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MIGRATE

Detailed Description

MIGRATE

MIGRATE





MIGRATEv9 INTRODUCTION

Using the MIGRATEv9 program, contaminant transport from multiple sources either at the surface or buried, can be quickly and accurately modeled in two dimensions. In addition to advective-dispersive transport, the program can consider sorption, radioactive and biological decay, and transport through fractures.

Unlike finite-element and finite-difference techniques, MIGRATEv9 does not require the use of a "time-marching" procedure. MIGRATEv9 uses a finite-layer technique to model contaminant migration. This technique provides numerically stable and accurate results while requiring relatively little computational effort. One or more landfills, buried waste sites, disposal ponds, or spills may be modeled in two dimensions. These sources may be adjacent or offset from each other. Model properties may be either constant or transient with the concentrations calculated at specified times, depths, and distances.

MIGRATEv9 DATA ENTRY

Using the main menu bar at the top of the screen, datasets can be created, edited, and executed. The output from these datasets can then be graphed, displayed, or printed.

MIGRATEv9 Constant Properties

Datasets can be created or edited using the Data Menu. A dataset can have either constant properties or time-varying properties. In addition, there are menu items that allow the quick entry of standard landfill designs. Constant Properties datasets are the simplest to enter. In this type of dataset, the hydrogeology and functioning of the engineered systems do not change with time. Only the concentration in the source and soil layers changes with time.

MIGRATEv9 Time Varying Properties

General Data
Each constant properties dataset is composed of: general data (e.g., number of landfills, layers), top and bottom boundary conditions (e.g., finite mass), and layer data (e.g., porosity and diffusion coefficient).

First, general data is entered about the model such as:

  • Title of the Dataset
  • Number of Landfills (each landfill is offset linearly from the origin)
  • Number of Soil Layers (each layer can have different properties)
  • Laplace Transform and Gauss Integration parameters to be used in the inversion of the data

Boundary Conditions
There are two boundaries for every dataset, at the top and at the bottom of the layers. The top boundary is usually the point of contact with the contaminant source (finite mass or constant concentration). However, if the source is buried, the top boundary may represent either an impermeable surface or a surface where the contaminants are being continually removed. The bottom boundary represents the contact between the soil layers and a permeable aquifer, impermeable bedrock, or a layer where the concentration is kept at zero (e.g., in a lab experiment).

Top Boundary Condition
The finite mass top boundary condition can be used to represent a landfill or other source where the mass of the contaminants is limited. Properties such as the offset distance, size, mass of contaminants, half-life, and volume of leachate collected are specified for each landfill. For each of these parameters, the user can select from various types of units.

Bottom Boundary Condition
This boundary condition may be used to represent an aquifer below the soil layer. The concentration in this aquifer will vary with time as mass is transported into the aquifer from the layers above and is then transported away by the horizontal velocity in the base strata.

Layer Data
After specifying the boundary conditions, the properties of each soil layer should be specified. These properties include the thickness, porosity, dry density, sorption (distribution) coefficient, half-life, horizontal and vertical diffusion coefficients and velocities, and type of fractures (if any). In addition, the layer can be used as a leachate collection system. Any or all of the layers may be fractured. These fractures may be one, two, or three dimensional. In a fractured layer, the program considers advective-dispersive transport along the fractures coupled with diffusion into the matrix on either side of the fracture.

Time Period Data
Time-Varying Properties, General Data
The time-varying properties option is used to specify models whose hydrogeologic or engineered systems properties vary with time. For example, this option can be used to simulate the progressive failure of the leachate collection system in a landfill, the expansion of a landfill after several years or a change in source and aquifer properties. The data for a time-varying properties model is divided into general data, boundary conditions, layer geometry, and time period data. General data for the model is the same as for constant properties models and also includes the number of time periods during which the properties vary.

In this type of model, time is divided into periods during which the properties of the model are constant. There may be any number of time periods and the time periods can be of any length. The contaminant concentration in the source may be specified at the beginning of each time period or the concentration at the end of the previous time period can be used to model a depleting source.

During each of these time periods, the data for each layer must be specified. This data may change for each time period and will default to the data for the previous time. In addition, the data for the top and bottom boundary conditions can be entered for each time period. The data for the landfills may change for each time period.

MIGRATEv9 Leakage Rate (Subtitle D) Landfill
There are also options to create and customize predefined models quickly and easily. These models include the following.

Primary Liner (Subtitle D) Landfill
There are also options to create and customize predefined models quickly and easily. These models include landfills with primary leachate collection and composite liners (Subtitle D).

Primary and Secondary Liner Landfill
There are also options to create and customize predefined models quickly and easily. These models include landfills with primary leachate collection and composite liners (Subtitle D), and landfills with primary and secondary leachate collection and composite liners (Subtitle C). In this quick landfill entry option, layers such as the geomembrane, clay liner, aquitard, and aquifer can be included or discarded simply by selecting 'Yes' or 'No' beside the layer name.

Leakage Rate (Subtitle D) Landfill
The leakage rate through the composite liner may be calculated using the method proposed by Giroud et al., 1992, and Giroud and Bonaparte, 1989. These calculations consider leakage due to permeation and defects in the geomembrane.

Geomembrane Hole Data
In addition to the type of contact between the geomembrane and the clay liner, the leakage will also depend on the type, size, and frequency of the defects.

Finite Mass Source
The landfill contaminant source can be either finite mass or constant concentration. If the source type is finite mass, the waste thickness and density, infiltration through the cover, and percentage of mass can be specified for the contaminant.

Primary Clay Liner of GCL
For each layer present in the model, the parameters may be specified in any units; the program will automatically convert all units to either SI or US. The liner can be either clay or a geosynthetic clay liner.

Aquifer
If an aquifer is present beneath the landfill, the thickness and porosity of the aquifer can be specified. The program will automatically calculate the minimum outflow velocity in the aquifer; this value or a higher value can be specified.

MIGRATEv9 MODEL EXECUTION

Concentrations

After the dataset has been created, the model can be executed. The concentration of the contaminant can be calculated at any number of specific times and distances. The model can be executed interactively or in batch mode. Output from the execution is stored in an output file specified by the user.

MIGRATEv9 RESULT OUTPUT

After the model has been executed, the output file can be displayed, graphed, and printed. Graphs can be of concentration versus time, depth, or distance. In addition, graphs can have the concentrations plotted in color. All of these graphs can also be easily printed on several types of printers.

Concentration vs. Time
Default values for the graphs are automatically determined. These values can be easily changed to allow complete customization of the graph. For the concentration versus time graph, one or all of the depths and distances may be plotted on the same graph.

Concentration vs. Depth
Concentration versus depth graphs show the change in contaminant concentration with depth, either for a specific time and distance or for all the times and distances calculated.

Concentration vs. Distance
Graphs of concentration versus distance show the variation in the concentration of the contaminant with distance at a specified depth and time or at all depths and times.

Concentration vs. Time and Depth
The change in concentration with time and depth graph can be used to illustrate the movement of the contaminant plume into deeper depths over time.

Concentration vs. Distance and Depth
The change in concentration with distance and depth for a selected time graph can provide a view of the spread of contaminants in the soil at a specified time.

MIGRATEv9 PRINT OPTIONS

Any of the graphs can be printed by pressing 'P' while they are displayed. Many of the features of the printed graph can be controlled such as the size, titles, and fonts.

MIGRATEv9 TOOLS

Tools are available to aid in creating and checking datasets. These tools include a facility to import POLLUTEv6 data files, a calculator, context-sensitive help, and future add-on tools. Context-sensitive help can be accessed any time by pressing by the F1 key; the information displayed contains many cross-references which can be displayed by clicking on the highlighted text.

MIGRATEv9 PREFERENCES

The display colors, mouse buttons, and program environment can all be adjusted as desired.

Many of the program features can be customized such as directories, file extensions, screen display time, and printer type. Most popular screen types are supported including SUPER VGA, VGA, and EGA, or the screen type can be automatically detected.

A large variety of printers are also supported such as Epson 9 and 24 pin, HP LaserJet, HP Pen Plotters, HP Paint Jet, and Postscript. Graphs can also be converted into PCX file format.

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