In the decade since I published the first edition of this book, the field of geochemical reaction modeling has expanded sharply in its breadth of application, especially in the environmental sciences. The descriptions of microbial activity, surface chemistry, and redox chemistry within reaction models have become more robust and rigorous. Increasingly, modelers are called upon to analyze not just geochemical but biogeochemical reaction processes.

At the same time, reaction modeling is now commonly coupled to the problem of mass transport in groundwater flows, producing a subfield known as reactive transport modeling. Whereas a decade ago such modeling was the domain of specialists, improvements in mathematical formulations and the development of more accessible software codes have thrust it squarely into the mainstream.

I have, therefore, approached preparation of this second edition less as an update to the original text than an expansion of it. I pay special attention to developing quantitative descriptions of the metabolism and growth of microbial species, understanding the energy available in natural waters to chemosynthetic microorganisms, and quantifying the effects microorganisms have on the geochemical environment. In light of the overwhelming importance of redox reactions in environmental biogeochemistry, I consider the details of redox disequilibrium, redox kinetics, and effects of inorganic catalysts and biological enzymes.

I expand treatment of sorption, ion exchange, and surface complexation, in terms of the various descriptions in use today in environmental chemistry. And I integrate all the above with the principals of mass transport, to produce reactive transport models of the geochemistry and biogeochemistry of the Earth's shallow crust.