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6. The temporal variation of these properties,
7. The fundamental knowledge on the kinetics and mechanisms of
important water-rock reactions,
8. The kinetics of sorption processes, and
9. The degradation pathways of organic matter.

Inverse Geochemical Modeling

Plummer (1992)* explains that "Inverse geochemical modeling combines
information on mineral saturation indices with mass-balance modeling to
identify and quantify mineral reactions in the system." The mass-balance
modeling requires (Plummer, 1992):

1. Element mass balance equations,
2. Electron conservation equations,
3. Isotope mass balance equations, when applicable,
4. Aqueous compositional and isotopic data, and
5. Mineral stochiometry data for all reactants and products.
Plummer (1992)* warns that "The inverse-modeling approach is best
suited for steady-state regional aquifers, where effects of hydrodynamic
dispersion can often be ignored."

Forward Geochemical Modeling

The objective of the forward geochemical modeling is to predict
mineral solubilities, mass transfers, reaction paths, pH and pe by using
available solid-aqueous data in aqueous specification models (Plummer,
1992). Some of the important features of the advanced forward geo-
chemical models are cited by Plummer (1992) as:
1. Access to a large thermodynamic data base,
2. Generalized reaction-path capability,
3. Provision for incorporation of reaction kinetics in both dissolution
and precipitation,
4. A variety of activity coefficient models,
5. Treatment of solid solutions,
6. Calculation of pH and pe,

* Reprinted from "Water-Rock Interaction," Proceedings of the 7th international symposium,
WRI-7, Park City, Utah, 13-18 July 1992 Kharaka, Y. K. & A. S. Maest (eds.), 90 5410
075 3, 1992, 25 cm, 1730 pp., 2 vols., EUR 209.00/USS246.00 GBP147. Please order
from: A. A. Balkema, Old Post Road, Brookfield, Vermont 05036 (telephone: 802-276-
3162; telefax: 802-276-3837; e-mail: info@ashgate.com).