Dissolved Oxygen
Oxygen is essential to life in water as it is to life on land. Oxygen availability determines whether an aquatic organism will survive and affects its growth and development. The amount of oxygen found in water is called the dissolved oxygen concentration (DO) and is measured in milligrams per liter of water (mg/l) or an equivalent unit, parts per million of oxygen to water (ppm).
DO levels are affected by:
- Altitude
- Water agitation
- Water temperature
- Types and numbers of plants
- Light penetration
- Amounts of dissolved or suspended solids
As water low in oxygen comes into contact with air, it absorbs oxygen from the atmosphere. The turbulence of running water and the mixing of air and water in waterfalls and rapids, add significant amounts of oxygen to water.
Dissolved Oxygen Level Breakdown
This chart shows the breakdown of water quality classes by dissolved oxygen content. As you can see, fish and other creatures that live in the water need lots of oxygen to live, just like we do
(Source: WAC 173-201A).
Dissolved Oxygen Fact Sheet
State Standards
| Class AA (Extraordinary) |
Exceeds 9.5 mg/l |
| Class A (Excellent) |
Exceeds 8.0 mg/l |
| Class D (Good) |
Exceeds 6.5 mg/l |
| Class C (Fair) |
Exceeds 4.0 mg/l |
(Source: WAC 173-201A-030)
Factors that affect dissolved oxygen concentrations
- Since dissolved oxygen concentrations are directly affected by water temperature, (the warmer the water, the lower the amount of oxygen present), then all factors that affect water temperature affect dissolved oxygen concentrations. (Such as over-grazing and warm water releases from industrial power plants).
- Overgrowth of streamside vegetation will deplete water of the small amount of oxygen present for other aquatic life. This overgrowth can be stimulated by nitrate rich livestock waste and other fertilizer applied to crops.
Dissolved Oxygen Test Instructions
This test measures the amount of oxygen that is dissolved in the water and can be used by aquatic life. Follow these simple steps using the chemicals and containers found in the HACH Kit.
- Step 1:
- Fill the round glass bottle (with the glass stopper) with the water to be tested by holding the bottle in the stream for about 2 minutes. Remove air bubbles trapped in the bottle.
- Step 2:
- Take one plastic pillow of Dissolved Oxygen (#1) chemical from the plastic box. Cut the end of the pillow with the fingernail clipper or scissors and pour the powder into the Dissolved Oxygen Water bottle. Take on of the plastic pillows of Dissolved Oxygen (#2) chemical and add its powder to the water sample. Carefully place the stopper in the bottle to prevent air bubbles from being trapped. To prevent air bubbles, tilt the bottle slightly and quickly insert the stopper. Hold the bottle and stopper firmly. Then shake rapidly to mix water and chemicals. (It's okay if some powder remains on the bottom of bottle after shaking.) A cloudy mixture, called floc or precipitate, will form. If oxygen is present the floc will be orange-brown in color.
- Step 3:
- Allow the water sample to stand until the floc has settled into the lower half of the upper half clear. Again shake the bottle. Again let the bottle stand until the upper half of the bottle is clear. In cold weather this will take longer.
- Step 4:
- Remove the stopper from the bottle. Take one plastic pillow of Dissolved Oxygen (#3) chemical from the container. Cut end with clippers and add the powder to the water. Carefully re-stopper the bottle and shake to mix. The floc will disappear and a yellow color will form if oxygen is present.
- Step 5:
- Take the (PLASTIC) test tube and the SQUARE GLASS MIXING BOTTLE. Fill the PLASTIC test tube up to the top with the treated water sample. Pour this water into the square mixing bottle.
- Step 6:
- Take dropper bottle of Sodium Thiosulfate Standard Solution. Swirl the water sample in the square bottle, then add the Sodium Thiosulfate Solution drop by drop to the water sample. HOLD THE DROPPER VERTICAL. Count each drop. Swirl, add a drop, swirl, add a drop until the water sample changes from yellow to colorless. Pause for at least 2 seconds between each drop.
- Step 7:
- Record the number of drops you had to get a colorless liquid. This will be the PARTS PER MILLION (PPM) of Dissolved Oxygen in the water sample. Record your r reading on the data sheet.
