No matter how clean a river might look, pollution is an ever present threat. Rivers are wild habitats, and as such they will never be totally sterile – but there are lots of sources which contribute to pollution above and beyond what would naturally occur. This page summarises just a few of these sources.
Rivers are part of nature, and raw or untreated river water is not safe to drink. Rivers contain microbial life and pollutants from a range of natural sources; for example, defecation by wild animals and dead and decaying organisms in and around the river.
Arable and livestock farming can contaminate rivers when manure, topsoil, pesticides and fertilisers get washed off the land and into streams, rivers and aquifers. When livestock animals are kept in the vicinity of a river, and have direct access to the river, livestock can defecate directly into the waterway. Even without direct access, rainwater can mix with manure and other animal waste, before draining into the river as runoff. In addition, decomposing animal carcasses near or in the river can severely damage water quality.
Urban and Transport
Pollution caused by urban land-use and transport may be due to pollutants entering a watercourse via urban runoff, road drainage, or atmospheric deposition (a fancy way of saying that particles in the atmosphere get returned to the land when it rains).
Mining and Quarrying
Active and abandoned mines can cause problems for river water quality due to exposed pollutants, such as heavy metals and acidic water, being washed into local watercourses.
Untreated and treated sewage pollution
Untreated Sewage: During periods of heavy rain, Storm Overflows (SOs) from treatment works and Combined Sewer Overflows (CSOs) from the sewer network are used as a safety valve for the sewer system, preventing our homes from flooding. Unfortunately, when these overflows kick in, rainwater mixed with raw sewage is discharged into our rivers. This means that the majority of what gets flushed down the toilet or poured down the sink – including non-biodegradable items like wet wipes – ends up polluting the river.
In theory, the rainwater should dilute the raw sewage and screens should remove gross solids — however, the existing infrastructure can’t cope with our rapidly increasing population, leading to many discharging more often than they should.
Treated Sewage: Domestic and industrial sewage has to be treated before it can be discharged to the environment. In built up areas, sewage is piped to the nearest water company sewage (also called wastewater) treatment works, where it is filtered and undergoes chemical and biological treatment to remove contaminants before releasing as treated ‘effluent’ back to rivers or the sea. Unless there is a designated bathing water or shellfish water nearby, there is no requirement to sterilise the effluent, so most sewage effluent discharges to rivers still contain high levels of bacteria and pathogens. Domestic properties and small businesses which are not connected to water company sewage networks will have their own local sewage treatment facilities, such as septic tanks or ‘package’ treatment works. The locations of all of these are not on the public register so we can’t map the risk, and the estimated half a million homes with non-mains drainage are likely to be a significant source of localised pollution, particularly in rural areas.
Next, on to misconnections. In the UK, it is estimated that there are between 150,000 and 500,000 houses with drain misconnections. Within a modern property, there are usually two sewers: a waste water sewer and a surface water sewer.
- A wastewater sewer is where household water (e.g. from showers, sinks, toilets, etc.) is sent to be treated at a wastewater treatment plant, before being released into the environment
- A surface water sewer is where rainwater and runoff go to be directly released into rivers or streams
Sometimes the two become misconnected, meaning that untreated waste water is released into rivers, posing a serious threat to the environment and the health of river users.