GQA Chemical Classification System

Last updated: 11 September 2009

The chemical grade of the General Quality Assessment (GQA) scheme is defined by the level of Biochemical Oxygen Demand (BOD), ammonia and dissolved oxygen in the water. These have been selected as indicators of the extent to which waters are affected by waste water discharges and rural land use run-off containing organic, biodegradable material.

The quality of rivers is affected by discharges from sewage treatment works and industries, and by drainage and discharges from agriculture.

These three factors give the most useful chemical assessment of river water quality for the purposes of the GQA.Their use provides some continuity with the National Water Council’s (NWC) classification system used in the past.

Chemical Classification

A summary of the class limiting criteria is given in Table A. The overall class assigned to a river reach is determined by the worst of the three classes for the individual elements.

Table A: Standards for the Chemical GQA

GQA

Class

Dissolved Oxygen

(% saturation)

10-percentile

BOD

(mg/l)

90-percentile

Ammonia

(mg N/l)

90-percentile

A (Very Good)

B (Good)

C (Fairly Good)

D (Fair)

E (Poor)

F (Bad)

80

70

60

50

20

less than 20

2.5

4

6

8

15

-

0.25

0.6

1.3

2.5

9.0

-

The new classes are defined in terms of the 90-percentile for BOD and ammonia and the 10-percentile for dissolved oxygen; in other words, the river reach should contain less than the specified levels of BOD and ammonia for at least 90 per cent of the time, whilst the level of dissolved oxygen must not fall below the prescribed limit for more than 10 per cent of the time.

Chemical Class

The likely uses and characteristics of water in the various chemical classes are set out below:

Likely uses and characteristics1
  
A (Very Good)
  • All abstractions
  • Very good salmonid fisheries
  • Cyprinid fisheries
  • Natural ecosystem
B (Good)
  • All abstractions
  • Salmonid fisheries
  • Cyprinid fisheries
  • Ecosystem at or close to natural
C (Fairly Good)
  • Potable supply after advanced treatment
  • Other abstractions
  • Good cyprinid fisheries
  • A natural ecosystem, or one corresponding to a good cyprinid fishery
D (Fair)
  • Potable supply after advanced treatment
  • Other abstractions
  • Fair cyprinid fisheries
  • Impacted ecosystem
E (Poor)
  • Low grade abstraction for industry
  • Fish absent, sporadically present, vulnerable to pollution2
  • Impoverished ecosystem2
F (Bad)
  • Very polluted rivers which may cause nuisance
  • Severely restricted ecosystem

1 Provided other standards are also met
2 Where the class is caused by discharges of organic pollution