Modeling tracers in intact columns: A study of the influence of fracture aperature on solute transport

Abstract

This study created one and two-dimensional fracture flow and transport models that were compared to the transport properties of a known fracture system. In a cooperative study with Dr. Gerilynn Moline at Oak Ridge National Laboratory, Oak Ridge Tennessee, her lab data and my modeling were compared. Dr. Moline excavated columns of saprolite from Oak Ridge, and packed them in PVC pipe. The saprolite columns were flushed with tracers to study the effects of fracture flow and solute transport. Measurements taken of the column by Dr. Moline as she dissected it indicate that the fracture system was constructed of mostly small fractures (aperture = 0.02 cm), spaced close together (spacing = 0.75-1.0 cm). Tracer concentration information was collected, and Dr. Moline plotted breakthrough curves from the observed data. At Temple University, groundwater models of the saprolite columns were created using fracture flow transport programs (CRAFLUSH and FRAC3DVS). Both one and two-dimensional models were generated using data collected from Oak Ridge, and from sensitivity analyses performed at Temple. Breakthrough curves created from the modeled data were then compared to Dr. Moline's observed breakthrough curves. Results from one and two-dimensional models demonstrate that the observed fracture network does not recreate the breakthrough curves and is not representative of the dominant fracture network for the system. Modeled breakthrough curves demonstrate that the best fitting parameters create a fracture pattern made of a few large fractures (aperture = 0.16 cm) spaced farther apart (spacing = 1.5-2.0 cm), regardless of fracture lengths. The modeled fracture aperture and fracture spacing indicates that solutes flow preferentially through large fractures rather than through small disconnected fractures. This modeling method gives evidence that approximately 20% to 25% of the fractures within a system are influential in solute transport. Thus the fracture flow models calculate effective parameters that can be used to characterize the dominant fractures within a fractured porous medium.