The application of solution equilibrium models to ground water quality has had considerable appeal to theoretically minded investigators, although the mineral composition of the solids in such systems commonly is heterogeneous and poorly known. The activities of the solute species can be determined completely and because movement of water through the bedrock is slow, there is a considerable time span available for completion of slow chemical reactions. Presumably, any solution reaction that reasonably could be expected to reach equilibrium would do so in the usual groundwater aquifer system. However, in ground water systems where geologic strata of high and low permeability are mixed, a flow pattern can occur in which water movement is largely confined to the more permeable layers.
Differences between minerals composition in the layers may cause considerable variation in water quality with depth at any given site. Therefore, ground water sampling wells that penetrate several of these layers may yield water that is an enriched mixture of solutes from the more permeable geologic layers. A sampling well influenced by factors such as this is unlikely to give usable ground water quality information. Unfortunately, there is no easy way of evaluating the importance of these effects, nor sometimes of knowing for certain whether these permeable layers are present or absent.