Aluminum and Iron Equilibria in Soil Solutions and Surface Waters of Acid Mine Watersheds

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Abstract
The composition of soil solutions and surface waters emanating from unreclaimed or partially reclaimed stripmined watersheds with low buffering capacity in Kentucky were compared with soil solution compositions of unaffected strata in the watershed. The data suggest that almost 20 yr after mining, most soil solutions and surface waters of the disturbed areas still contain high levels of dissolved Al, controlled primarily by the solubilities of a jurbanite‐like mineral (upper limit) and alunite (lower limit). Soluble Al in solutions of undisturbed areas was consistent with the solubility of kaolinite or gibbsite. The absence of jurbanite x‐ray diffraction peaks suggested the presence of an amorphous or a mineral stoichiometrically similar to jurbanite. Despite greater contact times of soil solutions with the soil compared to surface waters, their compositional differences were insignificant. The control of soluble Al by basic aluminum sulfate minerals was not affected by the variable mineralogical and textural composition of soil and geologic strata in the watershed. Apparently, this is the result of low buffering capacity. At pH < 4, pH and sulfate activities can be used to accurately predict the levels of soluble Al3+ in surface and groundwaters of the watersheds. Similar predictions from pH and SO2−4 activities can also be made for dissolved Fe3+ levels, supporting the stoichiometry but a much higher solubility than that of jarosite.