Chemistry of natural waters—VI

Water Research Pergamon Press 1971. Vol. 5, pp. 943-956. Printed in Great Britain

CHEMISTRY

OF NATURAL

WATERS--VI

CLASSIFICATION OF WATERS P. H. KEMP Natal Regional Laboratory, National Institute for Water Research (S.A. Council for Scientific and Industrial Research), Durban, South Africa

(Received 19 November 1970) Abstract--A scheme is proposed for standardizing water analyses to make them directly comparable despite differences in TDS. This utilizes the molar concentrations of the acids and bases considered to be dissolved in the water and leads to a convenient diagrammatic representation of water analyses. Comparison of Natal river waters by this means shows that they possess various features in common and can be grouped together in a series of chlorided waters. Rain water and sea water also fit into this series, as do many American river waters. A number of American river waters do not fit into this series but rather appear to constitute a series of sulphated waters of rather different composition. The technological properties of waters (buffer capacity, phi, hardness, etc.) are strongly dependent on TDS and waters of either series can occur at any level of TDS. Since the chemical composition of a water is not arbitrary but rather appears to be governed by the series relationships, TDS is of greater importance than chemical composition in any classification of waters from a practical standpoint. Water quality is an ill-defined concept. It is often more useful to consider the probable utility of the water, and a broad assessment of this can be obtained from knowledge of the electrical conductivity of the water and the BeD. Taking into account various accepted specifications for water for particular uses, a scheme of classification of waters into five classes according to probable utility is proposed. This has a simple statistical basis and can be adapted, with scant loss in precision, for use where only few samples of the water have been studied. The classification leads to a clear and consistent picture of the present water sources of Natal. INTRODUCTION

PREVIOUSpapers of this series have discussed some of the fundamental ionic relationships arising in the inorganic chemistry of natural waters and various related technical parameters of importance in water chemistry. It is of interest to complete the discussion by investigating the application of the principles so considered to a practical scheme for classifying waters. It is clear at the outset that any useful classification scheme must take into account the TDS of the water, since this, as has been shown, has a major influence upon the hardness, buffering power, pH, and pH of natural waters. The actual chemistry of the water, i.e. the ratios subsisting between the concentrations of the various solutes, i n d e p e n d e n t o f the T D S , w o u l d seem to be o f e q u al i m p o r t a n c e . W e t h er ef o r e c o m me n ce by considering this m o r e c o m p l e x m a t t e r first. STANDARDIZED ANALYSES

To examine changes of chemical composition apart from those of mere dilution or c o n c e n t r a t i o n , a m e a n s o f eliminating the effects o f v a r y i n g T D S is required. O n e simple way o f a c c o m p l i s h i n g this is to utilize t he m o l a r c o n c e n t r a t i o n s o f the various acids an d bases dissolved in the w a t e r (see Part I). E a c h o f these can be expressed as a percentage o f t h e i r total, a n d the result is a set o f molar percentages t h a t is independent of the TDS. 943

944

P.H. KEMP

In most instances it is necessary to consider only carbonic, sulphuric and hydrochloric acids and the hydroxides of calcium, magnesium, sodium and potassium. All the concentrations except the first can usually be obtained directly from the analysis of the water, while that of carbonic acid can be found from the pH value and the total alkalinity, as described in Part III, if an actual determination has not been made. In exceptional cases, additional acids and bases that are present in significant concentrations may be included if necessary. Dissolved silica can also be included if present in ionic form. However, in most instances the silica will be in non-ionic form and moreover at a concentration independent of the TDS so that it may be neglected on the grounds that its percentage would bias the results so that they are no longer independent of the TDS. This procedure has the advantage that it does not make use of the experimentally determined TDS, which can be appreciably in error. It also avoids the difficulty that the solid material weighed during that determination does not coincide with the material originally present in solution (HEM, 1959), a difficulty which gives rise to such an anomaly as causing a sodium bicarbonate solution of 840 mg 1-1 to show the same experimental TDS as a sodium carbonate solution of 530 mg 1-1 owing to the loss of carbon dioxide during evaporation. A further advantage of the procedure described is that changes in the chemical composition of the water due to the addition of one or more salts result in closely related (though not exactly stoicheiometric) changes in the relevant molar percentages TABLE1. EXAMPLE

CALCULATION OF MOLAR PERCENTAGES

Original analysis: TDS pH value Ca Mg Na K Total alkalinity as CaCO3 SO4 CI Millimolar concentrations: Ca(OH)2 Mg(OH)2 NaOH KOH H2CO3 H2SO# HC1

100 7.50 21.2 3.5 6"0 2-2 61.5 12.6 5.9 0.530 0.146 0'261 0.0564 1.32 0.131 0.166

Molar percentages: Ca(OH)2 Mg(OH)2 NaOH KOH

20-3 5'6 10.0 2.2

H2CO3

50"5

H2S04 HC1

5"0 6"4

mg 1-1 nag 1-~ mg 1-1 mg 1-1 nag 1-I mg 1-~ mg 1-1 mg 1-1

Chemistryof Natural Waters--VI

945

This is not the case if weight concentrations (mg 1-1) are used to calculate the percentages instead of molar values. To illustrate the calculations, the first section of TABLE 1 gives a water analysis. The tabulated total alkalinity corresponds to 1.23 m equiv. 1-1 and from the given TDS value it is found that y = 0.952 (Part I) so that, from the pH and total alkalinity, the millimolar concentration of carbonic acid is 1.32 (Part III). The other molar concentrations are obtainable directly from the analytical data and are shown in the second section of the Table. The molar percentages are then as in the third section. In many instances the analysis of a given water sampled at different times will provide percentages that differ relatively slightly and in no systematic manner because, irrespective of changes in the TDS, the chemistry of the water remains reasonably constant. In other instances, the percentages may show systematic changes correlated with changes in the TDS, indicating that the water chemistry undergoes some kind of real variation. MOLAR COMPOSITION DIAGRAMS Comparison of molar percentage data is often a useful means of detecting the presence of pollution in natural waters and assessing the extent of seasonal changes in the water chemistry. A further useful application is the diagrammatic representation of the inorganic water chemistry. For this purpose the molar percentages may be taken in an invariable fixed order: Ca(OH)2 Mg(OH)2 Any other bases, in order of molecular weight NaOH KOH H2C03 Any other acids, in order of molecular weight H2SO4 HC1 Lines are drawn across a horizontal rectangle (whose long side represents 100 per cent) in such a way that the distance of each line from the one on its immediate left represents the molar percentage of one of the solutes according to the above order. The result will appear as in FIG. 1 and will resemble a line spectrum. It is convenient to use broken lines for the other acids and bases not specifically named in the above list, since this aids identification and comparison. With a little practice it will be found that the diagrams can be interpreted at sight, this being facilitated by the fact that the NaOH and KOH lines are usually closely spaced to form a "doublet" provided the specified fixed order is always observed. Indeed, keeping always to the same order is absolutely essential if the diagrams are to have any utility at all. For completeness it is useful to add the numerical value of the TDS of the water in mg 1-1 (or of its electrical conductivity in ~U cm-1 at 20°C) on the left of the diagram and its pH value on the right.

946

P.H. KEMP

z

o

[

o =T

•r

a

n o O L.

O~

N~ "tO

"1"

O~

0 "r oO Z ",,"

"

"1"

1 1

II II II

FIG. 1. General appearance of molar percentage diagram. The comparison of molar percentage diagrams drawn in this manner is often far more revealing that the comparison of tables of numerical data. Such diagrams can be used in exactly the same way as the similar yet simpler diagrams based on the weight concentration of the solutes (KE~n', 1971) but they are of a more general and adaptable nature. SURFACE

WATERS

OF

NATAL

On applying the above diagrammatic procedure to the results for many river waters obtained over the past seventeen years during the course of surveys carried out in Natal, it became apparent that the analyses of the waters showed some systematic variation. The molar percentage of each major solute in unpolluted waters was correlated with that of carbonic acid. Denoting the latter by x, the linear regressions shown in TABLE 2 were obtained, based on 50 water analyses. They are shown diagrammatically in FIG. 2, which is constructed so that the intersections of the regression lines with a horizontal line drawn through the appropriate molar percentage of carbonic acid give the positions of the lines in the corresponding molar percentage diagram, i.e. FIG. 2 is virtually a set of infinitely thin molar percentage diagrams mounted one above the other. It will be seen that, as the percentage of carbonic acid increases, those of N a O H and HCI decrease appreciably while those of Ca(OH)2 and Mg(OH)2 increase a little. The scatter of the fifty analyses on which the regressions are based is relatively large, as can be seen from the standard errors of estimate given in TABLE 2. The differences between waters of high and of low molar percentage of carbonic acid are, however, much greater than can be accounted for by experimental or random error. It therefore appears that all these waters collectively represent a real series of related analyses, and this series has been for convenience called the series of chlorided waters. The X2 statistic may be used to test whether or not a given analysis accords with this series: Expected) 2 X2 = x ~ ( Observed

the summation being extended over the six major solutes, excluding H2CO 3. With X2 > 15.03 there is no accord, whilewith X2 < 12.59 it maybe accepted that the observed analysis fits into the series (these are the values of X2 with 6 degrees of freedom at the 0.02 and 0.05 probability levels). It is of interest to note that the analysis of sea water (SvERDRUP et aL, 1942) and that of rain water provided by GOPa4AM (1955) accord very well with the chlorided series (X2 = 2.56 and 7.49 respectively). The analysis of TABLE 1 does not (X2 = 19.19).

Chemistry of Natural Waters--VI

947

=// ~- "°V

I

/

II

II

20

40 Molar per cent

60

80

I00

FiG. 2. The series of chlorided waters. The most interesting point, however, is that in all the waters examined, there is no correlation whatever between the molar percentage of carbonic acid (and hence of the TAeLE 2. REGRESSION EQUATIONS FOR. MOLAR.PERCENTAGES IN UNPOLLUTED NATAL RIVERS (CHLORIDED SERIES) Molar percentage

Ca(OH)2 Mg(OI-I)2 NaOH KOH HzSO, HCI

Regression on molar percentage of carbonic acid (x)

0.7 + 5.1 + 42"2 -0.2 + 3"0 -48'8 --

0.20x 0.08x 0"47x 0.02x 0"03x 0"80x

Standard error of estimate

1.92 1-94 3-23 1.52 2.32 2.81

other solutes) and the TDS. This finding is perhaps surprising in view of the comments o f CONWAY 0942), who considered that river waters did show systematic composition changes as the TDS increased. However, statistical support for that deduction appears to be lacking. The implications of the independence of composition from TDS will be considered below. AMERICAN SURFACE WATERS The analyses of 98 American river waters have been given by NOROELL(1951). The data do not include p H values, but it is clear from Part III of the present series of papers that no very great error is involved by assuming that the molar concentration o f carbonic acid is approximately equal to the equivalent concentration of total alkalinity. In addition, sodium and potassium were not itemized separately, but since the molar percentage o f potassium hydroxide in surface waters is always low a value of 2 per cent may reasonably be assumed.

948

P.H. KEMP

On calculating the molar percentages for all analyses (KEMP, 1969) it was found that, although many of them accorded quite well with the series of chlorided waters of FIG. 2, in forty instances there was no agreement. Consequently it appears that many American river waters are also members of the chlorided series but that many others are not. By comparing the forty discrepant cases with each other in the same way as before, it was again found that there was systematic variation, the molar percentage of each major solute being correlated with that of carbonic acid as shown in TABLE 3. The standard errors of estimate were rather greater than in TABLE 2, as would be expected in view of the fact that the analyses were less precise. These relations are shown diagrammatically in FIG. 3, which is constructed in the same way as FIG. 2. This diagram differs from the preceding one in showing rather greater percentages of Ca(OH)z, Mg(OH)2 and (most notably) H2SO4, with lower percentages of HC1. Using the X2 test as described above, analyses that fit FIG. 3 may 60

o~

°~

~-~

~'~/

~,/

ro

o

40

Eo o

20

I

20

I

I 40

~lV

I 60

t

II 80

I

tO0

M o l o r per c e n t

FIG. 3. The series of sulphated waters. be matched against those of similar carbonic acid percentage that fit FIG. 2 and vice versa. It is then found that the analyses represented by the two diagrams are statistically quite distinct provided the molar percentage of carbonic acid is less than about 50; at higher values no distinction is possible. TABLE 3. REGRESSION EQUATIONS FOR MOLAR PERCENTAGES IN AMERICAN RIVERS (SULPHATED SERIES)

Molar percentage Ca(OH)2 Mg(OH)2 NaOH KOH H2SO, HC1

Regression on molar percentage of carbonic acid (x) 15.5 + 6.1 + 27.8 -2.0 29.3 -19.3 --

0.07x 0.06x 0.36x 0.46x 0.3Ix

Standard error of estimate 2.81 2-97 4.58 -5.09 4.61

Chemistry of Natural Waters---VI

949

This suggests that, as well as the series of chlodded waters represented by FIG. 2, there exists what may be termed a series ofsulphated waters as represented by FIG. 3, although these are not well represented among the unpolluted river waters of Natal. Polluted Natal river waters, especially those receiving the sulphate-rich drainage water from coal mines, do however belong to the sulphated series. As for the Natal waters, no correlation between the TDS and any of the molar percentages of the American waters could be established. These results do not constitute a proof, but they strongly suggest that the chlorided and sulphated water series are two distinct types of waters that can be identified in nature. The origin of these water types cannot as yet be accounted for, but it is clear that any theory of the chemical composition of surface waters must take account of these results. Waters that CLARKe (1924) reported as distinct types are brought together in these two series, although it is not to be expected that the chlorided and sulphated water series are the only types of natural water to exist. Clarke himself listed many other kinds of waters, and it will be of great interest if comparisons of the sort described above can be made for other waters in other parts of the world by those who have sufficient data available. CHARACTERISTICS OF THE TWO WATER SERIES It is fairly simple, utilizing the equations given earlier in this series of papers, to compare the main properties of the chlorided and sulphated water series. The chlorided waters have a positive soda alkalinity if the molar percentage of carbonic acid exceeds about 40, the sulphated waters if it exceeds about 50. In consequence, both series show relatively low permanent hardness at high carbonic acid percentages. The chlorided waters have a higher sodium content for a given TDS value, and the sodium percentage (equivalents per litre of sodium as a percentage of the other cations) can exceed 60 when the molar percentage of carbonic acid falls below about 20. These waters may therefore present a sodium hazard when used for irrigation, but the waters of the sulphated series in general should not show this (see KLINTWORTH, 1952 and WILCOX, 1955--the so-called sodium absorption ratio is considered a better index of sodium hazard, and for the chlorided waters this is some 2 or 3 times greater than for the sulphated waters). The chlorided waters have slightly higher pHs values (by about 0.4) than those of the sulphated waters of comparable TDS. Most waters of either series will possess scale-dissolving properties when the TDS is about 100 mg 1-1 or less, and scale-forming properties when it is about 300 mg 1-1 or more. Their pH values are very similar. In fact, differences of TDS influence the properties of these waters far more than do differences of chemical composition. This is quite evident from FIGS 2 and 3, which show that the chemical changes encountered in passing along either of the water series are of a much lower order of magnitude than the range of TDS values found in natural waters (from about 20 mg 1-1 to 2000 mg l- ~ or more). Admittedly, waters of one or other of the two series do show some degree of variation. For example, a chlorided water with 50~o (molar) of carbonic acid will not always show exactly 18"7~o of sodium hydroxide, as is shown by the standard error in TABLE2. But it is clear that, if most waters belong to one or other of the two series, waters of outstanding chemical peculiarities will very rarely be encountered. In other words, the chemical composition of a surface water is not in general arbitrary but

950

P . H . KEMP

rather appears to be governed by the appropriate series relationships. Moreover, the same comment also applies to underground waters, since published data (BOND, 1946; NORDELL, 1951; VAN WYK, 1963) shows that these also generally accord with the chlorided or sulphated series. Consequently, from the practical point of view it appears to be more realistic to base a classification of water upon TDS rather than upon differences in chemical composition. Low TDS invariably goes with low pH, poor buffering powers, high phi, scale-dissolving properties and low total and permanent hardness, while high TDS is always accompanied by high pH, strong buffering powers, low phi, scaleforming properties and high total and permanent hardness, and reasonable differences in chemical composition do not change this general pattern. WATER QUALITY

AND UTILITY

There is a real need for a practical means of classifying waters to enable planners and water engineers to form a clear idea of their suitability for use, especially in any developing country deliberately setting out to exploit and simultaneously safeguard its water resources. It is usual to speak rather vaguely about "water quality" and to suppose that the most useful classification of water sources is one based upon this ill-defined concept. Water quality cannot logically be discussed without reference to water use (HoAK, 1953) and it has no necessary connection with either purity or pollution, but it is clear that many diverse factors must be taken into account before attempting to pronounce upon the quality of a given water. These include: (a) The concentrations of organic and inorganic materials dissolved in the water, both major constituents and also minor ones such as heavy metals, insecticides, detergents, etc. (b) Relationships between these concentrations, governing such things as corrosivity, buffer capacity, hardness and pHs as well as sodium percentage and related parameters. (c) The extent and frequency of the variations shown by all these concentrations and parameters in course of time, variations which in many instances are sutficient to make a water appear of radically different quality. (d) The nature, amount and variation of the suspended silt carried by the water, which determines in large degree the type of treatment to be applied or whether any treatment at all is possible. (e) The bacteriology of the water, especially as it relates to faecal pollution and the presence of pathogens. (f) The self-purification capacity, in cases where the envisaged use of the water is one of waste disposal (though this is a use which should be discouraged as much as possible). This in turn is dependent on a host of chemical, biological and physical factors. (g) The means to be employed for extracting the water from its source, for an adequately designed impoundment dam can smooth out variations so that a permanent supply of good quality water can be guaranteed even though the dam may be fed by a river that is too saline for part of the year, (h) The treatment proposed for the raw water, for any water, no matter how poor in

Chemistry of Natural Waters--VI

951

quality, can be improved by suitable treatment so that it is unfair to condemn it out of hand without considering this matter adequately. The factors that merit consideration are evidently so numerous that it is quite impractical to combine them all into a comprehensive scheme for classifying waters according to suitability for different uses. This necessarily means that no unambiguous and comprehensive definition of water quality can ever be formulated, and this in turn suggests that water quality is not a scientific concept, no matter how useful it may be as a phrase for use in broad discussions. For practical purposes it seems preferable to disregard the ill-defined concept o f water quality when attempting to classify waters and to devise a scheme based more directly upon water use. Preferably such a scheme should entail the measurement o f a few relatively simple chemical parameters so that it can be useful to and understandable by people with little technical knowledge. A satisfactory scheme can in fact be constructed by using just two parameters, the electrical conductivity and the 5 day 20°C biochemical oxygen demand (BOD). Most specifications for particular water uses make direct reference to the TDS o f the water. The WORLD HEALTH OR~A~ZATION (1963) has laid down that water suitable for drinking should have a TDS not exceeding 500 mg 1-1, although this can be relaxed to 1500 mg 1-1 provided no alternative source of supply is available. These requirements follow those of the U.S. PUBLIC HEALT, SERVICE (1962) but are not to be taken too literally. WELSH and THOMAS (1960) have discussed their purport and made it clear that they were imposed so as to prevent people accustomed to drinking soft waters from experiencing temporary unpleasant effects if they move to a district where the water is harder. There is no known effect attributable to water just in consequence of high TDS, and even constituents such as sulphate, chloride, alkalinity and magnesium can be tolerated at very high concentrations before they are physiologically harmful. Water with a TDS exceeding 1500 mg 1-1 is therefore not to be taken simply as unsuitable for drinking; rather it is to be regarded as unsuitable as a source of municipal supply, which is a much weaker limitation. Water of much higher TDS, provided this is not accompanied by unacceptably high concentrations of physiologically harmful substances, can be drunk by individuals without any danger. Water used for drinking by stock is subject to the same limitations as water for human consumption, although the level of tolerance is higher and a TDS of up to 5000 mg 1-1, other things being equal, is acceptable here (VAN WYI(, HAMMAN and MYBURGI-I, 1969). The TDS of water used for irrigation is rather more critical (KLIWrWORTI~, 1952; WILCOX, 1955). Less than 500 mg 1-x is usually satisfactory, between 500 and 1500 mg 1-1 requires special management, while above 1500 mg 1-1 is not suitable for irrigation at all except under very special circumstances. Matters such as the sodium and boron contents must also be taken into consideration, as is well known. Since TDS and conductivity are correlated for most waters, the latter being far more simply and expeditiously determined, it is convenient to base the water classification upon conductivity rather than TDS. Also, since both TDS and conductivity vary widely in a given water source, usually showing a marked seasonal variation with more erratic variations superimposed, it is clear that the scheme must be a statistical one. The BOD of a water, although its practical determination is open to a number o f W.R. 5110----L

952

P.H. KEMP

objections, is the most satisfactory parameter at the present time for characterizing the concentration of organic matter. The ordinary processes of water treatment can normally remove organic material from a raw water provided its concentration is not too great, and it is for this reason that the World Health Organization has imposed a limit of 4 mg 1-1 on the BOD of raw waters to be used for public supply. With higher concentrations than this, part of the organic material is likely to escape removal and pass into the distribution system, carrying with it bacteria and other organisms likely to be pathogenic or otherwise undesirable. Like the conductivity, the BOD of natural waters often shows a considerable variation with time. A ten-fold variation has been observed, for example, in some of the rivers of Natal, not showing a seasonal trend, although in others the BOD has been found to remain fairly constant. Hence although 4 mg I- 1 can be accepted as an upper limit for raw water to be used as a source of public supply, this must again be interpreted statistically. One important limitation of the BOD determination is that it will not give a reliable result if the water contains toxic substances inhibiting bacterial life. A low BOD value is therefore not necessarily a sign of a clean water and in consequence whenever BOD determinations are made it is essential to check that toxic substances are indeed absent. Spiking samples with known amounts of organic substances, using special bacterial tests, comparing BOD values with the results of related chemical tests and observing the fauna of the water concerned can all be applied for this purpose. The presence of toxins, besides indicating that the BOD result is spurious, also suggests that the water has been polluted in some way and hence that it may be dangerous as a source of supply. Such waters should therefore be ranked as of very low utility. Consideration of the conductivity and BOD together will often show immediately the uses for which a given water is not suited and hence, by inference, those for which it probably is suited. These two parameters can therefore be combined in the scheme of classification shown in TABLE4. The scheme recognizes five classes into which all waters (surface or underground) can be placed, each class being definitely unsuitable for some particular uses and probably suitable for others. To confirm that a certain water is really acceptable for some specific use, however, requires a far more detailed study of many other of its characteristics and reference to the relevant specification, if such has been drawn up. It is during this last stage of investigation that attention should be paid to such things as oxygen content, nitrogen, phosphate, heavy metals, trace elements, organics, etc. as well as the bacteriology. The scheme is a purely chemical one. It has deliberately been designed as such on the grounds that most of the other disciplines used in the study of waters reduce to no more than indirect ways of assessing the water chemistry. Moreover they are not particularly efficient at doing this. The fauna of a water source, for example, is affected by various factors other than and independent of the water chemistry, e.g. the ambient climate, the current speed and the nature of the stream bed, and experience has shown that biological and chemical assessments can radically disagree in some thirty per cent of the cases studied The scheme has nothing to do with pollution, for polluted waters can fall in any one of the five classes. Moreover it is not necessarily a classification of water quality, for reasons already discussed, though there must evidently be some correlation bet-

Chemistry of Natural Waters--VI

953

ween the (undefined) overall quality of a water and its class number. The scheme is to be regarded solely as a classification according to the probable utility of the water. TABLE4. CLASSIFICATIONOF WATERSACCORDINGTO THEIRPROBABLEUTILITY BOD

o

Low At least 95 % of the time less than 4 ppm

High More than 5 % of the time above 4 ppm

Low At least 95 ~ of the time below 750/~ O era- t

Class 1

Class 4

Intermediate

Class 2

High More than 95 % of the time above 2250 ~O cm -t

Class 3

Class 5*

* All toxic waters are also to be included here.

Class 1 Probably suitable as a source of municipal water supply and for most other uses. Class 2 Probably suitable as a source of municipal water supply provided it is abstracted by means of a suitably designed impoundment dam. Probably suitable for drinking by private consumers and probably for most other uses, but not for irrigation except in special circumstances. Class 3 N o t suitable as a source of municipal water supply nor for industrial use nor ordinarily for irrigation, but in m a n y instances suitable for drinking by private consumers and for watering cattle if the conductivity is not excessive. Class 4 Probably suitable for irrigation, but not for drinking, stock watering nor industrial purposes. Class 5 Unsuitable for almost every use except perhaps irrigation under special circumstances. Note:

A water described as probably suitable for some specific use must not in fact be accepted for that use until further details of the relevant specification have been studied and other matters considered.

P R A C T I C A L A P P L I C A T I O N OF T H E SCHEME OF C L A S S I F I C A T I O N Rigidly to apply the above scheme of classification requires that at least twenty samples of the water concerned must be examined in order to obtain an adequate picture of the statistical distribution of the results. The samples should be taken over the course of a full year so as to cover the whole range of variation of the classifying parameters. This is often impractical, but it is usually quite satisfactory to take a limited number of samples, some at the height of the low-flow season and some at the height of the

954

P.H. KEMp

season of high flow but at least one for each season, and to treat the results in the semiempirical manner set out in TABLE 5. This assumes that the conductivity and BOD values over the full ranges of variation are normally distributed, an assumption which, if not strictly true, appears to be a reasonable approximation. Random statistics can be used for the BOD, since there is no evidence that this parameter varies seasonally under ordinary circumstances. The seasonal variation of the conductivity, however, does not permit this. Instead, reliance is placed on the facts that the standard deviation of a normal distribution is equal to approximately 0.4 times the difference between the values of the variate for the 12.5 and 87.5 percentiles, and that the conductivities of the high-and low-flow samples approximate to these values. In fact, the ranges of conductivity values used to distinguish the water classes in TABLE4 are so great that it is only rarely that statistical treatment is needed at all. The use of a small number of samples occasionally leads to a different classification than if twenty or more samples had been examined. The difference is not usually serious, however, and discrepancies like this are unavoidable. They represent the price to be paid for economies in the sampling. In those few cases where the calculations of TABLE 5 leave the classification uncertain it is necessary to examine additional samples. One single sample is, of course, quite inadequate to characterize any water, no matter what scheme of classification is used. WATERS OF NATAL It is of interest to conclude by summarizing, in terms of the proposed scheme of classification, the knowledge built up over the course of the last seventeen years of the waters of Natal. Most of the Natal river waters (which are mainly of the chlorided series) fall into Class 1 and are probably suitable for almost any use. Some of the rivers of Zululand (the northern part of the Province) must be placed in Class 2 because they are to some extent fed by underground water of high conductivity (VAN WYK, 1963) and because use of their waters for irrigation also results in increased salinity (WILCOX, 1962). The water of these rivers is still suitable for many uses, however, especially when impounded by properly designed dams. Most of the small streams of the coastal region fall into Classes 4 and 5 since they are organically enriched. Pollution of the rivers is often associated with the cities and larger towns and occurs also at a few isolated points where particular industries have become established. It is usually responsible for the appearance of waters of Classes 3, 4 and 5 of restricted utility. The coalmining areas of the Province carry many streams of Classes 2 and 3 as a result of pollution from working and disused mines and dumps, while the sugar mills of the coastal region are usually responsible for organic pollution which degrades the water to the Class 4 level. In principle it should be possible by reasonably simple and economic means to abate organic pollution so that many of the polluted Class 4 waters can be restored to Class 1 level and the Class 5 waters upgraded mostly to Class 2. Mineral pollution due to coal mines is more difficult to combat, however, and unless some far-reaching regional water scheme can be designed and implemented there is little hope of improving the Class 2 and Class 3 waters of the coalfields nor of preventing a progressive

955

Chemistry of Natural Waters--VI TABLE 5. CLASSIFICATION BASED ON SMALL NUMBERS OF SAMPLES

The total number of samples used is n. These are taken at the heights of the dry and rainy seasons---at least one sample for each season. B O D values are treated by the usual random sample statistics: m = mean BOD of all samples (5 days, 20°C, in ppm) s = standard deviation of BOD

where the xt are the individual BOD results (if n = 2, s = 0'707r where r is the range of the observed BOD values). Conductivity values are not randomly measured and need different treatment: D = average dry season conductivity (tz• cm -1 at 20°C) R = average rainy season conductivity M = estimated mean conductivity S = estimated standard deviation of conductivity D a n a R are taken as approximating to the 87"5 and 12.5 percentiles so that: M = O.5 (D + R)

S =0.4ID--RI The scheme of classification is then as follows:

M M M M

+ + ---

1"645 S 1"645 S 1-645 S 1-645 S

< > < >

750 750 2250 2250

m + 1"645s < 4.0

m + 1.645s > 4.0

Class 1

Class 4

Class 2 Class 3

Class 5*

* All toxic waters are also to be included here. increase in the n u m b e r o f rivers with such waters as m i n i n g contains. U n h a p p i l y , t h e coalfields are situated in one o f the driest regions o f N a t a l where further d e t e r i o r a t i o n o f the surface waters c o u l d be m o s t serious. T h e larger rivers o f the coalfields are n o t yet m u c h affected because their flows are so great, a n d it is u n l i k e l y t h a t t h e y will ever b e c o m e so c h a n g e d as to m o v e o u t o f Class 1. U n d e r g r o u n d waters in N a t a l often fall in Class 2, t h o u g h t h o s e o f Classes 1 a n d 3 a r e also c o m m o n . These waters a r e u s u a l l y free f r o m o r g a n i c m a t e r i a l so t h a t Classes 4 a n d 5 are virtually u n k n o w n a m o n g them. It m u s t n o t be forgotten, however, t h a t such a s u m m a r y does n o t give a c o m p l e t e picture. F o r example, m a n y o f the surface waters o f N a t a l are scale-dissolving with quite high pHs, m a n y are corrosive to metals a n d a l m o s t all o f them, d u r i n g the r a i n y season, c a r r y e n o r m o u s l o a d s o f s u s p e n d e d silt. A l g a l b l o o m s are c o m m o n . The selfp u r i f y i n g p o w e r s o f the rivers are high on a c c o u n t o f their shallow d e p t h , swift t u r b u l e n t flow a n d prolific b i o l o g i c a l activity but, as is the case with all shallow streams (EDWARDS a n d OWENS, 1965), self-purification proceeds m a i n l y on the river b e d after d e p o s i t i o n o f the organic m a t e r i a l a n d hence gives rise to relatively deep b u t n o t extensive oxygen sags. T h e u n d e r g r o u n d waters are usually quite d e v o i d o f dissolved oxygen a n d s o m e t i m e s c a r r y u n a c c e p t a b l y high c o n c e n t r a t i o n s o f nitrates a n d fluorides. T h e p r o p o s e d scheme o f classification, t h o u g h r e a s o n a b l y simple, is clearly o f p r a c t i c a l value. But a n y water source can only be assessed a d e q u a t e l y b y m a k i n g it the subject o f i n d i v i d u a l s t u d y o f very b r o a d scope. Its suitability for specific uses can

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t h e n and only then be fully deduced, using the whole o f the i n f o r m a t i o n t h a t has been gathered.

Acknowledgements--Much of the content of this series of papers is based upon a Ph.D. thesis accepted by the University of Natal. Grateful acknowledgements are made to Professor J. W. BAxn_~ESof that University, and to Dr. G. J. STANDER,Director of the National Institute for Water Research of the South African Council for Scientific and Industrial Research, for their guidance and advice during the writing of the thesis; also to the Natal Town and Regional Planning Commission for permission to make use, where necessary, of the results of river surveys carried out in Natal on their behalf. Thanks are also offered to the CSIR for permission to publish these papers. REFERENCES BOND G. W. (1946) A Geochemical Survey of the Underground Water Supplies of the Union of South Africa. Union of S.A., Dept. of Mines, Geol. Survey Mem. 41. CONWAYE. J. (1942) Mean geochemical data in relation to oceanic evolution. Proc. Roy. Irish Acad. B48, 119-159. EDWARDSR. W. and OWENSM. (1965) The oxygen balance of streams. 5th Symposium of the British Ecological Society, Ecology and the Industrial Society, pp. 149-172. Gogru~M E. (1955) On the acidity and salinity of rain. Geochim. Cosmochim. Acta 7, 231-239. HEM J. D. (1959) Study and Interpretation of the Chemical Characteristics of Natural Waters. U.S. Geol. Survey, Water Supply Paper 1473. HOAK R. D. (1953) Water supply and pollution control. Sewage Ind. Wastes 25, 1438-1439. KEMp P. H. (1969) The Chemistry of River Waters. Ph.D. thesis, University of Natal. KEMP P. H. (1971) Standardized water analyses and their application to river surveys in Natal. Water Research 5, 291-295. KtaNTWORTH H. (1942) The Use of Brak Water for Irrigation. Union of S.A., Dept. of Agriculture (Chem. Series No. 185), Bull. 328. NOROELL E. (1951) Water Treatment for Industrial and Other Uses. Reinhold Publishing Corp., New York. SVERDRtWH. et al. (1942) The Oceans. Prentice Hall, Anglewood Cliffs, N.J. U.S. PtmLIC HEALTHSERVlCE(1962) Drinking Water Standards. U.S. Dept. of Health, Education and Welfare, Washington, VAN WYK W. L. (1963) Ground-Water Studies in Northern Natal, Zululand and Surrounding Areas. Republic of S.A., Dept. of Mines, Geol. Survey Mem. 52 VAN WYK W. L., HAMMArqP. F, and MwtmGr~ R. I. D. M. (1969) Ondergrondse Water in SuidwesAfrika. Simposium oor Grondwater in Suidelike Afrika. Pretoria. WELSH G. B. and THOMASJ. F. (1960) Significance of chemical limits in USPHS drinking water standards. J. Am. Water Wks Ass. 52, 284-300. WILCOXL. V. (1955) Classification and Use of Irrigation Waters. U.S. Dept. of Agriculture, circ. no. 969. WILcox L. V. (1962) Salinity caused by irrigation. J. Am. Water Wks Ass. 54, 217-222. WORLD HEALTHORGANIZATION(1963) International Standards for Drinking Water. Geneva.