The DSP: GWT Dispersion Package pane is used to activate the Dispersion package. It is on the MODFLOW Packages and Programs dialog box under Groundwater Transport| | GWT: Groundwater Transport.
When solutes are transported by groundwater, the speed and direction of transport of individual molecules can vary based on the speed and direction of the water flow in which they are dissolved. For example, one molecule may happen to follow a very convoluted flow path through small pores where the flow rate is slow. Another close-by molecule may happen to flow through large pores that are well aligned with the direction of flow resulting in a faster speed. These differences in flow rate can cause an initially sharp solute front to spread out as transport proceeds. The spreading occurs in both the direction of flow and perpendicular to the direction of flow. Molecular diffusion also contributes to the spreading of the solute.
This spreading or dispersion phenomenon is modeled as being analogous to molecular diffusion even thought the actual process is not primarily due to molecular diffusion. Two different types of dispersion are recognized: longitudinal dispersion and transverse dispersion. Longitudinal dispersion is dispersion in the direction of flow and transverse dispersion is perpendicular to the direction of flow.
In MODFLOW models, horizontal flow is distinguished from vertical flow. Horizontal flow typically occurs between cells with similar properties whereas vertical flow is often between cells with contrasting properties. This has implications for how dispersion is modeled.
In MODFLOW 6, there are two components of longitudinal dispersivity. One component is for horizontal flow and the other is for vertical flow. If flow is strictly horizontal or strictly vertical, only one of these components applies. If the flow has both horizontal and vertical components, the two components of longitudinal dispersivity are combined to determine the total longitudinal dispersivity. In the MODFLOW 6 Dispersion package, these two components are the ALH and ALV data sets respectively. For strictly horizontal flow, there are two components of transverse dispersivity; one is for horizontal dispersion and one for vertical dispersion. These are the Dispersivity package data sets, ATH1 and ATH2 respectively. For strictly vertical flow only a single transverse dispersivity parameter is used: ATV. For flow that has both horizontal and vertical components, all three components are used in computing the transverse dispersion.
The Dispersion package has one additional data set, DIFFC, that is used to account for molecular diffusion.
There are a few limitations of how dispersion is modeled in MODFLOW 6. First, barriers in the Horizontal Flow Barrier package impede flow across the barrier but do not impede dispersion across the barrier. Second, sloping layers are common in MODFLOW models. You might expect that flow within layers would be considered horizontal flow for purposes of calculating dispersion and flow across layers would be treated as vertical flow but this is not the case. Horizontal flow is flow within the X-Y plane even if there are sloping layers and vertical flow is flow in the Z direction. Third, the treatment of dispersion as analogous to molecular diffusion tends to produce unrealistic back dispersion, and the dispersion coefficients are scale-dependent, particularly in applications involving estimation of effective dispersion coefficients in heterogeneous aquifers (Langevin and others, 2022, Konikow, 2010).
The DSP Package is based on the mathematical formulation presented for the XT3D option of the NPF Package available to represent full three-dimensional anisotropy in groundwater flow. XT3D can be computationally expensive and can be turned off to use a simplified and approximate form of the dispersion equations. For most problems, however, XT3D will be required to accurately represent dispersion.
Activating XT3D RHS adds xt3d terms to right-hand side, when possible. This option uses less memory, but may require more iterations.
There are several optional data sets that can be used with the Dispersion package. The other options on the Dispersion Package pane determine whether or not these data sets will be generated.
