Geochemists increasingly find a need to better understand the distribution of microbial life within the geosphere, and the interaction of the communities of microbes there with the fluids and minerals they contact. How do geochemical conditions determine where microbial communities develop, and what groups of microbes they contain? And how do those communities affect the geochemistry of their environments? In this chapter we use multicomponent chemical reaction modeling combined with thermodynamically consistent kinetic expressions to explore how microbially catalyzed reactions proceed in the laboratory and in nature.

A reactive transport model is a simulation of the movement of chemical species in a flow regime, in which the species are allowed to react chemically. In this chapter, we derive a finite difference model of reactive transport in flowing groundwater, introduce the concept of a Damköhler number in the context of our model, show how the model can be evaluated numerically, and provide example calculations showing how such a model behaves.

In this chapter we construct multicomponent chemical reaction models of how an aqueous fluid might react with the minerals it contacts, according to kinetic rate laws. We show that the fluid can be in equilibrium or disequilibrium with respect to the minerals, and how the fluid chemistry can approach an apparent equilibrium that is in fact a steady state, rather than a thermodynamic equilibrium. We further construct example calculations that demonstrate the basis of Ostwald’s step rule and the nature of the incongruent dissolution of albite feldspar.

We construct in this chapter reactive transport models of the mobility of heavy metals in an aquifer containing a complexing surface. The modeling shows that surface complexation can play a controlling role in the fate and transport of heavy metal contamination in the environment. Surface complexation causes not only retardation, as might be expected from simple sorption theory, but a pronounced tailing that hinders remediation by pump-and-treat methods.