Measurement of River Water Quality
This Briefing Sheet describes how the water quality of rivers is assessed using two different types of data, biological and physico-chemical, to give as complete a picture as possible.
THE BIOLOGICAL ASSESSMENT OF WATER QUALITY
Biological surveillance is the only means whereby changes to the riverine ecology can be detected, and it forms the essential complement to the longer-established physicochemical monitoring of water quality. Broadly, it may be said that:
- physicochemical monitoring will measure the causes of pollution and the quantity of pollutants,
- biological surveillance will measure the effects of pollution.
Biological monitoring procedures for rivers are based principally on the examination of suitable resident organisms. Most widely used in river quality investigations are the group of larger and readily visible invertebrate animals which colonise the substrata of all rivers. These animals are collectively referred to as macroinvertebrates, the main constituents of which are young aquatic stages of certain insects.
The sensitivity and tolerance to pollution shown by different members of this animal group vary considerably from species to species. Some species are very sensitive to reductions in dissolved oxygen and will not be found in areas where the oxygen levels are not consistently high. The opposite is true of other animals, and there is a whole range of species with responses in between.
A characteristic feature of polluted environments is a reduction in overall community diversity in line with increasing pollution severity, and also an increase in the density of tolerant species. A temporary improvement in conditions will not be reflected immediately in the macroinvertebrate community because the life cycles of the individual species, in contrast to micro-organisms, are relatively long. It follows that recolonisation of areas which were, but are no longer polluted, will take time.
Unlike fish which are mobile and may avoid polluted areas, the macroinvertebrates are unable to avoid pollution in this way, so that the community has to endure the full range of environmental conditions which affect its particular habitat. The composition of a community from any given point in a river therefore reflects the average water quality at that particular point. It is this fact which makes the macro invertebrate assessment procedure such a valuable tool in water quality assessment.
THE BIOLOGICAL ASSESSMENT PROCEDURE OF THE EPA
Biological water quality assessments by the Environmental Protection Agency are based on the composition of the macroinvertebrate communities which inhabit the substratum of rivers and streams. These comprise, in the main, immature aquatic stages of insects, together with crustacea (shrimps), mollusca (snails and bivalves), oligochaeta (worms), and hirudinea (leeches). Shallow, fast-flowing, well-aerated stretches of river "riffles" are sampled in preference to "nonriffle" areas, as they show most clearly the water quality status and effects of pollution.
For assessment purposes the communities have been divided arbitrarily into four groups - sensitive, less sensitive, tolerant and very tolerant forms. The relative proportions of the various organisms in a sample are determined, and the water quality status is then inferred by comparison with the expected ratios in unpolluted habitats of the type under investigation. The assessment procedure also takes into account other relevant factors such as the intensity of algal and/or weed development, water turbidity, bottom siltation, nature of the sub-stratum, speed of current, and water depth. The biological information is then condensed to readily understandable form by means of a 5-point biotic index (Q values), an arbitrary system in which community composition and water quality are related:
| Biotic Index (Q value) |
Water Quality |
| 5 (diversity high) |
good |
| 4 (diversity slightly reduced) |
fair |
| 3 (diversity significantly reduced) |
doubtful |
| 2 (diversity low) |
poor |
| 1 (diversity very low) |
bad |
Intermediate values e.g. Q3-4 or Q1-2, are used to describe conditions where appropriate. Also, where toxic influences are suspected the suffix 0 is appended to the relevant Q rating, e.g. Q 1/0 or Q 1-2/0. In the interests of simplicity four main classes of water quality have been defined. These relate to the Q Value scale and indicate the degree of pollution as follows:
| Quality Ratings |
Category of River Water Quality |
| Q5, Q4-5, Q4 |
unpolluted |
| Q3-4 |
slightly polluted |
| Q3, Q2-3 |
moderately polluted |
| Q2, Q2-1, Q1 |
seriously polluted |
THE PHYSICO-CHEMICAL DETERMINATION OF WATER QUALITY
It is important in water management to know the concentrations of the various constituents (natural or pollutional) of water. In the most commonly met form of pollution, by organic biodegradable wastes (see Briefing Sheet 11), physicochemical assessment of water quality is usually based on five parameters: Biochemical Oxygen Demand (BOD), Ammonia, Nitrate, Phosphates and Dissolved Oxygen (DO).
The last-mentioned, discussed in Briefing Sheet 11, is of cardinal importance and is always determined in the physico-chemical surveillance of rivers, normally by a portable meter. The parameters, ammonia - which is toxic - and nitrate and phosphates, both of which are nutrients and hence key factors in eutrophication, have all been discussed in Briefing Sheet 11, leaving BOD to be commented on here.
In the BOD test the rate of loss of oxygen in a sample incubated under standard conditions is measured. The oxygen uptake is due to the activity of micro-organisms in breaking down the organic matter present in the sample. The greater the rate of loss of dissolved oxygen the greater is the amount of organic matter present. Thus the test provides a useful general measure of the level of contamination of the sampled water by biodegradable wastes.
Important as they are, this limited range of parameters does not comprise the only constituents of water analysed in cases of pollution. Other parameters such as colour, pH (which indicates the degree of acidity or alkalinity) and chloride (a major constituent of sewage and other wastes) are all determined almost routinely on river waters.
In cases of industrial or mining pollution a most important group of pollutants which must be analysed are metals iron, manganese, copper, lead, zinc, cadmium, chromium, nickel, etc. All are undesirable and some, e.g., cadmium, copper and zinc, are very toxic to fish. There are other pollutants whose presence or absence must be ascertained organic micropollutants, highly undesirable compounds such as pesticides which can have very serious effects even in trace amounts, hence the term, micropollutant.
In contrast to the biological assessment, the physico chemical examination relates to discrete samples of water and thus to conditions at the precise moment of sampling. In order to obtain a reliable assessment of physico-chemical the sampling must be repeated at regular intervals. The frequency of sampling required is governed by the range of conditions liable to be encountered. Variability is likely to be greatest in reaches subject to waste discharges, particularly where these are intermittent.
In practice, sampling programmes designed to monitor physico-chemical characteristics are carried out at frequencies between weekly and monthly. It is advisable, however, that such regular sampling be augmented by more detailed surveys, e.g. of 24-hour duration, carried out in the more important reaches at critical periods. The most important period is likely to be in the summer or autumn months when river flows are at their maximum, water temperatures are at their maximum and dissolved oxygen levels are at their lowest.
COMPARISON OF PROCEDURES
The physico-chemical techniques have the merit of being precise, discriminatory and quantitative. They are 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 data required by licensing authorities for the assessment of compliance by licensed discharges with prescribed standards. However, with regard to general quality monitoring, especially in Ireland where there is a large number of clean rivers to be monitored, costs may be prohibitive in view of the need to repeat the sampling to gain sufficient data for an assessment.
Instead, a winter and a summer biological assessment would normally provide a reasonably accurate indication of average water quality, whereas a considerably greater number of physico-chemical samples would usually be required to achieve the same degree of confidence. Furthermore, if a discharge is irregular or surreptitious there is a good chance that routine physico-chemical monitoring would not detect it; the probability is the opposite for biological analysis.
The properties of the techniques described above are summarised in the following table, which shows that the most satisfactory approach to river water quality surveillance is to combine both procedures, which jointly offer the most benefits.
| Parameter |
Monitoring Performance |
| Chemical |
Biological |
| Precision (i.e. assessment of pollutant concentration) |
Good |
Poor |
| Discrimination (what type of pollution) |
Good |
Poor |
| Reliability (how representative is the limited number of samples) |
Poor |
Good |
| Measure of effects |
No |
Yes |
| Relative Costs |
High |
Low |