Title: Date: October 21, 1997
Author: MIT
Last saved by: fredjo

DEPARTMENT OF CIVIL AND ENVIRONMENTAL ENGINEERING
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
1.72 Groundwater Hydrology
Lab #2: Visual MODFLOW

Due Nov. 24, 1998

The purpose of this lab is to explore the fundamentals of well dynamics and the sensitivity of flow systems to input parameters.  This lab will also introduce you to MODFLOW, one of the most commonly used groundwater flow models in the hydrologic community.  MODFLOW is a public domain, 3-D, finite difference solution to the groundwater flow equation.  You may find more information about MODFLOW, as well as the source code, on the web at http://water.usgs.gov/software/modflow96.html.  Everything you will need to complete the lab is provided in this packet and on the class web page at http://web.mit.edu/1.72/www.  Documentation for Visual MODFLOW is on reserve in Barker Engineering Library, and another copy of the manual is in the MEng room (1-143).

You may work on this lab in pairs.  However, each person must hand in their own discussion of the lab, including tables, plots, and/or figures to illustrate your work.  Please indicate your partner’s name on your write-up.  

THE PROBLEM

You will be modeling an area called Remson Gulch, located between a lake (Lake Loague) and a wetland (Wildfowl Wetland), as shown in Figure 1. The Gulch is approximately 1.25 by 1 kilometers in size.   The homogeneous, isotropic, unconfined aquifer in the Gulch has a hydraulic conductivity of 10^-5 meters per second and receives annual recharge of 2x10^-8 meters per second.  The Gulch is flanked on either side by relatively impermeable fractured granite (the Coast Hills and the Grandview Mountains).  The elevation of Lake Loague is 100 meters (msl), and the elevation of the Wildfowl Wetland is 95 meters (msl).  You should assume that seasonal variation in the water table and recharge may be neglected (i.e., this is a steady-state simulation).  The bottom aquifer elevation is approximately 75 meters (msl).  This set of parameters describing the natural system in Remson Gulch will be called the base case scenario.

Your task is to assess the effects of housing development upon Remson Gulch—consider yourself a high-paid consultant (use imagination).  In Phase I of the developer’s plan, 100 new homes will be built.  In order supply water and sewer utilities to these homes, a municipal water supply well will be installed near Lake Loague and a treated wastewater infiltration field will be built near the Wildfowl Wetland.  Neither the water in Lake Loague nor the water in the wetland is safe for drinking, so you want to insure that the well does not draw from either of these sources.  In addition, it would be bad if the water supply well intercepted the wastewater from the infiltration field (although treated, also not good for drinking).  Phase II of the development involves building 100 more homes, and thus requires a larger water supply and greater sewer capacity.  The goals of modeling this area are:

Figure 1: Plan view of Remson Gulch with coarse modeling grid superimposed.  
Each grid cell is 50 m by 50 m.

1.  To understand the sensitivity of the Remson Gulch system to model input parameters,
2.  To assess the feasibility of developing the Gulch according to the proposed plan, and 
3.  To analyze the effects of development of the Wildfowl Wetland, especially in terms of   
     water quality.

MODEL LOGISTICS

1. The software
Visual MODFLOW has been installed on the CEE-NT network, a network of PCs located in building one (1-051, 1-252, 1-341)—including the MEng machines (1-143).  We have 10 licenses for this software, meaning 10 people should be able to use it at one time.   You can start MODFLOW from the start menu by selecting:  
start => Programs => Visual MODFLOW
2. Inputs
Since Visual MODFLOW makes creation of input files so easy, you will be creating them yourself.  However, we have given you a head start.  Download remson.zip from the course web site http://web.mit.edu/1.72/www.  Unzip remson.zip to unleash the full slew of Visual MODFLOW input files.
	3. Outputs
Visual MODFLOW is also a post-processor for MODFLOW.  You may output any figures you create to a file that may be incorporated into an MS Office document (export => .emf) or send them directly to a printer.

Time to get started!

BASE CASE SCENARIO

From the main Visual MODFLOW menu, open the given input files.  This includes a site map and layer surface information (as shown in Figure 1):
	File => new
You will now complete the input files and run a base case simulation.  

•	Import sitemap
After naming your new model files, a window will appear.  Choose create model using base map and select the file sitemap.bmp.  Make your model 25 columns by 20 rows by one layer and enter a zmax of 150.  Hit create.  You will be asked to georeference the bitmap.  Select OK.  Click on the lower left corner of the sitemap and enter the coordinates x_1 = 0, y_1 = 0. Now click on the upper right corner of the sitemap and enter the coordinates x_2 = 1000,  
y_2 = 1250.  In the start point box, enter x = 0, y = 0, angle = 0.  In the model coordinates box, enter x_1 = 0, y_1 = 0, x_2 = 1000, y_2 = 1250.  Select OK.  You should now have a sitemap that looks like Figure 1.

•	Model grid
Select Input from the main menu.  To refine the model grid, select Grid from the input menu.  From the side-menu, you may use edit row and edit column to add, delete, or move grid lines.  If you right-click on your mouse, you may insert evenly spaced gridlines (as we did in the class demo).  Use view column/view row to look at slices through your model.  Now you will add the third dimension to your model.  MODFLOW itself only uses the bottom layer elevation in the simulation, but Visual MODFLOW allows us to keep track of the ground surface too. Select import surface from the side menu.  Use the file surface.asc for the ground surface elevation.  Perform the same procedure for the bottom elevation using bottom1.asc. You may want to adjust the vertical exaggeration (at the bottom of your screen).

•	Inactive cells/no flow boundary
After you are satisfied with the model grid, you need to indicate which model cells are inactive.  This is how geometry is incorporated into the finite difference model (i.e., you indicate which cells do not contain aquifer material—like the mountain areas).  We are assuming that the Coast Hills and Grandview Mountains are “no-flow” boundaries.  From the side menu, select inactive cells.  If you choose mark single from this menu you can “paint” individual cells as inactive with the mouse (you can also play with the other methods of marking cells if you want).

•	Aquifer Properties
Now select Properties => conductivity from the top menu.  Since you have not yet entered any values, a window will pop up asking for default conductivity and storage values.  We are assuming this system is homogeneous and isotropic, so K_x = K_y = K_z = 10^-5 meters per second everywhere in the model domain.  You will also need to provide S_s = 10^-5 m^-1, S_y = 0.2, and     n = n_e = 0.25 even though our simulations won’t be using these values (since we are only running steady-state flow).

•	Boundary Conditions
Set the boundary conditions for the model by selecting Boundaries => constant head from the top menu.  Use the side menu to indicate the constant head boundaries of the wetland (95 meters) and the lake (100 meters).  Use a stop time of 100 days (yes, we are running steady-states, but Visual MODFLOW needs this just in case we change our minds).  Now add recharge over the model area by selecting Boundaries => recharge.  Set the recharge to 630.72 mm/year, set the stop time to 100 days, again this is extraneous information for our purposes.

•	Set up Zone Budget
This handy feature of Visual MODFLOW lets us specify zones in which we’d like to see the fluxes in and out of cells.  This is especially handy if, for example, we wanted to know how much groundwater was flowing into a constant head boundary (like a wetland).  Select Zbud from the top menu.  Visual MODFLOW considers the entire region to be zone 1.  Make two new zones, one for the lake boundary cells and one for the wetland boundary cells.

Now you are ready to run MODFLOW.  Go back to the main menu and select Run.  We are running a steady-state simulation.  Go to layers and designate our single layer as Type 1: Unconfined.  Go to recharge and select recharge is only applied to top grid layer.  For everything else, we will use the default options.  Select Translate/Run and choose MODFLOW and Zone Budget.  Hit the translate/run button.  The numerical engine window will appear and track the progress of the solver.  When it has finished, select exit from the engine window (the one with the nice little engine graphic in it).  You should be back in the main menu now.  Select output to take a look at your results.  You have options of contouring the head and/or showing velocity vectors.  If you select Zbud and then Zbud output, Visual MODFLOW will show you the mass balance for the simulation, and the fluxes into and out of each zone.

Compare your results to the attached base case Visual MODFLOW output (Figure 2).  Notice the groundwater divide in the middle of the Gulch.  [Q1] How much of the recharge flows into the lake?  Into the wetland?  [Q2] Based on the hydrology of the natural (undisturbed) system, do you think the developer’s plan could be feasible?  Make sure that you understand how to run MODFLOW before you continue!

SENSITIVITY ANALYSIS

Analyze the sensitivity of the model to the hydraulic conductivity value, the recharge amount, and the thickness of the aquifer.  Perform each of the runs suggested.  You do not need to produce a plot for each run, but please comment on what you learn about each parameter and answer all questions.

	1.  Hydraulic conductivity 
Run the model with a high K of 10^-4 m/s and then a low K of 10^-6 m/s.  [Q3] How would an order of magnitude uncertainty in the value of K affect your confidence in the model results?  [Q4] What would be a logical upper bound on the height of the water table?  

2.  Recharge 
Run the model with no recharge (this would make the constant head boundaries the only forcing in the model).  Now run the model with a slightly higher  (than the base case) recharge of 693.8 mm/yr and then with an order of magnitude higher recharge of 6307.2 mm/yr. [Q5] Do you notice similarities in the results between changes in K and changes in recharge?  [Q6] What would happen if you increased both K and recharge by an order of magnitude over the base case?

3.  Aquifer thickness 
Run the model with the aquifer bottom elevation at 50 meters.  You can make this change by importing the surface bottom2.asc as the bottom layer elevation of your model.  [Q7] Under what circumstances could we reasonably model this system as a confined aquifer?  [Q8] How do the Dupuit Forcheimer assumptions (used by MODFLOW) affect the position of the water table?

You should now have a fairly good understanding of how sensitive the Remson Gulch system is to our assumed parameter values.  

PHASE I DEVELOPMENT ANALYSIS

Using your base case scenario, now install the pumping well that will supply water to the housing development.  From the input menu, select wells => pumping wells and then Add Well (from the side menu).  When you click the position of the well, a well edit window will appear.  Type in a well name (Visual MODFLOW requires this) and select the screen all button to make the well fully-penetrating.  Under pumping schedule use 100 days for the stop time and input the pumping rate of -259.2 m^3/day.  The minus sign indicates that we are pumping from the well.  You will also need to check that this well doesn’t go dry.  

There are two ways in which a modeled pumping well can go dry.  First, the entire cell around the well can go dry.  This is particularly problematic in MODFLOW because once a cell goes dry, MODFLOW does not allow it to wet again.  A re-wetting package (see the manual) has been developed to handle this problem.  The second way for a well to go dry is for the model cell to have an average water level above the base of the aquifer, but the water level in the real, much smaller well casing drops below the aquifer bottom.  Normally, you would have to perform a hand-calculation to check on this possibility since MODFLOW works with average water levels in each cell.  Fortunately, Visual MODFLOW will take care of this for you.  Select the Use as Obs button.  Another window will appear.  You will have to supply another name.  Select an observation point near the bottom of the well (just to be sure).  You need to enter an observed head value so Visual MODFLOW will provide the calculated value (I used 90 meters).  Exit out of this window and back to the well edit window.  Enter the well radius of 0.457 meters.  Now if you ask Visual MODFLOW to show you the head in the pumping well, it will calculate a water level based on the Theim equation for an unconfined aquifer:

where h_w is the estimated head in the well, h is the head in the model cell, r_e is the radial distance from the well where head is measured, and r_w is the well radius. [Q9] What assumptions in the Theim equation will affect the accuracy of this method in calculating well water levels?

Next tell your model about the wastewater infiltration field.  From the top menu, select Boundaries => Recharge.  Assign a new recharge property to the model cells that cover the infiltration field (about 2,500 m^2).  The recharge for these cells should be 19,552.32 mm/yr (630.7 mm/yr from precipitation plus 18,921.6 mm/yr from wastewater).  We assume that wastewater production is equal to half the water supplied to the housing development.

Run the model.  Comment on changes in the hydraulic gradient and flow rate as compared to the base case scenario.  Provide at least one head contour plot of your results.  The button calc vs obs on the side menu will provide information about the water level at the pumping/observation well. [Q10]  Is there excessive mounding (i.e., above the ground surface) at the infiltration field?  [Q11] What is the steady-state water level in the pumping well?  In the pumping well cell? [Q12] How are flows to Lake Loague and the Wildfowl Wetland altered?  [Q13] Does the Phase I proposed development plan seem feasible?

PHASE II DEVELOPMENT ANALYSIS

Phase II doubles the number of houses in the Gulch, creating a corresponding increase in water demand and wastewater generated.  Thus you want to pump a total of 518.4m^3/day of clean water and dispose of 259.2 m^3/day of wastewater.  The developer has a few ideas about how to accomplish this:

1.  Push it to the limit
Increase the pumping rate at the original well to -432 m^3/day (and introduce severe water conservation laws in the housing development).  Increase the recharge rate over the infiltration field to 32,166.7 mm/yr.  Remember to check for excessive mounding and the well pumping dry.  Comment on how this plan changes the flow rate and hydraulic gradient in Remson Gulch.  Provide a head contour plot for this scenario.
[Q14] Does this scheme seem feasible? [Q15] Can this well be pumped at -518.4 m^3/day?

2.  Add a well
Even your client knows that eventually, the residents of Remson Gulch will demand a larger water supply.  Add the phase 2 well in the location shown on the sitemap.  Pump BOTH wells at a rate of  -259.2 m^3/day. Increase the recharge rate over the infiltration field to 38,473.9 mm/yr. Comment on how this plan changes the flow rate and hydraulic gradient in Remson Gulch.  Provide a head contour plot for this scenario.  [Q16] Does this scheme seem feasible? [Q17] How are the flows to Lake Loague and the Wildfowl Wetland affected?

	3.  Move the well
You suspect that there may be a better location for the new pumping well.  Change the coordinates of the well to (525,1025) in (x,y) coordinates.  Comment on how this plan changes the flow rate and hydraulic gradient in Remson Gulch.  Provide a head contour plot for this scenario.  [Q18] Does this scheme seem feasible? [Q19] How are the flows to Lake Loague and the Wildfowl Wetland affected?

MODEL SOPHISTICATION/COMPLEXITY

Thus far, we have not truly taken advantage of the three dimensional capabilities of MODFLOW.  As it happens, a local wetland conservation society has been protesting Remson Gulch development.  The organization's graduate student geologist has argued that the developer's (your) model is too simplistic.  She believes that based on paleoclimatic evidence, a freakish small inland sea briefly covered a section of the Gulch, leaving behind an incomplete layer of non-conductive sediments.  Modify your model to include a confining layer (K = 10^-9 m/s) in the Northern half of the Gulch.  The easiest way to do this is to split your one layer model into two equal layers, and then add a thin layer between them.  Assign the low conductivity value to half of this layer.  You will also need to determine how best to screen your pumping wells.  Try pumping first from the lower aquifer and then from the upper.  [Q20] How does the introduction of the confining layer affect the way your modeled area responds to development?

FURTHER INVESTIGATION

As a thorough consultant, you will naturally propose additional work to the client (which naturally requires additional resources).  [Q21] Pose a question about groundwater flow and the development of Remson Gulch.  Explore and answer your question by modifying and interpreting your model. 
*   *   *   *   *
Please turn in your comments/observations on all model runs, answers to all questions asked in this lab, and any relevant or illustrative plots.  Don’t forget to label all plots/figures.  You are encouraged to use quantitative information in your analysis.  Brief answers are greatly appreciated.  This lab should not have to be long to be insightful.

HAVE FUN!
(points docked for not enjoying this assignment)

Figure 2: Modeling results for Remson Gulch base case scenario.