Popular Science Monthly/Volume 32/April 1888/The Chemistry of Underground Waters
|THE CHEMISTRY OF UNDERGROUND WATERS.|
TO understand the chemical composition of subterranean waters, we must inquire anew of the geological constitution of the country, and it will usually answer with precision and certainty.
Water does not have to remain long in the soil to dissolve and remove various substances from the rocks. Chemical analysis has already shown that such substances exist in the Water-sheets of alluviums in more notable proportions than in the neighboring rivers. The difference is sufficient to explain why in Hungary, Egypt, India, and China, river-water is preferred to well-water for culinary uses. In the subsoil of inhabited places, water is not charged with mineral substances only; but the liquids of manure-heaps and other elements of corruption are transmitted to it by sewers, factories, and cemeteries. The unhealthy effect of the impurities thus conveyed to the wells has been frequently recognized; and it would be surprising if water exposed for centuries to such infiltrations did not cease to be potable.
The most common bodies found to be contained in subterranean waters are oxygen, nitrogen, and carbonic-acid gases, chlorides, carbonates, silicates, and sulphates of lime, magnesia, and soda, and organic substances, in the presence of excessive proportions of which water may cease to be drinkable, or even fit for domestic uses.
Water sometimes acquires also useful properties during its subterranean course. Springs which are employed as therapeutic agents are endowed with mineral qualities. The name of mineral is sometimes extended to other springs the high temperature of which makes them susceptible of similar uses, even when the amount of foreign matter they contain is inferior to what is included in many potable waters.
Chloride of sodium, or sea-salt, is sometimes present in so feeble proportions as not to be perceptible to the taste. It is derived from very widely distributed rocks, which contain traces of it. In other springs it exists in much stronger proportions. Such springs derive their salinity from beds of rock-salt, which it has been found profitable to mine or bore for directly.
Gypsum is dissolved in water under similar conditions. It is present in large masses and in a very fine state of division in the Parisian Tertiary beds, and, being freely soluble, gives hardness to many springs. It is often associated with other substances, which give therapeutic qualities to water, as in the cold springs of Contrexeville and Vittel in the Vosges, and the hot springs at Ago vie and Schinznach in Baden. The mineralization in these cases is due to the presence of the soluble sulphates of lime and magnesia, which are furnished by the gypsiferous beds and dolomites of the Trias. At Birmenstorf the natural leaching of the gypsum rocks is imitated artificially; and either salt is dissolved according as it predominates in the particular stratum on which the process is performed; so that two kinds of medicinal waters are prepared. The purgative springs of Sedlitz, Seidschütz, and Püllna in Bohemia, derive their sulphate of magnesia from the Tertiary marl which they have traversed; and, this fact once recognized, similar waters have been produced for half a century by washing the rock — an experiment which was the starting-point of the artificial mineral-water industry. The origin of most of the sulphureted waters, or, to speak more exactly, of their sulphuret of calcium, is explained by the facility with which the sulphates can yield their oxygen to organic matters; a thing which occurs notably when carbonaceous matters, lignite, stone coal, or bitumen, are found associated with gypsum.
The origin of gaseous or acid springs, which constitute one of the most important families in a hygienic aspect, is connected with exhalations of carbonic acid, which are in turn one of the most remarkable phenomena of the globe's interior economy. The emanations of carbonic acid, as well as the springs to which they give character, are most commonly grouped near volcanoes, active or extinct, and ancient volcanic rocks, basalts, and trachytes. The granitic table-land of central France, in the chain of the Puys, as in the masses of Mont-Dore, the Cantal, and the Nivarais, exhales daily torrents of the gas, either dry or in solution, from more than five hundred springs. There are Royat, with its tumultuous ebullition. Saint-Allyre at Clermont, Saint-Nectaire, where all the oozings of the ground, even the road-ditches, boil with gas. Disengagements of carbonic acid are frequent in the mines of Pontgibaud, which are situated on the side of a crater and an ancient lava-flow, and where veins of silver-bearing lead-ore furnish conduits for the asphyxiating gas.
Countries where the volcanic rocks do not appear on the surface, but which are broken by deep dislocations, may also be the seat of exhalations of carbonic acid. The gaseous springs of Pongues, and some others of the Niévre, are situated upon simple faults. In northern Germany, on the left bank of the Weser, the country is riddled with fractures which give passage to abundant disengagements of carbonic acid, especially upon the plateau of Paderborn, and in the neighborhood of Pyrmont, Disburg, and Meinsberg. Acid springs occur under nearly the same conditions of stratification as the warm springs of which we are about to treat; and, like them, they reach the surface with the assistance of quartz or other mineral veins.
Sometimes the carbonic acid is abundant enough to make the waters in which it is incorporated spurt up violently; as at Montrond, Loire, where the intermittent eruptions from a depth of five hundred metres attain a great height; and at Nauheim, in Vétéravie, where a column of salt water has been spouting for forty years. As a counterpart to these phenomena of solution, waters form deposits which are interesting for more than one reason. Reaching the open, thermal and gaseous springs meet conditions of pressure and temperature different from those which ruled in the depths, and are consequently subjected to reactions to which the oxygen of the atmosphere frequently contributes. The deposits which they make are observed chiefly on the surface of the ground.
Lime, which is very abundant in the condition of a carbonate, is always in solution, in small quantities at least, and in larger quantities when free carbonic acid is present to assist the process. The conditions which provoke the disengagement of the gas at the same time determine the precipitation of the calcareous salt; and this is why calcic carbonated waters often give rise to important deposits. The ancients were impressed by the stalactites of caverns, and by petrifying springs which covered plants and other bodies immersed in their basins with a stony precipitate. Industry has profited by the property, and has obtained in petrifactions bas-reliefs and images of a very delicate molding. In many places the deposit has become extensive enough to constitute a rock formation like the travertine at Tivoli, which furnished building-stones for Rome.
Silica, although regarded as insoluble in water, may become associated with it by the aid of intermediate agencies, and form combinations which are even the predominant elements of some springs, as at Plombières, Bagnères-de-Luchon, Ax, Saint-Sauveur, and Amélie-les-Bains. Sometimes silica is so abundant that it is isolated as opal on coming in contact with the air. The basins of many geysers are thus carpeted with it.
Iron-ore, or limonite, is also constantly in formation in such quantities that the beds can be worked. It is known, according to the conditions under which it is deposited, under the names of bog-ore, field-ore, or lake-ore. It is generally buried at slight depths below the surface, forming thin beds. Its modern origin is demonstrated by the presence of products of human industry, such as fragments of pottery and utensils, which are met in the massive blocks; and it is, moreover, sometimes renewed in places where it has recently been worked. More than a thousand lakes in Sweden, Norway, and Finland supply this mineral in rounded and separated globules. Although its formation is in constant continuation, the cause of it has been for a long time misapprehended. It is a result of slow dissolutions which have been frequently observed in arenaceous clays. Rain-waters traversing them, having seeped along roots undergoing decomposition, take from them an acid principle, and thus acquire the power of dissolving oxide of iron as they go. Reappearing in the air, they abandon the hydrated peroxide of iron, leaving it as a brown, gelatinous precipitate. Organic substances in this way contribute to the formation of mineral matters. According to recent investigations, the almost boiling Steamboat Springs in the United States precipitate, besides sulphur, small quantities of gold, mercury, silver, lead, copper, and zinc, which, by the aid of certain salts and their high temperature, they hold in solution. These deposits appear to be the continuation of those which in the same region, as at Sulphur-bank, have formed beds which are mined for their mercury.
What we have been able to observe from the surface of the ground gives a very limited and imperfect idea of the actions which excavations made to secure some particular thermal waters have revealed to us. The bottom of the basin of the principal spring at Bourbonne-les-Bains, where the temperature reaches 68° C, has furnished some very remarkable facts relative to the formation of minerals. The place was a flourishing station in the Roman period. In draining an ancient well a blackish mud was reached which contained fragments of wood, acorns, thousands of filberts which had become black like lignite, and numerous medals. The washing of four cubic metres of the mud yielded more than five thousand pieces of money, mostly of bronze or tin, but some of silver and gold. The four coins of the last metal bore the images of Nero, Hadrian, Faustina the younger, and Honorius. Twenty of the silver pieces belonged to the Gallico-Roman period, while the other coins were consular or imperial pieces, mostly of the first centuries of the empire; but some were as recent as the Lower Empire. The bronze pieces of the medium and smaller sorts were likewise of different ages, but three types of Augustan coins predominated. Many of them had been cut in two, doubtless to prevent their being taken out and used again, they having been cast in as offerings to the health-giving fountains. Ex-voto offerings were also recovered, including a bronze statuette of a man whose leg had been hurt.
Some of the coins had been so corroded by the action of the hot water that the figure on them could not be discerned. Others had been further corroded into holes and notches. Many others had been wholly dissolved, but had engendered, at the expense of their bronze, new and solidly agglutinated combinations. The species thus originated were identical in their crystalline forms and general characters with similar natural minerals—sulphuret of copper, copper pyrites, and variegated copper-ore. The most numerous crystals are regular tetrahedrons, like those of the mineral called gray antimonial copper, of which they have also the composition, the luster, and other properties. In some of the coins the tin of the bronze has passed into the state of an oxide, and has formed a white superficial crust. A real separation has therefore been produced between the metals of the alloy by the different workings of their chemical affinities. It seems as if in all of these transformations Nature, claiming her rights upon what human industry had taken out of her domain, had been pleased, with the aid of the mineral water, to recover her property, and reconstitute exactly the ores of copper and tin which the miner's operations had taken from her, and from which the furnaces of the metallurgists had laboriously extracted the two metals of bronze.
Lead pipes, of which there were a great number in connection with a white marble piscina, had suffered a no less energetic alteration. They were deeply corroded and perforated, and had by solution formed minerals with bases of lead—the sulphuret, or galena, and the chlorocarbonate, or phosgenite.
Among the iron compounds the bisulphuret, or pyrites, is of special interest, on account of its abundance in the crust of the earth. At Bourbonne, and in the basins of other thermal springs at Aix-la-Chapelle, Bourbon-Lancy, Bourbon-l'Archambault, and Saint-Nectaire, pyrites has been detected in course of formation, but only in the deeper parts of the basin remote from atmospheric oxygen.
In view of these changes, wrought by thermal water on inorganic bodies, it is not surprising that the same agent should also act upon organic bodies. The wood of the piles supporting a masonry-work, while it has perfectly preserved its texture, has become hard and heavy with the mineral matter which it has absorbed. The original substance has almost disappeared, and given place to carbonate of lime, which has penetrated, as theshows, to the most minute interstices of the vegetable cells.
These springs of Bourbonne have given rise, within a very limited space, to not less than twenty-four species of crystalline minerals, in combinations which, accumulated and grouped as they are, closely resemble the ancient metalliferous beds—in detail as well as in generaL
Other evidences of the mineralizing power of thermal waters have been exhibited in their operation for considerable distances below the surface of the ground. Penetrating the subsoil at Plombières, they have since the Roman period engendered a series of species no less remarkable than the preceding series, although they did not attract attention by a metallic luster; they are silicates of the zeolite group, opal, and chalcedony. When we try to apply the experimental method to the reproduction of geological phenomena, we are met, among other difficulties, by the brevity of human life, which is very short in comparison with the immense periods which have presided over the formation of the crust of the earth. Such facts as these, fortunately, make up for this inability, and put us in the presence of experiments that are forbidden to our laboratories, from which we learn what can be effected by very weak actions prolonged through ages. By these synthetic demonstrations, carried on during twenty times the duration of human life. Nature teaches us that she is still employing processes similar to those which she used in the most remote epochs.
We come now to see how subterranean waters obtain a heat which makes them thermal springs, and which connects them by intermediate agents with volcanic phenomena that are still, at first sight, so different and almost contrasted. The temperature of springs is generally nearly equal to the mean temperature of the ground from which they issue. But there are some exceptions to this usual condition, which are called thermal; a term which should be applied, not only to waters manifestly hot or warm, but also to those which by thermometrical indications differ by only two or three degrees from the normal temperature. Thermal springs are not always, therefore, distinctly separated from ordinary springs.
The extreme variations of temperature which we feel so vividly, according to the seasons, penetrate the ground very slowly and gradually subside, till they become insensible at a depth which is measured at Paris as of about twenty-five metres. Below this stratum of invariable temperature, the heat gradually increases as we descend; a fact which is not confined to temperate regions, but has been observed in countries near the equator and near the poles. It has been demonstrated by observations made in mines, in tunnels, and in artesian wells.
It is evident that this internal heat can not emanate from the sun nor from any cause exterior to our globe, for, if it did, it would not increase as we descend. It appears to be the resultant and continuation of the heat through which our planet has formerly passed. In radiating toward the celestial spaces, which are colder than anything of which we know, the outer masses are necessarily consumed first, while the heat continues intense in the central masses. In consequence of this general increase of heat, there are present in all parts of the interior of the globe, even far away from active volcanoes, rocks, contact with which heats water in a greater or less degree.
We have now to examine the various ways in which the structure of the rocks permits water, after it has descended to great depths, to return to the surface. The simplest way is by a turning back of the strata. The water of the artesian wells of Paris has been forced, having entered at the outcrops of the beds, to descend, between impenetrable strata, to a depth of which it has taken the temperature. The existence of a vast thermal bed under a part of the north of France would not have been revealed without the borings which have opened a way of return to its waters. But if the strata to which we refer, instead of being disposed in a vast concave basin, are subjected to a bending which would bring them up again to the surface, their thermal water would return with them, as if drawn through a siphon. This is the kind of a disposition which Nature has made real in countries where the strata have been bent under strong mechanical action. Such a structure may be recognized in the cases of the springs of Barbotan, Baden, Schinznach, Aix-la-Chapelle, Bercette, and in the Appalachian Mountains in Virginia.
Another mechanism of Nature, yet more closely resembling a siphon, is furnished by the large, nearly vertical fractures called faults, which descend to an indefinite depth, far below any point which it is possible for us to reach. Generally, they serve only for the direct descent of waters which, having been swallowed into them, find a little farther down an outlet from which they issue still cold. But it also happens, and the fact should be kept clearly in mind, that a fault offers a return route to water which has been heated at great depths. This is the case at Bourbonne-les-Bains. In the Alps, according to M. Lory, the same fault feeds the thermal springs of Monestier, Briançon, Brides, and Salines near Montiers. The fault which cuts and terminates the chain of the Alps near Vienna in Austria is the emissary of a considerable number of springs. Most of them are cold; but some hot springs, as those of Baden and Vöslau, are ranged along a distance of eleven kilometres.
The return branch of these natural siphons is often filled up and obstructed by incrustations which the water has formerly made, so as to form metalliferous veins. If the obstruction is not complete, or if it has been opened by raining excavations, these veins may be still serving for a way of ascent. A passage pierced at Plombières some thirty years ago in the granitic flank of the valley for the regulation of the warm waters, cut several veins of quartz and fluor-spar, along the sides of which springs gushed out forcibly. A similar incident took place at Lamalon, in Hérault, where it became necessary to stop the working of the veins of copper and lead, in order not to compromise the existence of the thermal establishment, the source of which was only a few dozen metres away. In the Comstock lode torrents of water having a temperature of 158° Fahr. gave out such heat that the workmen had to use ice to cool their shafts; and after twenty years of most profitable operation the mines became the object of great expenditures to obviate this difficulty.
It is for the most part in the neighborhood of extinct volcanoes and rocks of a volcanic nature that faults produce thermal spoutings. While such springs are usually wanting in the greater portion of the French central granitic plateau, they abound in those regions of the same plateau which are traversed by volcanic rocks.
There is no reason, therefore, to be surprised at finding the region of active volcanoes itself rich in emanations of this kind. Puzzuoli, Baiæ, and the baths of Nero, are situated near the solfataza of Puzzuoli and the ancient craters of Agnano and Lake Avernus. On the island of Ischia, as at Guadeloupe, the hot waters gush copiously from the flanks of the volcanoes.
We also observe near volcanoes boiling water violently projected into the air by torrents of vapor. The noise they make, which is like that of a steam-boiler, has caused the name of "Steamboat Springs" to be given to a group of this kind in the State of Nevada. Such springs are in close analogy with others in which the water is forced up, in the shape of a tall column, by intermittent eruptions. The latter have received the generic name of geyser, from the Icelandic word signifying to spout. One of the most remarkable geyser regions in the world is in the western part of the United States, near the borders of Wyoming Territory, where are grouped together more than two thousand very hot springs, which we might imagine to have been engendered by some vast steam-furnace.
Waters may also acquire a high temperature by borrowing from eruptive rocks which have been thrown up from greater depths and still retain a part of their primitive heat. They generally rise by the force of hydrostatic pressure, as in the artesian wells, while the expansive force of vapor is sometimes the elevating agent. Volcanoes, the eruptions of which suggest only the idea of fire, constitute, in fact, gigantic intermittent springs of water, the temperature of which surpasses everything that we can comprehend.
Thus, the vapor of water not only forms the most abundant and most constant product of eruptions, but it seems even to be, through its enormous tension, the mother of them. From the very beginning of the crisis it bursts out in enormous spurts, dragging matter of every kind up the subterranean conduit. This vapor produces a vertical column which spreads out in the upper regions of the atmosphere in the shape which in Italy Pliny has compared to that of a pine-tree. It is sometimes blackened, especially at the beginning of an eruption, by solid dejections of cinders or lapilli. The watery column may reach a considerable height if it is not carried away or dissolved by aerial currents. Torrential rains frequently fall from the clouds engendered by these exhalations.
Impossible as it may appear, water is incorporated in fused and incandescent lavas, and consequently participates in a temperature exceeding 1,000°; but when it is vaporized its temperature falls at once to the boiling-point.
The water expelled from volcanoes gives only a very limited idea of the importance of the domain of that fluid in the depths of the earth. When we consider how many opportunities it finds to penetrate by capillarity and other means into interior regions of a very high temperature, we can not doubt that these regions contain superheated water. Imprisoned within rocky walls that offer an enormous resistance, it acquires a tension which recent experiments show to be of marvelous power.
Water also contributes invisibly to mechanical actions. In view of the immense force it exhibits in eruptions, we have a right to suppose that in regions where it has no outlet it may, under the force of its enormous pressure, be also an effective cause of the most formidable earthquakes, which are simply volcanic eruptions without outlet. These agitations are produced more especially in countries the ground of which is dislocated and has most recently acquired its present relief. Such a geological constitution, which is recognized as connected with earthquakes, would have the precise effect of favoring the admission of water through its large fractures to deep and hot regions. Conditions of this kind are realized in all the parts of the basin of the Mediterranean which have been so frequently and violently agitated within historic times.
The facts that subterranean waters have thus taught us for the present epoch will aid in giving an idea of what they have effected in the extremely remote times of the geological periods. Minerals, which are their work, permit us to follow the track which they have left behind them through thousands of centuries.—Translated for the Popular Science Monthly from the Revue des Deux Mondes.