Monday, May 7, 2012
Tracer tests are a powerful tool for quantifying transport and hydraulic subsurface parameters. Anderson (2005) suggests that hydrogeologists should capitalize on recent improvements in temperature sensors, and that heat data should be collected and interpreted more widely. Heat data are increasingly used in groundwater-surface water interaction studies, and have proven useful in calibrating numerical models. It would appear that there is an opportunity for more widespread use of heat as a tracer in hydrogeology, but the use of heat is currently inhibited by a lack of quantitative guidance and intuition on situations where heat is an appropriate tracer. Furthermore, it is not immediately obvious what advantages heat tracers offer over solute tracers. These two tracer types have not been systematically evaluated within the same framework to allow for a rigorous quantitative comparison. The most fundamental differences between solute and heat tracers are that thermal diffusivity is 3-4 orders of magnitude greater than molecular diffusion, and heat can be transported through both the connected pore spaces and the porous medium. How this affects flow rate determination using the tracers in both homogeneous and heterogeneous geologic settings requires exploration. In this study, we use a numerical model to compare and contrast solute and heat tracers in order to develop quantitative guidance for the applicability of these tracers in natural hydrogeologic settings. A numerical model is an efficient tool to perform this analysis because the problem can be well constrained, and flow velocities are known. Results suggest that the uncertainties and bias which arise from using tracers are greater compared to hydraulic methods. Uncertainties and bias are lower for heat tracers compared to solute tracers; however heat tracer tests require a longer duration due to thermal retardation, which under some circumstances may be a significant drawback.