After months of relentlessly miserable weather for most of the UK, spring brings renewed enthusiasm for spending time outdoors hiking, wild swimming, paddling or on walks.
Millions of people visit lakes and riversides every year. Yet with constant, and sadly necessary, reminders about sewage and water pollution , it's not surprising that people are increasingly worried about whether the water they see is safe.
Cocktails of contaminants created by sewage systems , agricultural pollutants and urban runoff are currently at the forefront of public, scientific and regulatory focus.
Not one UK river was free from chemical contamination, and only 14% were classified as having "good ecological status" at the last assessment .
In 2024 alone, raw and partially treated sewage was discharged in to watercourses for more than 3.6 million hours . With around 15,000 sites regularly discharging effluent, in addition to ongoing inputs from agriculture, transport and other industries , the 2025 results due to be published this year are not expected to show significant improvement.
Given this, many people who spend time around rivers want to know how to identify pollution.
It's worth remembering that pollution isn't always visible.
River contaminants take many forms . Some, like oil sheens, excrement, sanitary products and fly-tipping are visible and often odorous. Others such as nutrients including phosphates and nitrates, heavy metals , microplastics , and chemicals like ammonia and Pfas "forever chemicals" are invisible to the naked eye.
Pollutants may enter rivers from points such as discharge pipes, but also from farmland or roads, making them difficult to trace. Assuming you're not armed with specialist pollution testing kit , identifying contamination often relies on observing indirect signs. Here are five indicators to look out for along with the conditions in which they are most likely to appear:
1. Sewage fungus
Look out for a pungent, gelatinous, fuzzy carpet along the river bed, often found downstream of sewage outfall pipes.
Although not technically a fungus , these slimy, brownish-grey growths of bacteria and microorganisms thrive in nutrient-rich waters. Sewage fungus reduces oxygen levels in water, suffocating and physically smothering aquatic life.
2. Algal blooms
Look out for thick green mats or scum on the surface of the water, often looking like pea soup or spilled paint in green, blue-green, or brownish-black colours. Odours may be musty, earthy or grassy.
Algae is a natural part of aquatic ecosystems . Warm and dry weather combined with overloads of nitrogen and phosphorus nutrients, can trigger excessive algal growth. Algal blooms block light and use up available oxygen during their decomposition in a process known as eutrophication . The effects of algal blooms, especially blue-green algae can be catastrophic to aquatic life, and toxic to humans and pets.
3. Murky water
Look out for colours that are different to what's expected, and cannot be easily explained by what you know about the local landscape.
Sewage tends to present as grey, cloudy or milky , sometimes accompanied by bubbles, foam and a foul odour. Green or blue-green is indicative of algal blooms. Unusual colours like orange , red, yellow, or black may suggest industrial leaching or chemical dumping . Soil erosion can cause large quantities of suspended solids giving rivers a murky brown appearance that reduces light penetration and can suffocate organisms.
However, not all colours are a cause for concern. Heavy rain will naturally suspend particles and transport sediments giving rivers a temporary murky appearance. Local geology and soils may result in tea-coloured water caused by high iron content, or oily sheens caused by aerobic iron-fixing bacteria in boggy areas. You may even spot harmless dyes used by water companies and contractors to trace drainage leaks and misconnections.
4. White foam
Look out for bright white or milky-grey foam that doesn't easily disperse. It is often accompanied by a perfume, soapy or detergent-like smell.
Man-made foam from sewage, detergents, fire-fighting activities, pesticide runoff and industrial processes causes oxygen depletion in water. They can contain noxious forever chemicals known to cause serious health conditions in humans and wildlife.
Understanding river foam requires a discerning eye - not all foam is bad . Natural foam, while aesthetically unpleasant, can be formed by decaying leaves and plants. White-to-brown in colour, natural foam smells earthy or slightly fishy. It can collect in large clumps and is common on windy days, following heavy rain and in turbulent and nutrient-rich waters. In the absence of other environmental impacts like dead fish, algal blooms, or obviously stagnant water, natural foam is rarely problematic.
5. Aquatic life
Look out signs of distress including fish gasping at the surface, dead fish , or unusual behaviour from animals that live in and around the river.
Pollution causes a decrease in dissolved oxygen, which alongside drought and temperature extremes are leading causes of fish deaths and wildlife distress.
The presence of species such as kingfishers, water voles, frogs and riverflies indicate a cleaner river environment. A prolonged and noticeable absence of wildlife using the river is a concerning sign.
Interpreting the signs and taking action
Even with these indicators, identifying pollution is not always clear-cut. Many of these signs can overlap or have natural explanations, making it difficult to draw firm conclusions based on appearance alone.
To build a clearer picture, it's important to consider additional information. For example:
- Are there unusual smells?
- Is there a visible discharge point nearby?
- Have there been reports of people or animals becoming ill after contact with the water?
- Has there been recent sewage discharge in the area? In England, water companies provide near real-time data on sewage discharges through monitoring systems. However, these show how long spills occur, not the volume or concentration of pollutants, so they offer only part of the picture.
If you suspect serious pollution, it should be reported immediately to the relevant environmental authority .
While the current state of our waterways is concerning, understanding the signs of pollution and taking action are important steps toward improvement. Being well informed could help avoid turning your day out on the water into an unpleasant and potentially unhealthy experience for you and your friends and family.
Jess Neumann works at the University of Reading as an Associate Professor of Hydrology. She is a trustee of River Mole River Watch, a water quality charity who work with, advise, and receive funding from environmental and conservation organisations and agencies, water companies, commercial services, local authorities and community groups. She is a Director of the UK Chapter for the International Association for Landscape Ecology.
