Where Does All the O2 Go?

Dissolved oxygen concentrations are affected by factors such as temperature, salinity and dissolved organic matter. Dissolved organic matter enters Puget Sound in

• sewage
• waste water from sewage treatment plants
• urban and agricultural runoff
• wastes from food processing industries, meatpacking plants, and dairies
• wastes from lumber mills, paper plants and other industries, and
• decaying aquatic plants and animals

1. Look at the list of sources for dissolved organic matter. An increase in any of these factors tends to increase/decrease the amount of oxygen the water can hold.
   

   (Circle the word which correctly completes the sentence.)

2. What are two sources of dissolved organic matter in runoff from urban areas?
   

   a.
   
   b.
   
   

3. What are two sources of dissolved organic matter in runoff from agricultural areas?
   

   a.
   
   b.
   
   

4. Each element in the model system you will be using represents a component of Puget Sound. Use arrows to connect the model elements with the components they represent.
   
   

Start by making a milk solution. In a small jar, add two level teaspoons of powdered milk to 40 ml of water. Label this jar milk.
In another jar, make a yeast solution by mixing two level teaspoons of dry yeast with 40 ml of water. Be sure the yeast is well mixed. Stir it again just before you use it. Label this jar yeast.
Now label three clean test tubes 1, 2, and 3 and place them in a test-tube rack.
Using two different syringes, or graduated cylinders, add the amounts of water and milk called for in Table 1 to the three test tubes.
Add 20 drops of methylene blue solution to each test tube and mix thoroughly.
Set up a data sheet like the one shown and RECORD the data and observations from this step. Add four ml of yeast solution to test tube 1, mix it well, and RECORD the exact time. Return the first test tube to the rack and do not disturb it (otherwise, air may get mixed with its contents). Follow the same procedures for the other two tubes. Observe carefully and continuously and RECORD the exact time when any changes in the tubes take place. This part of you experiment may take about 20 minutes.
While you are waiting, think about what you have been doing and make some predictions about what will happen.

1. a.  Complete table 1 by recording the ml. of yeast added and by calculating the percent of milk contained in each of the three test tubes.
   

2. Which test tube contained the most food for the decay organisms (decomposers)?
   
   
   
3. Which tube had the least food?
   
   
   
   

2. Your test tubes may have had a ring of blue color at the surface even after the remainder of the liquid had changed color. What might account for this?
   
   
   
   
   
   
   
3. In which tube did the change occur most rapidly? Is this what you expected? Why?
   
   
   
   
   
   
   
4. Develop an explanation that accounts for the differences in time needed for the change to occur in each of the three tubes.
   
   
   
   
   
   
   

Cover the tops of the tubes and shake them rapidly so that air is mixed with the liquid.

1. Each of your test tubes represented a sewage-waste-Puget Sound-water system. Each one contained a different concentration of sewage (milk). What effect would an increase in temperature have on these mini-systems? How could you find out?
   
   
   
   
   
   
2. How would your systems have been affected if more decomposers (yeast) had been present at the beginning of your experiment?
   
   
   
   
   
   
3. Decay organisms often reproduce rapidly when conditions are favorable and nutrients are plentiful. What do you think would happen to the supply of dissolved oxygen in a river if the decay organisms began to multiply?
   
   
   
   
   
   
4. Sewage usually contains many substances. Try enriching your mini-system with other ingredients. Try soda, sugar, vinegar, baking soda, honey, Karo syrup, etc. For each substance, compare your results with those you obtained with milk.
   
   
   
   
   
   
5. Visit a nearby sewage plant. What kind of sewage does it receive? How does it treat sewage? What happens to the treated sewage?
   
   
   
   
   
   
6. Carry out the above experiment in flasks using four times the amount of each ingredients. Monitor the dissolved oxygen concentration in the flasks. Does the methylene blue correctly indicate the presence or absence of oxygen?

As noted above, Biochemical Oxygen Demand (BOD) is an important measure of the health of Puget Sound waters. The following procedure will enable you to measure BOD in water which you sample.

1. Obtain a sample of the water to be tested. Record the depth at which the sample was taken. Remember, samples taken at or near the bottom may hold more oxygen-demanding materials and organisms. If you will only be sampling at one depth it is best to sample midway between the surface and bottom away from shore to obtain a representative sample. Be sure your sample is large enough to run at least two BOD tests.
   • If you will not be testing for dissolved oxygen immediately, wrap the sample container with foil or black electrical tape to shield the sample from light.

   • Like the Dissolved Oxygen tests, it is important to run all tests for comparison at the same time of day.

1. Fill two glass-stoppered DO bottles (one clear and one black) with sample water, again holding them for two to three minutes halfway between the surface and the bottom if sampling by hand.

2. Stopper the bottles underwater if sampling from shore, ensuring no air bubbles are present in the bottles.

3. Prepare the clear sample bottle according to the directions for the Dissolved Oxygen test. Determine the DO level of this sample.

4. Place the black sample bottle in the dark and incubate for five days at 68 F°. This is very close to room temperature in many buildings. If there is no incubator, place the blackened sample bottle in a "light-tight" drawer or cabinet. After five days, determine the level of dissolved oxygen of this sample by following the steps of the DO testing procedure.

5. The BOD level is determined by subtracting the DO level of the blackened sample from the original DO level found in the first sample above (taken five days previously).

   BOD = dissolved oxygen (original sample) minus the dissolved oxygen (after incubation).  Record Your Answer
   
   
   
   The BOD measure is, the amount of oxygen consumed by organic matter and associated microorganisms in the water over a five-day period.

   BIOLOGICAL OXYGEN DEMAND DATA SHEET
   Test Tube	Time Mixing Started	Time Change Occured	Amount of Time for Change
   1	BR	BR
   2	BR	BR
   3	BR	BR
   Description of change: