Dames & Moore, 1999 - USDA Forest Service

calcite were being dissolved, one ion of oalciurn or two ions of potassium would match one ion of sulfate.
Comparison of the ratios of sulfate to other elements is used as a tool to understand which minerals may
be contributing to mine water chemistry. Two different waters may have dissimilar concentrations of
sulfate and other elements, but if the ratios are similar, a common source process is implied. In the above
example, comparison of ratios of calcium to po&s& would indicate the relative contributions of calcite
and potassium feldspars to water chemistry.
63.2 Formation of Secondary Minerals
Subsection 6.1.2 also described the types of secondary minerals that have been observed within the
Holden Mine (Table 6.1- 1). These minerals are formed, in part, when the water can no longer contain the
cdmponents that form the mineral. The simplest example is the fonnation of sodium chloride crystals
when a solution of common salt is heated, causing water to be evaporated. The evaporation causes the
concentrations of sodium and chloride to increase in the water. Eventually, the laws of thermodynamics
dictate that salt crystals will form as the concentrations increase. As the crystals form, sodium and
chloride are removed from the water and the concentrations of sodium and chloride in the water remain
steady.
At hard rock metal mine sites, the formation of secondary minerals from water can be predicted or
confirmed using the same principals. It is useful to predict the precipitation of secondary minerals
because it indicates where metals are being removed from solution, and how these minerals may be
preventing pH from increasing. Increase in pH allows heavy metals to precipitate. Further discussion of
secondary mineral formation is provided in Subsection 6.3.3.
Several models are available to evaluate water chemistry for evidence of formation of secondary
minerals. These models include MTNTEQA2, WATEQF, PHREEQC and EQ3. The differences between
the models include the database of elements and minerals, the ability to model mixing of waters
(PHREEQC), and the ability to model saline solutions (EQ3). For this project, the MMTEQA2 (Allison
et al. 1991) model was selected but with the database developed by Nordstrom et al. (1990). The reasons
for this selection are as follows:
The model was developed by the US Environmental Protection Agency and is probably
the most widely used of its type.
The Nordstrom et al. (1990) database is larger than the original MMTEQA2 database.
Individual waters rather than mixed waters are evaluated.
The waters at the Holden Mine are relatively dilute and do not require the use of EQ3.
The inputs to MTNTEQA2 (and all comparable models) are the dissolved concentrations of all analyzed
parameters, and other conditions such as temperature, pH, Eh and gas concentrations. The outputs from
the model are:
Ion (charge) balances (expressed as Cations-Anions/Total Ions, in meq/L)
Mass distribution of dissolved ions and ion complexes