16.1 Introduction

To date, our treatment of groundwater flow has some implicit assumptions concerning flow in a porous medium. Basically, when we talk of flow in porous media, we are referring to water moving through small, evenly distributed pores, with laminar flow driven by a hydraulic gradient where the flow is linearly and directly proportional to the gradient.

However, this is often not the case—aquifers are not always geologic layers with small, evenly‑distributed pore spaces. Groundwater flow in many geologic units is controlled by discontinuities in the rock or sediment—cracks, bedding planes, breccia zones along faults, and conduits formed by dissolution. The physics of groundwater flow in these discontinuities is different from flow in a porous media system. Fractures or conduits often control the direction and magnitude of flow in a groundwater system. Flow in these discontinuities, or in non‑porous media, is the subject of this section, which focuses on fracture flow. Many of the concepts apply to conduit flow as well. More specific information related to conduit flow is provided by Kuniansky and others (2022) in Introduction to Karst Aquifers↗ and by Stevanović and others (2024) in Karst: Environment and Management of Aquifers↗, both are available as free downloads from The Groundwater Project.

For our purposes, a fracture is defined as any relatively planar discontinuity in a rock or sediment. We tend to think of fractures as a break in a rock, but for hydrogeologists, a bedding plane acts the same as a stress‑release joint in bedrock. When we look at rocks in nature, we find that all rocks exposed at Earth’s surface are fractured to some degree. In many rocks, the fractures are not a significant part of the flow system, and it is often a valid assumption to ignore them or to assume that flow in them is acting the same as flow in a porous medium. Fractures and planar discontinuities, however, are frequently a significant part of an aquifer flow system.