GEOCHEMISTRY OF ALUMINUM IN SURFACE AND GROUND WATERS AFFECTED BY ACID ROCK DRAINAGE

New data from the Questa, NM, baseline and pre-mining project combined with older data from Leviathan, CA, Summitville, CO, and Iron Mountain, CA mine sites and interpreted with speciation calculations elucidate the major geochemical processes controlling dissolved aluminum in surface and ground waters. Acid-sulfate waters (typically with pH=1 to 3, from pyrite oxidation) dissolve aluminum from aluminosilicate minerals, primarily feldspars and clays. As these waters travel downgradient, dilution has no effect on the Al/SO4 ratios provided the pH is less than 4 for ground waters and less than 5 for surface waters, indicating aluminum behaves conservatively. Once ground waters reach a pH of 4, aluminum begins to precipitate, probably as an aluminosilicate gel or clay, if sufficient dissolved silica is present. For surface waters, pH values approaching 5 are needed to initiate precipitation. The pH dependence for aluminum precipitation is related to the pKa1 = 5.0 for hydrolysis. The lower pH of aluminum precipitation for ground waters is caused by greater residence time of the water in contact with larger mineral surface areas and coprecipitation with silica. Aluminum-rich acid waters seeping into a neutral stream such as the Red River, New Mexico, continually precipitate aluminum. The concentration and load of particulate/colloidal aluminum in the Red River increase while the dissolved aluminum concentration remains relatively constant. For pH values of 5 to 7.5, speciation calculations suggest that a microcrystalline to amorphous form of Al(OH)3 limits the aluminum concentration. Actual precipitates are amorphous to X-ray diffraction and approximate a basaluminite composition. At pH values from 7.5 to 8.5, speciation forces an incorrect slope on a pAl3+ vs. pH plot because it assumes hydrolysis where there appears to be little or none. In this pH range, dissolved aluminum concentrations are nearly constant and independent of pH, suggesting stabilization by organic complexes.