GGU-CONTAM-FE: Mechanisms relevant to contaminant propagation
Five important mechanisms describe contaminant propagation in groundwater and must be entered as initial data for propagation simulation:
Convection
is the migration of contaminants in the direction, and with the apparent flow velocity, of groundwater flow. This propagation mechanism is normally dominant in a flowing system but is superimposed upon and influenced by others. Mathematically, this mechanism enters the differential equation in the shape of the apparent flow velocities ux and uy.Dispersion
includes the local deviation from the mean flow velocity. One differentiates between dispersion caused by the grain skeleton, which describes, amongst other factors, deviations in the flow course around the grain skeleton, and macro-dispersion, which is the dominant mechanism in wide-area contaminant propagation, e.g. in sand lenses and layering. This mechanism is described by the mathematical values of longitudinal (in flow direction) dispersivity αL (m) and transverse (perpendicular to flow direction) dispersivity αT (m).Diffusion
This type of propagation is determined by the concentration gradient. It is normally only effective in systems with extremely low velocity. In water at 10 °C, the diffusion coefficient Dm is 10-9 m/s².Adsorption
expresses the inhibition of contaminant migration due to adsorption onto the grain skeleton and is described by the sorption constant sorp (-). The sorption constant is the product of grain density and the adsorption coefficient. Contaminant propagation is retarded by adsorption.Decay
As a result of physical, chemical and biological reworking, contaminants are eliminated from the groundwater. This process is described by the decay constant λ (l/s).
A mass balance equation and a space- and time-dependent, analysable differenzial equation results from these values:
Mass balance:
Increase in dissolved contaminant mass =
net input by convection
+ net input by diffusion and dispersion
+ input from contaminant sources
- extraction from wells
- loss through decay reactions
- adsorption onto the grain matrix