Rock-water interactions controlling zinc, cadmium, and lead concentrations in surface waters and sediments, US Tri-State Mining District. 2. Geochemical interpretation

We have studied principle rock-water interactions that control trace metal concentrations in a complex geochemical environment containing multiple contaminants and multiple solid phases by combining kinetic and thermodynamic evaluation of the water chemistry with spectroscopic analyses of the sediments. This approach allows the number of geochemical reactions needed to model and predict trace metal mobility over a range of natural settings to be greatly constrained. In the U.S. Tri-State Mining District (Kansas-Missouri-Oklahoma) the most important geochemical interactions are degassing of CO(2)(g)-rich waters, the shortterm uptake and release kinetics of zinc, cadmium, and lead; competition between iron oxyhydroxides and carbonates for zinc, cadmium, and lead; and catalysis of sulfide dissolution by iron in near-neutral waters. In our field study, degassing of CO(2)(g) waters is responsible for the range of pH measured at each site over the 1-year field study. Trace metal release and uptake kinetics by iron oxyhydroxides and carbonates are driven by changes in pH. Aqueous metal concentrations and pH of pond water and streamwater in contact with high-iron sediments suggest that 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. Cadmium 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 carbonate 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 effects among multiple metals for carbonate and iron oxyhydroxide phases and to quantify reaction rates of metal release and uptake from sulfides and secondary phases in the presence of dissolved iron.