Interpretation of a network-scale tracer experiment in fractured rock conducted using open wells

Abstract

The presence of fractures in bedrock allows for rapid aqueous contaminant transport through complex pathways and for diffusion of solutes between the fractures and the matrix. To better understand transport in these settings, tracer experiments are a commonly used tool. The need for expensive multi-level wells to obtain depth-specific concentrations, however, significantly limits the cost efficiency. The primary objective of this study is to develop a method whereby a discrete fracture network approach can be used to simulate the results of a divergent tracer experiment conducted using open observation boreholes in a well-characterized dolostone over distances of 55 m to 242 m. The experiment was conducted using a fluorescent tracer which allowed for continuous concentration measurement with depth in each open observation well. Two numerical models were employed in the interpretation of the experiment. The first was a 1-D finite difference model focused on flow and transport in the observation wells and the second was a 3-D control-volume finite element model capable of simulating the entire fracture network. Through fitting the experimental data to simulations, the most important fractures for transport in the system were identified. The number of fractures that participated in transport was few relative to the number of fractures observed in core and in constant head test results. Heterogeneous distribution of the fracture apertures was determined to be the likely cause of the highly tortuous transport observed at the site. This study demonstrates that tracer experiments conducted using open observation boreholes and a downhole fluorometer can improve our understanding of large-scale transport in fractured rock, especially when analysed with multiple models, and compared to other measured properties such as matrix porosity, hydraulic aperture, and fracture orientation.