The Geology of Aquitards in Alluvial Aquifers: A Predictive Approach Based on Facies Models

A sophisticated understanding of the three-dimensional distribution of silt- and clay-rich bodies of strata (elements) in aquifers is critical given that they not only have the potential to act as aquitards or semi-confining units and vertically partition groundwater flow into separate aquifer zones, but also provide lateral barriers to groundwater flow, impacting contaminant distribution and groundwater flow dynamics. Additionally, when in prolonged contact with dense nonaqueous phase liquid (DNAPL) or contaminated groundwater, fine-grained elements may become storage zones for contaminant mass via matrix diffusion and thus serve as long-term secondary sources of contamination to groundwater that can confound remediation strategies and render remedy performance projections unreliable. The stratigraphic architecture of aquifer systems, including fine-grained facies architecture, is complex but is not random and can be effectively predicted through application of facies models. This paper reviews depositional models ("facies models") for common depositional environments with a focus on alluvial end-members of braided fluvial, meandering fluvial, and alluvial fan facies models. We examine the facies models from the perspective of aquitards and present case studies to provide an overview of the expected aquitard dimensions and characteristics. The critical yet underappreciated role of the paleosol as a potential aquitard is also examined, and basic criteria for differentiating ancient floodplain clay units with high lateral continuity from other laterally discontinuous clay units are provided.