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Chemistry classification method

We test samples for ammonia, biochemical oxygen demand (BOD) and dissolved oxygen. The results for each site are averaged and percentiles are calculated. We compare these with limits set for each of the six grades in the table below. We assign a grade to the river according to the lowest grade achieved in any of the three tests. For example if the site gets a grade A for ammonia and dissolved oxygen but a grade B for BOD, then we give the site a grade B.

Chemistry classification method

Classification

Likely uses and characteristics *

A - very good

All abstractions
Very good salmonid fisheries
Cyprinid fisheries
Natural ecosystems

B - good

All abstractions
Very good salmonid fisheries
Cyprinid fisheries
Ecosystems at or close to natural

C - fairly good

Potable supply after advanced treatment
Other abstractions
Good cyprinid fisheries
Natural ecosystems, or those corresponding to good cyprinid fisheries

D - fair

Potable supply after advanced treatment
Other abstractions
Fair cyprinid fisheries
Impacted ecosystems

E - poor

Low grade abstraction for industry
Fish absent or sporadically present, vulnerable to pollution **
Impoverished ecosystems **

F - bad

Very polluted rivers which may cause nuisance
Severely restricted ecosystems

 

*providing other standards are met
**where the grade is caused by discharges of organic pollution