17.2 Basic Classification of Hydrogeologic Environments

Groundwater occurs everywhere in the subsurface, and almost any geologic setting can host an aquifer even if it only provides small volumes of water. However, there are certain geologic environments that tend to have sufficient porosity and permeability to support a substantial aquifer. Keep in mind that both parameters are necessary—a clay or vuggy basalt unit may have lots of porosity and hold a lot of water, but if the pores are too small or not connected, permeability will be low and well yields will not be adequate. Conversely, a set of fractures in a granite may have an extremely high permeability—because it only takes a small aperture to get a very permeable fracture—but if the matrix porosity is low, a well will quickly drain the fractures and will not yield much water over time.

To understand these systems, we classify five basic geologic settings—with variations within each setting—plus a few special cases.