Advective-Dispersive Transport in Pipes

The simplest application of a Pipe pathway is to use it to simulate one-dimensional advective-dispersive transport.

The first step in defining a Pipe pathway is to specify its geometry. The geometry of a Pipe is defined by specifying a length, a cross-sectional area, and a perimeter.

You then specify the flow rate of the Reference Fluid in the pathway (by defining one or more advective mass flux links from the pathway) and a dispersivity. The velocity in the pathway is computed as a function of the flow rate, the cross-sectional area, a user-specified saturation level, and the porosity of the porous medium (if any) which is specified to fill the pathway.

GoldSim then solves the one-dimensional advection-dispersion equation analytically to compute flux rates exiting the pathway. Pipes explicitly represent the decay of species as they flow through the pathway, as well as the ingrowth of daughter products from parent species. Parents and daughters can have completely different transport properties which will be explicitly represented within the pathway.

Two types of linear retardation processes can be represented within a Pipe:

• equilibrium partitioning between the fluid in the pathway and an infill material; and

• equilibrium partitioning between the fluid in the pathway and a coating material (around the perimeter of the pathway).

If you wish to simulate retardation due to partitioning onto an infill material, you must specify a porous Solid medium to represent the infill. Similarly, if you wish to simulate retardation due to partitioning onto a coating material, you must specify a Solid medium to represent the coating (which can be different from the infill medium), and a coating thickness. Fluid within the pore volume of the coating material is assumed to be immobile.

The mobile fluid in the pore volume of the pathway, the immobile fluid within the coating material, the infill material and the coating material are all assumed to be at equilibrium at all times. The effective retardation factor is therefore computed as a function of the properties of the infill and coating media (bulk densities, porosities, and partition coefficients) and the geometry of the pathway (saturation level, cross-sectional area and perimeter).