User's guide of Proceessing Modflow 5.0

290 Processing Modflow
6.6.2 Two-Dimensional Transport in a Uniform Flow Field
Folder: \pm5\examples\transport\transport2\
Overview of the Problem
In this example, transport of solute injected continuously from a point source in a steady-state
uniform flow field should be simulated. The available parameters are listed below:
Layer thickness = 10 m
Groundwater seepage velocity = l/3 m/day
Effective porosity = 0.3
Longitudinal dispersivity = 10 m
Ratio of transverse to longitudinal dispersivity = 0.3
3
Volumetric injection rate = 1 m /day
Concentration of the injected water = 1000 ppm
Your task is to construct a 2D-model and use MT3DMS to calculate the concentration
distribution at the end of a 365-day simulation period.
Modeling Approach and Simulation Results
A numerical model consisting of 46 columns, 31 rows and 1 layer was constructed to simulate
the problem. A regular grid space of 10 m is used for each column and row. The configuration
of the model is shown in Fig. 6.59. The model layer is simulated as a confined layer. The top
and buttom of the model layer is at an elevaiton of 10 m and 0 m, respectively. To simulate the
groundwater seepage velocity of 1/3 m/day, fixed-head boundaries (with h = 11 m and h = 10m)
are assigned to the west and east side of the model. The horizontal hydraulic conductivity is 45
m/day.
The flow field was first calculcated by MODFLOW. The third order TVD scheme was
used in the simulation for the advection term; and the GCG solver is used to solve the dispersion
term. The contour map of the concentration field at the end of the 365-day simulation period
obtained for this example is shown in Fig. 6.60. An analytical solution for this problem is given
by Wilson and Miller (1978). The analytical solution is applicable only under the assumption
that 1) the aquifer is relatively thin, so that instantaneous vertical mixing can be assumed; 2) the
injection rate is insignificant compared with the ambient uniform flow.
Fig. 6.61 shows the breakthrough curves at an observation well located 60 m downstream
to the injection well. The analytical solution is obtained by using the computer program
6.6.2 Two-Dimensional Transport in a Uniform Flow Field