Investigation into Dense Non-aqueous Phase Liquid (dnapl) Transport and Remediation in Vertical Fractures

This thesis investigates the transport behavior of a common category of industrial pollutants, known as Dense, Non-Aqueous Phase Liquids (DNAPLs). These organic chemicals are more dense than water and relatively insoluble. A typical release of DNAPL at the ground surface will migrate downward, relatively unhindered, until it reaches an impermeable clay layer or bedrock, where a pool begins to form. If the clay or bedrock is fractured, DNAPL can invade the fracture system and continue to migrate. Once present in a fracture network, DNAPL can act as an ongoing contaminant source to groundwater, often rendering drinking water supplies unusable. The work described in this thesis is part of a larger project designed to ultimately predict DNAPL behavior in real, rough-walled fracture systems, toward the aim of developing better schemes to locate and remediate it. The work presented in this thesis examined the invasion behavior of DNAPL in watersaturated, smooth-walled, planar-shaped, vertical fractures. Experiments were performed at a reduced scale using a geotechnical centrifuge. Experimental results demonstrated that a simple static model can reasonably predict the DNAPL pool height that can be developed before fracture invasion occurs. In addition, a dynamic model derived from the Navier-Stokes equations was also shown to reasonably predict the asymptotic displacement behavior of the invading DNAPL, and its behavior at the fracture exit. This thesis also examined the behavior of DNAPL plugs in water-saturated, smoothwalled, circular-shaped, vertical fractures under upward displacement conditions. These experiments were performed at full-scale in the laboratory. The results demonstrated that a theoretical model based on a pressure balance reasonably predicts the pressure gradient that must be created in the system to mobilize a DNAPL plug trapped in the fracture. Experiments also confirmed that there was no observable water channeling along or through the DNAPL in the smooth-walled fracture, and that the DNAPL displaced as a plug. The results obtained in these experiments suggest that hydraulic flushing might be developed into a feasible remediation technique for removing residual DNAPL from fractured rock or clay, if the fracture roughness does not significantly alter the flow dynamics. Thesis Supervisor: Patricia J. Culligan-Hensley Title: Associate Professor of Civil and Environmental Engineering