ChemPath is a three-dimensional groundwater transport model. ChemPath is designed to take the groundwater flow solution from a groundwater model and solve the contaminant transport problem with the user-defined initial and boundary conditions for the contaminant.
ChemPath takes the groundwater flow solution in the form of pathlines starting from designated points in the flowfield. Each pathline can be considered as the center line of a streamtube. For example, a set of pathlines starting from a contaminant source area represent the groundwater flow from the source area in the form of several stream tubes.
A number of groundwater models create the pathlines, e.g., FLOWPATH, Micro-Fem, GMS, Visual MODFLOW, ModIME, etc. Some of these models produce pathlines in a steady-state flow (e.g., Micro-Fem, Visual MODFLOW, FLOWPATH), while others (e.g., ModIME) can also produce pathlines in a transient flow. ChemPath can solve the transport problem with either kind of pathlines which can be 2- or 3-dimensional. The pathline file produced by any of the above models can be directly imported into ChemPath. The pathlines generated by any other model or by flow-net analysis will have to be preprocessed to the generic format.
ChemPath solves for the contaminant concentration along these solute pathlines instead of the 2 or 3-D grid or mesh. Owing to this innovative approach, ChemPath is free from numerical dispersion and is capable of handling complex solute-aquifer interactions, e.g.,
- Zero or first-order decay for Dissolved and Sorbed phase
- Equilibrium Sorption - Linear, Freundlich, and Langmuir Isotherms
- Rate-limited Sorption - Linear and Langmuir Isotherms
ChemPath includes a graphical user interface to define the following types of initial and boundary conditions:
- Uniform and Nonuniform Initial Conditions
- Steady-state and Time-varying Concentration Boundary Conditions
- Steady-state and Time-varying Flux Boundary Conditions
The usual steps involved in transport modeling with ChemPath are:
- Input relevant aquifer and contaminant information with the help of Input Wizards.
- Diagnose your data sets with the diagnosis routine.
- Run the calculation routine to calculate the transient concentrations along the pathlines.
- Postprocess the results from the calculation routine to create color-coded concentration plots.
ChemPath - Assumptions and Simplifications
ChemPath assumes that the individual pathlines are independent from each other. In other words, ChemPath ignores the transverse dispersion of the contaminant in the subsurface. The transverse dispersion is equal to transverse dispersion coefficient multiplied by the concentration gradient in the transverse direction. The transverse dispersion coefficient is one order of magnitude less than the longitudinal dispersion coefficient and in most contaminant situations, the transverse concentration gradients are very small. Therefore, ignoring the transverse dispersion of the contaminant is a valid assumption in most contaminant situations.
ChemPath - Strengths
Since ChemPath solves for the concentration along each individual travel path, ChemPath is capable of including fairly complex chemical reactions. Since the pathlines are one-dimensional in mathematical terms, it is fairly straightforward to include reactions like kinetic nonlinear sorption, two-site sorption (combination of equilibrium and kinetic sorption), etc.
In addition, ChemPath takes a few minutes to run even for a very complex situation as opposed to hours taken by other three-dimensional transport models. Therefore it is very easy to calibrate and run the model for various scenarios.
ChemPath was designed for the Windows environment and uses the state-of-the-art, easy-to-use concept of Input Wizards. Creating and editing the input is accomplished with extraordinary ease. Also, the initial and boundary conditions can be defined graphically. The Postprocessor lets you see the output in several different ways, e.g., the Dissolved and Sorbed Concentration along the pathlines in various specified depth ranges, Dissolved and Sorbed Concentration versus Distance or Travel Time along each pathline, and Travel Time versus Distance along each pathline. You can print the output on any Windows-compatible printer to produce a full-color/B&W presentation or export it to SURFER or DXF format.
ChemPath - Limitations
ChemPath ignores interaction among adjacent pathlines which can be important in certain situations. ChemPath results will not be very useful in those situations. Those situations include:
- extremely long travel distances compared to the spatial extent of contamination, and
- very complex geometry of the plume (especially if the user is interested in the details of the plume geometry rather than the concentration history at the extraction wells).
Comparison between MT3D and ChemPath
Strengths of MT3D
1. MT3D is a fully three-dimensional model whereas ChemPath is a pathline based 3-D finite difference model.
2. MT3D is completely compatible with MODFLOW so running transient flow and transport is easier. On the other hand, ChemPath needs pathline solutions from MODPATH or PATH3D.
Strengths of ChemPath
1. ChemPath is free from numerical dispersion whereas MT3D has large numerical dispersion.
2. ChemPath allows equilibrium as well as rate-limited sorption whereas MT3D only allows equilibrium sorption.
3. ChemPath takes minutes to run whereas MT3D can take hours or sometimes days to run.
4. Capture zone analysis is easier with ChemPath whereas MT3D is not geared towards capture zone analysis.
5. ChemPath works with various flow models such as FLOWPATH, Micro-Fem, and even FlowNet analysis. On the other hand, MT3D works only with MODFLOW.
6. The ChemPath solution is almost always stable whereas MT3D solution is unstable unless one has strict control over grid dimensions, chemical parameters and solution method.
7. ChemPath is extremely easy to set up and run.
Answers to Frequently Asked Questions
How exactly does ChemPath solve for chemical concentrations in the subsurface? ChemPath takes the travel path information which can be generated using PATH3D, MODPATH, Micro-Fem, Visual MODFLOW, ModIME, GMS and FLOWPATH, or simple flow-net analysis. ChemPath then solves for the chemical concentrations along a number of these travel paths. The snapshots of chemical concentration in the subsurface are constructed from the collection of the concentration information on the travel paths.
Is ChemPath a transient model? Yes, it is a transient model. It accepts transient boundary conditions for all the travel paths. The travel path of the solute can be traced using either a steady-state or transient flow model.
How is the numerical dispersion compared to other models? ChemPath is completely free from numerical dispersion.
What are typical run times for ChemPath? ChemPath runs in minutes even for a very complex three-dimensional situation.
What chemical fate and transport processes are included in ChemPath? Since ChemPath solves for the concentration along each individual travel path, it is capable of including fairly complex chemical reactions. At present ChemPath includes:
- Advection
- Dispersion
- Zero/first-order decay of the dissolved phase
- Zero/first-order decay of the sorbed phase
- Equilibrium and kinetic linear sorption
- Equilibrium and kinetic Langmuir sorption
- Equilibrium Freundlich sorption
Is ChemPath a multiphase model? No. ChemPath is a single-phase model.
How does ChemPath obtain the groundwater flow solution? The pressure or groundwater flow solution comes from other groundwater flow models, e.g., PATH3D, MODPATH, Micro-Fem, Visual MODFLOW, ModIME, GMS and FLOWPATH. Flowlines (X, Y, Z, and Travel time) generated by any other model can be imported into ChemPath; flowlines can even be generated by simple flow net analysis.
What adjustments are made to the constants to correct for temperature and ionic strength? The temperature correction has to be applied to the constants outside ChemPath. ChemPath does not vary the thermodynamic constants as a function of ionic strength.
Do you have a kinetic database? Do you have a database of radioactive decay constants ? Can I easily input my own thermodynamic constants into the ChemPath database? We have a database of kinetic coefficients and radioactive decay constants. It is not integrated with ChemPath yet, we are planning to do it in future. Until then, we can provide you with information about the database and a shareware utility to search that database. You can also add more data to that database.
Are ChemPath upgrades free? All minor upgrades to ChemPath (Version 1.1, 1.2, etc.) are going to be free for the registered users. Major upgrades (such as Version 2.0) will be available to the registered users for a charge less than the regular price.
Does ChemPath do Monte-Carlo simulations? We don't have Monte-Carlo simulation capability in ChemPath right now. We may do it in future. We feel that it should not be difficult to do this even manually, since
- ChemPath takes such a small time to run a simulation, and
- It is so easy to change the input variables.
ChemPath Requirements: PC 486/Pentium running Microsoft Windows 3.1 or Windows95 with 6 MB RAM and 20 MB hard disk space. |