4. Right click grid ( ) and select New MODFLOW, then select Packages, enabling both the optional Well and Recharge option.

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1 Groundwater Hydrology CE Example 1 -- Grid Approach Building a grid is the first step in creating a groundwater model using GMS, and it requires the following components: top and bottom elevations, hydraulic conductivity values, and starting head values. Recharge values and wells can also be added, among other physical features. 1. Open a new file in GMS by selecting File à New. 2. Let s change the units of Length from ft to m in Edit à Units. 3. Right click anywhere inside the left hand portion of the screen (in the project explorer) and select New à 3D Grid. Under the X- and Y-Dimension, change number of cells to 31. Under the Z-Dimension change the length to 30 m and the number of cells to 3. Click OK. (Note: you can change the view in the toolbar next to the save and print commands in the top left corner: ; if you want to rotate the grid, choose Display à General Mode, then either choose the oblique view option or rotate using at the right of the project explorer. You can view different layers of the model using near the top.) 4. Right click grid ( ) and select New MODFLOW, then select Packages, enabling both the optional Well and Recharge option. 5. First we will define the starting heads for each cell. For a steady state simulation (the default in GMS), the simulation will converge quicker if starting head values are chosen wisely. Let s assume a constant head boundary condition both at the top and bottom row of layer 1. To do this make sure the select cells option is on ( ), then simply left click, hold, and scroll over the top row of cells on layer 1. Right click, then select Properties. Change the IBOUND to Specified Head, and enter 30 m for the Starting head for the top row. Repeat the same process for the bottom row and impose a starting head of 24 m. In the project explorer you can also select MODFLOWà Globalà Starting Heads and either manually change the values or import a matrix from excel or Matlab. We have to manually specify the starting head values for all other cells.

2 Under MODFLOWà Globalà Starting Heads, for layer 1, select Constant Grid à Apply to variable head cells only, and input 25 m. Note: if your imposed starting head is lower than the bottom elevation of the layer, you might get an error and it will be harder for the simulation to converge. Your grid should now look like this in Plan View specified head): (where the orange diamonds indicate a cell with 6. Check that the top and bottom elevations are correct in MODFLOW à Global Options à Top Elevation and going through the layers. 7. Specify Horizontal Hydraulic Conductivity in either the project explorer (under LPF à HK) or the LPF Package in the MODFLOW tab on top. Let s set every cell equal to 20 m/day using the Constant à Grid tab. 8. Lastly, we will impose a recharge rate by selecting RCH in the project explorer (Grid à MODFLOW). Select the Constant à Array tab and input 0.01 m/day. 9. We can check for errors before we running the simulation by selecting MODFLOW à Check Simulation. You can run the simulation by selecting in the top middle of the screen. Here s what layer 1 should look like in Plan View after the simulation has run:

3 Note: You can change the contour settings by selecting Display à Contour Options. 10. Now, let s add a well to see how that would impact the head distribution. On layer 1, select the cells with I,J,K coordinates (16,16,1), which is displayed under the grid once the cursor is over the appropriate cell. Make sure the select cells option is on. Right click and select Sources/Sinks à Wells à Add BC (on the bottom), then impose a volumetric flow rate of -500 m^3/day (where the negative sign indicates you are extracting) in the All row. 11. Re-run MODFLOW. Below, the contour options are changed to Color Fill and Linear in Display à Contour Options. You can add labels as shown below in Label Options.

4 12. Now let's change the left and right boundaries to constant head and change the Starting head of the bottom layer. We can do this all in one step, by highlighting the left column, holding Ctrl and then highlighting the bottom row, and then the right column. Right click, then select Properties. Change the IBOUND to Specified Head, and enter 30 m for the Starting head. 13. Re-run MODFLOW and see how the boundary conditions changed the head distribution. Flip through the layers and see the distribution for each layer. Layer 1 is shown below:

5 14. Let's impose this set of boundary conditions on the other two layers. Go to MODFLOW à Global Options à Starting Heads. Then left click and then right click on the grey square in the top left corner and go to Copy. Now go to layer 2, left click and then right click on the same grey square and go to Paste. Do the same for layer 3. Next go to MODFLOW à Global Options à Ibound. Go through the exact same process of copying the values for layer 1 onto layers 2 and Now we're going to replace the well we have with a well screened the entire thickness of the aquifer. First, right click on the cell that the current well is on and go to Sources/Sinks à Delete All BCs. Now go to layer 3, right click on cell (16,16,3) and go to Sources/Sinks à Wells à Add BC (on the bottom), then impose a volumetric flow rate of -500 m^3/day in the All row. This well is screened on the bottom of layer 3. Now for each of cells (16,16,3), (16,16,2), and (16,16,1) we are going to change the vertical anisotropy to.001 (effectively creating a well that is screened the entire thickness of the aquifer by letting the vertical hydraulic conductivity be 1000 time the horizontal hydraulic conductivity). In order to do this, right click on the cell that the well is in (16,16,3) and go to Properties à VK/VANI and type.001. Now you can either do the same thing for the other two cells, or you can go to MODFLOW à LPF à VK/VANI and copy the data from layer 3 onto layers 1 and 2 as we did before with the Ibound and Starting heads. 16. Re-run MODFLOW again. Now all of the layers will have head distributions similar to that of layer 1 from the last run.

6 GMS Homework 1 In this project, you will use the grid you built in class to study three different types of heterogeneity. In all 3 grids 1/3 of the cells will have hydraulic conductivity values of 8 m/day, 1/3 with 20 m/day, and 1/3 with 50 m/day. You will build grids with horizontal layers, vertical layers, and no layering (random). Download and save the files 'verthk.dat' and 'randomhk.dat' from your (files are also available on the course website). In GMS To begin we want to make 3 copies of the work we did in class, so go to File à Save as to create copies named 'horizlayers', 'vertlayers', and 'random'. Horizontal Layers 1. Open the project file 'horizlayers.gpr' that you just created. Everything in the project explorer on the left side of the screen will be collapsed. 2. Go to 3D Grid Data à grid à MODFLOW à LPF à HK. 3. Change layer 1 to 50 m/day by going to Constant -> Layer and typing in 50. Change layer 3 to 8 m/day. 4. Run MODFLOW Vertical Layers 1. Open the project file 'vertlayers.gpr' that you just created. Everything in the project explorer on the left side of the screen will be collapsed. 2.Right click on Go to 3D Grid Data à grid and go to Import Data Set and open find 'verthk.dat' 3. Go to 3D Grid Data à grid à MODFLOW à LPF à HK. 3. Go to 3D Data Set -> Grid and select grid à verthk 4. Run MODFLOW Random (no layers) 1. Open the project file 'random.gpr' that you just created. Everything in the project explorer on the left side of the screen will be collapsed. 2.Right click on Go to 3D Grid Data à grid and go to Import Data Set and open find 'randomhk.dat' 3. Go to 3D Grid Data à grid à MODFLOW à LPF à HK. 3. Go to 3D Data Set -> Grid and select grid à randomhk 4. Run MODFLOW

7 Questions 1. Which of the simulated aquifers you made do you think is the most like a real aquifer (homogeneous, horizontal layers, vertical layers, or random)? Why? 2. For each grid, find the head at 2 different radii from the well (I recommend using cells (1,16,6) and (1,16,15), but you can use different cells as long as they're neither on the well nor on the boundary and they're in the same layer). You can estimate from the picture, or you can go to 3D Grid Data à grid à 'project name' (e.g. random) à Head. Then click on View Values and find the cell you want and find the head values under the 'f' column. Download the MATLAB file 'GMS_K_check.m' and put in your values to calculate the effective hydraulic conductivity. Note that only the ratio of the radii is used, so you can calculate the radii in terms of cell lengths (e.g. If you used cells (1,16,6) and (1,16,15), then r1 = 1 and r2 = 10). 3. Calculate the arithmetic, geometric, and harmonic means of the hydraulic conductivity values (8, 20, and 50). Which grid corresponds to which average?