Cracks in the Clay: The Role of Fractures and Macropores in Critical Zone Hydrology

Fine-grained geologic deposits often contain extensive networks of fractures, root holes and other macropores which can strongly influence groundwater flow and contaminant transport. The extent and depth of these features varies greatly according to the origin and geologic/pedologic history of the material. Rootholes typically persist to depths of only a few meters, although in some clays they can be found at much greater depths. Desiccation fractures, which are common in glaciolacustrine deposits, also tend to rapidly decrease with depth, but fractures caused by sub-glacial stresses may be pervasive throughout thick till seququences.

Recent research in weathered clay-rich residuum developed on sedimentary rocks in east Tennessee show evidence of fractures and fracture-induced flow to depths of up to 40 m. Fractures and macropores can also act as pathways for transport of natural and anthropogenic constituents to underlying aquifers. Solutes are transported by advection along the fractures/macropores but can also be strongly attenuated by diffusion into the fine pore structure. In contrast, mineral colloids and microorganisms, are largely size-excluded from the fine-pore structure and hence can travel at much faster rates than solutes.

Field tracer experiments in fractured clays in Canada, Denmark and Tennessee showed colloid transport rates of a few m/day to >100 m/day at sites where solute tracers were transported at  rates that were 100s of times slower. Immiscible phase liquids, such as industrial solvents or coal tar, can enter some fractures or macropores, even in relatively low hydraulic conductivity materials and can lead to extensive contamination. These immiscible liquids dissolve and diffuse into the fine pore structure, where they can act as long term sources of contamination to adjoining streams or underlying aquifers.

Although there has been substantial progress over the past 25 years in developing a better understanding of the role of fractures in controlling flow and transport in clay-rich deposits, considerable work remains to be done. This includes better education of geo-environmental researchers and professionals, as well as development of better conceptual and numerical models of fracture origin, vadose and saturated zone flow, and contaminant transport.