamic evaluation of the water chemistry with spectroscopic an alyses of the sediments. This approach allows the number of geochemica l reactions needed to model and predict trace metal mobility over a ra nge of natural settings to be greatly constrained. In the U.S. Tri-Sta te Mining District (Kansas-Missouri-Oklahoma) the most important geoch emical interactions are degassing of CO2(g)-rich waters, the shortterm uptake and release kinetics of zinc, cadmium, and lead; competition b etween iron oxyhydroxides and carbonates for zinc, cadmium, and lead; and catalysis of sulfide dissolution by iron in near-neutral waters. I n our field study, degassing of CO2(g) waters is responsible for the r ange of pH measured at each site over the 1-year field study. Trace me tal release and uptake kinetics by iron oxyhydroxides and carbonates a re driven by changes in pH. Aqueous metal concentrations and pH of pon d water and streamwater in contact with high-iron sediments suggest th at oxidation-reduction reactions involving iron accelerate sphalerite dissolution kinetics in near-neutral waters. This study clearly shows that zinc is preferentially partitioned into secondary zinc hydroxide or iron oxyhydroxide, but in the same sediments cadmium is not. Cadmiu m is the most mobile element because it dissolves from sulfide and is taken up by calcite only in waters with pH > 7. Lead is taken up by ca rbonate or iron oxyhydroxide and is extremely insoluble in these near- neutral waters. To accurately predict trace metal mobility in complex environments, laboratory studies are needed to quantify competition ef fects among multiple metals for carbonate and iron oxyhydroxide phases and to quantify reaction rates of metal release and uptake from sulfi des and secondary phases in the presence of dissolved iron.