Investigation of DNAPL Dissolution in a Three-Dimensional Fracture Network

Dense non-aqueous phase liquid (DNAPL) present in fractured bedrock settings introduces remediation challenges that are dramatically different from porous media, yet the dissolution characteristics of DNAPL in a fractured network setting have not yet been fully investigated. Accurate characterization dissolution kinetics in a fractured network setting may yield a better understanding of removal both the free and dissolved phases in field settings.  This research investigates dissolution behavior of DNAPL in a three dimensional (3-D), fractured, low porosity sandstone experiment using tertrachlorethylene (PCE) as the contaminant of interest. To understand dissolution in the fractured system, DNAPL was emplaced directly into fractures and effluent concentrations of PCE were monitored over the course of the experiment. Conservative tracer tests were conducted prior to DNAPL emplacement to understand tracer behavior in a “clean” system. The impacts of fractures perpendicular to primary flow paths on conservative tracer transport lead to tailing of breakthrough curves. Earlier studies have attributed this primarily to diffusion into and out of the rock matrix; however, matrix diffusion testing confirmed that small matrix porosity (less than three percent) of the sandstone eliminates the impact of matrix diffusion processes operating in the experimental system while, discrete fracture samples confirmed dead end fractures as the dominant feature for diffusion-based processes observed in the system.  Following DNAPL emplacement conservative (bromide)/non-conservative (sodium dodecylbenzenesulfonate) tracer testing was conducted directly following DNAPL emplacement and over the course of the experiment to understand the temporally evolving interfacial area of emplaced DNAPL. Effluent PCE data were used to determine effective dissolution rates and the dependence on DNAPL interfacial area and groundwater velocity.  The DNAPL dissolution rates and measured interfacial areas observed in the 3-D experiment are compared to those measured in an analogous experiment performed in a one-dimensional experimental fracture system, and to values measured in porous media systems.