ARCHIVE: e-Digest Statistics about: Inland Water Quality and Use
Freshwater quality
Nutrients
The presence of various nutrients in excessive quantities in water, combined with particular weather conditions i.e. warm sunny conditions, low rainfall resulting in low river flows, can lead to eutrophication. This is nutrient enrichment of waters causing an accelerated growth of algae and higher forms of plant life to produce an undesirable disturbance to the balance of organisms present, and to the quality of the water. It is a complex phenomenon and the challenge is to find suitable measures of state and process by which to assess it. In the UK, nitrates and phosphates are generally the nutrients contributing to eutrophication. The limiting factor in freshwaters is generally phosphorus, whilst in marine and coastal waters, nitrates tend to limit the process There is ongoing concern about nitrate leaching to waters because of eutrophication, other environmental effects associated with high nitrate levels and the risk that high nitrate levels in drinking water may present to human health.
The main source of nitrate in UK freshwater is leaching from agricultural land although treated sewage discharges are also a factor for rivers. The main phosphorus sources are outputs from sewage treatment works and diffuse pollution from agriculture. As point source pollution has been gradually improved in recent years, diffuse pollution has become more evident, especially from agricultural sources. It is estimated that over 70 per cent of nitrates, and over 40 per cent of phosphates in English waters are derived from agricultural inputs. Despite recent improvements in sewage treatment, the proportion of phosphorus in English rivers has remained relatively constant over the last ten years. Addressing diffuse pollution from agriculture is the single biggest future challenge for improving water quality. [18]
Under the Urban Waste Water Treatment Directive about 10,800 km of river lengths in the UK are designated as Sensitive Areas (Eutrophic). The Nitrates Directive [8] provides for the prevention of agricultural nitrate pollution through the designation of Nitrate Vulnerable Zones, which were significantly increased during 2002. In recent years, there has been evidence of blue-green algal blooms at over two thousand different standing water sites such as lakes. To address the associated risks and impacts of eutrophication, the Environment Agency published a national eutrophication strategy [9] in 2000. Proposals for a strategy to control nutrient enrichment of Northern Ireland were published in 1999.
Table 9a shows information on nitrate levels at tidal limits of the 159 major GB rivers monitored as part of the HMS for 1980-2004. Levels of phosphorus (both total phosphorus and orthophosphate) have also been monitored, the most comprehensive data available being concentrations of orthophosphates. These data (in terms of phosphorus) have been used as this gives a good indication of the trends in levels for total phosphorus. Average concentrations at tidal limit sites reflect levels at lower reaches of the rivers, but are not necessarily representative of levels found further upstream or in rivers further inland.
The rivers have been broadly grouped into three categories according to the type of landscape through which the river mainly flows. Using the Institute of Terrestrial Ecology (ITE) landscape classifications, 'lowland arable' covers rivers flowing mainly through landscapes classified by ITE as 'arable', 'lowland pastural' covers those flowing mainly through 'pastural' landscapes, and 'uplands' covers those flowing mainly through 'marginal upland' or 'upland' landscapes. For each broad landscape group, the annual averages of the site means have been calculated as above (see Harmonised Monitoring Scheme). Lowland arable rivers generally have higher nitrate and phosphorus levels than lowland pastural rivers, which in turn have higher levels than upland rivers. Arable rivers in England have higher levels of both nutrients than in Scotland; pastural rivers in England have higher levels of nitrates than in both Wales and Scotland, and pastural rivers in Scotland have higher phosphorous levels than England and Wales.
Table 9b shows the annual average concentrations and the maximum and minimum site means of nitrate in rivers for 1980-2004. Table 9c shows the annual average concentrations and the maximum and minimum site means of orthophosphate in rivers for 1980-2004. Table 9d shows the percentage of river lengths with concentrations of over 30 mg/l of nitrates and percentage river lengths with concentrations of phosphates over 0.1 mg/l. A level of 50mg/l of nitrates is the EU maximum admissible concentration for drinking water, to be met by 95 per cent of samples, and also the threshold established by the World Health Organisation for health protection. This standard is also required under the Nitrates Directive [8] for all surface waters and groundwater. A lower level of nitrates was selected (30mg/l) in Table 9d to allow for comparison of regional differences and clear identification of future trends.
The River Water Quality Database now provides data on nitrate and phosphate concentrations as well as chemical and biological quality of monitored rivers for each Government Office Region and Local Authority in England and Wales.
In Detail:
- Harmonised monitoring scheme
- River Water Quality Database providing data on chemical and biological river water quality and nutrient concentrations for each Government Office Region and Local Authority in England and Wales.
- Chemical river quality surveys
- Biological river quality surveys
- Groundwater quality
Further Information:
- Press Release:
- River Water Quality framework indicator for Sustainable Development: 2005, 17 August 2006 (384/06)
Data Tables:
- References, further reading and links to other resources:
- [8] Reducing Water Pollution from Agriculture (Defra internet)
- [9] Aquatic Eutrophication in England and Wales: a management strategy. Environment Agency, August 2000
- [18] Defra, Water Strategy: Directing the Flow - priorities for future water policy, November 2002 (Defra internet) pages 31 to 34
- Internet Links:
- For further details of national monitoring programmes:
- Environment Agency for England and Wales: River quality
- Scottish Environment Protection Agency Environmental Data : Classification Scheme
- Northern Ireland: Environment and Heritage Service River Water Quality Monitoring
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Page last modified: 17 August 2006
Page published: 17 August 2006