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12 <br />Combining the two steps gives the same as reaction 17. <br />The hydrogen ions catalyze or speed up the dissolution reac- <br />tion 17 without being consumed. Such secondary weathering <br />reactions are of great importance in establishing the water <br />quality. Iron dissolved in water is more of a nuisance and <br />can be tolerated to a fairly high concentration as seen in <br />the drinking water standards listed in Table I. In contrast, <br />Cd, As, Pb and Ag are quite toxic and have low maximum per- <br />missible concentrations in drinking water. <br />High levels of iron will not remain dissolved in surface <br />waters for very long because the pH has to be at an uncommonly <br />low value of 3 or lower to keep mechanism 15 from proceeding <br />to the precipitation of Fe(OH)3. Is this true for the other <br />• base metals dissolved in water? Unfortunately, some metal <br />such as Zn, Cd and Mn can remain dissolved in water even <br />when the pH is raised to values around 7. Boyles and co- <br />workers (21) performed studies which show quite well <br />the relation of pH to dissolved metal concentration. They <br />increased the pH of Argo Tunnel mine drainage by controlled <br />increments from its initial value of 2.8 to pH=8. At each <br />step an analysis was done of the metals still dissolved <br />in the water. Their results are reproduced in Figure 1. <br />The important point to note here is that Fe is precipitated <br />from the water quite early but the other base metals stay <br />dissolved. The conclusion is that precipitation of iron <br />from the water does not imply that mine effluents are of <br />•