Particle Distribution Overlay MODPATH requires the initial location of particles for tracking. These locations are set with the Particle Distribution overlay window. This version of Graphic Groundwater includes Ring, Cell Faces, and Cell Volume distribution options. The first is new and probably unique to Graphic Groundwater. The other two operate as designed by Pollock (1989). A Ring distribution is a collection of points along a circle (or ring) in a horizontal plane within a cell. This type of distribution is most useful for reverse tracking in order to determine the capture zone of a well. Three location parameters; the relative diameter of the ring of particles, the relative position of the horizontal ring in the cell, and the number of particles defines a ring distribution. A relative location is specified with a number ranging from 0 to 1. For example, a relative diameter of 1 represents a ring of particles located in a circle circumscribed in the cell. A relative vertical position of 0 places the ring in a horizontal plane at the bottom of the cell; a relative vertical position of 0.5 places the ring in a horizontal plane in the middle of the cell; a relative vertical position of 1 places the ring in a horizontal plane at the top of the cell. Graphic Groundwater evenly distributes the number of particles around the ring. Particle Distributions are set for a particular cell or group of cells, not in every cell in the model. In addition, like a Cell Stress overlay, a Particle Distribution overlay can have more than one distribution of particles per cell. For example, a single cell could have a ring of particles at the top and bottom of the cell. With the Cell Faces option, the user specifies the face of the cell as well as the number of divisions in two directions parallel to that face. MODPATH splits the face into a grid with rows and columns equal to the number of specified divisions and places particles in the center of the grid cell. With the Cell Volume option, the user specifies the number of divisions parallel to all three spatial coordinates. MODPATH splits the interval volume of the cell into a three-dimensional grid with rows and columns and layers equal to the number of specified divisions and places particles in the center of the grid cell. The Particle Distribution Overlay window has a look and feel similar to that of the Cell Stress Overlay window. The interface was developed in order to allow more than one distribution of particles to be entered into a cell if desired. The window includes three text boxes for entering and displaying location parameter values, Batch, Slice, and Other drop down lists, a Color command, and Add Batch, Delete Batch, Clear Batch, Set Batch, Transient Options, and Finished buttons. You must first add a batch of particles. A batch is defined as one or more cells that share the same slice (consult Chapter 4 for a discussion of slices), color, and distribution parameters. For example, a batch of ring particles would be one or more cells operating in the same slice, with the same distribution parameters, and with the same color for the particle tracks. After you add a batch, select one or more cells where this batch exists. Choose the slice, enter the value or values for the location parameters in the text boxes, select a color for the particle paths, then click on the Set Batch button. Graphic Groundwater indicates the change by displaying a red inner border and the number of the batch in the selected cells. You can change the location parameters, the color, or the layer, but you must click on the Set Batch button for the change to take effect. You do not have to select the cells, just make the changes in the text boxes, drop down lists, and option buttons. You can add cells to a batch by selecting one or more cells and then clicking the Set Batch button. The cells set earlier still remain in that batch. To remove a particular cell or cells from a batch, first select the cell or cells, then click on the Clear Batch button. A click on the Delete Batch button removes the batch and renumbers all remaining batches. Clearing all the cells in the batch will also delete the batch. Graphic Groundwater displays all cells containing a particular distribution of particles in the grid. Choosing a batch in the Batch drop down list highlights all the cells in that batch and resets the color box, the Layer drop down list, and the location parameter text boxes to the values for that batch. If one or more batches share one or more cells with the batch highlighted in the Model window, the Other drop down list is activated and displays the number of the other batch or batches in that group of cells. Graphic Groundwater only displays the number of the first batch set for that cell. If a cell has more than one batch, another red border is displayed, nested inside the original red border. You navigate between the different batches of particles with either the Batch drop down list or Other drop down list (if enabled). In addition, selecting a cell containing a batch of particles will cause the Batch drop down list to display the batch number and changes the Slice drop down list and the values displayed in the text boxes to those for that particular batch of particles. The Transient Options button displays a pop-up window for specifying options for particle tracking under transient conditions (see the Particle Tracking window). This window includes Period and Step drop down lists, text boxes, and slider controls. You set the start time and ending time in units of cumulative model time. If you know when the particle should start and/or end, just enter these values directly into the respective text boxes. If you want the particle to start or stop at the beginning of a time step, just select the stress period and time step with the drop down lists. Graphic Groundwater will enter the cumulative model time into the text box for you. If you want the particle to start or end at some point within the time step, use the slider control. Moving the slider index to the middle of the bar would start (or end) a particle half way into the time step; moving the index to the right end of the slider control would start (or end) the particle at the end of the time step. Graphic Groundwater adjusts cumulative model time displayed in the text box as you move between stress periods and time steps or adjust the slider control. No values are set for transient tracking until you click the Set Batch button in the main Particle Distribution overlay window. All particles in a batch must share the same start and end times. The default start time for transient models is the beginning of the simulation and the default end time is the end of the simulation, so you do not need to open the Transient Options pop-up window in most cases. The Release Interval text box provides data that Graphic Groundwater uses to create new particles during tracking. Be careful with this option. In some cases a very large number of particles could be created during tracking. For example, suppose you have a transient model with a total simulation time of 1 year and you create a ring batch with ten particles. If you specify a release rate of 0.00274 years (1 day), a beginning time equal to zero and an ending time equal to one year, then specify a tracking duration of 1 year and a tracking step of 0.00274 years (1 day) in the Particle Tracking window, Graphic Groundwater will generate a new batch of ten particles every tracking step, producing a total of 3,650 particles by the end of tracking. Since the time it takes to track particles is a function of the number of particles, tracking can slow to a crawl if you accidentally create an excessive number of particles. A value of zero (the default) in the text box indicates that no new particles should be created during tracking. Particles created during tracking have the same initial location as specified in the Particle Distribution overlay window. See Pollock (1994) for examples and a discussion of the benefits of transient particle tracking.