PHREEQC has capabilities for geochemical inverse modeling, which attempts to account for the chemical changes that occur as a water evolves along a flow path (Plummer and Back, 1980; Parkhurst and others, 1982; Plummer and others, 1991, Plummer and others, 1994). In inverse modeling, one aqueous solution is assumed to mix with other aqueous solutions and to react with minerals and gases to produce the observed composition of a second aqueous solution. Inverse modeling calculates mixing fractions for the aqueous solutions and mole transfers of the gases and minerals that produce the composition of the second aqueous solution. The basic approach in inverse modeling is to solve a set of linear equalities that account for the changes in the moles of each element by the dissolution or precipitation of minerals (Garrels and Mackenzie, 1967, Parkhurst and others, 1982). Previous approaches have also included equations to account for mixing, conservation of electrons, which forces oxidative reactions to balance reductive reactions, and isotope mole balance (Plummer and Back, 1980; Parkhurst and others, 1982; Plummer and others, 1983; Plummer, 1984; Plummer and others, 1990; Plummer and others, 1991; and Plummer and others, 1994).