CASE STUDY: Dissolved Oxygen in a River

Source:

Principles of Surface Water Quality Modeling and Control, Thomann, Robert, and Mueller, John, Harper Collins, 1987.

Background:

Dissolved oxygen is a measure of the amount of oxygen that is found in bodies of water, and is a key indicator of the "health" of a water resource. DO values below 5.0 mg/liter indicate a fundamentally unhealthy river or lake -- fish die, the water has a noticeable odor, and the water is not drinkable. Dissolved oxygen levels are influenced by five factors:

Oxygen absorbed from the atmosphere that is produced by the photosynthesis process
Oxygen absorbed from the atmosphere that comes from natural re-aeration (strongly impacted by turbulence in moving bodies of water, such as rivers. Turbulence increases the opportunity for water and air to mix).
Oxygen that is used up by organic pollutants, primarily sediment or sewage dumped into the river.
Oxygen used up by the respiration of organisms, such as fish.
Excess oxygen that is released to the atmosphere.

Modeling Task:

Build a computational model that evaluates changes in dissolved oxygen levels in a river over a 48-hour period. Once your model is operational, experiment with various input parameters to help in your understanding of the causal effects of pollution and turbulence on the health of a river.

For this model, two of the factors above increase the level of dissolved oxygen, and three reduce the level. You will need several equations for this model:

Average daily O2 produced by photosynthesis = -5sin(2p/24*Time-of-day) + 6.7

For this equation, you will need to use three built-in functions: TIME, SIN, and PI. They can be found alphabetically under the Built-In Functions list.

Oxygen released to atmosphere = IF DO_in_River > 9 THEN (DO_in_River - 9)/DT else 0

You will need to use the built-in function DT for this equation

Sewage load and turbulence are measured by using a relative scale -- from 1 - 7 (low to high) for sewage load, and 1-6 (calm water to Class IV whitewater) for turbulence. Depending on the value chosen a DO "load" is chosen from a graphical input:

Turbulence	DO Load
1	0.6
2	2.0
3	3.0
4	7.77
5	23.0
6	37.0

Sewage Load	Oxygen Used by Sediment or Sewage
1	0.07
2	0.5
3	1.5
4	1.5
5	4.0
6	7.0
7	8.0

Other beginning parameters are as follows:

Initial DO in river = 6.7

Respiration by organisms = 2.8

Sewage Load = 4

Turbulence = 3

Developed by
Graphic of Shodor Logo