Water Quality

The water quality of the Severn Estuary is an important indicator of the overall health of the Estuary’s ecosystem and also an important factor in influencing tourism, recreational activities and the commercial/industrial sectors. Water quality in the Estuary, as with other estuaries, is complex; with a large variety of inputs from numerous different sources and complex interactions between contaminants and ‘master variables’ such as salinity and dissolved oxygen. Due to the high levels of suspended sediment and the tendency of many contaminants to associate with particulate matter due to their low solubility, sediment quality is also an important issue in the Estuary.

The levels of most contaminant inputs to the Estuary are much lower than they were 25 years ago, following the closure of major industries and the introduction of stricter pollution control1. This has resulted in improvements to both overall water and sediment quality; though discharges of contaminants continue, along with the legacy of past pollutants still present in sediments in certain areas.

For many years, the Estuary and the rivers feeding it have been widely used for the disposal of many forms of treated and untreated waste products. Until quite recently, the Estuary received sewage and industrial effluents amounting to 800,000m3 a day and 200,000m3 a day, respectively2. The major industries discharging into the Estuary include (or have included until recently) the smelters, incinerators, fertiliser and numerous other chemical plants in the Avonmouth area; coal and steel industry, paper mills, chemical and pharmaceutical manufacturers in south Wales; and nuclear power plants at Hinkley, Berkeley and Oldbury. Sewage from the urban centres of Bristol, Gloucester, Newport and Cardiff adds directly to the pollutant load, as do domestic and agricultural sources to the large numbers of tributaries entering the Estuary3.

Contaminants from these activities enter the Severn Estuary either from point sources (as discharges, such as from wastewater treatment works and industries), or from non-point sources (diffuse pollution) such as atmospheric deposition (power stations and vehicle emissions), shipping (hull coatings) and storm water runoff from urban and agricultural areas.


Water Quality standards, Regulation and Monitoring

Water quality within the Estuary has to conform with legislation. European Directives set out standards for water quality and impose monitoring requirements3. The Environment Agency is responsible for achieving the water quality objectives in the Estuary and undertakes extensive monitoring to assess compliance with Directives (and other obligations and to inform consenting decisions). In particular, the Water Framework Directive provides a major focus for much of the biological and chemical monitoring currently undertaken4. Some of the key directives are briefly discussed below:

Water Framework Directive
This Directive was transposed into UK legislation in 2003 and is the most substantial piece of water legislation to date. It requires the protection of the ecological status of all inland and coastal water bodies from deterioration and, where necessary and practicable, the restoration of water bodies to “Good Environmental Status” by 2015. In doing so, it promotes the sustainable use of water as a natural resource; the conservation of habitats and species which directly depend upon water; the progressive reduction or phasing out of the release of pollutants that present a significant threat to the aquatic environment and the management of the effects of floods and droughts. The Directive is being implemented using Management Plans that operate at the River Basin scale. These River Basin Management Plans (RBMPs) contain the main issues for the water environment and the actions required to achieve the objectives of the Directive.

The area covered by the Severn Estuary and Bridgwater Bay is situated within the Severn River Basin Management Plan. The River Basin is subdivided into management catchments and then into ‘water bodies’. The aim of achieving at least ‘Good Environmental Status’ applies to each water body.

The status of water bodies is judged by a number of measures, including chemical and biological quality. The Severn Estuary is currently classified as a heavily modified water body (due to the extensive flood defences that constrain its natural development) with moderate ecological status/ potential, though it has been assessed as achieving good chemical status.

Dangerous Substances Directive
The aim of this Directive is to improve water quality through the elimination and/or reduction of discharges of dangerous substances. The Directive established two lists of substances; List 1 substances are particularly dangerous because of their toxicity, persistence and bioaccumulation. Pollution by these substances must be eliminated. List II substances are less dangerous but have a deleterious effect on the aquatic environment. Input of these substances must be reduced. Environmental quality standards (EQSs) that identify limits have been established for all substances3.

Data collected by the Environment Agency generally indicate universal compliance with Environmental Quality Standards for all parameters monitored at the majority of sites. An exception is for the monitoring points for certain substances, particularly metals located in the Avonmouth area. However, when averaged across the year as required by the EQS, compliance for all parameters is achieved3.

Urban Wastewater Treatment Directive
The objective of this Directive is to protect the environment from the adverse effects of sewage discharges and, therefore, it sets treatment levels on the basis of size of sewage discharges and the sensitivity of waters receiving the discharges3. The ongoing improvement programme of the water utility companies has resulted in major improvements to the quality of sewage discharges to the Severn Estuary.

Bathing Waters Directive
This Directive is concerned with the quality of bathing waters for the purpose of protecting public health and requires the monitoring of principally microbiological parameters (see section on Bathing Waters).


Estuarine master variables

Dissolved Oxygen (DO)
DO in the water is one of the most basic and important measures of a water body’s health, affecting a wide array of aquatic plants and animals. Low DO has immediate and long-term effects, ranging from shifts in biological communities, disruption of fish migration, and (under the worst conditions) fish kills5. The level of DO in the Estuary decreases when concentrations of oxygen-demanding organic materials are too high.

Dissolved oxygen levels in the Estuary are generally high, with levels above 8 mg/l throughout the whole Estuary and concentrations above 95% at the seaward end4. During 2004 and 2005, monitoring of DO saturation found that saturations were greater than 90% at most sites, including bottom waters. The lowest surface value, 72.4% was just downstream of Lower Parting, Gloucester (in August 2005). Overall, DO saturation values suggest no widespread severe oxygen depletion, either at the surface or at depth in the Estuary2.

The Severn Estuary is a vertically well mixed estuary. A characteristic feature is a persistent north-south salinity gradient, with the lowest salinities occurring along the Welsh Coast6,7, due to greater input of freshwater. Salinities within the Estuary vary considerably over the tidal cycle and in response to changes in river flow. At high water on mean spring tides (with a typically low river flow) the freshwater saline interface is located approximately 8km downstream of Lower Parting, Gloucester. From here there is a steep salinity gradient until 50km, beyond which the salinity increases more gradually2 until west of the Holm Islands, with a maximum salinity (30‰).

Suspended particulate matter (sediments)
The Estuary’s ‘brown’, turbid waters are due to the vast volume of fine sediment held in suspension as a result of the extremely high energy environment. This suspended particulate matter is 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.

It has been estimated that on spring tides over 30 million tonnes of sediment are in suspension and around 4 million tonnes on smaller Neap tides8. Concentrations vary considerably – vertically, laterally and longitudinally. Median values along the length of the Estuary range from 81- 336mg/l, with the highest values occurring in the mid-Severn Estuary region (between Sharpness and Avonmouth8,9) and with extremely high suspended sediment concentrations near the Estuary bed, often exceeding 20,000mg/l8.

The most significant input contributors of nutrients to the Estuary are rivers, largely through agricultural run-off and urban wastewater discharge outfalls. Nutrients such as nitrogen and phosphorous, are critical to supporting aquatic life. However, an overabundance of nutrients can lead to excessive plant and algal growth, which causes dissolved oxygen to drop when plant and algal matter decay5. This phenomenon known as ‘eutrophication’, can impair aquatic life and lead to fish kills5.

The Severn Estuary and Bristol Channel provide one of the highest UK inputs of nitrogen and phosphorus to the marine environment, reflecting the Estuary’s size, the location of human settlements and the intensity of agricultural land use. Concentrations of dissolved inorganic nitrogen are reported to have doubled over the past 20 to 25 years4. Due to the highly turbid waters that permit little light penetration, algal productivity is generally low and eutrophication is not considered a major issue within the Estuary.

The pH of water in the Estuary increases from around neutral at its freshwater end to approximately slightly alkaline in the outer Estuary. Recent surveys by the Environment Agency showed that all pH results were within the EQS range of 6.0 – 8.5 (annual average) to protect fish and the EQS range of 7.0-8.5 (75 percentile) to protect shellfish4. There are however, a number of outfalls such as from some sewage treatment works and from Aberthaw Power Station, which have consented pH levels that are outside the normal consented values of pH 5.5 to 94. Such discharges result in very localised modifications to pH and the high buffering capacity of seawater and the available dilution ensures that pH quickly returns to the normal range.


The list of potential chemical pressures on water quality is substantial and includes metals, organometals, hydrocarbons, nutrients, solvents, mineral acids, biocides, fungicides, flame retardants, polychlorinated biphelnyls (PCBs), pesticides and radionucleides3. The decline in heavy industry and introduction of pollution control in the later part of the 20th century has seen a general downward trend in contaminant inputs. However a number of water quality issues continue – some of these are described below.

The main sources of trace metals to the Severn Estuary are from rivers and wastewater treatment works (WWTW). Trace metals are transported through the water body either in solution (dissolved metals) or adsorbed onto suspended sediments4. There has been a marked overall decline in both dissolved and adsorbed metal sediment concentrations over the past 30 years. This has been attributed to a combination of reduced industrial activity and improvements in emission controls.

Based on the review of consented discharges by the Environment Agency10, the average concentrations of dissolved metals such as cadmium, copper, nickel, lead and zinc (see Fig. 3) are all below Environmental Quality Standards (EQS) thresholds. However, at near-shore locations in the vicinity of outfalls and rivers, elevated concentrations of contaminants have been observed with isolated (i.e. one-off) exceedences of EQS for various metals. However these did not result in EQS exceedence when expressed as an annual average4.

The concentration of trace metals in the sediments of the Estuary is relatively uniform9, largely due to the strong tidal mixing which disperses contaminants from their source. Despite this dilution, contamination levels tend to be highest in fine sediments (primarily located between Avonmouth and Severn Beach; Caldicot flats; River Parrett and outer Bridgwater Bay; and between the mouths of the Rivers Usk and Taff). Enhancement in these fine sediments is still observed for a number of metals3.

There are currently no formal sediment quality standards applying to the UK, although certain Directives contain standstill provisions (i.e. concentrations in sediments should not increase). A study has shown that sediments at a number of sites in the Estuary contained levels of metals likely to exert pressures on organisms, though such effects were probably chronic and not acute4,9.


Trace organics

A wide range of organic substances have been detected in water, sediment and organisms in the Estuary including organotins, hydrocarbons, polyaromatic hydrocarbons (PAHs), pesticides, herbicides and polychlorinated biphelnyls (PCBs).

The vast majority of data for toxic organic compounds in the Severn Estuary have been found to be below the limit of detection. An exception to this is tributyltin (TBT) (a contaminant associated with antifouling paint). Maximum levels of TBT of 0.003µg/l have been recorded in the Severn Estuary. Concentrations of TBT in the sediments have been found to be in the higher ranges in the vicinity of the ports of Newport and Cardiff. The global ban on the use of TBT should result in reductions in the concentrations of organotins in such sediments over time, although in some,
degradation can take many years4.

Surveys in the 90s recorded concentrations of individual PAHs that were generally below the Water Framework Directive Priority Substances standards (annual average EQS of 50ng/l)11. However, more recent studies have shown that the maximum allowable concentration of 10ng/l for all PAHs was frequently exceeded12. The highest concentrations occur at certain sites located close to the coast where the past export of coal is considered to have made a significant contributing to the PAH contamination of the sediments.

Synthetic compounds have historically been an issue and monitoring has indicated several EQS exceedences for pesticides and herbicides in the upper Estuary. Recent data however, indicates that exceedences of EQS’ now rarely occur. Evidence therefore suggests continuing improvements in water quality. Concentrations of PCBs generally fall below the detection limits – this is thought to be due to their affinity for and persistence in sediments3.

Radiological Contaminants

Inputs of radiological contaminants into the Estuary are very low. The Environment Agency reported in 200713 that none of the key establishments discharging radioactive discharges into the Estuary resulted in exposure radiation that was more than 5% of the total exposure limit of 1mSv/year. In recent years, a major Estuary hotspot has been in the waters off Cardiff due to the discharges from the GE Healthcare’s facility in Cardiff; though in 2006, the total dose from this site was approximately 1% of the dose limit13. The total dose is likely to have reduced further due to reductions in discharges of tritium in liquid wastes4.

Within the Severn Estuary and its tributaries, contaminant levels (trace metals, trace organics and radiological contaminants) all currently comply with Water Framework Directive standards for Priority and Priority Hazardous Substances and Annex VIII pollutants (substances for which the UK Government has set EQS at national level)4. Each of the three water bodies (upper Estuary, lower Estuary and Bridgwater Bay) achieve Good Chemical Status and the concentrations of Annex VIII pollutants do not compromise achievement of Good Ecological Status4.

For more information see the following links:

  1. Langston, W.J., Jonas, P.J.C. & G.E. Millward, 2010. Editorial / Marine Pollution Bulletin 61 (2010) 1-4
  2. Jonas, P.J.C. & G.E. Millward, 2010.Metals and nutrients in the Severn Estuary and Bristol Channel: Contemporary inputs and distributions.
    Marine Pollution Bulletin 61 (2010) 52-67
  3. Bristol Deep Sea Container Terminal. Environmental Statement. July 2008. Royal Haskoning
  4. Department of Energy & Climate Change, 2010. Severn Tidal Power – SEA Topic Paper Marine Water Quality. April 2010
  5. State of the Delaware Estuary 2008. Vol.18, Issue 3
  6. Owen, A., 1980. A Three-Dimensional model of the Bristol Channel. Journal of Physical Oceanography, 10:1290-1302 – as shown in DECC
    Severn Tidal Power – SEA Topic Paper. Marine Water Quality. April 2010
  7. Uncles, R.J., 1983. Modelling tidal Stress, Circulation and Mixing in the Bristol Channel as a Pre-Requisite for Ecosystem Studies. Can. J. Fish. Sci. 40 (supplement 1): 8-19 – as shown in DECC Severn Tidal Power – SEA Topic Paper. Marine Water Quality. April 2010
  8. Kirby, R., Henderson, P.A. & R.M. Warwick, 2004. The Severn, UK: Why is the estuary different? Journal of Marine Science and Environment No C2 pp1-17 as shown in DECC Severn Tidal Power – SEA Topic Paper. Marine Water Quality. April 2010
  9. Langston, W. J., Chesman, B.S., Burt, G.R., Hawkins, S.J., Readman, J., and Worsfold, P. 2003. Site Characterisation of the South West European Marine Sites – Severn Estuary pSAC, SPA. A study carried out on behalf of the Environment Agency, English Nature and the CCW
  10. Environment Agency 2009. Severn Estuary/ Môr Hafren SAC/SPA. Review of Consents Appropriate Assessment Stage 3 Report: Water Quality Technical Annex
  11. Law, R.J., Dawes, V.J., Woodhead, R.J. & Matthiesson, P., 1997. Polycyclic aromatic hydrocarbons (PAH) in seawater around England and Wales. Marine Pollution Bulletin 34: 306-322 as shown in DECC Severn Tidal Power – SEA Topic Paper. Marine Water Quality. April 2010
  12. CEFAS, 2007. A Review of the Contaminant Status of SEA 8 Covering the Western Approaches, Celtic Sea and English Channel. CEFAS Report C3006/01 – as shown in DECC Severn Tidal Power – SEA Topic Paper. Marine Water Quality. April 2010
  13. Environment Agency, 2007. Fate and Transport of Particles in Estuaries. Volume I: Summary and Conclusions. Scinece Report SC000002/SR1. March 2007
Last Updated 2011

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