11.4 A Little about Chemical Reactions
The following is a basic overview of the types of reactions that can occur in groundwater systems. We will not get into the specific details of any of the reactions. However, a basic understanding of the nature of these reactions, and how we study them, is important.
Most of the reactions that affect groundwater chemistry involve reversible reactions, like precipitation and dissolution of mineral phases and certain oxidation‑reduction reactions. Reversible reactions are reactions in which the system reaches a dynamic equilibrium. For example, consider a small piece of the mineral calcite—the primary mineral in limestone—completely submerged in a glass of water. The water will dissolve the calcite until it becomes saturated with the ions in the calcite mineral. At that point, the system is at equilibrium; however, the reactions don’t stop; what happens is that calcite is precipitating and dissolving at the same rate, so the concentration in the water doesn’t change. This is the dynamic part of the equilibrium, the reactions (i.e., mineral precipitation and dissolution) still occur but are proceeding in both directions at an equal rate. The characteristics of this equilibrium—for example, how much is in solution and how much is in the mineral, are based on the thermodynamics of the system. In other words, these reactions are all about changes in energy in the system, and they are all about the system trying to reach a state of minimum energy. Several widely used mathematical models that represent these systems are based on the thermodynamics of equilibrium. These models and approaches to groundwater chemistry ignore reaction rates—in other words, if we assume that a system is at equilibrium, we know nothing about how long it took to get there or how fast the reactions are proceeding.
When we strive to understand the dynamics of geochemical systems, we often assume that the system is at equilibrium. This is like a lot of other assumptions that we make—it simplifies things for us and turns a complicated system into something that we can understand, while in reality, it is wrong. Natural aqueous systems are rarely at equilibrium; instead, they are in a state of moving towards equilibrium at a rate that is related to the reaction rates. In some cases, it may be more appropriate to use a kinetic approach to modeling the chemistry. Kinetic models are appropriate for reactions that proceed slowly.
Some reactions in groundwater systems are irreversible; they go in only one direction. Examples include radioactive decay, some oxidation‑reduction reactions, and degradation of organic contaminants. When dealing with irreversible reactions, the concept of equilibrium does not apply, and we are required to use a kinetic approach.
For more details and information about geochemical reactions, students are encouraged to review The Geochemistry of Natural Waters↗ by Drever (1997), which is available as a free download from The Groundwater Project.