CHAPTER III. LAKES AND RIVERS." ORIGIN. When rain falls upon the surface of the earth, bringing with it the impurities noted in the preceding chapter, part of it sinks deeply underground to reappear in springs. Another part runs off directly into streams, a part is retained as the ground water of soils and the hydration water of clays, and a portion returns by evaporation to the atmosphere. According to an estimate by Sir John Murray, the total annual rainfall upon all the land of the globe amounts to 29,347.4 cubic miles, and of this quantity 6,524 cubic miles drain off through rivers to the sea. A cubic mile of river water weighs 4,205,650,000 tons, approximately, and carries in solution, on the average, 762,587 tons of foreign matter. In all, nearly 5,000,000,000 tons of solid substances are thus carried annually to the ocean. Suspended sediments, the mechanical load of streams, are not included in this estimate; only the dissolved matter is considered, and that represents the chemical work which the percolating waters have done. Although the minerals which form the rocky crust of the earth are relatively insoluble, they are not absolutely so. The feldspars are especially susceptible to change through aqueous agencies, yielding up their lime or alkalies to percolating water and forming a residue of clay. Rain water, as we have already seen, contains carbonic acid in solution, and that impurity increases its solvent power, particularly with regard to limestones. The moment that water leaves the atmosphere and enters the porous earth its chemical and solvent activities begin, and continue, probably without interruption, until it reaches the sea. The character and extent of the work thus done varies with local conditions, such as temperature, the nature of the minerals encountered, and so on; but it is never zero. Sometimes larger and sometimes smaller, it varies from time to time and place to place. The entire process of weathering will be considered more fully later; we have now to study the nature of the dissolved matter alone, or, in other words, the composition of rivers and lakes. The data are abundant, but unfortunately complicated by a lack of uni • Excluding those belonging to closed basins. 53 formity in the methods of statement, which latter are often unsatisfactory and even misleading. The analysis of a water can be reported in several different ways, as in grains per gallon or parts per million; in oxides, in suppositious salts, or in radicles; so that two analyses of the same material may seem to be totally dissimilar, although in reality they agree. Before we can compare analyses one with another we must reduce them to a common standard, for then only do their true differences appear. The task of reduction may be tedious, but it is profitable in the end. STATEMENT OF ANALYSES. In the usual statement of water analyses an essentially vicious mode of procedure has become so firmly established that it is difficult to set aside. For example, a water is found to contain sodium, potassium, calcium, magnesium, chlorine, and the radicles of sulphuric and carbonic acids; or, in ordinary parlance, three acids and four bases. If these are combined into salts at least twelve such compounds must be assumed, and there is no definite law by which their relative proportions can be calculated. A combination, however, is commonly taken for granted, and each chemist allots the several acids. to the several bases according to his individual judgment. The twelve possible salts rarely appear in the final statement; all the chlorine may be assigned to the sodium and all the sulphuric acid to the lime, and the result is a meaningless chaos of assumptions and uncertainties. We can not be sure that the chosen combinations are correct, and we know that in most analyses they are too few. But are the radicles combined? This is a point at issue. Although no complete theory, covering all the phenomena of solution, has yet been developed, it is the prevalent opinion, at least among physical chemists, that in dilute solutions the salts are dissociated into their ions, and that with the latter only can we legitimately deal. Whether this theory of dissociation shall ultimately stand or fall is a question which need not concern us now; we can use it without danger of error as a basis for the statement of analyses, putting our results in terms of ions which may or may not be actually combined.a Upon this foundation all water analyses can be rationally compared, with no unjustifiable assumptions and with all the real data reduced to the simplest uniform terms. We do not, however, get rid of all difficulties, and some of these must be met by pure conventions. For example, Is silica present in colloidal form, or as the silicic ion SiO,,? Are ferric oxide and alumina present as such, or in the ions of their salts? The iron may represent ferrous carbonate, the alumina may In The ionic form of statement has been used in the Survey laboratory since 1883. Europe it has had strong advocacy from Prof. C. von Than, Min. pet. Mitth., vol. 11, p. 487, 1890. It is now rapidly supplanting the older system. be equivalent to alum; but as a rule the quantities found are so trivial that the true conditions can not be determined from the ratios between acidic and basic radicles. The unavoidable errors of analysis are commonly too large to permit a final settlement of these questions; and only in exceptional cases can definite conclusions be drawn. For convenience, then, we may regard these substances as colloidal oxides and tabulate them in that form. The procedure may not be rigorously exact, but the error in it is usually very small. If we consider an analysis as representing the composition of the anhydrous inorganic matter which is left when a water has been evaporated to dryness, the difficulty as regards iron disappears, for ferrous carbonate is then decomposed and ferric oxide remains. A similar difficulty in respect to the presence of bicarbonates also vanishes at the same time, for the bicarbonates of calcium and magnesium can only exist in solution and not in the anhydrous residues. If, in a given water, notable quantities of lime, magnesia, and carbonic acid are found, bicarbonic ions must be present, for without them the bases could not continue dissolved; but after evaporation only the normal salts remain. Sodium and potassium bicarbonates are not so readily broken down; but even with them it is better to compare the monocarbonates, so as to secure a uniformity of statement. In fact, some analysts report only normal salts, and others bicarbonates; so that for the comparison of different analyses we are compelled to adopt an adjustment such as that which is here proposed. In other words, we eliminate the variable factors, and study the constants alone. One other large variable remains to be considered-the variation due to dilution. A given solution may be very dilute at one time, and much more concentrated at another, and yet the mineral content of the water is possibly the same in both cases. For example, average ocean water contains 3.5 per cent of saline matter, while that of the Black Sea carries little more than half as much; and yet the salts which the two waters yield upon evaporation are nearly if not quite identical. In some cases, as we shall presently see, it is desirable to compare waters directly; but in most instances it is also convenient to study the composition of the solid residues in percentage terms. In that way essential similarities are brought to light and the data become most intelligible. Before proceeding farther, it may be well to consider a single water analysis, in order to illustrate the various methods of statement. For this purpose I will take W. P. Headden's analysis of water from Platte River near Greeley, Colorado," which he himself states in several forms. In the first column of the subjoined table the results are given in oxides, etc., as in a mineral analysis, and in Bull. No. 82, Colorado Agric. Exper. Sta., p. 56, 1903. grains to the imperial gallon. In the second column they are stated in terms of salts, and I have here recalculated Headden's figures into parts per million of the water taken. Finally, in a third column, I give, as proposed in the foregoing pages, the composition of the residue in radicles or ions, and in percentages of total anhydrous inorganic solids. So far as appearance goes, these statements might represent three different waters; and yet the analytical data are the same. A change in the last column of SiO, into the radicle SiO, would affect the other figures but slightly. The compactness and simplicity of the ionic form of statement are evident at a glance. Under it, as “salinity,” I have given the concentration of the water in terms of parts per million. One million parts of this water contain in solution 1.014 parts of anhydrous, inorganic, solid matter. SPRINGS. When water first emerges from the earth as a spring its mineral composition is dependent upon local conditons. Some spring waters are exceedingly dilute; others are heavily charged with saline impurities. To the subject of "mineral" springs, a separate chapter will be given, and only a few analyses of spring water, all taken from the records of the United States Geological Survey, need be given here. They represent the beginnings of streams, and are therefore significant in this connection. All these analyses are reduced to a uniform standard, in accordance with the rules laid down in the preceding pages. Analyses of spring water. A. Spring near Magnet Cove, Arkansas. Analysis by H. N. Stokes. B. Spring 1 mile west of Santa Fe, New Mexico. Analysis by F. W. Clarke. Some of these waters yield carbonates on evaporation, one yields mainly sulphates, and between the two extremes the carbonic and sulphuric ions vary almost reciprocally. One water is characterized by its high proportion of chlorine, and another by its large percentage of silica; but in all of them calcium is the dominant metal. In salinity they differ somewhat widely, but the most concentrated example contains only 925 parts per million, or 52 grains to the United States gallon, of foreign solids. It will be seen as we go farther that carbonate waters are the most common, for the reason that rain water brings carbonic acid from the air, and that substance is most active as a solvent of mineral matter. CHANGES OF COMPOSITION. As spring water flows from its source it rapidly changes in character. It receives other water in the form of rain or of ground water flowing from the soil, and it blends with other rivulets to produce larger streams. Under certain conditions a part of its dissolved load may be precipitated, and the composition of a river as it approaches the sea represents the aggregate effect of all these agencies. A river is the average of all its tributaries, plus rain and ground water, and many rivers show also the effects of contamination from towns and factories. Small streams are the most affected by local conditions, and show the greatest differences in composition; large rivers, as a rule, resemble one another more nearly. How rapidly and how profoundly the composition of a river may be modified are well illustrated in Headden's bulletin, which I have already cited. Cache la Poudre River in Colorado flows first through a rocky canyon, over bowlders of schist and granite, and thence Bull. No. 82, Colorado Agric. Exper. Sta., 1903. |