Tales of the Riverbank

Some oil-like films, coatings, and slimes, although they may look bad, are natural phenomena. These phenomena can be caused by bacteria such as Thiobacillus ferrooxidans (Iron Bacteria).

Slimes, oil-like films, and rock coatings are often made by bacteria that are reacting to the presence of minerals in the water. Bacteria live in wet areas, including: on the water surface, in the water column, and in the sediment. These bacteria are of no threat to human health and have been involved in the iron and manganese cycles for millions of years. Some bacteria are very useful because they remove harmful materials from water.

Bacteria create oil-like films when they attach themselves to the water surface. Sunlight bounces off the films, giving them an oily appearance. To test the difference between a bacterial film and oil floating on the water, break the film. If the film stays broken, it is a natural bacterial film. If it flows back into place, it is an immiscible hydrocarbon based substance, which indicates pollution.

Bacteria that precipitate copper minerals may make turquoise blue films. Green and purple bacterial slimes may appear when sulphur is present, while white slimes occur in the presence of aluminium, sulphur, or calcium minerals.

In many parts of the country, but especially in areas that are/have been mined for coal, you may seen some watercourses that are bright red or orange.

Whilst the discharge itself is usually crystal clear, the bed of the watercourse below the discharge can be coated in a rusty deposit.

What is this rusty deposit! ?

Well to be frank it is just that! – Iron Oxide.

What causes this and why does the water look clear when it discharges into the watercourse?

In areas where there is coal the ground geology tends to contain high concentrations of Iron (sometimes in the form of Iron Pyrites “Fools Gold”). When coal is extracted, the mineral veins of Iron are exposed to the air.  When groundwater flows through these coal workings, often after they are abandoned, the water will pick up these oxides of Iron in a Ferrous form. Although the water is rich in Iron it can still appear crystal clear at this point.

The Iron rich water, which can be slightly acidic, will then flow out into a watercourse, which is sometimes called “rebound” and once exposed to more oxygen and / or a pH increased in a fast flowing stream, it will rapidly deposit Ferric Iron Oxide.

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What can be done to prevent it?

The impact of these type of discharges can be varied, and in many cases, improvement work is being considered to treat the water and prevent the Iron Oxide being deposited in the watercourse.

If you want more information about Iron Ochre in the UK, you can download a report on Abandoned Mines and the Water Environment here:

our publication:  Abandoned mines and the water environment

If a watercourse is polluted with oil, you will see different visible effects depending on the amount.

Small amounts of oil on a river, canal or lake will appear as a thin dull layer on the surface. Generally, this means that the quantity of oil is very small, but we will still investigate. It is possible that there will be no smell of oil in the air unless you are close to the source of the pollution.

The next visual clue to the presence of oil is a “rainbow” sheen which we call irridescence. This may be visible across all or part of the water. At levels of oil contamination such as this we will install some absorbent booms to collect the floating oil whilst we try to identify the source of the pollution.

Larger amounts of oil will also show as a dull layer, and may be coloured with a bluish or reddish tint if it is Diesel. If you were to throw a pebble into the water, the layer would seperate, briefly showing rainbow colours and then the oil will re-combine. Sometimes there may also be patches of rainbow colours in faster flowing sections of the water. Generally at these levels of oil you will undoubtedly be able to smell it and even identify its type. These oil pollutions are likely to be serious and can cause significant damage to water birds, marginal plants and potentially kill fish.

Lastly, you occasionally may see a dullness and/or rainbow effect on the water which isn’t due to oil pollution. Its caused by a naturally occurring bacteria which i will describe in more detail in a later post.

As always, if you suspect that a watercourse is polluted, give us a call straight way on our freephone number 0800-807060.

More information on oil pollution can be found on our website:

Oil Pollution – The Environment Agency’s view

Different types of fish need varying amounts of oxygen in the water. Trout and Salmon are very active fish and generally thrive best in clean rivers with a high level of dissolved oxygen. Course fish such and Chub & Carp will survive in water containing lower oxygen levels.

If a pollution incident causes the oxygen levels in a river, stream or canal to fall too low, the early warning signs will be fish at the water surface seemingly “gasping for air”. If there is a clean feeder stream coming into the main channel they may well be congregating at this point to take advantage of some much needed oxygenated water.

If you see fish “mouthing” at the surface, it is vital that you call us without delay and explain what you have seen.

We have the ability to supplement the oxygen levels in a watercourse or carry out a fish rescue. Time is of the essence and you can inform us of any such issues 24/7 on 0800 807060.