16 Special Considerations for Characterizing Low-Permeability Systems, Aquitards

Saturated geologic materials that do not yield enough water to be used as a reliable water supply are generally referred to as low‑permeability units and, when they confine an aquifer, aquitards (Woessner & Poeter, 2020). Characterization of these systems can be more difficult because in situ pumping tests require low yields and have small zones of influence.

All low‑permeability units that would not be considered aquifers or aquifuges are referred to using the general term aquitards in this section. Aquitards retard the flow of groundwater. Units that almost totally preclude water movement can be referred to as aquicludes, and if totally impermeable, aquifuges (Woessner & Poeter, 2020). Though these terms imply sequentially decreasing permeability, the terms are only qualitative. Generally, characterization of low‑permeability units is difficult, and in some cases, standard pumping or slug tests is impractical because water does not flow to or from the unit at a high enough rate to conduct such tests.

Two publications provided on the gw-project.org web‑site specifically relate to hydrogeologic characterization of aquitards. These include Contaminant Transport Through Aquitards: Technical Guidance for Aquitard Assessment↗ by Bradbury and others (2006) and Contaminant Transport Through Aquitards: A State‑of‑the‑Science Review↗ by Cherry and others (2006). Though these reports focus on contaminant transport they include detailed information about the character of aquitards and the tools used to characterize hydrogeologic properties of these units. These publications are noted because they can broaden the reader’s understanding of aquitard characterization beyond that provided in this book. This section provides a general overview of methods to determine transmission and storage properties of aquitards.