Permeability Versus Depth in Earth’s Upper Crust: An Overview

Permeability is a key parameter controlling the transport of water, heat, and solutes in Earth’s upper crust, and is thus an important factor in several fundamental earth processes. Crustal permeability is highly heterogeneous, determined by multiple widely-varying geologic features such as lithology, fracture intensity, local stress state, etc. However, overall, permeability decreases with depth due to increasing lithostatic pressure and the resulting reduction of porosity and fracture apertures. Studies published within the past 15 years that compile available permeability estimates at various depths have discovered fairly uniform broad-scale relationships between depth and permeability for both lower-permeability crystalline rocks and higher-permeability sedimentary rocks (sandstones and limestones). Depletion and degradation of shallow groundwater resources in the face of a growing population and warming climate are driving a new interest in the prospect of finding deep groundwater resources. Modern hydrologic science has focused almost exclusively on groundwater flow at shallow depths (<1 km), and our understanding of deeper groundwater flow systems remains limited. Usable aquifers must have sufficiently high permeability to allow both high rates of extraction and active groundwater flow (flushing that reduces salinity) enabled by a hydraulic connection to the surface. This talk reviews published depth versus permeability relationships, along with the few available deep groundwater modeling studies, and makes a first-order attempt to address the basic question: How deep in the crust should we expect to find usable aquifers?