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UNC-Chapel Hill GEOG 110 - Lab 6 – Modeling Surface Water Contamination

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GEOG 110 – Lab #6 – Modeling Surface Water ContaminationGEOG 110 – Lab #6 – Modeling Surface Water ContaminationDue Date: 11:59pm, Friday, December 9, 2005Objectives: Familiarize yourself with the dynamic dissolved oxygen model describedin Chapter 5 of the course text, construct a model that makes use of themathematical relationships and system structure as they have beendescribed, and model surface water contamination in the system, such thatyou can examine the impact of adding a wastewater treatment facility tothe system.Background: Managing surface water contamination is an activity that is critical tohuman life in the modern age: As we live in greater and greaterconcentrations in urban environments, we are faced with the dualproblems of maintaining a safe drinking water supply upstream of thepopulation, while still managing and disposing of wastewater downstreamin a fashion that both does not adversely affect water quality andunreasonably increase the risk of contamination for those that live furtherdown the river. As you might guess, addressing such a problem can beaccomplished through a simulation approach, as contamination in asurface water system can exhibit complex behavior. We need some meansof assessing the likely impact of management decisions before we decidewhere to build a wastewater treatment plant, and modeling can providethat means.Resources: This lab exercise does not include the background and theory required foryou to understand contamination in a surface water system. For thatinformation, you should look to material from the lectures and the entiretyof Chapter 5 (pp. 113-126) of the course text, paying special attention toSections 5.2.2 through 5.3 where the system structure and mathematicalrelationships are described. At this point, you should have alreadysuccessfully completed five labs, and you should be getting verycomfortable with using STELLA, and building models in STELLA torepresent systems based solely on a description of that system. You canrefer back to the course text’s background material on perturbationexperiments and sensitivity analysis (Section 3.5, pp. 77-87), if you shouldchoose to use these approaches to examining the behavior of the modelyou create. This lab continues the trend in the lab sequence, where eachsuccessive lab’s procedure becomes a little less step-by-step, and a littlemore open-ended. You have already constructed one model from firstprinciples in Lab 5, and you will need to do the same in this lab, using thebackground on contamination in surface water systems in the text to guideyou through the process.1Procedure: Be sure to read Chapter 5 before attempting this lab, in order to familiarizeyourself 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 tothe 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 tryconstructing this model yourself, based on the system’s structure andmathematical relationships as they are described in the text. You cancheck your work against the Chap5a.stm model provided on the discthat accompanies the textbook.3. Let us begin by examining the model in its basic form, initially usingthe parameters provided on page 124 of the text, and then perform asensitivity analysis (3 runs, varying the initial value by 50%) on eachof the six specified parameters to get a sense of how the systembehaves when those parameters are changed. Run through thesensitivity analyses to get some sense of how the system functions,describe them in your lab report and answer question 1) in theexercises below.4. Now we are ready to examine the impact of adding a wastewatertreatment plant to the system. The most straightforward way toaccomplish this without modifying the model structure would be toassume that the treatment plant effluent will mix with river water, anduse some data about the characters of the sources of water being mixedto 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 themass balance equations provided in Chapter 5 along with the provideddata to arrive at the new initial DO and BOD values that we will usefor our simulation that includes the wastewater treatment plant.2Answer question 2) in the exercises below, showing your calculationsthat you use to arrive at your new DO and BOD values. Based on thedistance and velocity to determine the simulation time.5. Now, run the simulation again, keeping all the other values as theywere set previously, and change the DO and BOD initial values tothose from your calculations in order to incorporate the presence of thewastewater treatment plant into the system. Repeat the sensitivityanalyses 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 forthose variables and the current values for the others (remember despitethe fact that only the initial values of DO and BOD changed, otherparameters may display different behavior across their range), andanswer questions 3) and 4) below, noting the changes in modelbehavior now that the initial DO and BOD reflect the presence of thewastewater treatment plant.Exercises: In addition to providing the full lab report format described below, alsoanswer the following questions in your write-up:1) If you change a parameter and then let the model continue to run, does theDO value move towards equilibrium point? Is it a “stable” model? Besure to answer this question for every parameter you examine, and be sureto examine any and all parameters that might have an impact on thestability of the model. For each simulation (or group of simulations if youuse sensitivity analysis) that you ran, explain what you did and what yourresults were. Is there any variable that seems to “drive” this system?2) Using the mass balance equations provided in Chapter 5, and the


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UNC-Chapel Hill GEOG 110 - Lab 6 – Modeling Surface Water Contamination

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