In the presence of pollution, characteristic and well-documented changes are induced in the flora and fauna of rivers and streams.
Particularly well documented are the changes brought about by organic pollution in the macroinvertebrate community i.e., the
immature aquatic stages of aerial insects (mayflies, stoneflies etc.) together with Crustacea (e.g. shrimps), Mollusca (e.g. snails
and bivalves), Oligochaeta (worms) and Hirudinea (leeches). The changes which occur are due to the varying sensitivities of the
different components of the community to the stresses caused by pollution. It is known that similar organisms inhabit similar
habitats and that the most sensitive species inhabit the riffle areas. It is also well known that community diversity declines in
the presence of pollution and that sensitive species are progressively replaced by more tolerant forms as pollution increases.
Ideally, all the components of the aquatic biota (the micro-and macro-fauna and flora) should be utilised but in practice
macroinvertebrate community analysis is found to be satisfactory for routine water quality monitoring purposes.
For the purposes of the EPA assessment procedure benthic macroinvertebrates have been divided into five arbitrary
'Indicator Groups' as follows: Group A, the sensitive forms, Group B, the less sensitive forms, Group C, the tolerant forms,
Group D, the very tolerant forms and Group E, the most tolerant forms. These groups, and their relationships with the Biotic
Index (Q values) are set out below.
In contrast to physico-chemical surveys which extend throughout the year, biological surveys are usually undertaken in
the summer-autumn period (June-October) when flows are likely to be relatively low and water temperatures highest. Surveys
during this period are likely, therefore, to coincide with the worst conditions to be expected in those reaches affected by waste
inputs. Biological material for examination is obtained by a 'kick' sampling technique in the faster- flowing areas (riffles) of the
river or stream and the examination and assessment of water quality is made on site. Having determined the relative proportions
of the various organisms in a sample, water quality can be inferred by a comparison of this data with that which might be expected
from unpolluted habitats of the type under investigation. Other relevant factors such as the intensity of algal and/or weed
development, water turbidity, bottom siltation, substratum, current speed and water depth, DO saturation and water
temperature, are also taken into account in the assessment procedure.
Relationships between water quality and macroinvertebrate community structure are usually described by means of a numerical
scale of values. Such a compression of biological information inevitably results in a loss of meaningful information but some such
procedure is essential if this information is to be meaningful to non-biologists. The EPA scheme of Biotic Indices or Quality (Q) Values
and its relationship to water quality is set out here.
'Q' Value Community Diversity Water Quality Condition * Q5 High Good Satisfactory Q4 Reduced Fair Satisfactory Q3 Much Reduced Doubtful Unsatisfactory Q2 Low Poor Unsatisfactory Q1 Very Low Bad Unsatisfactory
* 'Condition' refers to the likelihood of interference with beneficial or potential beneficial uses.
The intermediate indices Q1-2, 2-3, 3-4 and 4- 5 are also used to denote transitional conditions. The scheme
mainly reflects the effects of biodegradable organic wastes (i.e. deoxygenation and eutrophication) but where a toxic
effect is apparent or suspected the suffix '0' is added to the biotic index (e.g. Q 1/0, 2/0 or 3/0) and attention is sometimes
drawn to siltation or atypical effects by appending an asterix to the biotic index. The scheme may be further simplified as
shown by the classification set out below:-
Biotic Index Quality Status Quality Class Q5, 4-5, 4 Unpolluted Class A Q3-4 Slightly Polluted Class B Q3, 2-3 Moderately Polluted Class C Q2, 1-2, 1 Seriously Polluted Class D
Class A waters are those in which problems relating to existing or potential uses are unlikely to arise; they are, therefore,
regarded as being in a 'satisfactory' condition. Classes B, C and D are to a lesser or greater extent 'unsatisfactory' in this regard.
For example, the main characteristic of Classes B and C waters is eutrophication which may interfere with the amenity, abstraction
or fisheries uses of such waters.
For the assessment of organic pollution the more commonly measured parameters include DO, BOD, Ammonia, Oxidised
Nitrogen (Nitrites plus Nitrates) and Phosphates. Continuous records of concentration and flow would form the ideal basis for
water quality assessment but in practice this is impossible for financial, technical and logistical reasons. Reliance must,
therefore, be placed on discrete samples because such samples constitute only a minute fraction of the whole body of water
under investigation and because they are only representative of conditions at the particular time of sampling the interpretation
of data arising from such samples requires great care.
Unlike the biological assessment of water quality, where the incidence and intensity of pollution is based on the degree to
which the chosen organism association deviates from its expected natural diversity, the physico-chemical assessment is
usually based on a comparison of the measurements made with water quality criteria or with standards derived from such
criteria. The setting of national standards for water, sewage and other effluents by the Minister for the Environment is provided
for under the Local Government (Water Pollution) Act, 1977 and the Environmental Protection Agency Act, 1992. Water quality
guidelines have been issued by the Minister on the advice of a technical committee (Technical Committee on Effluent and Water
Quality Standards, 1979). In addition, legally binding standards for water quality in Ireland arise from various EC Directives. Of
particular relevance in the present context are the 'Surface Water' and 'Freshwater Fish' Directives (C.E.C., 1975, 1978). The
former deals with the quality requirements of waters used as sources of public supply while the latter sets standards for waters
harbouring game or coarse fisheries, although these are legally binding only in the case of 'designated' waters. Both of these
Directives are now the subject of National Regulations, (Minister for the Environment, 1988, 1989). A digest of these standards
and guidelines for the more important of the physico-chemical parameters of pollution by organic wastes as appropriate to fishery
salmonid waters is set out below:
Water Quality Guidelines 50% of samples > 9mg/l O2
95% of samples > 6mg/l O2
No sample < 4 mg/l O2
< 5 mg/l < 4mg/l < 0.02 mg/l NH3 or < 0.02 mg/l NH3
< 0.016 mg/l N* < 0.02 mg/l N*
< 0.8 mg/l N**
Freshwater Fish Regulations
*=unionised **=total
These limits are more stringent than those applicable to the same parameters in abstraction waters
receiving standard treatment, as set out in the 'Surface Water' Regulations. The same position holds in the case of
most other water quality parameters so that the suitability of waters for fisheries is usually a good assurance of their
suitability for abstraction and for many other uses. The major exceptions are nitrate and microbiological quality in
which cases even high levels of contamination will not directly affect fish life.
Physico-chemical techniques have the merit of being precise, discriminatory and quantitative and they are, therefore,
essential if unpolluted waters are to be chemically typed or if pollutants in water are to be identified and their concentrations
quantified. Information of this type is essential to good water management as it provides the basic information required by
licensing authorities for the assessment of compliance by licensed discharges with prescribed standards. With regard to
general water quality monitoring, however, and particularly where a large number of clean rivers are to be monitored - as
in this country - a distinct disadvantage of a purely chemical approach is the cost: whereas just two biological samples
per annum (winter and summer) would normally provide a reasonably accurate assessment of average water quality, a
considerably greater number of physico - chemical samples would normally be required to achieve such an assessment
with the same degree of confidence.
A knowledge of the types of pollutants likely to be present is a prerequisite for effective chemical monitoring. With the
increasing complexity of many industrial effluents this may prove difficult if not impossible in certain circumstances.
Furthermore, if a discharge is irregular or surreptitious there is a good chance that it will not be detected at all by routine
chemical monitoring programmes. Since benthic macroinvertebrate communities respond to a wide range of water quality
characteristics and pollutants and because they can reflect the effects of mixed pollutants these shortcomings can often be
overcome by biological analysis.
A disadvantage of the biological approach is that, although capable of detecting ecological change, indicative of water quality
change, it does not identify the specific cause of a change: for this physico-chemical analysis is essential, especially in the case
of toxic pollution. It should also be pointed out that whilst water indicated to be of poor quality on biological grounds is suspect
for most uses, water indicated to be of good quality on such grounds, although acceptable for most uses including fisheries, may
not always be free from pathogens or harmful trace organics and may not therefore be acceptable as drinking water. Assessment
of this aspect requires specific microbiological and physico-chemical tests. Finally, in assessing water quality from data involving
benthic communities, due recognition must be given to the influences of other ecological factors such as depth and flow rate,
substratum type, the influence of shading and seasonal changes in life cycle.
From the foregoing it may be appreciated that both physico-chemical and biological water quality assessment techniques
have their own particular applications, advantages and disadvantages so that only by a combination of both may the limitations
of each be overcome and a thorough understanding of the total situation be gained. The advantages and shortcomings of the
two approaches are summarised below.
|
REALM |
PERFORMANCE |
|
|
|
Chemical |
Biological |
|
Precision |
Good |
Poor |
|
(Pollutant concentration assessment) |
|
|
|
Discrimination |
Good |
Poor |
|
(Pollutant identification) |
|
|
|
Measure of Effects |
No |
Yes |
|
Cost |
High |
Low |
|
Single Sample Value |
Poor |
Good |
|
Quality Classes |
Class A |
Class B |
Class C |
Class D |
||
|
Quality Ratings |
Q5 |
Q4 |
Q3-4 |
Q3 |
Q2 |
Q1 |
|
Pollution Status |
Pristine, Unpolluted |
Unpolluted |
Slight Pollution |
Moderate Pollution |
Heavy Pollution |
Gross Pollution |
|
Organic Waste Load |
None |
None |
Light |
Considerable |
Heavy |
Excessive |
|
Maximum B.O.D. |
Low (< 3mg/l) |
Low (< 3mg/l) |
Occasionally elevated |
High at times |
Usually High |
Usually very high |
|
Dissolved Oxygen |
Close to 100% at all times |
80%-120% |
Fluctuates from <80% to >120% |
Very unstable, Potential fish-kills |
Low, sometimes zero |
Very low, often zero |
|
Annual median PO4 |
0.015 mg/l |
0.03 mg/l |
0.045 mg/l |
0.07 mg/l |
> 0.1 mg/l |
> 0.1 mg/l |
|
Siltation |
None |
May be light |
May be light |
May be considerable |
Usually heavy |
Usually very heavy and anaerobic |
|
'Sewage Fungus' |
Never |
Never |
Never |
May be some |
Usually abundant |
May be abundant |
|
Filamentous Algae |
Limited Development |
Considerable growth, diverse communities |
Luxuriant growths, typically Cladophora |
Excesssive growths, typically Cladophora |
Usually abundant |
None |
|
Macrophytes |
Diverse communities, limited growths |
Diverse Communnities, Considerable Growths |
Reduced diversity, luxuriant growths |
Limited diversity, excessive growths |
Tolerant species only, may be abundant |
Most tolerant forms, minimal diversity |
|
Water Quality |
Highest quality |
Fair Quality |
Variable quality |
Doubtful quality |
Poor quality |
Bad quality |
|
Abstraction Potential |
Suitable for all |
Suitable for all |
Potential problems |
Advanced treatment |
Low grade abstractions |
Extremely limited |
|
Fishery Potential |
Game fisheries |
Good game fisheries |
Game fish at risk |
Coarse fisheries |
Fish usually absent |
Fish absent |
|
Amentiy Value |
Very high |
High |
Considerable |
Reduced |
Low |
Zero |
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