Chemistry, Third edition

WATER POLLUTION

Importance of dissolved oxygen in water

The dissolved oxygen concentration in water is vital for the support of fish. The

lower the level, the more polluted is the sample. The dissolved gas is used by micro-

organisms to oxidize organic matter in sewage. Unless the water can restore its dis-

solved oxygen concentration – for example by the turbulent flow of shallow waters –

the water will no longer support many organisms. Typically, fish growth is inhibited

below a dissolved oxygen concentration of 6 mg dm^3.

It is not generally realized how little oxygen is present dissolved in a sample of

‘pure’ water. A litre of water at 5 °C, in free contact with the atmosphere, contains only

about 9 cm^3 of oxygen, weighing 13 mg. As the temperature rises, the oxygen concen-

tration falls so that, at 20 °C, it is only about two-thirds of the level it was at 5 °C (see

page 185).

Oxygen reaches the water in two main ways. First, it dissolves at the surface from

the atmosphere. Still water takes up oxygen slowly, whereas turbulent water takes it

up more rapidly since bubbles are often submerged. The second source of oxygen in

water is from photosynthesis. Where there are many green plants present, the water

often becomes supersaturatedwith oxygen during the hours of daylight. However,

after dark, photosynthesis stops but the plants continue to respire and actually

reduce the amount of dissolved oxygen. Therefore, during a 24 hour period, some

waters have a considerable range of dissolved oxygen levels. A single determination

is therefore of little value in assessing the general condition of a body of water. Sev-

eral determinations, at various depths, locations and times need to be made.

The amount of oxygen used up in microbial oxidation is – the biochemical oxygen

demand(BOD) – another important water-quality indicator. The BOD is taken as a

realistic measure of water quality – a ‘clean’ river would have a BOD value of less than

5 ppm, whereas a very polluted river could have a BOD value of 17 ppm or more.

A BOD determination takes a few days, so another parameter called the chemical

oxygen demand(COD) is sometimes measured. In a COD determination, acid

dichromate is used to oxidize the organic matter in a sample of water. The measure-

ment takes only a couple of hours.

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Drinking water

Fluoride

Soluble fluoride is often added to drinking water to bring it up to a concentration of

1 ppm or 1 mg dm^3. This concentration is within agreed safety limits and has been

shown to protect teeth against decay.

High concentrations of fluoride are poisonous and are harmful to bones and

teeth at levels over 10 ppm (mg dm^3 ).

Lead

The limit for the concentration of lead ions in drinking water is 50 ppb ( gdm^3 ). A

survey in 1990 found that the ‘first draw’ of water from the taps of a significant pro-

portion of homes in the UK exceeded this limit, mainly because of the use of lead for

pipes, or solder, in areas where the water is relatively acidic. It is a good idea not to

drink the water that has been standing overnight in an older plumbing system, until

you have run out the water for at least a minute.