FLUX AND RESIDENT INJECTION IN GASEOUS ADVECTION EXPERIMENTS

The occurrence of gaseous pollutants in soils has stimulated many expe rimental activities, including forced ventilation in the field as well as laboratory transport experiments with gases. The dispersion coeffi cient in advective-dispersive gas phase transport is often dominated b y molecular diffusion, which leads to a large overall dispersivity gam ma. Under such conditions it is important to distinguish between flux and resident modes of solute injection and detection. The influence of the inlet type oil the macroscopic injection mode was tested in two s eries of column experiments with gases at different mean flow velociti es nu. First we compared infinite resident and flux injections, and se cond, semi-infinite resident and flux injections. It is shown that the macroscopically apparent injection condition depends on the geometry of the inlet section. A reduction of the cross-sectional area of the i nlet relative to that of the column is very effective in excluding the diffusive solute input, thus allowing us to use the solutions for a f lux Injection also at rather low mean flow velocities nu. If the whole cross section of a column is exposed to a large reservoir like that o f ambient air, a semi-infinite resident injection is established, whic h can be distinguished from a flux injection even at relatively high v elocities nu, depending on the mechanical dispersivity of the porous m edium.