Dames & Moore, 1999 - USDA Forest Service

Co-precipitation.
Sorption.
633.1 Description of Processes
Dilution
During the mixing of waters of different origins, the mass of some chemical parameters in solution may
be conserved due to the lack of processes resulting in loss from solution. The concentration in the mixed
solution reflects that the total mass is constant but contained in a different volume of water. The resulting
concentration, C, produced can be calculated from:
The concentrations and water quantit,ies for the two mixed solutions are CA, CB, and QA, QB, respectively.
Conservation of mass allows the origin of mixed waters to be evaluated. It can also be used to evaluate
the reliability of the water balance by performing a mass balance.
Chemical parameters for which the above calculation is reliable are referred to as "conservative." No
parameters are conservative under all conditions since mixing may produce a change in pH or Eh
(oxidation/reduction potential) that could conceivably cause precipitation. However, in the context of
mining environments, some elements can be regarded as conservative. Under oxidizing, relatively dilute
conditions, sulfate is conserved. Sulfate is non-conservative when an acid water containing high
. concentrations of sulfate mixes with a solution containing high concentrations of calcium. Hydrated
calcium sulfate (or gypsum) is precipitated when both calcium and sulfate are removed from solution.
Similarly, calcium is removed from solution, and is non-conservative. Calcium is even less conservative
than sulfate because it is readily dissolved from a variety of common rock types containing carbonate.
Magnesium also suffers from similar limitations to calcium except that its sulfate (epsomite) is more
soluble than calcium sulfate and is less likely to be precipitated from solution containing high sulfate
concentrations. It is also less common than calcium as a readily soluble constituent in common rock
types though it also occurs in carbonate rocks as dolomite and magnesite.
, Anions of the halogen elements (fluoride, chloride, bromide, iodide) can also be conservative due to the
relatively high solubility of their compounds. The main limitation is that their concentrations are usually
not high enough in mine waters to be detected.
Most heavy elements are not conservative because other fate processes described below affect them. Zinc
often shows quasi-conservative behavior because its hydroxide is relatively soluble under typical natural
water conditions. However, it is co-precipitated when hydroxides of iron and aluminum precipitate.
For the Holden Mine, magnesium has been identified as a useful conservative element downstream of
potential mine-influenced sources.
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