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Orthophosphate Concentrations in Rivers 11: 1993-2005
Percentage of sites 7,12 within band
Band | 1993 | 1995 | 2000 | 2003 | 2004 | 2005 |
|---|
=250 µg P/l | 5.2 | 8.3 | 4.1 | 3.6 | 1.3 | 1.4 |
|---|
125 to <250 µg P/l | 7.3 | 8.9 | 10.4 | 5.7 | 6.2 | 4.2 |
|---|
50 to <125 µg P/l | 23.6 | 21.4 | 19.8 | 16.7 | 19.4 | 17.3 |
|---|
25 to <50 µg P/l | 20.4 | 18.2 | 19.3 | 18.2 | 15.0 | 17.8 |
|---|
<25 µg P/l | 43.5 | 43.2 | 46.4 | 55.7 | 58.1 | 59.3 |
|---|
Raised levels of orthophosphate in freshwaters may lead to eutrophication. The main sources of orthophosphate are outputs from sewage treatment and diffuse pollution from agriculture.
Between 1993 and 2001 there was some fluctuation in the distribution of mean orthophosphate concentrations, with reduced precipitation in 1996 and 1997 increasing the percentage of sites with mean concentrations =250 µg P/l. Over the period 1993-2001 the percentage of sites with mean concentrations
<25 µg P/l averaged 45% and the percentage of those =125 µg P/l averaged 16%. By 2005, the percentage of sites with mean concentrations <25 µg P/l had increased to 59% and the percentage of sites =125 µg P/l had fallen to 5.6%. This reduction in orthophosphate concentrations is in part due to the installation of nutrient removal facilities in sewage treatment works.
Under the Urban Waste Water Treatment Directive ( UWWTD) (91/271/ EEC), catchments where nutrient levels are considered to be high are designated as sensitive areas. Discharges into waters that have been designated as sensitive require additional treatment to remove nutrients.
SOURCE: SCOTTISH ENVIRONMENT PROTECTION AGENCY
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