Here's another example: A soil system that contains a certain amount of naturally occurring lead. The question is whether the lead is going to remain on-site or dissolve and gradually move with the groundwater into surrounding streams and wells. Generally this would be answered by placing monitoring wells throughout an area and measuring the chemical concentration of lead. If, in addition to this monitoring, one were to apply the principles of chemical equilibrium, it would be possible to develop deeper insight into the chemical mechanisms controlling the system. A monitoring program answers what is happening in the system. Chemical equilibrium answers why.

For instance, if in the course of your monitoring program you were to notice that the concentration of lead was decreasing as you moved farther down a watershed, it would be useful to know what happened to the lead. The chemical equilibrium approach would tell you if lead solids were forming or if an observed change in pH was due to the addition of a new ion to solution. If most of the lead is calculated to form a particular solid phase, if might be interesting to see how small perturbations in the system effects the dissolution of lead. The net effect of applying chemical equilibrium modeling is that the  underlying mechanism is clearer , and the ability to make decisions about this system is improved.