Aquitards in Groundwater Systems: Groundwater Special Issue

Low-permeability geologic layers within groundwater systems, commonly referred to as aquitards or confining layers, are important controls of groundwater flow. Aquitards generally restrict groundwater flow and, importantly, can limit contaminant migration to adjacent aquifers. However, they are rarely spatially uniform or isotropic, varying in thickness and lithologic composition. At regional scales, most aquitards contain some type of heterogeneity, such as fractures or erosional windows, that affect groundwater flow and contaminant transport. Many groundwater investigations focus on aquifers, while the physical and chemical characteristics of aquitards, and their role in flow systems, may be over-simplified. Aquitards add complexity to almost all aspects of flow and transport in groundwater systems. This Groundwater special issue, Aquitards in Groundwater Systems, has its roots in a recent Geological Society of America conference. A hydrogeology session included a presentation that referenced “aquifers, aquitards and aquitardifers,” which induced giggles from some audience members and affirming head nods from others. The diversity of listener reactions suggested an opportunity to pursue a compilation of topical papers that span the current breadth and depth of aquitard studies. In this issue, Runkel and Meyer take on the hydrogeologic lexicon with formal introduction of “aquitardifer.” The term encompasses the anisotropy they document within low-conductivity formations. Such anisotropy can lead to restriction of flow in one direction while providing transmissivity in other directions. This explains production wells that are successfully completed in formations generally characterized as confining units. By explicitly identifying significant anisotropy in low-permeability sedimentary units, Runkel and Meyer advance the field of aquitard science. Aquitard science is not consistently integrated within hydrogeologic curriculum, and students may not be prepared to recognize flow systems where aquitards play a significant role. Meyer and others discuss teaching activities (lab exercises and field measurements) that help students develop understanding and intuition of the role aquitards play in flow systems. Methods to investigate and characterize aquifer heterogeneity are plentiful in the scientific literature but are not yet fully explored within the realm of aquitards. Van Leer and others model hydraulic response to pumping in an idealized, layered system of aquitards and aquifers. By illustrating key principles in pumping test design, they inform our understanding of drawdown in a heterogeneous aquitard. The mechanical properties of aquitards can also present challenges. Hinton and others look at the increasing risk of landslides as the geochemistry of a marine clay aquitard is altered, from the initial mix of glacial and seawater pore fluids, by modern-day recharge. The change in pore fluid reduces the strength of the clay, creating conditions favorable for landslides. The imprint of aquitard geochemistry on well water quality has practical implications for public water supplies and public health. Heller and others evaluate groundwater flow and chemistry within discrete intervals of aquitard/aquifer sequences. Focusing on arsenic, iron, manganese, and nitrate, they evaluate the role of fine-grained intervals in determining groundwater quality in public supply wells. Too often geologic knowledge is not fully applied to field sites, especially those with complex mixes of coarse- and fine-grained sediments. Questions such as whether fine-grained sediment encountered in a borehole is continuous across a site, or how sediments are related between boreholes, may depend on a consistent depositional model for the site. Shultz and Plank provide guidance and examples of how hydrogeologists can apply depositional models to specific settings. They demonstrate the use of braided fluvial, meandering fluvial, and alluvial fan facies models to create better conceptual site models. Finally, this issue features two papers that address properties of low-permeability sediments through examination of land subsidence processes. Subsidence, which is generally attributed to groundwater extraction, results from compaction of sediment related to a reduction in hydraulic head. A global-scale geohazard, land subsidence exacerbates flooding and sea level rise in coastal areas and, in developed areas, can wreak havoc with valuable infrastructure (e.g. foundations, bridges, pipelines). Han and others consider aquitard compaction and drainage, developing a technique to account for changes in hydraulic conductivity as compaction alters the thickness and reduces porosity of fine-grained sediment. In the second of these papers, Guo and others evaluate the potential for osmotic effects to contribute to subsidence in coastal regions. Their simulations of aquifer-aquitard systems demonstrate that osmotic effects driven by seawater intrusion are an important mechanism of land subsidence in coastal regions. This special issue collects recent research on aquitards and their role in groundwater flow and contaminant transport. While not exhaustive of all current studies of low-permeability environments, these papers provide breadth and depth across several important aspects of subsurface processes. We hope this collection provides readers with a new and richer understanding of the significance of low permeability materials in groundwater systems. This editorial does not reference data.