41. MT3DMS Problem 7
In a previous problem titled “two-dimensional transport in a uniform flow field” concentrations were compared between MODFLOW 6 and MT3D-USGS for a relatively thin aquifer (10 \(m\)) wherein instantaneous vertical mixing was assumed. In order to test transport simulation in a thicker aquifer, where all three spatial dimensions are required to adequately simulate the movement of solute, the current problem was devised. (Hunt, 1978) provides an analytical solution, which is used to verify MT3DMS in (Zheng & Wang, 1999), but not shown here. Instead, only the MODFLOW 6 and MT3DMS solutions are compared here.
Problem dimensions and aquifer properties are given in Table 41.1. The point source is located in layer 7, row 8, and column 3
Parameter |
Value |
|---|---|
Number of layers |
8 |
Number of rows |
15 |
Number of columns |
21 |
Column width (\(m\)) |
10.0 |
Row width (\(m\)) |
10.0 |
Layer thickness (\(m\)) |
10.0 |
Top of the model (\(m\)) |
0.0 |
Porosity |
0.2 |
Horizontal hydraulic conductivity (\(m/d\)) |
0.5 |
Volumetric injection rate (\(m^3/d\)) |
0.5 |
Longitudinal dispersivity (\(m\)) |
10.0 |
Ratio of transverse to longitudinal dispersitivity |
0.3 |
Ratio of vertical to longitudinal dispersitivity |
0.3 |
Simulation time (\(days\)) |
100.0 |
An analytical solution for this problem was originally given in (Hunt, 1978). Both numerical solutions invoke their respective TVD schemes. Moreover, MODFLOW 6 is using the XT3D package for simulating dispersion. The MODFLOW 6 solution shows great agreement with the MT3DMS calculated concentrations for the three layers displayed in Figure 40.1.
Figure 41.1 Comparison of the MT3D-USGS and MODFLOW 6 numerical solutions for three-dimensional transport in a uniform flow field. The analytical solution for this problem was originally given in (Hunt, 1978) and is not shown here
41.1. References Cited
Hunt, B. (1978). Dispersive sources in uniform ground-water flow. Journal of the Hydraulics Division, 104(ASCE 13467 Proceeding). Retrieved from https://trid.trb.org/view/72451
Zheng, C., & Wang, P. P. (1999). MT3DMS—a modular three-dimensional multi-species transport model for simulation of advection, dispersion and chemical reactions of contaminants in groundwater systems; documentation and user’s guide.
41.2. Jupyter Notebook
The Jupyter notebook used to create the MODFLOW 6 input files for this example and post-process the results is: