Chapter 3* - SELECTION OF WATER QUALITY VARIABLES

3.1. Introduction

The selection of variables for any water quality assessment programme depends upon the objectives of the programme (see Chapters 1 and 2). Appropriate selection of variables will help the objectives to be met, efficiently and in the most cost effective way. The purpose of this chapter is to provide information which helps the appropriate selection of variables. Each variable is discussed with respect to its origins, sources, behaviour and transformations in the aquatic system, the observed ranges in natural and polluted freshwaters, the role of the variable in assessment programmes, and any special handling or treatment of samples that is required. The final section of this chapter suggests some combinations of variables which might be used for different water quality assessment purposes. These can be used as a basis for developing individual programmes.

The methods employed to measure the selected variables depend on access to equipment and reagents, availability of technical staff and their degree of expertise, and the level of accuracy required by the objectives of the programme (see Chapter 2). A summary of the principal analytical methods for major variables is given in Table 3.1 and a summary of pre-treatment and storage of samples for different analyses is given in Table 3.2. Detailed descriptions of sampling and analytical methods are available in the companion volume to this guidebook by Bartram and Ballance (1996) and in a number of standard reference guides published by various international organisations and programmes, or national agencies (e.g. Semenov, 1977; WHO, 1992; NIH, 1987-88; Keith, 1988; APHA, 1989; AOAC, 1990). In addition a world-wide federation of national standards bodies and international organisations, the International Standards Organization (ISO), publishes a series of approved “International Standards” which includes methods for determining water quality. Further detailed information on the study and interpretation of chemical characteristics in freshwaters is available in Hem (1989), Environment Canada (1979) and many other specialist texts.

3.2. Hydrological variables

Determining the hydrological regime of a water body is an important aspect of a water quality assessment. Discharge measurements, for example, are necessary for mass flow or mass balance calculations and as inputs for water quality models.

Table 3.1. Analytical methods for determination of major chemical variables

Table 3.2. Pretreatment and storage requirements of samples for laboratory determination of chemical variables (see text for further details)

Where no indication is given under column headings, no special conditions of pretreatment or storage are necessary.

Sample bottles for many variables require special cleaning, particularly those for trace metals and organic micropollutants. Requirements for special cleaning are described in operational manuals for analytical methods (e.g. WHO, 1992).

DO Dissolved oxygen
POC Particulate organic carbon
TOC Total organic carbon
COD Chemical oxygen demand
BOD Biochemical oxygen demand

¹ When frozen

² NO₃^-, NH₄⁺, PO₄^3-, total P

3.2.1. Velocity

The velocity (sometimes referred to as the flow rate) of a water body can significantly affect its ability to assimilate and transport pollutants. Thus measurement of velocity is extremely important in any assessment programme. It enables the prediction of movement of compounds (particularly pollutants) within water bodies, including groundwaters. For example, knowledge of water velocity enables the prediction of the time of arrival downstream, of a contaminant accidentally discharged upstream.

Water velocity can vary within a day, as well as from day to day and season to season, depending on hydrometeorological influences and the nature of the catchment area. It is important, therefore, to record the time when measurements are taken and every attempt should be made to measure velocity at the same sites as other water quality samples are collected. Velocity is determined (in m s^-1) with current meters or tracers, such as dyes. Measurements are usually averaged over a period of 1-2 minutes.

3.2.2. Discharge

The discharge is the volume flowing for a given period of time. For rivers, it is usually expressed as m³ s^-1 or m³ a^-1. The amount of suspended and dissolved matter in a water body depends on the discharge and is a product of the concentration and the discharge. Natural substances arising from erosion (suspended matter) increase in concentration exponentially with increased discharge (see Figure 6.11A and section 6.3.3). Substances introduced artificially into a water body, such as trace elements and organic matter, tend to occur at decreasing concentrations with increasing river discharge. If a pollutant is introduced into a river at a constant rate, the concentration in the receiving water can be estimated from the quantity input divided by the river discharge (see the example in Figure 6.13). Sedimentation and resuspension (see Chapter 4) can, however, affect this simple relationship.

Discharge can be estimated from the product of the velocity and the cross-sectional area of the river. It should be measured at the time of sampling and preferably at the same position as water samples are taken. As cross-sectional area varies with different discharges, a series of measurements are needed in relation to the different discharges. Measurements of depth across a transect of the water body can be used to obtain an approximate cross-sectional area. Specific methods for calculating discharge are available in WMO (1974, 1980).

3.2.3. Water level

Measurement of water level is important to determine the hydrological regime of lakes, reservoirs and groundwaters and the interaction between groundwaters and surface waters. Measurement of water level is necessary for mass flow calculations in lakes and groundwaters and must be measured at the time and place of water sampling.

Water can flow to or from an aquifer which is in continuity with a river, depending on the relative water levels in the river and aquifer. Low water levels in the river can induce groundwater flow to the river, and high water levels can reverse the flow and produce losses from the river to the aquifer. Similarly, when groundwater levels are low (or deep) surface water infiltrates downwards to the water table (see Chapter 9). Depending on the relative water levels in the aquifer and river, stretches which gain or lose may occur in the same river. Also a particular stretch may be gaining at one time of year and losing at another, as river levels change with the seasons. As the river water and groundwater may be of very different qualities, significant variations in water quality may be experienced in wells close to rivers, and in the river itself. Measurement of groundwater levels is particularly important in relation to saline intrusion.

3.2.4. Suspended matter dynamics

Suspended particulate matter consists of material originating from the surface of the catchment area, eroded from river banks or lake shores and resuspended from the bed of the water body. Measurement of suspended matter transport is particularly important where it is responsible for pollutant transport and in such cases its measurements should be undertaken frequently (see Chapter 4). Usually sediment concentration and load increase exponentially with discharge (see Figure 6.11A). Particles may also settle, or be resuspended, under different discharge conditions.

Suspended matter concentrations should be measured along with the other hydrological variables. In rivers of uniform cross-section, a single sample point may be adequate, whereas for other rivers, multiple point or multiple depth, integrated sampling is necessary. Such samples should be taken at the same points as water velocity measurements and other water quality samples. In addition to analysing suspended matter as described in sections 3.3.4 and 3.3.5, grain size should be determined. Whenever possible, samples from bottom sediments should also be examined.

3.3. General variables

3.3.1. Temperature

Water bodies undergo temperature variations along with normal climatic fluctuations. These variations occur seasonally and, in some water bodies, over periods of 24 hours. Lakes and reservoirs may also exhibit vertical stratification of temperature within the water column (see Chapters 7 and 8).

The temperature of surface waters is influenced by latitude, altitude, season, time of day, air circulation, cloud cover and the flow and depth of the water body. In turn, temperature affects physical, chemical and biological processes in water bodies and, therefore, the concentration of many variables. As water temperature increases, the rate of chemical reactions generally increases together with the evaporation and volatilisation of substances from the water. Increased temperature also decreases the solubility of gases in water, such as O₂, CO₂, N₂, CH₄ and others. The metabolic rate of aquatic organisms is also related to temperature, and in warm waters, respiration rates increase leading to increased oxygen consumption and increased decomposition of organic matter. Growth rates also increase (this is most noticeable for bacteria and phytoplankton which double their populations in very short time periods) leading to increased water turbidity, macrophyte growth and algal blooms, when nutrient conditions are suitable.

Surface waters are usually within the temperature range 0° C to 30° C, although “hot springs” may reach 40° C or more. These temperatures fluctuate seasonally with minima occurring during winter or wet periods, and maxima in the summer or dry seasons, particularly in shallow waters. Abnormally high temperatures in surface water can arise from thermal discharges, usually from power plants, metal foundries and sewage treatment plants. Ground-water usually maintains a fairly constant temperature which, for surficial aquifers, is normally close to the mean annual air temperature. However, deep aquifers have higher temperatures due to the earth’s thermal gradient.

Temperature should be measured in situ, using a thermometer or thermistor. Some meters designed to measure oxygen or conductivity can also measure temperature. As temperature has an influence on so many other aquatic variables and processes, it is important always to include it in a sampling regime, and to take and record it at the time of collecting water samples. For a detailed understanding of biological and chemical processes in water bodies it is often necessary to take a series of temperature measurements throughout the depth of the water, particularly during periods of temperature stratification in lakes and reservoirs (see Chapters 7 and 8). This can be done with a recording thermistor linked to a pressure transducer, directly reading temperature with depth, or by reversing thermometers built into a string of sampling bottles, or by direct, rapid measurements of water samples taken at discrete depths.

3.3.2. Colour

The colour and the turbidity (see section 3.3.5) of water determine the depth to which light is transmitted. This, in turn, controls the amount of primary productivity that is possible by controlling the rate of photosynthesis of the algae present. The visible colour of water is the result of the different wavelengths not absorbed by the water itself or the result of dissolved and particulate substances present. It is possible to measure both true and apparent colour in water. Natural minerals such as ferric hydroxide and organic substances such as humic acids give true colour to water. True colour can only be measured in a sample after filtration or centrifugation. Apparent colour is caused by coloured particulates and the refraction and reflection of light on suspended particulates. Polluted water may, therefore, have quite a strong apparent colour.

Different species of phyto- and zooplankton can also give water an apparent colour. A dark or blue-green colour can be caused by blue-green algae, a yellow-brown colour by diatoms or dinoflagellates and reds and purples by the presence of zooplankton such as Daphnia sp. or copepods.

Colour can be measured by the comparison of water samples with a series of dilutions of potassium chloroplatinate and crystalline cobaltous chloride. The units are called platinum-cobalt units based on 1 mg l^-1 Pt. Natural waters can range from < 5 in very clear waters to 300 units in dark peaty waters. The total absorbance colour (TAC) method measures integrated absorbance of the filtered sample (pH 7.6) between 400 and 700 nm and the true colour (TUC) is determined by measuring the absorbance at 465 nm. One TAC unit is equivalent to the colour of 2 mg l^-1 Pt. The TAC units range from 1 to 250. As the compounds determining the colour of the water are not very stable, measurements should be made within two hours of collection.

3.3.3. Odour

Water odour is usually the result of labile, volatile organic compounds and may be produced by phytoplankton and aquatic plants or decaying organic matter. Industrial and human wastes can also create odours, either directly or as a result of stimulating biological activity. Organic compounds, inorganic chemicals, oil and gas can all impart odour to water although an odour does not automatically indicate the presence of harmful substances.

Usually, the presence of an odour suggests higher than normal biological activity and is a simple test for the suitability of drinking water, since the human sense of smell is far more sensitive to low concentrations of substances than human taste. Warm temperatures increase the rate and production of odour-causing metabolic and decay products. Different levels of pH may also affect the rate of chemical reactions leading to the production of odour.

Odour can be measured in terms of the greatest dilution of a sample, or the number of times a sample has to be halved with odour-free water, that yields the least definitely perceptible odour. The former method is known as the Threshold Odour Number (TON) and the latter method as the Odour Intensity Index (OII). Both methods suffer from the subjective variability of different human judges.

3.3.4. Residue and total suspended solids

The term “residue” applies to the substances remaining after evaporation of a water sample and its subsequent drying in an oven at a given temperature. It is approximately equivalent to the total content of dissolved and suspended matter in the water since half of the bicarbonate (the dominant anion in most waters) is transformed into CO₂ during this process. The term “solids” is widely used for the majority of compounds which are present in natural waters and remain in a solid state after evaporation (some organic compounds will remain in a liquid state after the water has evaporated). Total suspended solids (TSS) and total dissolved solids (TDS) correspond to non-filterable and filterable residue, respectively. “Fixed solids” and “volatile solids” correspond to the remainder after oven-drying, and to the loss after oven-drying at a given temperature, respectively. The latter two determinations are now less frequently carried out.

Residue determination is based on gravimetric measurement after following the appropriate procedures, i.e. filtration, evaporation, drying and ignition. The results of residue determination depend on the precise details of these procedures. Total suspended solids are the solids retained on a standard filter (usually a glass fibre “GF/C” grade) and dried to a constant weight at 105° C (Bartram and Ballance, 1996).

To achieve reproducibility and comparability, care must be taken in following the appropriate methods. For further details see WHO (1992) and Bartram and Ballance (1996). Samples should preferably be kept in hard-glass bottles until analysis can be performed, although polythene bottles can be used if the suspended material does not stick to the walls of the bottle. To help prevent precipitation occurring in the sample bottles they should be completely filled and then analysed as soon as possible after collection.

3.3.5. Suspended matter, turbidity and transparency

The type and concentration of suspended matter controls the turbidity and transparency of the water. Suspended matter consists of silt, clay, fine particles of organic and inorganic matter, soluble organic compounds, plankton and other microscopic organisms. Such particles vary in size from approximately 10 nm in diameter to 0.1 mm in diameter, although it is usually accepted that suspended matter is the fraction that will not pass through a 0.45 µm pore diameter filter (see Chapter 4). Turbidity results from the scattering and absorption of incident light by the particles, and the transparency is the limit of visibility in the water. Both can vary seasonally according to biological activity in the water column and surface run-off carrying soil particles. Heavy rainfall can also result in hourly variations in turbidity. At a given river station turbidity can often be related to TSS, especially where there are large fluctuations in suspended matter. Therefore, following an appropriate calibration, turbidity is sometimes used as a continuous, indirect measurement for TSS.

Transparency can be measured easily in the field and is, therefore, included in many regular sampling programmes, particularly in lakes and reservoirs, to indicate the level of biological activity. It is determined by lowering a circular disc, called a Secchi disc, on a calibrated cable into the water until it just disappears. The depth at which it disappears, and just reappears, is recorded as the depth of transparency. A Secchi disc is usually 20-30 cm in diameter (although the result is not affected by the disc diameter), and coloured white or with black and white sectors.

Turbidity should be measured in the field but, if necessary, samples can be stored in the dark for not more than 24 hours. Settling during storage, and changes in pH leading to precipitation, can affect the results during storage. The most reliable method of determination uses nephelometry (light scattering by suspended particles) by means of a turbidity meter which gives values in Nephelometric Turbidity Units (NTU). Normal values range from 1 to 1,000 NTU and levels can be increased by the presence of organic matter pollution, other effluents, or run-off with a high suspended matter content. A visual method of determination is also available in Jackson Turbidity Units (JTU), which compares the length of the light path through the sample against a standard suspension mixture.

3.3.6. Conductivity

Conductivity, or specific conductance, is a measure of the ability of water to conduct an electric current. It is sensitive to variations in dissolved solids (see section 3.3.4), mostly mineral salts. The degree to which these dissociate into ions, the amount of electrical charge on each ion, ion mobility and the temperature of the solution all have an influence on conductivity. Conductivity is expressed as microsiemens per centimetre (µS cm^-1) and, for a given water body, is related to the concentrations of total dissolved solids and major ions (see Figure 10.14). Total dissolved solids (in mg l^-1) may be obtained by multiplying the conductance by a factor which is commonly between 0.55 and 0.75. This factor must be determined for each water body, but remains approximately constant provided the ionic proportions of the water body remain stable. The multiplication factor is close to 0.67 for waters in which sodium and chloride dominate, and higher for waters containing high concentrations of sulphate.

The conductivity of most freshwaters ranges from 10 to 1,000 µS cm^-1 but may exceed 1,000 µS cm^-1, especially in polluted waters, or those receiving large quantities of land run-off. In addition to being a rough indicator of mineral content when other methods cannot easily be used, conductivity can be measured to establish a pollution zone, e.g. around an effluent discharge, or the extent of influence of run-off waters. It is usually measured in situ with a conductivity meter, and may be continuously measured and recorded. Such continuous measurements are particularly useful in rivers for the management of temporal variations in TDS and major ions.

3.3.7. pH, acidity and alkalinity

The pH is an important variable in water quality assessment as it influences many biological and chemical processes within a water body and all processes associated with water supply and treatment. When measuring the effects of an effluent discharge, it can be used to help determine the extent of the effluent plume in the water body.

The pH is a measure of the acid balance of a solution and is defined as the negative of the logarithm to the base 10 of the hydrogen ion concentration. The pH scale runs from 0 to 14 (i.e. very acidic to very alkaline), with pH 7 representing a neutral condition. At a given tempe