17.7 “Hard” Rocks

Granites, basalts, and metamorphic rocks are usually not very good aquifers. These rocks tend to have very low primary porosities, and in the ones that do have significant porosity—for example, vuggy basalts—the porosity is often not well connected. There are exceptions to this; some granites will have a small amount of porosity along the grain boundaries that are significant in a large enough thickness of the material. However, in general, productivity of these aquifers is almost always dependent on fractures and on weathered zones at the base of the overlying soils.

Fractures in igneous rocks can come from cooling stresses, from exfoliation and release of overburden stress, as well as from tectonic stresses. The key to understanding these systems is determining the distribution of the fractures. In regions dominated by regional tectonic processes, the fractures will often follow distinct orientations based on the structural history of the area. Cooling joints will tend to be vertical and fairly evenly spaced, although variations in spacing do occur.

Volcanic deposits can be stratified like sedimentary deposits, and the various layers will have different properties. Often basalt flows are interbedded with ash‑flow tuffs and other ash deposits. These will have different hydraulic properties and will respond to fracturing differently, and as a result cooling joints in the more brittle basalts will generally not propagate into the tuffs.

In some active volcanic areas, the basalt bedrock can have extremely high permeability. The basalt bedrock of the Hawai’ian islands in the US was created by flows of low‑viscosity lavas that tend to leave behind lava tubes and large open spaces. These lava tubes create conduits that produce very high permeability. The aquifers of Hawai’i, however, tend to contain mostly seawater with only a thin layer of fresh water in the uppermost part of the saturated zone. The Columbia River Basalts in the northwestern USA are interbedded with sediments that accumulated between lava flows resulting in very productive zones near the top of each of many basalt layers. Wells in the area produce high-quality water at rates up to 5000 gpm (~19,000 liters per minute) as documented by Whitehead (1994).

More information about aquifer systems in general and an overview of the largest and most significant aquifer systems can be found in Large Aquifer Systems Around the World↗ by van der Gun (2022), which is available as a free download from The Groundwater Project.