TWODAN, two-dimensional analytic flow modeling software from Fitts GeoSolutions, was just upgraded to a 32-bit Windows application (Version 5.0). This adds a simple and intuitive interface to TWODAN's advanced analytic modeling capabilities.

User Interface

TWODAN 5.0 has a seamless, Windows-standard user interface. It has been designed to be very simple and fast to use. Most modeling operations are executed at the push of a button on the main screen. Instead of using tedious data-entry forms, model input data and plot settings data are accessed directly in spreadsheet-like grids. You can quickly edit all aspects of the model input in the model input screen and all the plot settings from the plot settings screen. The input data for a model is stored in one file while the settings for a particular plot are stored in another. This separation is efficient, allowing you to push one button to repeat a plot with the same contours, pathlines, window coordinates, etc.

Main Screen.

Model Input Screen.

Coordinates are digitized using the mouse and a DXF basemap overlay. You can zoom or pan to a different view during the middle of a digitizing operation and you can continuously digitize multiple points, lines, or circles. TWODAN and the Windows Clipboard can be used for general-purpose digitizing, providing coordinates to other Windows applications.

TWODAN plots can be output to any device supported by the Windows operating system. Graphic plots may directed to the screen, Windows printer devices, bitmap (*.bmp) files, the clipboard, Surfer GRD files, or to DXF files. In addition, the data contoured and the pathline coordinates may be exported to comma-delimited ASCII files for GIS processing. Plots can include any combination of: (1) contours of head, potential, or stream function; (2) pathlines traced upstream or downstream from a point, line, or circle; (3) DXF basemap; and (4) layout of model elements. Printer plots may be scaled automatically to fit the page, or manually to a specific scale (e.g. 1 inch = 500 ft). You may print a plot with landscape or portrait orientation, and you may add a border box and up to three lines of title text.

TWODAN has extensive indexed and searchable on-line help, and a manual that covers installation, a tutorial, the method employed, and 14 checks of the program.

Modeling Capabilities

TWODAN has a suite of advanced analytic modeling features that allow you to model everything from a single well in a uniform flow field on up to complex remediation schemes with numerous wells, barriers, surface waters, and heterogeneities. The aquifer modeled by TWODAN can consist of one or two hydraulically connected layers, it can be confined and/or unconfined, and it can be homogeneous or heterogeneous. The analytic implementation of barrier elements gives much greater accuracy than is possible with numerical methods. TWODAN is also capable of optimizing discharges of steady wells based on specified head and aquifer discharge conditions. In addition, Both discharge- and head-specified linesinks are implemented and Vertical infiltration or leakage to or from the aquifer can be modeled as uniform or as locally variable.

TWODAN gives the usual compilation of calibration statistics, but it also plots the spatial distribution of (observed head - modeled head). Each difference is plotted at the coordinate of the observation, with different colors for positive and negative values so you can quickly see spatial patterns.

Application to Remediation Design with Barriers and Trenches

The following plots illustrate TWODAN's heterogeneity and barrier capabilities and some ideas regarding the use of barriers and high conductivity (K) zones in remediation schemes.

Two barriers with high K reaction zone.

Close up of barrier ends and high K reaction zone.

The above plots show how groundwater can be funneled into a high (K) reaction zone. The zone of water captured and funneled through the reaction zone is much narrower than the width of the upstream barrier opening. Substantial flow avoids the bottleneck in the funnel by flowing around the outside of the barriers.

An alternative concept is illustrated in the following figure.

High K reaction zone in the direction of flow.

In this scheme, there are no barriers, and the high K reaction zone is elongate in the direction of flow. Compared to the configuration with the two barriers, it captures a slightly greater amount of flow and the groundwater residence times within the reaction zone are larger. The construction cost of this scheme would probably be less than the scheme with barriers, and imperfections in construction have little consequence compared to the consequences of imperfect barriers.

SYSTEM REQUIREMENTS

• Intel 80486 based computer
• Windows95/NT

Author: Charles Fitts (University of Southern Maine)