65. Forward Particle Tracking, Structured Grid, Transient Flow
This example demonstrates a MODFLOW 6 particle tracking (PRT) model by reproducing example problem 3 from the MODPATH 7 (D. W. Pollock, 2016) example problems document (D. Pollock, 2017). An equivalent MODPATH 7 model is constructed for comparison, though only PRT results are shown.
65.1. Example description
PRT/MP7 Example 3 modifies the flow system from PRT/MP7 Example 1 with three stress periods: first a steady-state period with a single time step, length 100,000 days, then a transient period with 10 time steps, each with length 36,500 days, and lastly a steady-state period with a single time step lasting 100,000 days.
Boundary conditions are also modified in this example. There is not one but two wells, one in the first layer and one in the third layer. There is also a drain in the first layer, extending from roughly the center of the grid to the river on the grid’s right boundary. Both wells are inactive for the first stress period, then begin to pump as the 2nd stress period begins (after 100,000 days), and continue to pump at a constant rate for the rest of the simulation. Model parameters for this example are summarized in Table 65.1.
Particles are released in batches from a 2x2-cell square (4 total cells) in the upper left quadrant of the grid. Ten batches are released in total: the first batch is released at 90,000 days, after which batches are released every 20 days until 200 days have elapsed.
Parameter |
Value |
|---|---|
Number of periods |
3 |
Number of layers |
3 |
Number of rows |
21 |
Number of columns |
20 |
Column width (\(ft\)) |
500.0 |
Row width (\(ft\)) |
500.0 |
Top of the model (\(ft\)) |
350.0 |
Layer bottom elevations (\(ft\)) |
220.0, 200.0, 0.0 |
Horizontal hydraulic conductivity (\(ft/d\)) |
50.0, 0.01, 200.0 |
Vertical hydraulic conductivity (\(ft/d\)) |
10.0, 0.01, 20.0 |
Recharge rate (\(ft/d\)) |
0.005 |
River stage (\(ft\)) |
320.0 |
River bottom (\(ft\)) |
317.0 |
River conductance (\(ft^2/d\)) |
1.0e5 |
Soil porosity (unitless) |
0.1 |
65.2. Example Results
In this example a MODFLOW 6 particle tracking (PRT) model runs in a separate simulation from the groundwater flow (GWF) model (fig Figure 63.2). Intercell flows are read by the PRT model from the binary budget file written by the GWF model.
Path points on a 2000-day interval are visualized in plan view in fig Figure 65.2 and in 3D in fig Figure 65.3. Release and termination points are colored by destination in fig Figure 65.4.
Figure 65.1 Head simulated by the MODFLOW 6 groundwater flow (GWF) model.
Figure 65.2 2000-day particle path points. Points are colored by travel time.
Figure 65.3 Three-dimensional perspective of pathlines and 2000-day points. Points are colored by destination.
Figure 65.4 Particle release and termination points, colored by destination.
65.3. References Cited
Pollock, D. (2017). MODPATH version 7: Example problems.
Pollock, D. W. (2016). User guide for MODPATH Version 7—A particle-tracking model for MODFLOW. https://doi.org/10.3133/ofr20161086
65.4. Jupyter Notebook
The Jupyter notebook used to create the MODFLOW 6 input files for this example and post-process the results is: