GROUNDWATER FLOW AND CONTAMINANT TRANSPORT IN FRACTURED AND WEATHERED CLAY-RICH DEPOSITS

Most exposed clayey deposits contain extensive networks of fractures and macropores. These features can strongly influence groundwater flow and contaminant transport, but there are often misconceptions about their role. For example, many consider them to be strictly a near-surface feature. Rootholes and fractures caused by dessication (for example in glaciolacustrine deposits) tend to rapidly decrease in frequency with depth and, where clays are more than about 15 m thick, the deposits are often effective aquitards. However, fractures/macropores caused by sub-glacial stresses or by volume loss during weathering of carbonate rocks may persist to much greater depths. Similarly, there is a common myth that fractures aren't significant in very fine-grained clays or in swelling clays. Clay-rich residuum formed from weathering of carbonate rocks in east TN contains as much as 90% clay-sized particles (half of which are < 0.1 microns), yet we've observed evidence of fracture-flow to depths of up to 40 m. Similarly, water pouring out of fractures is commonly observed in excavations in the swelling clay soils (vertisols) of east Texas, demonstrating that swelling alone doesn't guarantee closure of the fractures. There are also misconceptions about contaminant transport in these materials. It's widely recognized that solutes can be retarded relative to flow in the fractures/macropores by diffusion into the fine-pore structure (i.e., matrix diffusion). However, this doesn't imply that dissolved contaminants are permanently immobilized by matrix diffusion or that other types of contaminants (colloids and DNAPLs) are immobile. In summary, 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 researchers and groundwater professionals, development of conceptual fracture origin/flow/transport models for a broader range of clay-rich deposits, and development of better tools and procedures for addressing fundamental questions, such as how deep are the fractures? how much recharge occurs through the clay layer? and does this layer provide adequate protection against migration of contaminants to the underlying aquifer?