Environmental Engineering Reference
In-Depth Information
Figure 16.15.
Contribution of advection/diffusion/dispersion to contaminant migration
in terms of concentration and flux [MAN 97b]
In fact, at low seepage velocity pure diffusion prevails; whereas at higher
seepage velocity advection is the main influence on contaminant concentration and
flux. The dispersive transport becomes noticeable at seepage velocities where
advective transport is orders of magnitude higher than both diffusive and dispersive
transport. Therefore, also within the high seepage velocity range, the dispersive
transport is only significant compared to diffusive transport.
The variation in laboratory test methods for the determination of diffusion-
dispersion parameters is wide. A detailed description of equipment and testing
methods is given in [JES 94b, ROW 95, SHA 91a, SHA 91b]. The methods
described by the different authors include transient and steady-state conditions for
solute migration, with or without advective transport, and with constant or
decreasing influent concentration. Among the different kinds of test, the decreasing-
source-single-reservoir-diffusion test and column test considering all its possible
variations (i.e. constant hydraulic gradient, constant solution flow rate via flow
pump device, etc.) are the most popular. The use of these tests can give indications
in terms of both diffusion-dispersion and sorption parameters. For these tests the
scale effect is not as important as in the case of determining hydraulic conductivity
[MAN 94b]. On the other hand, the boundary conditions imposed in the different
types of laboratory tests must be carefully considered in order to obtain a reliable
interpretation of each test in terms of both concentration and contaminant flux.
