SAND2007-7091
Unlimited Release
Printed November 2007
GEOQUIMICO:
An Interactive Tool for Comparing
Sorption Conceptual Models (Surface
Complexation Modeling versus KD)
Glenn E. Hammond
Hydrology Group
Pacific Northwest National Laboratory
Richland, Washington 99352
Randall T. Cygan
Geochemistry Department
Sandia National Laboratories
Albuquerque, New Mexico 87185-0754
Abstract
Within reactive geochemical transport, several conceptual models exist for simulating sorption
processes in the subsurface. Historically, the KD approach has been the method of choice due
to ease of implementation within a reactive transport model and straightforward comparison with
experimental data. However, for modeling complex sorption phenomenon (e.g. sorption of
radionuclides onto mineral surfaces), this approach does not systematically account for
variations in location, time, or chemical conditions, and more sophisticated methods such as a
surface complexation model (SCM) must be utilized. It is critical to determine which conceptual
model to use; that is, when the material variation becomes important to regulatory decisions.
The geochemical transport tool GEOQUIMICO has been developed to assist in this decision-
making process. GEOQUIMICO provides a user-friendly framework for comparing the accuracy
and performance of sorption conceptual models. The model currently supports the KD and SCM
conceptual models. The code is written in the object-oriented Java programming language to
facilitate model development and improve code portability. The basic theory underlying
geochemical transport and the sorption conceptual models noted above is presented in this
report. Explanations are provided of how these physicochemical processes are instrumented in
GEOQUIMICO and a brief verification study comparing GEOQUIMICO results to data found in the
literature is given.

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