What is it?
MODPATH is a particle-tracking model for MODFLOW built by the USGS.
Note: MODPATH version 7 works with MODFLOW-2005, MODFLOW-USG, and MODFLOW-6. Support for unstructured grids in MODFLOW-USG and MODFLOW-6 is limited to smoothed, rectangular-based quadtree and quadpatch grids with north-south orientation.
Source: USGS MODPATH PAGE
How to use it:
1) Specify Aquifer Parameters
Several aquifer parameters that are not contained in the MODFLOW files are needed to track particles.-Porosity [decimal]: The fraction (0.0-1.0) of pore space in the aquifer and affects the velocity that particles travel for a given flow rate [default 0.3].
-Retardation [decimal]: Factor (0.0-1.0) of particle flow to groundwater flow rates [default 1.0]. While these are currently set to apply to the entire model domain, enventually the user will be able to specify different zones.
2) Specify Simulation Parameters
-Tracking direction: track particles forward or backward delative to groundwater flow direction [default forward]. Use forward tracking to project plume migration or backward tracking to deliniate well protection areas.
-Weak sink/source options: particles can be designated to pass through or stop at weak sink or source cells [default pass through].
-Tracking simulation stop [dropdown]: this option determines what triggers the MODPATH model to end. If the "specified time" option is used a Specified stop time must also be provided.
-Release sets of particles [integer]: particles can be released in waves. This is useful if flow conditions change. If more than 1 set of particles is released, a interval between sets must be specified.
-Generate [dropdown]: choose one of four MODPATH output options here [default pathlines and endpoints]. If a timeseries output is chosen, the user must specify a time interval for the particle location timeseries. Start with a time interval that is 1-2 orders of magnutude smaller that the total travel time.
-save Particle locations for VTK analysis: use this option to save particle locations to a .ptls file that can be viewed with IGW-NET VTK tools. Choose to save entire timeseries (if available) or just endpoints.
3) Specify Initial Particle Locations
Initial particle locations can be set in a variety of ways.a) User Defined Option
The user can draw a shape (rectangle, polygon, line, or point) to define where to release particles. If the model is three dimensional, the user can also set the vertical position using layer fractions.To release particles from a rectangle, i) check the box next to "Release from zone", ii) if needed expand "Define particle Zone" and selet "Rectangle" from the dropdown, iii) Click "Draw Shape" to start drawing, then click on the image in two locations to define the diagonal of the release rectangle, iv) click "Save shape" to complete the shape.
To draw a polygon, line, or point the method is similar.
b) Model Feature Option
The user can pick this option to release particles from a set of features in the MODFLOW model. For example, this option can be used with a "backward" tracking simulation to determine well capture areas.c) File Upload Option
Upload a IGW-NET .ptls file to specify initial conditions.d) Continue Option
If you have already run a MODPATH simulation with the MODFLOW model, use the "Continue" option the use the endpoints from the last simulation as initial locations.4) Edit Display Options
Click on the "Display Options" button to open options to change pathline and particle size and color.Note: MODPATH display options can also be set in the "Particle Overlay Display" tab of the main "Display Options" window.