Table 5.8 - Descriptive statistics of annual mean physical and chemical variables at European river stations  | Number of river stations | Percentage of river stations with concentrations not exceeding |
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  | 25% | 50% | 75% |
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pH | 717 | 7.5 | 7.8 | 8.0 |
Total alkalinity (meq/l) | 274 | 1.0 | 2.5 | 4.0 |
Chloride (mg Cl/l) | 442 | 17.3 | 26.5 | 68.3 |
Organic matter and oxygen level (mg O2/l) |
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Biochemical oxygen demand | 645 | 1.9 | 2.8 | 4.7 |
Chemical oxygen demand | 470 | 7.8 | 15.0 | 25.0 |
Dissolved oxygen | 620 | 8.4 | 9.7 | 10.7 |
Nitrogen (mg N/l) |
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Ammonium | 580 | 0.1 | 0.2 | 0.4 |
Nitrate | 654 | 0.7 | 1.8 | 3.9 |
Total nitrogen | 329 | 0.8 | 2.1 | 4.5 |
Phosphorus (µg P/l) |
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Dissolved orthophosphate | 412 | 45.0 | 124.0 | 286.0 |
Total phosphorus | 546 | 59.0 | 170.0 | 366.0 |
Heavy metal (µg/l) |
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Copper | 192 | 1.0 | 4.0 | 8.0 |
Zinc | 176 | 5.0 | 10.0 |   |
Note: The following example for dissolved orthophosphate shows how to read the table. For this compound annual mean concentrations have been gathered for 412 European river stations. Of these, 25 per cent (103 stations) had concentrations below 45 mg P/l; 50 per cent (206 stations) were below 124 mg P/l and 75 per cent (309 stations) were below 286 mg P/l, respectively.