In this example, inverse modeling, including isotope
mole-balance modeling, is applied to the evolution of water in the Madison
aquifer in Montana. Plummer and others (1990) used mole-balance modeling to
quantify the extent of dedolomitization at locations throughout the aquifer.
In the dedolomitization process, anhydrite dissolution causes the
precipitation of calcite and dissolution of dolomite. Additional reactions
identified by mole-balance modeling include sulfate reduction, cation
exchange, and halite and sylvite dissolution (Plummer and others, 1990). Del
13C and del 34S data were used to corroborate the mole-balance models and
carbon-14 was used to estimate groundwater ages (Plummer and others, 1990).
Initial and final water samples were selected from a flow path that extends
from north-central Wyoming northeast across Montana (Plummer and others,
1990, flow path 3). This pair of water samples was selected specifically
because it was one of the few pairs that showed a relatively large
discrepancy between previous mole-balance approaches and the mole-balance
approach of PHREEQC, which includes uncertainties; results for most sample
pairs were not significantly different between the two approaches. In
addition, this pair of samples was selected because it was modeled in detail
in Plummer and others (1990) to determine the sensitivity of mole-balance
results to various model assumptions and was used as an example in the
NETPATH manual (Plummer and others, 1994, example 6). Results of PHREEQC
calculations are compared to NETPATH calculations. This example is also
discussed in Parkhurst (1997). The example can be run using the
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