GEOG 110 – Lab #6 – Modeling Surface Water Contamination
Due Date: 11:59pm, Friday, December 9, 2005
Objectives: Familiarize yourself with the dynamic dissolved oxygen model described in Chapter 5 of the course text, construct a model that makes use of the mathematical relationships and system structure as they have been described, and model surface water contamination in the system, such that you can examine the impact of adding a wastewater treatment facility to the system.
Background: Managing surface water contamination is an activity that is critical to human life in the modern age: As we live in greater and greater concentrations in urban environments, we are faced with the dual problems of maintaining a safe drinking water supply upstream of the population, while still managing and disposing of wastewater downstream in a fashion that both does not adversely affect water quality and unreasonably increase the risk of contamination for those that live further down the river. As you might guess, addressing such a problem can be accomplished through a simulation approach, as contamination in a surface water system can exhibit complex behavior. We need some means of assessing the likely impact of management decisions before we decide where to build a wastewater treatment plant, and modeling can provide that means.
Resources: This lab exercise does not include the background and theory required for you to understand contamination in a surface water system. For that information, you should look to material from the lectures and the entirety of Chapter 5 (pp. 113-126) of the course text, paying special attention to Sections 5.2.2 through 5.3 where the system structure and mathematical relationships are described. At this point, you should have already successfully completed five labs, and you should be getting very comfortable with using STELLA, and building models in STELLA to represent systems based solely on a description of that system. You can refer back to the course text’s background material on perturbation experiments and sensitivity analysis (Section 3.5, pp. 77-87), if you should choose to use these approaches to examining the behavior of the model you create. This lab continues the trend in the lab sequence, where each successive lab’s procedure becomes a little less step-by-step, and a little more open-ended. You have already constructed one model from first principles in Lab 5, and you will need to do the same in this lab, using the background on contamination in surface water systems in the text to guide you through the process.
Procedure: Be sure to read Chapter 5 before attempting this lab, in order to familiarize yourself with this model’s components and how they function.
1. Get STELLA started from the Citrix server.
2. For this lab, we are going to use a model that is substantially similar to the one depicted in Figure 5.5 on page 122 of the text.
In order for you to best understand the system, you might want to try constructing this model yourself, based on the system’s structure and mathematical relationships as they are described in the text. You can check your work against the Chap5a.stm model provided on the disc that accompanies the textbook.
3. Let us begin by examining the model in its basic form, initially using the parameters provided on page 124 of the text, and then perform a sensitivity analysis (3 runs, varying the initial value by 50%) on each of the six specified parameters to get a sense of how the system behaves when those parameters are changed. Run through the sensitivity analyses to get some sense of how the system functions, describe them in your lab report and answer question 1) in the exercises below.
4. Now we are ready to examine the impact of adding a wastewater treatment plant to the system. The most straightforward way to accomplish this without modifying the model structure would be to assume that the treatment plant effluent will mix with river water, and use some data about the characters of the sources of water being mixed to arrive at some new initial values of DO and BOD for our model. The course text provides the required values on pp. 125-126. Use the mass balance equations provided in Chapter 5 along with the provided data to arrive at the new initial DO and BOD values that we will use for our simulation that includes the wastewater treatment plant. Answer question 2) in the exercises below, showing your calculations that you use to arrive at your new DO and BOD values. Based on the distance and velocity to determine the simulation time.
5. Now, run the simulation again, keeping all the other values as they were set previously, and change the DO and BOD initial values to those from your calculations in order to incorporate the presence of the wastewater treatment plant into the system. Repeat the sensitivity analyses you did in step 3 above (each of the six parameters, 3 runs, +/- 50%), using the new values of DO and BOD as a starting point for those variables and the current values for the others (remember despite the fact that only the initial values of DO and BOD changed, other parameters may display different behavior across their range), and answer questions 3) and 4) below, noting the changes in model behavior now that the initial DO and BOD reflect the presence of the wastewater treatment plant.
Exercises: In addition to providing the full lab report format described below, also answer the following questions in your write-up:
1) If you change a parameter and then let the model continue to run, does the DO value move towards equilibrium point? Is it a “stable” model? Be sure to answer this question for every parameter you examine, and be sure to examine any and all parameters that might have an impact on the stability of the model. For each simulation (or group of simulations if you use sensitivity analysis) that you ran, explain what you did and what your results were. Is there any variable that seems to “drive” this system?
2) Using the mass balance equations provided in Chapter 5, and the data regarding the characteristics of both the river and wastewater treatment plant, calculate the new DO and BOD values we will need to model the system once the wastewater treatment plant has been built. Be sure to show your work!
3) Repeat the analyses you describe in question 1), using the new DO and BOD values that reflect the presence of the wastewater treatment plant in the system. Compare and contrast the behavior you see in the system now with that which you observed before you modified the values to include the wastewater effluent. Does the presence of the effluent shift the model output into a mode of behavior that is entirely different from that you observed previously or not?
4) What will the DO concentrations of the water be at the time that the discharge-river mixture reaches the fishing location 40 kilometers downstream? Are these concentrations on a downward, upward, or flat trajectory (i.e. can we expect the problem to get better, keep getting worse, or remain the same as the waters move downstream)?
What to do: Follow the procedures described above, saving your version of surface water pollution model in your course directory (e.g. mine would be davidten-lab6.stm). Write a lab report that includes the elements specified below, saving it in a Word document in your course directory, following the same naming convention as the model (e.g. mine would be davidten-lab6.doc).
The following should be included in your lab report, as has been required in previous reports. Be sure to include a complete section describing the models and the various ways in which you used them to examine the system behavior and answer the exercises. Do NOT repeat the same thing over and over when some aspect of the model remains unchanged between successive simulations, but DO point out the places where you do change the model between simulation runs and describe the changes fully:
1. Objectives
2. Models
2.1 A brief description of the model
2.2 The systems diagram with each component properly labeled
2.3 The system components:
a) Reservoirs:
b) Processes:
c) Converters:
d) Relationships: Define them mathematically, e.g. Births=Birth_rate*Population
2.4 Rate Equation for each reservoir: identify the impact of each converter on the rate equation.
3. Results: Put the model results in your lab report and provide a brief explanation for the model behavior as see in the graph.
4. Discussion: Answer the questions in the exercises in a Word document that you’ll also save in your course directory, following the same naming convention as the model (e.g. mine would be davidten-lab6.doc).
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