Water quality 101

Guest blogger Salka Hintikka is a Total Immersion Coach in Cumbria and a keen open water swimmer. Her background is in environmental engineering. When not coaching or swimming she works for the Environment Agency.

As swimming pools and many open water swimming venues are still closed, many of us keen swimmers have headed out to find new waters to swim in. This can come with a great sense of adventure and discovery, but also uncertainty about the water quality. Here’s a little “back to basics” on water quality and where to find more information in England.


There are many definitions of pollution, but let’s concentrate here on things that can make swimmers ill. Main culprits in the UK generally would be agricultural runoff and sewage.

How does it get to the river?

Point Source Pollution

Point source pollution is from a pipe, drain or other distinct source that discharges into a watercourse. It could be effluents from industry, sewage treatment works, road drainage etc. Generally, any industrial discharges need to be permitted by the Environment Agency and hence there should not be contaminants in such concentrations that would impact on the water quality or be unsafe to us swimmers.

Modern sewage treatment works (STW) do a great job in removing pollutants and pathogens but they cannot remove everything. However, the same natural “de-pollution” processes that are harnessed to use at the treatment works will also take place in the river. Hence, the further downstream you swim from the outfall, the cleaner the water.

But there also direct discharges from the sewerage network. These are called Combined Sewer Overflows (CSO). Besides the dirty water that we produce, sewerage networks traditionally are built to take rainwater from our streets, yards and roofs, hence the word “combined sewer”. It is not feasible to build enough capacity in the network for the most extreme rain events and as a result, there needs to overflow into watercourses otherwise the system would back up in our houses or flood the roads. These discharges are usually dilute as they are mostly rainwater, but there will still be some bacterial contamination.

Diffuse Pollution

Diffuse pollution, on the other hand, enters our watercourses more discreetly with runoff from the surrounding land area. It is more difficult to pinpoint or control and its effects are cumulative on a catchment scale. The main concern for us swimmers is agricultural runoff that could contain animal faeces.

Signs of Pollution

You might not be able to spot pollution easily. Surface water runoff and storm discharges generally last only a short while and won’t have long term impacts that you could see.

If there is long-term pollution you might spot “sewage fungus” in a river (see picture). This is a whitish-grey bacterial growth on rocks and riverbed and if you see it, you probably don’t want to swim in it. Sewage fungus occurs in water high nutrient and low oxygen concentrations. Pollution tends to provide the nutrients and the natural degradation of the pollutants causes oxygen levels to drop – perfect conditions for these bacteria to thrive. Despite its name, sewage fungus is not necessarily caused by sewage; slurry and silage effluent are other common causes. It is more visible in low flow conditions. As the flows get higher there is more dilution and aeration in the system to prevent this growth – in this case, the adage “dilution is the solution to pollution” is true.

Foam in rivers is usually natural, and harmless. But it could also be caused by pollution, mainly detergents. This leaflet gives you more information on types of foam.

Blue-Green Algae

Algae are present naturally in inland waters such as rivers, streams and lakes. An algal bloom can occur when conditions are right (warm temperatures and enough nutrients). During a bloom, the water becomes less clear and may look green, blue-green or greenish-brown. Scums can form during calm weather when several bloom-forming species rise to the surface. As a swimmer, you probably would choose not to swim through it. Whilst algal blooms are natural, they can be exacerbated by excessive inputs of nutrients from our activities (e.g. fertilisers, sewage and other effluents.)

Cyanobacteria or ‘blue-green algae’, a type of blooming algae, can produce toxins. These toxins can kill wild animals, livestock and pets. They can also harm people, producing rashes after skin contact and illnesses if swallowed, and even liver damage in worst cases. You can’t tell if an algal bloom is toxic just by looking at it, so it’s safest to assume it is.

If you spot something that could be blue-green algae, report it to the EA in England on (0800 807060). They’ll sample it and notify the landowner if it’s a toxic bloom. Landowners are responsible for putting warning signs up. This blog will give you more information on algal blooms.

Where can you find information on water quality?

The main thing to keep in mind is that water quality is not constant. It changes over time, even in the course of one day.

The Rivers Trust has produced a really helpful map where you can find out all permitted sewage discharges in England. This might influence your decision whether you fancy a swim in a certain stretch of river or not!

In England, the Environment Agency (EA) samples our rivers and lakes routinely. You can find the current and past overall classification of your swimming spot here. However, this classification does not tell you what the quality will be like on the day when you go swimming – and your main consideration should be whether it has rained recently. Note also that this classification does not include sampling for any bacteria, and they are the indicators for faecal matter contamination.

There is also a number of designated bathing waters that get sampled weekly during the summer season. The sampling is done to ensure the waters are safe to swim in, so it includes bacteria as well. If a sample has got too high a number of faecal bacteria, the local authority must advise the public immediately.

When is it safe to swim?

Unfortunately, there is no simple answer to this. As mentioned above, rain tends to increase inputs of “nasties” into the watercourses, so swimming during and after rain is always riskier. When is it safe again? There are no clear rules to it as every river is different. You might be able to assess whether the flow is “normal” by checking the river levels. Below you see a river near me last weekend when we had two days of heavy rainfall, causing the river level to rise. Once the rain has stopped, the river starts dropping, but this usually happens slower than the rise. In this case, once the river level is back down to about 0.5m you could think that the flows are back to “normal” and the risk of pollution is lower.

Another consideration with higher flows is that the current is going to be lot stronger than on a dry day making the swim riskier. Also, the runoff is likely to have made the water murkier, so if you’re in a new location you won’t be able to see what physical hazards there might be.

If you suspect that the water quality might not be as pristine as you’d like, try not to get it in your mouth or swallow it. Similarly, make sure you cover any wounds/cut and wash your hands afterwards.

What can you do?

As explained above, combined sewer overflows are one route for sewage to enter watercourses. They should only be activated during heavy rainfall, but they also start discharging if there is a blockage in the sewer network. These blockages are caused by fat (so-called “fatbergs”) or by “foreign objects”. If this happens during a dry period the discharge would be more concentrated untreated sewage than during a rainfall event. You can prevent these blockages by ensuring that you don’t pour any oils or fats down the drain. Just bin it!

Similarly, wet wipes, cotton wool, nappies, cotton buds and dental floss shouldn’t go down the toilet, regardless of what their packaging says. Your toilet’s diet should only consist of the three “p”s: pee, poo and (toilet) paper.

Also, make sure that all your dirty water is connected to the sewerage system – you’d be surprised how many washing machines and sinks have accidentally been connected to surface water drains.

If you live in a house with a septic tank, make sure it doesn’t discharge to a watercourse. Ensure it is emptied and maintained regularly.

If you notice pollution in the water when you’re out and about, report it to the Environment Agency, Natural Resources Wales or SEPA. They will investigate the problem and hopefully stop and prevent it from happening again. (EA and SEPA: 0800 807060, NRW: 03000 653000.)

Want to do more? Why not check your local Rivers Trust – they do great work improving our rivers.

About the writer: Salka Hintikka is a Total Immersion Coach in Cumbria and a keen open water swimmer. Her background is in environmental engineering. When not coaching or swimming she works for the Environment Agency.



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