Waterwatch Australia national technical manual

Module 4 - physical and chemical parameters
Waterwatch Australia Steering Committee
Environment Australia, July 2002
ISBN 0 6425 4856 0

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Methods (continued)

Dissolved oxygen

What is it and why does it matter?

Dissolved oxygen: a measure of the quantity of oxygen present in water (it has nothing to do with the oxygen atoms within the water molecules)

Oxygen is essential for almost all forms of life. Aquatic animals, plants and most bacteria need it for respiration (getting energy from food), as well as for some chemical reactions.

The concentration of dissolved oxygen is an important indicator of the health of the aquatic ecosystem. Persistently low dissolved oxygen will harm most aquatic life because there will not be enough for them to use.

In some circumstances, water can contain too much oxygen and is said to be supersaturated with oxygen. This can be dangerous for fish. Supersaturated conditions occur in highly turbulent waters in turbines and at spillways, because of aeration, and also on sunny days in waters experiencing algal blooms or with many aquatic plants, because of photosynthesis. In this supersaturated environment, the oxygen concentration in fishes' blood rises. When the fish swim out into water that has less dissolved oxygen, bubbles of oxygen quickly form in their blood, harming the circulation.

What factors affect dissolved oxygen?

The air is one source of dissolved oxygen, and aquatic plants, including algae, are another. The speed at which oxygen from the air enters and mixes through a waterbody depends on the amount of agitation at the water surface, the depth of the waterbody and the rate at which it mixes itself. As water temperature rises, oxygen diffuses out of the water into the atmosphere.

Shallow flowing waterways usually have high dissolved oxygen concentrations. In still waters, such as lakes, dissolved oxygen concentrations often vary from the surface to the bottom, with little dissolved oxygen in the deep, poorly mixed, layers.

Warm or saline water holds less dissolved oxygen than cold water or freshwater (see Table 4.6).

Table 4.6: Effect of conductivity and temperature on potential dissolved oxygen concentrations (mg/L) in waters at sea level
Temperature (°C) Conductivity µS/cm (salinity mg/L)
 
0
14400
28800
43200
57800
 
(0)
(9000)
(18000)
(27000)
(36000)
0
14.6
13.7
12.9
12.1
11.4
5
12.8
12.0
11.3
10.7
10.1
10
11.3
10.7
10.1
9.5
9.0
15
10.1
9.5
9.0
8.5
8.1
20
9.1
8.6
8.2
7.7
7.3
25
8.2
7.8
7.4
7.1
6.7
30
7.5
7.2
6.8
6.5
6.2
35
6.9
6.6
6.3
6.0
5.7

Dissolved oxygen concentrations change with the seasons, as well as daily, as the temperature of the water changes. At very high altitudes, the low atmospheric pressure means dissolved oxygen concentrations are lower. For example, at 1850 metres above sea level, the amount of dissolved oxygen in the water, in absolute terms (mg/L), will be only 80% of the amount at sea level in otherwise identical conditions.

Deep muddy lowland rivers, which contain more organic matter than upland streams, are likely to have lower dissolved oxygen concentrations than upland streams because bacteria are using the oxygen to break down the organic matter. Likewise, dissolved oxygen is usually lower than normal after storms have washed organic materials into any waterbody.

Aquatic plants photosynthesise during daylight and increase dissolved oxygen concentrations around them.

Figure 4.9 shows a hypothetical daily cycle for dissolved oxygen concentrations for both a river with much plant growth in it (eutrophic) and a normal river.

Figure 4.9: Variation of dissolved oxygen in a river with much plant growth compared with a clear river*
Figure 4.9: Variation of dissolved oxygen in a river with much plant growth compared with a clear river. *Few Australian rivers are 'clear' because of heavy turbidity.

In summary, dissolved oxygen concentrations are affected by:

Source: Waterwatch Queensland Technical Manual 1994

Suggested methods, equipment and reporting

Oxygen concentrations are expressed as milligrams per litre (mg/L), but percentage saturation (% sat) allows direct comparison between results from sites with different salinity and temperature values. However, remember that warmer water, even when it is 100% saturated, will have less absolute oxygen dissolved in it than cooler water at the same percentage of saturation.

Dissolved oxygen is best measured on-the-spot in the field, with a meter and probe. Meter readings are not affected by contamination or colouring in the water.

Alternatively, dissolved oxygen can be measured by Winkler titration of water samples fixed in the field. Titration can be completed using an eye-dropper (sensitivity = 1 mg/L), a syringe (sensitivity = 0.2 mg/L) or a digital titrator (sensitivity = 0.1 mg/L). The method you choose depends on the data quality you need.

It is important to record the time of day on your results sheet when you sample or make a field test for dissolved oxygen because of the increase and decrease in concentrations over 24 hours. It is best to try and make measurements at the same time each day.

If testing in estuaries, be aware of the tidal flow that may carry contaminants upstream from discharge points. When measuring dissolved oxygen in saline water, adjust the percentage saturation concentration according to your conductivity measurement.

Dissolved oxygen meter and probe

A dissolved oxygen meter is an electronic device in which oxygen diffuses across a membrane in a submerged probe, to complete an electrical circuit. It records the dissolved oxygen concentration in milligrams per litre or percentage saturation. Most meters also measure temperature. The advantage of this type of meter is that you can measure directly in the waterway.

Equipment

The equipment you will need for this method includes:

Procedure

  1. Turn the meter on and allow 15 minutes for the meter to reach equilibrium before calibrating.
  2. Calibrate the meter before each use, according to the manufacturer's instructions. It can also be checked against readings from the Winkler method.
  3. Place the probe in the stream below the surface (about wrist depth).
  4. Set the meter to measure temperature and allow the temperature reading to stabilise. Record temperature reading on a water quality results sheet.
  5. Switch the meter to read 'dissolved oxygen'. Record dissolved oxygen on the water quality results sheet.
  6. If testing saline waters, measure the electrical conductivity level and record on the water quality results sheet as well.
  7. Re-test water to obtain a field replicate result.

Calibration

Be sure to calibrate the meter according to the manufacturer's instructions, before each use. The calibration values for temperature and altitude should be printed in the manufacturer's instructions.

Calculating percentage saturation of dissolved oxygen

Refer to Figure 4.10.

Figure 4.10: Percentage saturation dissolved oxygen graph.
Figure 4.10: Percentage saturation dissolved oxygen graph (see below for instruction of how to use this graph).
  1. Mark your water temperature (°C) on the upper scale.
  2. Mark the water's concentration of oxygen (mg/L) on the lower scale.
  3. Hold a ruler between the two points.
  4. The point where the ruler crosses the middle scale is the % saturation.
  5. Record this result on the water quality results sheet.

For saline water samples (>1500 µS/cm) you need to know:

Use Table 4.6 to establish the potential dissolved oxygen:

Finally:

Winkler method

The Winkler method involves titrating a carefully taken and fixed sample. Titration involves the drop-by-drop addition of a reagent that neutralises the acid compound and causes a change in the colour of the solution. The point at which the colour changes is called the 'endpoint' and it indicates the amount of oxygen dissolved in the sample.

The sample can be fixed and titrated in the field at the sample site. It is also possible to fix the sample in the field and do the titration in the laboratory within 24 hours of sampling.

The low cost of this type of dissolved oxygen field kit is attractive if you are relying on several teams of samplers to sample a number of sites at the same time.

Equipment

The equipment you will need for this method includes:

Biological oxygen demand (BOD)

BOD is a measure of the amount of oxygen used by biological and chemical processes in a sample of stream water over a 5-day period. BOD bottles have tapered necks and a ground glass stopper or a special plastic lid.

Procedure

  1. Use glass BOD bottles which are free of contaminants. Rinse your sampling bottle in the water you are testing.
  2. Fill the bottle directly from the stream if it is wadable or accessible by boat, or use a deep-water sampler that is dropped on a line or extension pole from a bridge or boat (ask your Waterwatch coordinator about deep-water samplers).
  3. To stop surface scum entering the bottle, leave the lid on the sample bottle until the bottle is below the surface.
  4. Turn the bottle on its side and lower it into the water until the surface of the water is up to your wrists.
  5. When the bottle is below the surface, slowly remove the lid allowing the water to enter.
  6. Carefully turn the bottle vertically the right way up while it is below the surface to allow it to completely fill and release all trapped air.
  7. Recap the bottle while it is underwater, keeping it still - do not agitate the sample.
  8. Remove the bottle from the water and invert the bottle slowly to check that no bubbles have been trapped inside.
  9. When filling the dissolved oxygen bottle, take a water temperature reading at the same time and place.
  10. Repeat steps 1 to 9 to collect a replicate sample.
  11. Number and label both bottles and record the bottle numbers on the water quality results sheet.
  12. Follow the manufacturer's instructions for testing both samples for dissolved oxygen (see also, Testing in the laboratory below).
  13. If you are taking the sample to the lab for titration, you can store the fixed sample in the dark for up to 24 hours before completing the test.
  14. Record dissolved oxygen (mg/L), temperature (°C) and electrical conductivity (mS mg/L). Determine % saturation by using Figure 4.10.
  15. Record your readings on the water quality results sheet.

Testing in the laboratory

  1. To the sample collected in a 250 or 300 mL bottle, add 1mL of manganous sulfate (reagent no. 1) and 1 mL alkaline iodide azide (reagent no. 2). Mix by inverting the bottle until a precipitate forms and settles.
  2. When about half the bottle volume is occupied by clear liquid above the precipitate, add 1 mL of strong acid, usually concentrated sulfuric acid (reagent no. 3). Re-stopper the bottle and invert it several times until the precipitate has all dissolved. This step completes the 'fixing' of oxygen in the sample. The sample turns a yellow-brown colour, because free iodine has been released in it.
  3. Finally, titrate the sample (corrected for the volume lost by adding reagents) by adding sodium thiosulphate until the colour is that of pale straw. Then add a starch indicator and continue to titrate until the blue colour first disappears.

Maintenance

Safety with chemicals

The Winkler titration test uses a number of potentially hazardous chemicals so take care that chemicals are not flicked into eyes or spilt onto skin or clothes - wear safety glasses and rubber gloves. When testing, place the liquid waste bottle, paper towels and a squirt bottle of deionised water nearby, ready to wash or wipe off any chemicals that get onto skin or clothing or into the eyes or mouth.

Manganous sulfate (reagent no. 1) can irritate eyes and skin.

Alkaline potassium iodide azide (reagent no. 2) can cause severe burns, and is poisonous if swallowed.

Sulfuric acid (reagent no. 3) will cause severe burns, ingestion may be fatal, and inhalation can cause coughing and chest problems.

Data confidence

Interpreting your results

Increases in conductivity (salinity) reduce the maximum dissolved oxygen concentrations in water. For example, at 20°C and 7.3 mg/L dissolved oxygen, fresh water is 80% saturated but seawater is 100% saturated.

Dissolved oxygen concentrations should not fall below the 20th percentile of values typical for a waterbody in your region (ANZECC/ARMCANZ 2000). You may like to discuss this trigger value with your Waterwatch coordinator. A dissolved oxygen concentration of 2 mg/L will not support fish, and dissolved oxygen concentrations below 3 mg/L are stressful to most aquatic animals. At least 5-6 mg/L are required for fish growth and activity. Daytime concentrations of 6 mg/L are cause for concern as dissolved oxygen will decrease overnight.

Safety and waste considerations when measuring dissolved oxygen

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