Acidity of Acid Mine Water

we use the Bear Creek Uranium example discussed in 6.2 to calculate the acidity of contaminated groundwater and tailings fluids. If the total acidity of groundwater and contaminated sediments, as well as the neutralization capacity of the aquifer matrix are known, the distance that the acid plume will migrate can be estimated based on mass balance. 

The acidity of tailings fluids and contaminated groundwater comes from the sulfuric acid in the processing of the ore and from hydrolyzable cations such as Al3+, Fe3+, and Mn2+ in the solution. Titratable acidity is often determined by the Solotto method (Solotto et al., 1966). This procedure titrates an acid solution with NaOH to a pH of 7.3 and the total acidity is reported as mgL-1 or molL-1 of CaCC 3 equivalent. However, laboratory titration results are unreliable because of the sluggishness of hydrolysis and precipitation reactions of Fe3+ (Hem, 1985). The end-point of pH 7.3 does not have a theoretical basis. Another common method is the hot H2O2 method, which overcomes the kinetic problems (American Public Health Association, 1999). 

These modeling results can be compared with hand calculations. Drever (1988) showed calculations of mineral acidity in waters with high A1 concentrations. Hedin et al. (1993) used the formula 

Note that the formula above uses the free ions in calculations, ignoring the effects of speciation, and hence would overestimate the acidity. For example, our speciation modeling shows that the complex Fe3 (OH)4+ is dominant. Precipitation of 1 mole of Fe(OH)3(fl), therefore, only generates 1.6 moles of H+, compared with 3 moles for Fe3+. 

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This releases ion and contributes to the acidity of acid mine water. The Fe(OH)3 precipitate is a semigelatinous orange material that remains in and along stream beds as an unsightly deposit. The sulfuric acid from acid mine water is toxic to water animal and plant life and tends to leach Al ion from the geosphere. This species is phytotoxic (toxic to plants). 

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