The data and trends on the quality of rivers across London are outlined below as part of the London State of the Environment Report 2010.
About the data
The indicator uses data collected by the Environment Agency to assess fresh water quality, using the General Quality Assessment (GQA) scheme.
There are three types of GQA - chemical, biological and nutrient (which includes nitrate and phosphate).
Water quality is assessed for all designated river reaches, using representative sample points. These points can sometimes represent more than one reach. The chemical GQA network has reduced in recent years. The assessment now covers 209km of London’s watercourses.
GQA sampling is done 12 times a year, approximately once a month and the grades are calculated using three years’ data (36 samples). Therefore results for 2008 include samples taken in 2006, 2007 and 2008.
Each reach is assigned a grade according to the quality achieved. For the chemical and biological GQA the grades range from A for ‘very good’ to F for ’bad’ water quality. For the nutrient GQA there is no good or bad in the scale - the grades represent the level of nutrient concentrations (ranging from very low to excessively high).
Chemical GQA assesses freshwater quality based on two parameters: dissolved oxygen (DO) and ammonia. Biochemical Oxygen Demand (BOD) was removed from the calculation in 2007. As a result all the historic GQA grades have been recalculated to be assessed for dissolved oxygen and ammonia only.
Biological GQA assesses freshwater quality using surveys of macro-invertebrates. It is a good indication of any pollution that may occur infrequently or in low concentrations that may be missed through chemical monitoring.
The Water Framework Directive has introduced new mechanisms for measuring water quality that will replace the GQA scheme.
In recent years, the chemical and biological quality of freshwater in London has declined. There are some areas in the capital where quality has remained poor or even declined, particular in the Lower Lee catchment.
Chemical quality of rivers in London
The charts above show that there is an overall increase in the length of river achieving ‘good’ or ‘very good’ since 1990, from 29% to 44% in 2008. But, since a peak of 53% in 2001, there has been a continued decline.
There is a clear relationship between the trends in grades C and D, and grades E and F – when the % river length achieving grades C and D increases the length achieving grades E and F decrease, and visa versa.
The percentage of river length achieving ‘bad’ or ‘poor’ status in 2008 (10%) is just below the 1990 level (15%). The levels have fluctuated since 1990 with notable increases between 1997 and 1999 and in 2003, when river flows were lower due to drought conditions in the region.
There are no rivers with poor (grade F) chemical status in London
Since 2003, there has been an increase in grades C and D and a decline in grades E and F, showing an improvement in the worst quality rivers.
Particular improvement evident in the River Wandle (upstream of Beddington sewage works) from a grade C in 1990 to an A in 2008. Improvements since 2000 are evident in the Beverley Brook (Pyl Brook to the Tideway) which improved from a grade E to a C, and the Grand Union Canal (Cowley Reach from Iron Bridge to Brent) which improved from a grade C to an A in 2008.
Concern over chemical water quality in the Lower River Lee downstream of Tottenham Lock which has shown constant grade E quality, and also the Salmon Brook where quality declined to grade and has remained.
Biological quality of rivers in London
The chart above shows that current biological water quality is higher than in 1990 but a recent decline is evident, since 2002, when the length of river achieving higher quality (grades A and B) declined from 36% to 21% in 2008.
Between 2007 and 2008, the length of river achieving high biological quality has remained the same but the composition of grades A and B has changed – with grade A declining and B increasing. The decline in quality in the Fray’s River and New Years Green Bourne has caused this shift.
An increase in the river length achieving grades C and D since 2002, from 46% to 60% in 2008. This is alongside the decline in grades A and B, showing a decline in biological quality in London’s rivers.
There is a decline in grades C and D between 2007 and 2008, due a decline in quality in the Pool River and on part of the Grand Union Canal.
An occurrence of ‘bad’ quality (grade F) rivers, since 2004. The Grand Union Canal (Paddington Arm) only achieved grade F in 2004 and remained the same up to 2008. The Salmon Brook downstream of Deephams sewage works declined to grade F in 2007 and 2008.
Particular improvement in parts of the rivers Rom/Beam, Ravensbourne and Wandle (downstream of Beddington Sewage works) where biological quality has improved from grade E in 1990 to grade C in recent years.
Improvements in biological quality have also been recognised on the New Years Green Bourne, which improved from a grade E in 1990 to and B in 2008.
There is concern regarding:
- the River Brent, from Wembley Brook to the Tideway
- the Salmons Brook downstream of Deephams sewage treatment works,
- the River Ingrebourne downstream of Brentwood sewage works,
- the Lower Lee downstream of Tottenham Lock,
- parts of the Grand Union Canal (particularly the Paddington arm and Regents Canal) where the biological water quality has been historically poor and shown no improvements.
Nutrient quality of rivers in London
Nitrate levels are contributing to poor water quality in London’s rivers.
Since 1990 the percentage of designated river length with excessively high or very high levels of nitrate has declined from 57% to 52% in 2008.
There is no river length with ‘very low’ or ‘low’ levels of nitrate in London.
Phosphate is a major problem in London’s rivers.
Phosphate levels are ‘very high’ or ‘excessively high’ in just under 90% of London’s rivers. These levels have remained relatively constant since 1995 and represent the majority of designated rivers in London.
There are no rivers with ‘very low’ concentrations of phosphate and only around 8% have ‘low’ concentrations. Rivers with historically low concentrations are the River Cray, the River Darent and the upstream sections of the River Wandle.
