A comparison between solute transport in a discrete fracture and in a fracture network using a novel method for tracer detection

Abstract

Characterization of field-scale transport in bedrock aquifers is necessary due to the preponderance of groundwater contamination in these settings, and the increasing attention paid to these sites by regulatory bodies. However, as a result of the inherent complexity, and the consequent uncertainty in the dominant transport processes, large-scale transport in fractured rock is poorly understood.

In this study an investigation of large-scale transport was accomplished in part by conducting a radial-divergent tracer experiment in a 15 m thick section of aquifer with observations over a 245 m distance, using a novel tracer detection method capable of detecting breakthrough in individual fractures. The tracer experiment was conducted at a well-characterized field site in Smithville, Ontario, which is underlain by several large-scale bedding plane fractures, and used a submersible fluorometer to detect tracer arrival in-situ and to obtain vertical fluorescence profiles (VFPs) from observation boreholes.

To complete the investigation, hydraulic characterization data and VFPs were used to approximate the dominant transport pathways and a numerical model which solves for flow and transport in discrete fracture features (HydroGeoSphere) was used to simulate the tracer experiment. The results of the experiment and the modeling exercise were compared to those from a large-scale single fracture tracer experiment conducted previously at the same site, for which the modeling was revisited.

The experimental results of the fracture network experiment (FNE) were markedly more heterogeneous than those of the previously conducted single fracture experiment (SFE), with multiple peaks in the breakthrough curves, and scale dependent changes in breakthrough character. The VFPs illustrate that differences in the observed transport arise due to tortuous transport pathways within individual fracture features, and the combined effect of this tortuosity in the numerous fractures contributing to transport in the fracture network.

For the observation boreholes closest to the source (< 55 m), both the FNE and SFE models were capable of fitting the data using parameters within the range of values determined from prior lab and field experiments at the site. These fits became poorer over increased transport distances however, where the models used could not account for the increased effects of tortuosity.