Water quality: physico-chemical conditions
Standards have been developed for a number of indicators of water quality in each of the four main types of surface water bodies:, lochs, transitional waters (estuaries) and coastal waters, matched to biology.
This section sets out:
- Typologies used to classify rivers and lochs for water quality standards
- Details of the proposed numerical standards for the following conditions:
Water quality standards proposed in this paper |
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Standard | Rivers | Lochs | Transitional and coastal waters |
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Oxygenation | Dissolved oxygen and biochemical oxygen demand ( BOD) | Dissolved oxygen | Dissolved oxygen, including fundamental intermittent standards |
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Ammonia | Total ammonia | Not required by WFD | Not required by WFD |
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Acid conditions | pH | Acid neutralising capacity ( ANC) | Not required by WFD |
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Salinity | [future] | Conductivity | [future] |
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Nutrient conditions | Soluble reactive phosphorus | [future] | [future] |
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Rivers - water quality
1. River typologies for water quality
Overall:
For water quality, rivers are divided into types depending on their altitude and alkalinity:
Table A1: Basic typology for rivers |
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Site Altitude | Alkalinity (as mg/l CaCO 3) |
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< 10 | 10 to 50 | 50 to 100 | 100 to 200 | > 200 |
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Under 80 metres | Type 1 | Type 2 | Type 3 | Type 5 | Type 7 |
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Over 80 metres | Type 4 | Type 6 |
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When developing standards, UKTAG found that some types of river responded in similar ways, and so proposed standards for groups of these river types, as set out below.
Oxygenation and ammonia:
Standards are proposed for two broad types of river. These correlate quite well with those currently used by the environment agencies, including those for the Freshwater Fish Directive ( FWFD).
Table A2: River typology for oxygenation and ammonia |
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Proposed types | Existing types |
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Upland and low alkalinity (types 1,2,4,6) | Salmonid † |
Lowland and high alkalinity (types 3,5,7) | Cyprinid ‡ |
†Salmonid waters can support salmonid ('game') fish, i.e. salmon, trout and whitefish. ‡Cyprinid waters can support cyprinid ('coarse') fish, which include carp, barb, roach, and chubs. NB In any case where a lowland, high alkalinity water body is a salmonid river, the proposed standards for the upland, low alkalinity river will apply. This is because fish are more at risk from low dissolved oxygen standards than invertebrates, and so require tighter standards. |
Acid conditions:
The proposed standards are identical for all types of rivers.
Nutrient conditions:
The proposed standards are based on four different river types.
Table A3: River typology for phosphorus |
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Site Altitude | Alkalinity (as mg/l CaCO 3) |
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< 50 | > 50 |
Under 80 metres | Type 1n | Type 3n* |
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Over 80 metres | Type 2n | Type 4n |
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* There were concerns during development of the standards about some of the sites used to define the '3n' standards. Therefore the proposed standards for type '4n' will be used for type '3n' as well, until more data has been collected to allow reassessment of this classification. |
2. Oxygenation standards - dissolved oxygen and biochemical oxygen demand ( BOD)
Table A4: Dissolved oxygen (% saturation) - rivers |
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Proposed standard (10-percentile) | Existing standards |
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River type | High | Good | River Type | High (10-percentile) | Good |
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(10-percentile) | FWFD (5-percentile) |
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Upland and low alkalinity (types 1, 2, 4, 6) | 80 | 75 | 'Salmonid waters' (types 2, 4, 6) | 80 a | 70 a | 65 - 75 b |
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Lowland and high alkalinity (types 3, 5, 7) | 70 | 60 | 'Cyprinid waters' (types 3, 5, 7) | 45 - 55 b |
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a Standards for the highest two classes in the existing Scottish Rivers Classification Scheme. b Freshwater Fish Directive ( FWFD) standards are defined in mg/l dissolved oxygen (4 mg/l salmonid, 6 mg/l cyprinid), but have been transposed into % saturation values for comparison with the proposed standards. |
Table A5: Biochemical oxygen demand ( BOD) (mg/l) - rivers |
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Proposed standard (90-percentile) | Existing standards (90-percentile) [All river types] |
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River type | High | Good | High | Good |
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Upland and low alkalinity (shaded boxes above - types 1, 2, 4, 6) | 3 | 4 | 2.5* | 4* |
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Lowland and high alkalinity (white boxes above - types 3, 5, 7) | 4 | 5 |
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* Standards for the highest two classes in the existing Scottish Rivers Classification Scheme. |
3. Ammonia standards
Table A6: Total ammonia (mg/l) - rivers |
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Proposed standard (90-percentile) | Existing standards (90-percentile) [All river types] |
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River type | High | Good | High | Good |
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Upland and low alkalinity (types 1, 2, 4, 6) | 0.2 | 0.3 | 0.25* | 0.6* |
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Lowland and high alkalinity (types 3, 5, 7) | 0.3 | 0.6 |
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* Standards for the highest two classes in the existing Scottish Rivers Classification Scheme. |
4. Acid condition standards - pH
No new standards have been proposed in this paper - the existing standards used in SEPA's classification system will be retained for the first cycle of River Basin Management Planning.
Table A7: Acid condition (pH) - rivers |
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Proposed (and existing) standard
[All river types] |
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High (5- and 95- percentile †) | Good (10 percentile †) |
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 6 to  9
( i.e. range 6 - 9 inclusive) | 5.2 |
pH a measure of the concentration of H + ions - a low pH represents a lot of H + ions and therefore very acidic water. A pH of 7 is defined as neutral. Acidic water has a pH below 7; alkaline water has a pH over 7. † To meet the proposed 'good' standard, a river cannot have a pH lower than 5.2 for more than 10% of the time. To achieve the proposed 'high' standard, water pH cannot be lower than 6 for more than 5% of the time, or higher than 9 for more than 5% of the time. |
5. Nutrient condition standards - Phosphorus
The tables below compare the proposed standards with:
Table A8: values used under the Water Framework Directive for classification or characterisation purposes - also measured as soluble reactive phosphorus
Table A9: guidelines proposed by the Environment Agency and Countryside Council for Wales as part of a process of reviewing permit conditions to meet the requirements of the Habitats Directive - measured as total reactive phosphorus.
Table A8: Soluble reactive phosphorus (µg/l) - rivers |
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Proposed standard (annual mean †) | Standards used for characterisation
(annual mean †)
[All river types] |
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River type | High | Good | High a | Good b |
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1n | 30 | 50 | 20 | Sil c40 | 100e |
2n | 20 | 40 |
3n*, 4n | 50 | 120 | Cal d100 |
* see comments in 'typology' descriptions above † annual mean is the average of all measured values during a year a 'High' value used for characterisation and classification under WFD b 'Good' value used for characterisation under WFD c 'Sil' refers in WFD characterisation (Article 5 report) to a river in a siliceous-typed section ( i.e. flowing over rock with high silica content, such as sandstone) - low alkalinity river. d 'Cal' refers in WFD characterisation (Article 5 report) to a river in a calcareous-typed section ( i.e. flowing over rock with high calcium content, such as limestone) - high alkalinity river. e Existing standard in the Scottish Rivers Classification Scheme - applied to all rivers |
Table A9: Reactive phosphorus (µg/l) - rivers |
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Proposed standard (annual mean) - soluble reactive P | EA guidelines for meeting Habitats Directive (annual mean) - total reactive P |
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High | Good | River type | Natural (mid-High) | Guideline (mid-Good) | Threshold (just above Moderate) |
20-50 | 40-120 | Head waters | 0-20 | 20-60 | 40-100 |
Most rivers | 20-30 | 40-100 | 60-200 |
Large rivers | 20-30 | 60-200 | 100-200 |
The 'Natural' level is an estimate of water quality with no human impact. |
Lochs - water quality
1. Loch typologies for water quality
Overall:
A number of factors have a significant influence on the ecology of lochs, including geology, depth and altitude. However, for the water quality standards proposed in this paper, only very simple groupings of lochs are used.
Oxygenation:
Standards are proposed for two loch types, based on the species of fish they naturally support:
Salmonid waters - can support salmonid or 'game' fish, i.e. salmon, trout and whitefish.
Cyprinid waters - can support cyprinid or 'coarse' fish, which include carp, barb, roach and chubs.
Salinity:
A single standard is proposed for all lochs.
Acid conditions:
A single standard is proposed for all lochs.
2. Oxygenation standards - dissolved oxygen
| Table A10: Dissolved oxygen (mg/l) - lochs |
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Proposed boundary (mean in July-August)
[All loch types] | Standards currently used by Swedish Environment Protection Agency |
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Loch type | High | Good | Moderate | Poor | O 2 rich | Moderately O 2 rich | Moderately O 2 deficient | O 2 deficient | ~ no O 2 |
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Salmonid | 9 | 7 | 4 | 1 | 7 | 5 | 3 | 1 | <1 |
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Cyprinid | 8 | 6 | 4 | 1 |
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3. Acid condition standards - Acid Neutralising Capacity ( ANC)
The table below compares the proposed boundary standards with the standards for acidity currently used by SEPA.
Table A11: Acid Neutralising Capacity (µ equivalents /l) - lochs |
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Proposed standard
[All loch types] | Existing standards b
[All loch types] |
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High | Good | Baseline ANC0 | Present ANCt |
40 | 39 to 20 | 19 to 0 | -1 to -20 | -21 to -40 | < -40 |
> 40 | > 20 a | 40 | b. 1 | b. 2 | b. 2 | b. 3 | b. 3 | b. 4 |
20-39 | | b. 1 | b. 2 | b. 3 | b. 3 | b. 4 |
0-19 | | | b. 1 | b. 2 | b. 3 | b. 4 |
a where other evidence demonstrates that the loch had a historic (pre-industrial) value below 20, UKTAG recommends that 0 is used. b The existing SEPA scheme uses a look-up table. The current ANC value ( ANCt) is compared with a baseline value for that loch ( ANC0) to assign a band (class) from 1 to 4. The table shows how changes in the ANCt value relative to the baseline ANC0 value are related to band assignment. |
4. Salinity standards - conductivity
Table A12: Salinity - lochs (reported by proxy as µ siemens /cm) |
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Proposed boundary (annual mean)
[All loch types] |
Good |
1,000 |
There are no existing standards for salinity levels in Scottish lochs.
Transitional and coastal waters - water quality
1. Oxygenation standardsTable A13: Dissolved oxygen (mg/l) - transitional /coastal waters |
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Proposed standard
(5-percentile) |
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Status | Freshwater | Marine | Description |
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High | 7 | 5.7 | Protects all life-stages of salmonid fish |
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Good | 5 - 7 | 4.0 - 5.7 | Resident salmonid fish |
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Moderate | 3 - 5 | 2.4 - 4.0 | Protects most life stages of non-salmonid adults |
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Poor | 2 - 3 | 1.6 - 2.4 | Resident non-salmonid fish, poor survival of salmonid fish |
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Bad | 2 | 1.6 | No salmonid fish. Marginal survival of resident species |
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These proposed standards take into account recent studies on fish oxygen requirements. These found that an upper limit of 7 mg/l is enough for most requirements, but most resident fish species are stressed when only 2 mg/l available. |
Dissolved oxygen varies with salinity. Table A13 sets out proposed standards for dissolved oxygen in saline marine waters and in freshwaters. Values for transitional waters with intermediate levels of salinity should be read from the graph below.
Intermittent Standards
Additionally, UKTAG recommended the use of standards for intermittent discharges. These are proposed to protect transitional and coastal waters from extreme events that can cause prolonged periods of low oxygen, which can cause serious damage to biology 8.
Return periods define the minimum period allowed between episodes of low oxygen concentration - e.g. the dissolved oxygen level should not fall below the threshold level for more than one 6 hour tidal cycle in any 6 year period for 'good' status. Oxygen levels must remain above the threshold concentration for the rest of the time.
Table A14: Dissolved oxygen (mg/l) for action on intermittent discharges - transitional and coastal waters |
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Proposed Intermittent Standard (5-percentile) |
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Status | Minimum dissolved oxygen (mg/l) | Return period (years) |
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Good | 2 | 1 in 6 |
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Moderate | 2 | 1 in 3 |
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