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ACIDIFICATION

CLASSIFICATION

In acid waters with pH <7 and Ca <4 mgl an additional component is used for WFD river invertebrate status
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classification in Scotland and Wales to reflect impacts from acid deposition – see Section 2.4. It is based on
WFD AWICsp (see Chapter 5 Section 3.1.8).

WFD AWIC sp is an index of acid deposition. It was developed The WFD AWIC index, typology and classification are
sp
from previous versions of AWIC (Acid Water Indicator calculated using simple algorithms in a spreadsheet that can
Community). Unlike the WHPT indices, its reference values be downloaded from UK TAG’s website: http://www.wfduk.
are not derived from RIVPACS (Section 3.1.2 and 3.1.3) org/sites/default/files/Media/Characterisation%20
but are based on the likelihood of acid deposition. This is of%20the%20water%20environment/Biological%20
because acidification (acid deposition) is not the only source Method%20Statements/WFD-AWIC%20
of increased acidity: effluents from existing and historical Calculation%20sheet.xls
mining, particularly for metals, and some industrial effluents
can also increase acidity. The class boundaries are given in Table 3.8. At pH and
Ca levels in excess of those stated, WFD AWIC ceases to
Measures of confidence of class are not provided for respond to the primary environmental gradient and will give
this classification, which hinders its interpretation. The erroneous results.
acidification classification is used to report WFD status
in Scotland and Wales only, although the method can be
applied (with caution) to rivers elsewhere in the UK.

Table 3.8
The UK’s official river invertebrate acidification class boundaries (from government directions)

WFD AWIC (EQR)
WFD AWIC (EQR) WFD AWIC (EQR)
Class boundary Clear water, England
Humic water Clear water, Scotland
& Wales
High/Good 0.93 1.00 0.91

Good/Moderate 0.83 0.89 0.83
Moderate/Poor 0.77 0.78 0.72

Poor/Bad 0.73 0.67 0.66

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