Unit 9a. Urbanization and Pollution



The goal of this lab is to explore the relationship between urbanization, land use change, and pollution using STELLA. Runoff from urban areas is a very large source of water pollution. There are two types of pollution: non point source pollution, and point source pollution, which occurs from a specific source such as the effluent pipe of a waste water treatment plant.


Figure 9.a.1

Urban pollution


In this lab, we will explore non point source pollution, which occurs from runoff over urban and other disturbed areas. The most common form of non point source pollutants are sediment and nutrients from urban and agricultural runoff; non point source pollutants also include pesticides, pathogens (bacteria and viruses), salts, oil, grease, toxic chemicals, and heavy metals. These wash into water bodies from agricultural land, and other areas of disturbance. Non point source pollution results in beach closures, destroyed habitat, unsafe drinking water, fish kills, and many other severe environmental and human health problems (EPA 2005).


The Clean Water Act of 1972 helped clean up of many of our country's waters. However, approximately 36 percent of the Nation's surveyed river miles, 37 percent of its surveyed lake acreage, and 37 percent of its surveyed estuarine square miles are not safe for basic uses such as swimming or fishing to to high pollution levels (EPA 2005). To protect surface and ground water quality, urban development must be guided by plans that limit runoff and reduce pollutant loadings.


How Urban Areas Affect Runoff

(Excerpt taken from EPA 2005)


Increased Runoff

The porous and varied terrain of natural landscapes like forests, wetlands, and grasslands trap rainwater and snowmelt and allow it to slowly filter into the ground. Runoff tends to reach receiving waters gradually. In contrast, impervious surfaces in urban landscapes like roads, bridges, parking lots, and buildings prevent filtration into the ground. During a storm event, water accumulates, then quickly runs off in substantial quantities.


Cities generally have storm sewer systems that rapidly channel this runoff from roads and other impervious surfaces. Runoff gathers speed once it enters the storm sewer system. When it leaves the system and empties into a stream, large volumes of quickly flowing runoff erode stream banks, damage streamside vegetation, and widen stream channels. This results in lower stream water depths during non-storm periods, higher than normal water levels during wet weather periods, increased sediment loads, and higher water temperatures.


Increased Pollutant Loads

Urbanization also increases the variety and amount of pollutants transported to receiving waters (streams, rivers, lakes and estuaries). Sediment from development and construction contain toxic substances such as oil and grease; nutrients and pesticides from turf management and gardening; viruses and bacteria from failing septic systems; road salts; and heavy metals. Increased pollutant loads can harm fish and wildlife populations, kill native vegetation, foul drinking water supplies, and make recreational areas unsafe.


Model Urbanization and Pollution with STELLA

Create a working STELLA model of urbanization effects on the pollution levels in a hypothetical lake. First, determine the number of stocks in the pollution model and place them on your STELLA workspace. Next, make the relevant connections between the stocks using the flow symbols. Make sure that your flows are actually connected to your stocks. (To bend flow arrows, hit the Shift key where you want to insert a "kink" in the flow.) Then add the appropriate converters and connectors. When the model structure has been fully laid out, assign the values given below. Switch to modeling mode by turning the Globe into the X2 icon.


Change the Run Specs so that the simulation runs from 0 to 30, using a time step (DT) of 0.25 years. This shows what will happen in your model over a 30-year period. You are now ready to explore the effects of urbanization and land use change on lake pollution levels. Create a numeric display  for each stock in your model and determine how pollution changes over 30 years as urban land area increases and forested land area decreases proportionally.


Hint: Use curly brackets {} to keep track of units while modeling in STELLA


Stock #1: Urban Land Area

Initial Value = 5 {sq miles}





Stock #2: Forested Land Area

Initial Value = 100 {sq miles}





Stock #3: Pollution in Lake

Initial Value = 0 {kg/yr}











Question 9.a.1

How did Urban Land Area, Forested Land Area and Pollution in Lake change over the 30year time period of your simulation?


Figure 9.a.1

Child collecting water from drainage ditch


Question 9.a.2

Research urbanization rates around the world using the internet or the Inquries.mxd ArcGIS database. Are the results of Urban Land Area change realistic for rates of urbanization currently happening in countries around the globe? What trends are expected in the next 30 years? Write a paragraph describing the trends in current and future urbanization and its potential costs and benefits.


Question 9.a.3

Why is Forested Land Area connected to Lake Water Purification? What role do forests play in purification and protection of receiving waters?


Question 9.a.4

What is the difference between point source and non point source pollution? Add a source of point source pollution to your model. For example, add a paper factory, which released point source pollution into your lake. What happens to the pollution levels in the lake?


Question 9.a.5

How might preserving greenbelts or green-spaces improve pollution levels in your system? Research some real cities that have implemented greenbelt and green-space initiatives using the internet. What impacts do they have?


Question 9.a.6

What assumptions are we making in this model? Are they realistic?







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