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Dissolved Oxygen

Dissolved oxygen is the term for atmospheric oxygen that becomes mixed in water and occurs between the water molecules. The presence of oxygen in water is good. Dissolved oxygen is necessary for healthy lakes and rivers. Most aquatic plants and animals need oxygen to survive. Fish will drown in water when the dissolved oxygen levels get too low. The absence of dissolved oxygen in water is a sign of possible pollution.

Most dissolved oxygen gets into the water from contatct with the atmosphere. Waves on lakes and slow-moving rivers, water tumbling over riffles or waterfalls on fast- moving rivers mixes oxygen into the water. Anything that increase the surface contact of water and the atmosphere will increase oxygen in the water. Plants and algae also add oxygen to the water through photosynthesis. Because plants need light for photosynthesis, dissolved oxygen levels tend to be highest in the late afternoon and lowest at dawn.

Temperature has a very big affect on oxygen levels. In order to determine a Water Quality Index, it is necessary to determine the Percent Saturation. DO percent saturation It may seem strange, but cold water holds more dissolved oxygen than warm water. Think about it this way. If you opened two cans of pop and placed one in the refrigerator and left one at room temperature, which do you think would lose its fizz first? In the winter, dissolved oxygen levels are usually higher than in summer. That is why fish kills usually occur in late summer just before dawn.

Climate can affect oxygen levels in other ways. During dry seasons water levels decrease and the flow rate or discharge of a river is lower. As the water moves slower, it mixes less with the air; and the dissolved oxygen level goes down. During rainy seasons oxygen levels tend to be higher.

The main man-made factor causing dissolved oxygen levels to change in a negative way involves the build-up of organic wastes. Organic wastes are the remains of any living or once-living thing. Leaves, grass clippings, dead plants or animals, and sewage are examples of organic wastes that can lower oxygen levels in water. Organic wastes are decomposed by bacteria which take oxygen out of the water. When people dump organic wastes into lakes and streams it causes dissolved oxygen levels to decrease which can harm the aquatic life.

When dissolved oxygen levels get lower, they can cause major changes in the types and amounts of aquatic organisms found living in the water. Species that need high levels of dissolved oxygen such as mayfly nymphs, stonefly nymphs, caddisfly larvae, pike, trout, and bass will move out or die. They will be replaced by organisms such as sludge worms, blackfly larvae, and leeches which can tolerate lower dissolved oxygen levels.

Dissolved Oxygen Protocols Dissolved oxygen levels can be determined using a variety of tests. AccuVac checks, electronic monitoring, and drop titration are just a few of the methods available. Most tests involve a colormetric change.

Listed below are the instructions for using the Hach Company's OX-2P model 1469-00. It uses the Winkler drop titration method for determining DO amounts.

Remove the stopper and rinse the dissolved oxygen (DO) sample bottle (round, 60ml glass bottle with stopper) with water to be sampled. Empty the bottle.
Immerse the DO bottle beneath the river's surface away from the shore. Use gloves to avoid contact with the river.
Tilt the bottle upstream to fill and allow the water to overflow for two to three minutes. (This will make sure air from the bottle will not be mixed with the stream water.)
Stopper the bottle quickly making sure that you DO NOT TRAP ANY AIR BUBBLES. (The best way to do this is to place the sample bottle on a level, flat surface and then carefully drop the stopper into place.)
Use the clippers to open one Dissolved Oxygen 1 reagent silver foil packet and one Dissolved Oxygen 2 reagent white foil packet. Remove the stopper. Add the contents of each of the packets to the water sample.
Stopper the bottle carefully. MAKE SURE THERE ARE NO AIR BUBBLES. Grip the bottle and stopper firmly. Shake the bottle vigorously to mix. A flocculate (floc) precipitate will form if oxygen is present. The precipitate will be brownish-orange in color. A small amount of powdered reagent may remain stuck to the bottom of the bottle. This will not affect the results.
Allow the sample to stand until the floc has settled about halfway in the bottle. Shake the bottle again. Let it stand once more until the upper half of the sample is clear.
Use the clippers to open one Dissolved Oxygen 3 reagent powder pillow (found in the large round plastic container). Remove the stopper and add the contents of pillow 3. Carefully restopper the bottle. Check to make sure there are no air bubbles. Shake to mix. The floc will dissolve and an amber color will develop if oxygen is present.
Fill the small round plastic measuring tube to the top with the amber colored sample water. This tube measures the correct amount of sample to be tested. Put the stopper back on the DO bottle with the extra amber colored sample water and set it aside. This extra sample water will not be used.
Empty the tube of sample water into the square mixing bottle. Place the bottle on top of a white piece of paper so that the color change shows up better.
Unscrew the top of the brown sodium thiosulfate bottle. Fill the eyedropper with the sodium thiosulfate. Hold the dropper straight up- and-down over the square mixing bottle and add one drop at-a-time into the amber colored water sample in the square mixing bottle. Swirl the bottle after each drop. Count the number of drops it takes to turn the water sample from amber to clear.
Record the number of drops it took to turn the sample in the square mixing bottle to clear. Each drop is equal to 1 mg/1 of dissolved oxygen. For example, if it takes 8 drops to turn the sample clear, then there are 8 mg/1 of dissolved oxygen in the sample.
When finished, pour the treated samples into the waste water bottle. Rinse the bottles and tube. Put everything back into the DO kit and return the kit to its container.

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