16.7 Influence of Fractures on Fluid Flow and Mass Transport

Fractures affect fluid flow in several ways. The obvious way is that open fractures provide a conduit for water to move in the subsurface. Due to their planar nature, fractures can impart a significant anisotropy in an aquifer, which in turn affects flow paths in the aquifer. The preferred flow paths created by fractures can significantly affect the paths that contamination takes in the subsurface, and the high permeability associated with those features can drastically affect travel time for contaminants. In addition, fracture skins can influence transport of contaminants by impeding diffusion into the matrix or even facilitating retardation processes—for example, sorption of contaminants in the fracture skins. As a result of these factors, it is not uncommon for teams of hydrogeologists to invest a great deal of time and money characterizing a contaminated aquifer and developing predictions about contaminant transport directions and travel times, only to observe contamination showing up very quickly in a completely unexpected location. The observation of contaminants in fractured aquifers moving much faster than expected and appearing in places we do not expect is so common that it is known as “Black’s Law” (Black, 1993).

It should also be noted that fractures can act as barriers to flow in an aquifer as well as conduits. Fractures often contain precipitates of crystalline minerals or fault gouge—that is, ground‑up or broken rock and/or sediment—that can drastically reduce permeability. Faults can also juxtapose permeable beds against units with very low permeability, which in turn reduces the lateral extent of an aquifer.