CONCEPTUAL MODEL OF GROUNDWATER FLOW IN SEDIMENTARY AQUITARDS HAVING HIGH VERTICAL ANISOTROPY

The homogeneous, equivalent-porous-medium paradigm can be difficult to apply to fractured sedimentary aquitards. Commonly-used, parallel-plate conceptual models for flow in fractures do not generally account for layered geologic heterogeneity. However, in relatively undeformed rock of very low matrix hydraulic conductivity, geologic information such as the geometry of flowpaths in fracture planes and mechanical stratigraphy can be used to constrain conceptual models for flow and effective properties.

Fractures that can be important flowpaths form principally in response to tectonic stresses. In layered heterogeneous rock, fracture formation and extent are controlled by the relative stiffness of different layers. Published data and field observations indicate that fractures perpendicular to bedding are often offset by or contained within layers, but that layer-parallel fractures can be regionally continuous along planes of weakness between beds. Flow in fracture planes occurs in anastomosing pathways that follow regions of greatest aperture. The pattern of flowpaths in perpendicular, discontinuous, and offset planes in layered heterogeneous rock occupies three dimensions. The resulting transmissive fracture network is hydraulically much better interconnected in the horizontal than in the vertical dimension. Such anisotropic interconnection allows lateral flow in contrast to the typical assumption of vertical flow in aquitards.

This conceptual model for highly anisotropic flow in perpendicular discontinuous fractures in sedimentary rock, although usually not explicitly described, is borne out by many recent studies. It accounts for heterogeneous hydrogeochemical and hydraulic head data with depth in the Maquoketa Formation in southeastern Wisconsin. In other settings, it also explains the occurrence of isolated bedding-plane fracture zones that can be considered mini-aquifers in otherwise low-conductivity rock.