Water Quality

Water Quality

The water quality of the Severn Estuary is an important indicator of the overall health of the Estuary’s ecosystem. Keeping the water free of pollutants, contaminants and rubbish all acts to increase water quality. It influences tourism, recreational activities, and industry, where higher quality waters are sought out for these activities. Water quality in the Estuary is a complex and complicated system. With a large variety of inputs from numerous different sources and complex interactions between contaminants and ‘master variables’ such as salinity and dissolved oxygen, it is a huge task to monitor and control these. Due to the high levels of suspended sediment (silt suspended in the water column), contaminants often ‘attach’ to the sediment particles, meaning sediment quality is also an important issue in the Estuary.

The Severn Estuary’s waters are covered by several policies which help keep the water clean. As a result of these policies and the efforts of different relevant authorities around the Estuary, levels of contaminants released in rivers and the Estuary have seen reductions over the past 25 years, especially sewage and metals1. Until recently, the Estuary received sewage and industrial effluents amounting to 800,000m3 a day and 200,000m3 a day, respectively1. However, several water bodies have failed to meet high ecological status, and there are still many improvements to be made.

The Severn River Basin District Management Plan2 governs the area containing all of the water bodies which drain into the Estuary, which is termed a ‘catchment’.  The Severn’s catchment area covers 21,000 km2 across England and Wales combined, covering scenic rural landscapes and the busy cities of Bristol, Coventry, Cardiff, and Gloucester. There is a total of 401 natural and 43 man-made bodies of water in the Severn River catchment, including 358 rivers, canals, and surface water transfers3. Elements which can affect the quality of these water bodies include changes to water levels, substances from agriculture, wastewater from abandoned mines, sewage output from water industry, and recreational activites4. Contaminants from these areas enter the Estuary from point sources (concentrated from one event or location) or diffuse sources (pollutants which arrive from a large area, for example, agricultural fields).

Water Quality standards, Regulation and Monitoring

Water quality within the Estuary has to conform with standards set out by national legislation. The Environment Agency and Natural Resources Wales are responsible for achieving the water quality objectives in the Estuary and undertake extensive monitoring to assess compliance with Directives (and other obligations and to inform consenting decisions). Some of the key directives are briefly discussed below:

The Water Framework Directive

Set out by the EU, the Water Framework Directive was added into UK legislation in 2003 and is a substantial piece of water legislation. The main principle involves protection and improvement of the ecological and chemical health of our rivers, lakes, estuaries, coastal waters and groundwater5. This promotes the sustainable use of our water resources and ensures a future for the many species living within them. The Directive designates areas for drinking water, recreational waters, nutrient-sensitive areas and areas designated for habitat protection5.

Within this framework, water bodies are assessed and then ranked in terms of their ecological and chemical quality. The Severn Estuary is currently classified as a heavily modified water body, as man-made structures and alterations interact with water flow within the catchment. A water body is ranked from bad to high for ecological quality, and as a fail or good for chemical quality. The Estuary catchment has moderate ecological status and has achieved good chemical status – the exact numbers of which are shown in figure 1.

Figure 1 – Ecological and chemical 2015 classification for surface waters within the Severn Estuary Catchment (Environment Agency, 2016)

Bathing Waters Directive

This legislation acts to ensure high water quality of bathing waters around the coastline. This protects public health and enables many recreational activities to continue around the coastline safely. It covers a range of pollutants, including E.coli bacteria, cyanobacteria, excess amounts of algae and general waste7. For more information about bathing waters, see section on bathing water quality.

Urban Wastewater Treatment Directive

The objective of this Directive is to protect the UK’s water bodies from the negative impact of large sewage discharges. It sets out waste water treatment standards for urban areas and takes special care to protect especially sensitive areas7. These sensitive areas include water bodies that may become ‘eutrophic’ if protection is not given. Discharges of sewage can lead to water bodies becoming eutrophic, which is a phenomenon where input of sewage causes algae to bloom and multiply rapidly. Eventually, algae reach excessive levels and act as a blanket over the water, starving flora and fauna of vital oxygen supplies. Many sewage companies have greatly reduced their discharges over recent decades, though there are still sewage pollution incidents which can occur.


The list of contaminants which can affect water quality in the Estuary includes metals, hydrocarbons (e.g. oil), nutrients, solvents, biocides, fungicides, flame retardants, polychlorinated biphelnyls (PCBs), pesticides and radionuclides (the by-products from nuclear industry). The closure of many energy generating plants and increases in pollution control measures within the past decade has drastically reduced the numbers of these contaminants within the Estuary. Within the Severn Estuary, contaminant levels are in line with the Water Framework Directive5 (standards for Priority and Priority Hazardous Substances)

Estuarine master variables

A number of different factors, known as master variables, control the water quality of the Severn Estuary (summarised in Figure 2). These variables are affected by different contaminants entering the system, which then change the physical and chemical nature of the Estuary. Changes in these variables can positively or negatively affect water quality.

2. Summary of the master variables which affect water quality within the Estuary  

Dissolved oxygen

Dissolved oxygen (DO) refers to the amount of oxygen dissolved within water and is one of the most important factors affecting a water body’s health. Low DO has immediate and long-term effects, ranging from shifts in biological communities, disruption of fish migration, and (if DO drops too low) kills fish. The level of DO in the Estuary decreases when concentrations of oxygen-demanding organic materials are too high, for example, when eutrophication of a water body occurs. Dissolved oxygen levels in the Severn Estuary are generally high, with levels above 8 mg/l throughout the whole Estuary8.


Nutrients, including nitrogen and phosphorous, support the growth of aquatic life. These nutrients might be familiar to keen gardeners, where plant feed enriched with nitrogen and phosphorous makes up for any deficiencies a plant may have. When excess nitrogen and phosphorous leaches into the environment, this causes excessive plant and algal growth. The algae can cover the surface of the water like a blanket, and effectively smothers the water beneath by absorbing the dissolved oxygen. This phenomenon known as ‘eutrophication’, can impair aquatic life and cause the deaths of many organisms.

The Severn Bridge – showing the brown-tinged water clearly


The Severn Estuary is made up of a combination of freshwater and saltwater, which mixes at different points along the Estuary. Salinity is a measure of the salt content in a body of water, which can range from saline (salty) to brackish (between salt and fresh water) and fresh (very low salts). A characteristic feature of the Estuary is a north-south salinity gradient, with the lowest salinities occurring along the Welsh Coastdue to freshwater input from multiple rivers such as the Rhymney and Ebbw. Salinity along the Welsh Coast ranges from 20-30 g/kg (grams of salt per kilogram of water) at Swansea to 22-24 g/kg at Barry and only 1-12 g/kg at Cardiff9. At high water on spring tides (with a typically low river flow) the freshwater-saline interface is located approximately 8km downstream of Lower Parting, Gloucester. From here, the salinity increases moving towards the Bristol Channel and out to sea.

Suspended particle matter (sediments)

The Estuary’s brown tinged waters are due to the vast volume of fine sediment held in suspension (i.e. suspended in the water) as a result of the extremely high energy environment. These suspended salts and sands are not directly related to any form of pollution but can influence other aspects of water quality such as levels of dissolved oxygen and the adsorption of metals. For example, metals such as copper and zinc have a positive charge, and so are attracted to negatively charged particles of silt10, though other physical and chemical properties determine which contaminants are attracted to particle matter. These particles hold onto contaminants and can be ingested by marine organisms and accumulate in the food chain.


pH is a measure of the acidity or alkalinity of a substance, which ranges from the extremely acidic at pH 1 to the extremely alkaline at pH 14. The pH of water in the Estuary increases from around neutral at its freshwater end (around pH 7) to approximately slightly alkaline (around pH 8) in the outer Estuary8. There are, however, a number of outfalls such as from some sewage treatment works which have caused localised changes to pH outside of the normal slightly alkaline range8. These changes are normally reversed by dilution and the ability of water to act as a ‘buffer’ (to resist pH change).

For more information, visit:

  1. Metals and nutrients in the Severn Estuary and Bristol Channel: Contemporary inputs and distributions https://www.sciencedirect.com/science/article/pii/S0025326X09005153 (Jonas and Millward, 2010)
  2. Water for life and livelihoods Part 1: Severn river basin district River basin management plan https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/718336/Severn_RBD_Part_1_river_basin_management_plan.pdf (Environment Agency, 2016)
  3. Severn – Summary River Basin District https://environment.data.gov.uk/catchment-planning/RiverBasinDistrict/9/Summary (Environment Agency)
  4. Severn – Reasons for Not Achieving Good https://environment.data.gov.uk/catchment-planning/summarypages/summary/RiverBasinDistrict/9 (Environment Agency)
  5. Explanatory Memorandum to the Water Environment (Water Framework Directive) (England and Wales) Regulations 2017 No. 407 https://www.legislation.gov.uk/uksi/2017/407/memorandum/contents
  6. UK Statutory Instruments: The Bathing Water Regulations 2013 No. 1675 Part 1 https://www.legislation.gov.uk/uksi/2013/1675/part/1   
  7. European Commission: Urban Waste Water Directive Overview https://ec.europa.eu/environment/water/water-urbanwaste/index_en.html
  8. Severn Tidal Power – SEA Topic Paper Marine Water Quality https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/69903/40._Marine_Water_Quality_-_NTS.pdf (Department of Energy & Climate Change, 2010)
  9. Fouling communities and non-native species within five ports along the Bristol Channel, South Wales, UK. Estuarine, Coastal and Shelf Science, 252, p.107295. https://www-sciencedirect-com.abc.cardiff.ac.uk/science/article/pii/S0272771421001487  (Holmes, S. and Callaway, R., 2021)
  10. Adsorption of multi-heavy metals Zn and Cu onto surficial sediments: modeling and adsorption capacity analysis. Environmental Science and Pollution Research, 21(1), pp.399-406. https://link.springer.com/article/10.1007/s11356-013-1916-2  (Li, S., Zhang, C., Wang, M. and Li, Y., 2013)

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