Injection Mode Effects on Tracer Experiments in Columns

Abstract

In the study of solute transport through laboratory columns packed with natural porous media, tracers are commonly applied as a step input or as a square-wave pulse of several pore volumes duration. However, tracer applications such as a short-duration pulse (effectively simulating a Dirac input) should provide a more sensitive means of determining flow characteristics. We describe an experimental confirmation of this expectation using studies of tritiated water transport through soil columns. The porous media studied include both homogeneously packed sands and media with clearly defined paths of preferential flow. For all of the systems studied, the duration of the solute input pulse dramatically influenced our ability to discern the adequacy of a simple convection-dispersion model. In particular, the use of Dirac inputs was most effective at revealing situations in which physical/chemical processes other than convection and dispersion were present.