Water

WATER.

I. Physico-chemical properties and composition of water. The water expanses of the world ocean and seas constitute 361 million sq. km and occupy 71% of the entire Earth's surface. In a free state, water occupies the most superficial part of the Earth's crust, the so-called hydrosphere. The exceptional importance of water in a free state for life has long attracted attention to the relationships between free water and water bound in any physico-chemical compounds. Predictions about the dehydration of our planet were often made (E. Kant, 1750); however, geological study of the great movements of the oceans on the Earth's surface does not give indications of a decrease in the mass of the ocean's water, which is one of the constant values of our planet. The elementary chemical composition of water is expressed by the formula H₂O, showing that a molecule of water consists of two atoms of hydrogen (H₂) and one atom of oxygen (O). The weight composition of water is expressed as 11.19% H and 88.81% O. During electrolysis, water decomposes with the release of two volumes of hydrogen at the cathode and one volume of oxygen at the anode, which, when mixed, give three volumes of oxyhydrogen gas, which forms a strong explosion from an electrical spark or contact with a heated body, with the formation of two volumes of water vapor. The phenomena of the strong explosion of oxyhydrogen gas are explained by the enormous exothermicity of the reaction of water formation, which testifies to the very significant chemical affinity of its constituent elements and the strength of their compound. By chemical character, water is a completely neutral oxide H₂O, incapable of changing the color of neutral reactive litmus paper and having equal concentrations of dissociated hydrogen and hydroxyl ions (H⁺) = (OH^-) = 10^-7, or, which is the same, pH = pOH = 7. The H of the water molecule is capable of being replaced by metals. Metals of the alkali (K, Na) and alkaline-earth (Ca) groups, which can form oxides (alkalis) soluble in water, produce a violent release of H from water, with heating and sometimes with ignition of the released H. Many other metals are also capable of direct replacement of the H of water, especially at high temperatures, but this reaction is reversible, since hydrogen is capable of taking oxygen from metal oxides, reducing the metals (for example, iron—Ferrum hydrogenio reductum). The H of water can also be replaced by certain metalloids that have a significant affinity for O. Thus, at white heat, C easily displaces the H of water vapor, resulting in the so-called "water gas," consisting of equal volumes of H and carbon monoxide: H₂O + C = H₂ + CO. Water is the most universal solvent for solid, liquid, and gaseous substances. Dissolution is always accompanied by thermal phenomena: heating or cooling. A change in the concentration of solutions is also accompanied by the release or absorption of heat. The combination of water in crystals and hydrates also corresponds to its own heats of formation. In addition to thermal ones, light phenomena are also observed in many hydration processes (e.g., during the slaking of lime). The solubility in water of each solid and gaseous substance for given temperatures and pressures has its own strictly defined saturation limit; liquids either mix with water in all proportions (for example, ethyl alcohol, methyl alcohol, sulfuric acid) or have specific coefficients of mutual solubility (e.g., at 20° in 100 g of water, 7.51 g of ordinary ethyl ether dissolves, and in 100 g of ether—2.7 g of water). The boiling point of solutions of solid substances is higher than the boiling point of water, with the molecular increase in boiling point equaling 0.52°. The molecular decrease in the freezing point for water equals 1.85°. Connected with the dissolving power of water is its role as a medium or catalyst for most chemical reactions, which has long found its expression in the proposition "Corpora non agunt, nisi fluida sive soluta," i.e., bodies interact only when liquid or dissolved. Such an energetic element as Cl, in the absence of moisture, does not act on metals and even on dry ammonia gas. Obtaining chemically pure water is achieved with immense difficulties by distilling it in sealed platinum or gold apparatuses, in an airless space, from the middle fractions of carefully distilled water, previously treated by boiling with potassium permanganate while acidifying with sulfuric acid. In this case, it should be borne in mind that even such water may contain foreign impurities in the form of colloidal particles, easily detected in the Tyndall phenomenon when a beam of light from a projection lantern is passed through water in a dark room. To obtain optically empty, chemically pure water, distilled water must be subjected to energetic centrifugation or filtration through an ultrafilter prepared from collodion. Ordinary distilled water is only a certain approximation to chemically pure water. In recent years, the electrolytic method of desalination of water has been finding application, consisting of the electrolysis of water flowing between batteries of electrodes placed in special dialyzing partitions. Many physical properties and features of water serve as starting points for the production of various measurements. The melting point of chemically pure ice at normal pressure (760 mm Hg) is taken as the zero point. The melting point of ice is easily reproducible and one of the most constant points. With an increase in pressure by 1 atmosphere, the melting point of ice decreases by only 0.00753°, so that to lower the melting point of ice by 1°, an increase in pressure of 132.8 atmospheres is required. The temperature of steam of water boiling at the same normal pressure is taken as 100°, i.e., a degree of temperature is considered 0.01 of the entire difference between the melting point of ice and the boiling point of water. The weight of 1 cubic cm of water at 4° (the temperature of its greatest density), at normal gravity (i.e., at sea level and 45° geographical latitude of the locality), and when weighed in a vacuum, is taken as the unit of density, weight, and mass and is called a gram. The change in the weight of 1 cubic cm or the volume of 1 g of water with a change in temperature was the subject of careful study and showed that the coefficient of expansion of water undergoes noticeable fluctuations in different sections of the temperature scale. Having reached its greatest density at 4°, water expands upon further cooling, and upon freezing, turning into ice, it further increases its volume by more than 8%: the specific gravity of water at 0° is 0.99987, the specific gravity of ice at 0° is 0.91674. Such a significant increase in volume upon freezing causes the formation of an ice cover on the surface of water bodies in winter, protecting them from further cooling and freezing through. In standing water bodies, in the depths, water with the greatest density collects—about 4°. The peculiarities of the change in the specific gravity of water upon heating and cooling explain the summer direct and winter reverse stratification (alternation of layers) of lakes and other standing water bodies. Expansion upon freezing also has enormous geological significance, since the freezing of water in cracks and pores of rocky rocks serves as a powerful tool for their grinding, which is a necessary prerequisite for weathering processes, which play such a prominent role in soil formation, in the preparation of so-called soil solutions, and in the formation of the salt composition of natural fresh waters, these main nutritional juices of terrestrial and aquatic plant life. Along with high density, water possesses the greatest heat capacity of all liquids, which is not the same for different temperature sections. The amount of heat required to heat 1 g of water from 0° to 1°, under normal conditions, was taken as the unit of heat and is called a gram-calorie ("cal") or small calorie, in contrast to the large, or kilogram-calorie ("Cal" or "Kcal"). At the present time, the unit of heat is more often considered "cal₁₅" and "Kcal₁₅", i.e., the amount of heat required to heat 1 g and, respectively, 1 kg of water from 14.5° to 15.5°. For water, as for any liquid, each evaporation temperature corresponds to a limiting pressure of the so-called saturated vapor; with an increase in pressure by a decrease in volume, part of the vapor that saturated the given volume passes from a vaporous to a liquid state. On the other hand, by lowering the temperature of steam having a pressure lower than saturated, it is possible to bring its cooling to a saturated state at any pressure and cause its condensation into a liquid state, the onset of which is signaled by the appearance of dew or fog. Optics. In small volumes, water seems to be a colorless transparent liquid, but in a significant layer in transmitted light, chemically pure and optically empty water reveals a definite blue coloration, depending on the more significant absorption by water of the red and orange parts of the spectrum (wavelength λ=0.60–0.66 μ) compared to the blue part (wavelength λ=0.48–0.52 μ). With a sufficiently large layer, optically empty water would absorb rays of all wavelengths and would seem completely black both in transmitted and reflected light. The blue color of sea water is more intense and darker, the greater the transparency of sea water or the smaller, according to Shuleikin, the scattering coefficient. Conversely, the color of sea water is greener and browner, the less significant the depth of reflection of light rays, and the greater the scattering coefficient.

The inherent coloration of suspended particles also constitutes one of the components of the color of Water in natural bodies of water (the coloration of clay, loess, peat, and plant and animal plankton particles). By the composition of Water in hygiene, hydrology, limnology, and oceanography, one understands the complex of dissolved chemical substances and gases, mechanical impurities (mineral and organic detritus), and planktonic microorganisms and bacteria present in the sampled portion of Water. The well-known dissolving and destructive power of Water explains why chemically pure Water cannot exist under the natural conditions of our planet. Rising in the form of vapor during evaporation from the vast water expanses of oceans and seas, water, it is true, is freed from the vast majority of substances dissolved and suspended in it, but together with the vapor from water surfaces, certain volatile organic substances (such as, for example, methane and other hydrocarbons formed on the bottom as a result of the decay of dead organic matter) and mineral substances (such as, for example, CO₂, NH₃, and H, which are the result partly of physiological excretions, partly also of bottom and soil processes) pass into the atmosphere, which, along with the vapors, move with air masses at the will of atmospheric currents. Condensing, upon the cooling of the atmosphere below the saturation temperature, into the droplet-liquid state of clouds and fogs, this Water again dissolves the substances that flew off with it and also becomes saturated with atmospheric gases (N₂, O₂, Ar, He, CO₂, etc.). Falling in the form of rain, hail, or snow, the Water of atmospheric precipitation becomes enriched in the lower layers of the atmosphere with particles of terrestrial dust floating in the air, soluble salts (sodium chloride and sodium sulfate, etc.) remaining from dried water spray carried by winds from the crests of sea waves, bacteria and other microorganisms, nitrogen oxides always formed in the atmosphere under the influence of electrical phenomena, organic nitrogenous compounds, ozone, and H₂O₂. Rainwater exhibits some radioactivity, especially after a thunderstorm, and snow and hail possess it 3-5 times more. The saturation of atmospheric Water with atmospheric gases is governed by Dalton's law (1805): "Water in the presence of an atmosphere formed by several gases dissolves each of them as if it were alone under the pressure which it occupies in the mixture." According to Dalton's law, in 1 liter of Water at 0° and 760 mm, the following quantities of gases (in cubic cm) can be dissolved from atmospheric air: O₂ (21%), N₂ (78.06%), CO₂ (0.03%), Ar (0.94%) 10.27, 18.32, 0.5139, 7.98, 14.50, 0.3582, 6.52, 12.03, 0.2634, 5.48, 10.46. According to Bonjean, rainwater, on average, contains about 23 cubic cm of gases per liter, of which: Nitrogen (4/5).

According to Keilhack, rainwater, on average, contains about 0.0003 by volume of CO₂, i.e., 0.3‰—values close to the theoretical ones. Due to a higher solubility coefficient, water absorbs more O₂ from the air than N. The volumetric ratio of O₂ to N in atmospheric air, equal to approximately 1:4, changes for air dissolved in water to 1:2. For CO₂, which is present in atmospheric air in the amount of 0.03% by volume, the relative content in air dissolved in water increases to 1.78% by volume, i.e., its proportion increases almost 60-fold. The content of dissolved substances in rain, snow, and other meteoric water far from populated areas is small and does not exceed a few mg per liter—at the end of the rain less than at the beginning, on high mountains less than in valleys. Depending on local conditions (industrial cities), the salt content in meteoric water can reach several tens of mg. It is often contaminated by soot. In coastal areas, the content of chlorides predominates in meteoric water. The content of suspended impurities and bacteria in atmospheric water is also subject to fluctuation depending on local conditions. Miquel (1886) for the period 1883–86 calculates an average of 4.3 bacteria per 1 cubic cm for rural areas (Montsouris) and 19 bacteria for Paris. Yankovsky (1888) gives higher numbers for snow water—from 34 to 463 bacteria. However, only from the moment atmospheric precipitation hits the earth's surface and, primarily, when seeping through the pores of the soil, does the true formation of the composition of water begin. Rainwater or melted snow water that has fallen on the earth's surface partly evaporates back into the atmosphere, partly flows along the surface into open water bodies, and partly penetrates into the ground, soaking through the pores of the soil. The fate of precipitation that evaporates again, both directly from the earth's surface and through the upper layers during soil drying, differs little from the fate of water evaporating from water bodies. The difference may concern, mainly, volatile impurities rising from the soil with water vapor and soil air. Water flowing directly along the earth's surface into streams, rivers, or lakes is enriched on its terrestrial path, mainly, with mechanical impurities of soil, mineral, and organic detritus, the qualitative and quantitative composition of which depends on the composition and character of the soil surface, vegetation cover, terrain slope, etc. Along with detritus, directly flowing water is also enriched with various microorganisms and bacteria. The number of bacteria in 1 cubic cm of rain and snow flood water can reach hundreds of thousands. The content of soil detritus can fluctuate within very wide limits—from tens of mg to tens of g per liter, which determines the varying degree of turbidity, which also depends on the degree of fragmentation of the detritus. Spring floods on the Moscow River are characterized by a content of a maximum of several hundred mg of suspended substances and a drop in transparency to 7–8 cm. Floods in the basins of the Volga and Oka rivers give similar values. Rivers of the chernozem belt carry a significantly larger amount of soil detritus during floods. The content of organic substances, suspended, colloidal, and dissolved in waters of direct surface runoff, is usually significant and causes their high oxidizability. As for dissolved mineral substances, their amount in waters of direct surface runoff is usually small and does not exceed several tens of mg per liter. The reason for this is that 1) water of direct runoff is in contact with the washed soil surface for only a short time; 2) the soil surface is usually poor in directly soluble mineral substances, since before the formation of the flowing layer of water, it is subjected to the washing and dissolving action of the first portions of atmospheric water, wetting it and soaking through its pores; 3) surface runoff water, having a negligible amount of dissolved CO₂, saturating it under the negligible partial pressure of CO₂ of the free atmosphere, possesses only weak dissolving power in relation to silicates and aluminosilicates that have survived destruction in weathering processes. The composition of waters of direct surface runoff is very well reflected by the compositions of flood waters, streams, and small rivers (transparency 4–10 cm, suspended substances 100–300 mg, solid residue 50–70 mg, hardness not higher than 2 German degrees, oxidizability 10–20 mg O₂, color 30–60° on the platinum-cobalt scale, number of bacteria per 10⁶). The composition of lakes and similar standing water bodies, which are 80–90% fed by direct runoff water, is genetically identical in terms of dissolved ingredients to the composition of the waters feeding them. The difference lies in the release by settling from suspended turbidity and bacteria, and in the change in composition by biological factors (consuming CO₂, nitrogenous compounds, and other nutrient salts in summer and enriching the composition with the same compounds in winter as a result of regenerative processes in bottom silt deposits). Water flowing out of lakes represents the same open surface runoff, only slowed down by its stay in surface water bodies. The salt composition of that part of the water that has fallen on the earth and is absorbed by the soil reaches the greatest saturation. Seeping through the pores of the soil, this part of atmospheric water is freed by filtration from contamination by soil detritus and bacteria and, at the same time, is enriched with dissolved CO₂, which is constantly produced in living soil layers as a result of ongoing biochemical processes, the partial pressure of which in soil air causes an increase in the dissolving power of water and the formation in the soil of concentrated soil solutions of calcium bicarbonate, magnesium, and other salts. Living soil layers represent the environment in which the processes of weathering of mineral components and the processes of mineralization of organic substances accumulating annually in the soil (dead roots of vegetation cover, humic substances in various stages of decomposition, etc.) occur with the greatest intensity. These phenomena give rise to the constant formation of CO₂, accumulated in the soil in the form of organic matter, originating from the photosynthetic process of terrestrial plants. It is precisely in the form of bicarbonates and free CO₂ that the carbon of the accumulated dead organic matter is extracted from the soil by seeping water. The intensity of the biochemical processes of the soil, as a result of which free CO₂, bicarbonates, nitrates, ammonium, and other salts occur, is in general terms the determining factor for the magnitude of the salt composition of future groundwater, the further underground journey of which introduces only corrections to it depending on the composition and structure of the washed geological rocks. Hence WATER.

It is understandable why the fertilization and pollution of the soil cover, by intensifying weathering processes, contribute to an increase in the salt composition of water, a vivid illustration of which is provided by the increased salt compositions of drainage waters from irrigation fields.

The main "skeletal" part of the salt composition of natural fresh waters in general, and especially of groundwater runoff, is the bicarbonates of alkaline earth metals: calcium and magnesium. Bicarbonates of alkaline earth metals do not exist in the form of pre-prepared mineral substances, but arise as a result of the dissolving [CaCO₃ + CO₂ + H₂O = Ca(HCO₃)₂] and destructive (CaSiO₃ + CO₂ + H₂O = CaCO₃ + H₂SiO₃) power of CO₂ dissolved in water.

A feature of both bicarbonates is chemical instability, expressed in their tendency to dissociate into neutral carbonates and free CO₂. An indispensable condition for retaining these salts in solution in an undecomposed form is the equilibrium between the dissociation tension of the bicarbonate ion and the partial pressure of the dissolved CO₂, whereby this equilibrium part of free CO₂ does not possess "aggressive" properties, i.e., it loses its dissolving capacity in relation to marble or another type of calcium carbonate. The increasing bicarbonate composition is accompanied by progressively increasing amounts of this equilibrium CO₂ (Tillmans and Auerbach, 1913; Johnston, 1916; Kolthoff, 1922).

Penetrating into the ground, water quickly loses its supply of dissolved O₂, consumed in various biochemical reactions of the soil layers, so that underground water as a rule does not contain free dissolved O₂, instead of which water receives from the soil free and bicarbonate CO₂, salts of other oxygen acids, such as: nitrates, sulfates, etc. With prolonged underground existence, water can gradually lose even this bound O₂ of its salts, whereby nitrates are the first to be reduced, being replaced by saline ammonia (for example, Moscow artesian waters of the Carboniferous limestone of the Serpukhov horizon); it is not rare to encounter artesian water with signs of sulfate reduction, whereby the presence of H₂S, in the absence of gypsum-bearing rocks, can be explained by the reduction of water sulfates.

Emerging onto the earth's surface in the form of so-called underground runoff, water carries out from the soil in its composition significant quantities of highly valuable biogenic substances in a mineralized state and, depending on the time of year, either carries them away in winter along river systems into lakes and seas in an untouched and unchanged form or in summer feeds with them the phytoplankton (see Plankton) of lakes and the phytobenthos (see Benthos) of rivers. The salt composition of water in rivers and other bodies of water fed by underground runoff undergoes, depending on the vital activity or death of biological factors, deep in meaning and sometimes very significant quantitative changes, which include, for example, the depletion of water in the summer time of free and partly bicarbonate CO₂, nitrates, saline ammonia, and a certain increase in the content of albuminoid NH₃ due to the development of planktonic organisms.

The biochemical history of water from direct surface runoff is no less complex and diverse than that of underground runoff water. A sharp difference between them and underground water lies in the scarcity of salt composition and in the relative abundance of suspended turbidity and colloidal organic substances. Possessing negligible amounts of free CO₂, saturating it under a partial pressure of about 0.003 atmospheres, water from direct runoff in its short-term contact with the soil surface does not have time to become enriched at the expense of the biochemical products of its weathering. Entering the river from the entire river basin, water from open runoff is the cause of the usual flood pattern. Entering surface standing bodies of water (lakes), they form powerful reserves of surface water, which, along with underground runoff, take part in the constant feeding of rivers; the composition and properties of river water are composed of the variable quantities of this participation. Direct open runoff participates in the feeding of rivers only optionally, in short periods of floods.

In lakes during summer direct stratification, nutrient salts accumulate in the bottom zone, below the thermocline, from where they rise by diffusion into the overlying warmed layers, which, during daily thermal mixing, distribute these substances throughout the entire assimilating water column. Thanks to the development of phytoplankton and blooming, all CO₂ and the greater part of the salts are immediately assimilated in the photosynthetic vegetative process.

The composition of sea water differs from the composition of fresh waters both in quantity and quality of dissolved salts. The salinity of ocean water is 35 g per 1,000 g of water, whereas the salinity of river waters is usually below 0.35 g per 1,000 g of water. The predominant skeletal component (usually over 90%) in fresh waters is, as indicated above, the bicarbonates of alkaline earth metals. For sea water, the predominant skeletal component is chlorides (of sodium and magnesium). 1,000 g of ocean water contains: Table salt (NaCl) 27.1 g; Magnesium chloride (MgCl₂) 3.8 g; Magnesium sulfate (MgSO₄) 1.6 g; Gypsum (CaSO₄) 1.3 g; Potassium sulfate (K₂SO₄) 0.9 g; Calcium carbonate (CaCO₃) 0.2 g.

Biogenic substances brought by rivers into the seas (nitrates, ammonium salts, phosphates, silicic acid, iron, CO₂, free and bicarbonates, calcium) do not accumulate in sea water, as they are consumed biologically by plants and algae. As for calcium bicarbonate, besides biological consumption by algae and animals (skeletons, shells, carapaces), its excess, losing the free equilibrium CO₂ that accompanied it (losses to the atmosphere, assimilation in the photosynthetic process), dissociates into carbonic acid and poorly soluble calcium carbonate (16 mg CaCO₃ per 1 liter), which precipitates in an insoluble state in the form of tiny particles, which slowly and continuously settle to the bottom, forming so-called sedimentary rocks. Chlorides, which constitute a negligible content of fresh waters and are poorly extracted in the process of weathering of primary rocks, accumulate in the salt composition of sea water due to their significant solubility and insignificant biological consumption.

Lit.: Vilg O., Fundamentals of Physical Chemistry, SPB, 1910; Khvolson O., Course of Physics, vol. I, Berlin, 1923; Mendeleev D. I., Fundamentals of Chemistry, SPB, 1903; Vernadsky W., La geochimie, P., 1924; Chemiker-Kalender, Berlin, 1928; Hütte, p. 573, Berlin, 1927.

S. Ozerov. P. Physico-chemical examination of water. The results of a physico-chemical examination of water must establish: 1) the presence in the water of substances harmful to health; 2) the presence in the water of certain chemical compounds in the form of so-called pollution indicators, which in themselves, in the quantities in which they are found in water, are harmless, but the presence of which indicates a connection between the water body and one source of pollution or another; 3) the fluctuation of the physical properties and chemical composition of the water, which under certain conditions can serve as a clear indicator of the sanitary unsuitability of the source, and 4) the general habitus of the water, determined by its chemical composition. According to the goals pursued during the examination of water, water analyses are subdivided into three main groups: 1. Complete chemical analyses, performed in cases where an exhaustive, comprehensive characterization of the salt composition of the water of a given water source is required. This occurs when examining the water of mineral springs, where even the most insignificant content of individual compounds is of great importance. 2. Complete sanitary analyses - performed in cases where the water source is intended for relatively widespread use (installation of a water supply system, etc.). 3. Brief sanitary analyses - usually performed for the sanitary characterization of individual small water sources serving for small, limited water use. The scheme of a complete chemical analysis must include the determination of all elements that can be analytically proven in the water under investigation. The schemes of complete and brief sanitary analyses must satisfy the above-mentioned goals, and usually, in every country, they strive to standardize them by legislative means. Such standard schemes for complete and brief sanitary analyses of water in the USSR were developed by a commission under the Permanent Bureau of All-Union Water Supply and Sanitary-Technical Congresses. In 1927, it released "Standard Methods for the Examination of Drinking and Waste Waters" (publication of the Permanent Bureau, No. 75), which were recommended by the People's Commissariat of Health of the RSFSR for use by various sanitary-hygienic and sanitary-technical institutions (see table on p. 244); All determinations, with the exception of physical properties (odor, taste, turbidity, and sediment), are performed quantitatively. Of the listed determinations, the following are performed on-site immediately after taking the sample: source temperature, transparency, color, odor, and taste; on-site or immediately after delivery to the laboratory, the following are determined: reaction to litmus, nitrous acid, nitric acid, saline ammonia, H₂S, ferric and ferrous iron, oxidizability, free CO₂, and the water is fixed for the determination of O₂. The shorter the time separating the taking of the sample from the beginning of the analysis, the more reliable and trustworthy the results obtained. The following time limits are allowed before the start of the analysis: for unpolluted waters - 72 hours, for fairly clean waters - 48 hours, and for polluted waters - 12 hours. The collection of a water sample for chemical analysis must be carried out in compliance with conditions guaranteeing that: 1) all elements of randomness in the composition of the taken water are eliminated, 2) the water is taken truly from the place in the total mass of water that was intended, 3) the water did not undergo any changes in its composition during the sampling. For a general physico-chemical analysis, a 4-liter water sample is necessary. Individual samples are taken: a) in a 1.5-liter flask for the determination of the five-day oxygen consumption according to the English method, with this sample being transported in a special icebox in the summer; b) in two 1/4-liter flasks for the determination of O₂ according to the Winkler method, with manganese chloride and caustic potash being added to this sample on-site, and c) in a 200-400 cc flask for the determination of dissolved CO₂ on-site. All flasks are rinsed several times with the water being collected before filling. Each sample is accompanied by a form with the following information, which is absolutely necessary when sending water: 1) type and name of the water source, 2) exact address of the water source, characteristics of the location and the location of the nearest dwelling, 3) time of sampling (year, month, day, and hour), 4) purpose of sampling, 5) on whose instructions and by whom the sample was taken, 6) from what depth and point of the given water body the sample was taken, 7) depth of the well or other water body, 8) thickness of the water layer, 9) method of sampling, 10) quantity of the sample taken and number of samples, 11) color, taste, and odor at the moment of sampling, 12) transparency, turbidity, sediment, opalescence at the moment of sampling, 13) temperature of the source and air, 14) weather conditions, and 15) method of preservation. In the warm season, if the shipment of water to the laboratory requires more than a day, the water should be preserved: one portion - for the determination of oxidizability and ammonia nitrogen - by the addition of 2 cc of 25% sulfuric acid per 1 liter, and another portion - for the determination of the solid residue, suspended solids, loss on ignition, nitric and nitrous acids, and chlorides - by the addition of 2 cc of chloroform. The results of quantitative determinations of substances contained in the water under investigation are expressed in milligrams per liter of water at a temperature of 17.5°. Water hardness is expressed in German degrees, i.e., in milligrams of calcium oxide per 100 cc of water. Regarding the degree of precision of the expression of quantities, the following guidelines are usually followed: 1) for values over 10, the result is expressed as a number with two digits after the decimal point, 2) for numbers from 1 to 10, one decimal place is allowed, 3) for numbers from 0.1 to 1, no more than two decimal places are allowed, 4) for ammonia and nitrite nitrogen, the expression of values in tenths of a mg is allowed. Temperature is expressed in degrees Celsius, transparency in cm, concentration of hydrogen ions in pH values, alkalinity and acidity in cc of their normal solutions per 1 liter of water. Lit.: "Standard Methods for the Examination of Drinking and Waste Waters", publ. by the Permanent Bureau of Water Supply Congresses, M., 1927; Khlopin G., Methods for the Examination of Drinking and Waste Waters, M.-L., 1925; Gotschlich E., Handbuch d. hygienischen Untersuchungsmethoden, Jena, 1926.

S. Voznesensky.

III. Bacteriology of water. The bacterial content in waters—atmospheric, terrestrial, and underground—varies depending on the degree of contact of water with the living earth. 1. Atmospheric waters, which have not yet come into contact with the earth, contain few bacteria, which enter from the atmosphere with terrigenous dust. 2. Waters of direct surface runoff (rain and snow) are subjected to abundant bacterial contamination in their direct contact with the earth's surface (flood waters of rivers and streams). 3. Standing or slow-flowing surface waters of "indirect surface runoff" (lakes, dammed reservoirs, etc.), which manage in the process of settling and other types of self-purification to free themselves from the terrigenous turbidity and bacteria brought in by streams, usually possess sufficient purity in a bacterial respect. 4. Underground water, formed by percolation through the soil, is more or less free from bacterial contamination acquired on the soil surface, thanks to long, slow percolation through deeper soil and subsoil layers. 5. River water, which in its origin is a function of the changing ratios of the three types of water feeding the rivers: 1) direct surface runoff (flood), 2) "indirect" surface runoff (lake), and 3) underground runoff—is subject to the greatest fluctuations in bacterial composition. In inhabited areas, to the natural bacterial contamination of water through the soil are added numerous sources of fecal-sewage, domestic, and industrial, so-called cultural bacterial contamination, which is the most dangerous for water supply sources, because with these types of contamination, representatives of paratrophic forms enter the water, among which are the most dangerous pathogens of waterborne infectious diseases, such as: bacteria of typhoid fever, paratyphoid, dysentery, infantile diarrhea, and other groups of intestinal bacterial flora. Although pathogenic representatives of the paratrophic group are not habitual inhabitants of the aquatic environment, they nevertheless survive for a comparatively long time in water, especially at low temperature, and retain their virulence in it. Conradi (1904) considers the survival of typhoid bacilli in non-sterile drinking water to be over a year. According to Houston, upon artificial contamination of water, 99.9% of typhoid bacilli die within a week, although some survived up to 9 weeks. According to Winslow and Prescott, from the court case regarding the water supply conditions of the city of Jersey City and from observations of the sewage of the city of Chicago, one can draw the practical conclusion that any water after 4 weeks of storage is safe. Factors of self-purification of water from bacterial contamination are: settling, consumption by other microorganisms, light, the bactericidal action of O₂ released in the process of photosynthesis, a decrease in the nutrient value of the medium, a change in temperature, a change in reaction (pH), the magnitude of osmotic pressure, the phenomena of d'Herelle's bacteriophagy, and the phenomena of adsorption during percolation and filtration through the soil. Rain, lake, and artesian water contain from a few units to no more than a few tens of bacteria per 1 cubic cm. In uncontaminated springs, the number of bacteria usually also does not exceed a few tens per 1 cubic cm. In dug wells, the number of bacteria fluctuates within the range of a few hundred; in rivers—from a few tens and hundreds to hundreds of thousands per 1 cubic cm. The presence in water of the colon bacillus (Bact. coli comm.) is considered a sign of fecal contamination of water. According to Winslow and Prescott, the following groups of bacteria are common for water: 1) fluorescent; 2) chromogenic (violet, red, yellow); 3) the coli-aerogenes group; 4) the Proteus group; 5) non-gas-forming, non-chromogenic, non-spore-forming bacilli that do not produce Proteus colonies, that curdle or do not curdle milk, that liquefy or do not liquefy gelatin; 6) spore-forming of the Bacillus subtilis type; 7) white, yellow, and pink cocci. Bacterial analysis of water usually consists of 1) determining the number of colonies growing on solid nutrient gelatin or agar-agar after a certain period of time (48 hours) at a certain temperature (20-22° for gelatin, 25-28° for agar-agar) and 2) finding the coli-titer, i.e., the smallest volume of the water under investigation which gives a reaction for the presence of the colon bacillus. The determination of the coli-titer is based on the ability of representatives of the colon bacillus group to ferment glucose or mannitol with the formation of gas at a temperature of 45.5-46°C, which is inhibitory for other microorganisms that cause carbohydrate fermentation (Eijkman and Bulir tests). To establish the fecal origin of the colon bacillus, according to the German method, its identification with typical Bact. coli comm. is required. According to the American method, the establishment of the physiological characteristics of the coli subgroup from the entire extensive group of coli-aerogenes bacteria is required, because, according to American research, the entire coli subgroup is sufficiently typical for characterizing fecal contamination. The collection of water samples for bacterial research is carried out both with the help of special instruments into sealed, evacuated test tubes or flasks of the Roux instrument type, previously sterilized, and into ordinary types of vessels: sterile test tubes plugged with cotton, flasks with ground-glass stoppers, etc. Surface samples from open water bodies are taken from a depth of 10-15 cm from the surface. When taking samples from pipes, water taps, etc., it is necessary to flame the taps, pipes, etc., with the flame of a blowtorch and flush with a stream of water for 15 minutes. When examining the qualities and suitability of water for drinking, the amount of water for each sample must be no less than 500 cubic cm; for examining water for the presence of pathogenic bacteria—3 liters; for examining obviously contaminated waters—a sample of 10 cubic cm. It is desirable to perform the inoculation of the collected water samples at the site of sample collection; if this is impossible, then the transport of samples must be carried out under the following conditions: 1) the samples are kept in a special icebox at a temperature of 1-5°, 2) the cotton plugs must not be wetted, and 3) the samples must be delivered to the laboratory no later than 3 hours after their collection; if these requirements are not met, a corresponding reservation is necessary during the analysis. Biology of water - see Biological analysis.

S. Ozerov. IV. Sanitary evaluation of water. In the sanitary evaluation of water, the following are taken into account: a) physical and organoleptic properties of water, b) its chemical composition, c) the quality and quantity of microorganisms contained in water, and d) the sanitary conditions of the water body from which the water is taken. Water intended for the supply of populated areas, i.e., for drinking, for cooking, and for various household purposes, must first of all have good physical and organoleptic properties: it must be transparent, colorless if possible, cool, and have a pleasant taste without any foreign aftertaste or odor. Turbid, strongly colored water possessing any kind of aftertaste or odor may be permitted for water supply only in exceptional cases and on the indispensable condition of its preliminary purification. Favorable physical and organoleptic properties of water do not in themselves resolve the question of the suitability of water for drinking and household purposes. This question can be resolved satisfactorily only in connection with the chemical and bacteriological analysis of water, illuminated by a local survey of the water body from which the water originates. Sanitary requirements for the chemical composition of water boil down, mainly, to the fact that the water should not show signs of pollution and should not contain too many dissolved substances of mineral and organic origin. Among these substances, impurities clearly harmful to health are completely unacceptable: lead, copper, mercury, and other poisonous metals, arsenic, chromium, organic poisons, etc. A significant content of dissolved organic substances in water, in the presence of NH₃ and nitrous acid, indicates contamination of the water with nitrogenous waste and products of their decomposition. If a large amount of chlorides is simultaneously found in the water, a suspicion of urine contamination arises. A high content of lime and magnesium salts makes water hard. Such water, although it does not harm health, is poorly suited for many household purposes: meat, vegetables, and tea do not cook well in it; it requires excessive, useless expenditure of soap when washing clothes and bathing, and produces significant scale in samovars and steam boilers, etc. An excessive content of magnesium sulfate in water can cause a laxative effect in unaccustomed individuals. Repeated attempts by hygienists and chemists to precisely standardize the chemical composition of good-quality water have not had much success. Depending on local geographical and geological conditions, the developed standards proved suitable for some territories and completely unsuitable for others. Professor Erisman, when evaluating drinking water by chemical composition, recommends adhering to the following normative values (see article 249). The provided standards can serve as a valuable aid in the sanitary evaluation of water; however, the numerical values in the table should not be given a dogmatic, i.e., absolute, meaning. Considering the nature of local conditions, it is often possible to deviate from the limit values, not doing so only in relation to indicators of water contamination by decomposing waste of animal origin. Furthermore, it is also necessary to keep in mind that the use of limit values will be correct only if, when evaluating water, the totality of all analytical data is taken into account, and not just any one constituent part of the water. Limit standards for the chemical composition of drinking water (quantities in 1 liter of water): Solid residue after evaporation 500-600; Calcium oxide (CaO) 180-200 (including MgO no more than 40-50); Chlorine (Cl) 20-30; Sulfuric acid (SO₄) 80; Nitric acid (N₂O₅) 30-40; Nitrous acid (N₂O₃) traces; Ammonia (NH₃) 2-3; Oxidizability (amount of O₂ used for oxidation of organic substances dissolved in water) 18-20; Total hardness in German degrees. As for the sanitary evaluation of water in a bacteriological respect, a simple quantitative bacteriological analysis, i.e., the counting of colonies growing on gelatin or agar from 1 cubic centimeter of water, also yields results that are to a certain extent relative, due to the absence of firmly established quantitative standards for the bacteriological composition of water. Thus, Lubbert allows no more than 50-60 bacteria per 1 cubic centimeter of water, Plagge and Proscauer no more than 50-150, Koch no more than 300, and Pfeiffer and Mikel no more than 1,000 bacteria. A simple count of colonies grown on a nutrient medium from 1 cubic centimeter of water finds application, mainly, in the comparative evaluation of water from the same water body at different times or in different parts of it, as well as at water treatment plants for monitoring the operation of filters, ozonation devices, chlorination, etc. A qualitative bacteriological analysis of water, and mainly the presence in water of the colon bacillus (Bact. coli comm.) as an indicator of water contamination by human and animal excreta, has significantly greater sanitary significance. Due to the wide distribution of the colon bacillus in open natural water bodies (rivers, ponds, lakes), it is not always possible to demand its complete absence in water. Whipple established the following evaluation of water by the "titer" (quantity) of Bact. coli: Whipple's standards: Very clean water—Bact. coli in 100.0 and more cubic cm of water; Sufficiently clean—10.0-100.0; Doubtfully clean—1.0-10.0; Bad—0.1-1.0; Very bad—0.01-0.1. In some cases, especially during epidemics, it is important to verify the absence in water of pathogenic bacteria—typhoid, dysentery, and cholera vibrio. Water from small stagnant water bodies, as well as small streams and irrigation ditches flowing through populated areas, may require special examination for the presence of pathogens of parasitic diseases in humans or animals (see below). For a correct and comprehensive sanitary evaluation of water, it is very important to become acquainted with the water body from which the water is taken. It is necessary to conduct a careful sanitary survey of the water body and clarify the conditions that may influence the composition of the water in it. It is necessary to study the flora and fauna of the water body. If the inspection and biological survey show that there are no grounds to fear dangerous contamination of the water body, then one can be more lenient when evaluating the results of the water analysis. If, however, contamination of the water body already exists, or there is a possibility of such contamination, then any shortcomings of the water in physical properties, chemical and bacteriological composition must be treated very strictly. In these cases, any exceeding of the limit standards, especially in relation to nitrous acid, ammonia, organic substances, and the colon bacillus titer, must serve as sufficient grounds for rejecting the water. During the local inspection, the main attention must be paid to local topographical and geological conditions, to the proximity of cesspools and refuse pits, to dumps of sewage and garbage, to cattle yards, to sewage pipes for waste, to the discharge of factory waters, etc. It is also important to verify whether sanitary supervision of the water body exists and whether a sanitary protection zone has been established to protect the water body from dangerous contamination.

N. Ignatov. W. Water as a medium of life. Water, covering the greater part of the Earth's surface, is the medium of life for aquatic organisms, which in their aggregate constitute the main part of the biosphere. Among marine and freshwater organisms, there are those that can tolerate significant changes in chemical composition and salinity; for example, some crustaceans (from mysids, gammarids), mollusks (mussel-Mytilus), cockle (Cardium), many fish (e.g., gobies, eels, sturgeon) can live in fresh and marine water. Among various groups of freshwater organisms, there are so-called euryionic species capable of tolerating wide fluctuations in the active reaction of the medium (e.g., some larvae of aquatic insects, some species of rotifers and protozoa). Others, to which the majority of invertebrate inhabitants of the sea belong, are in a much closer dependence on the properties of the aquatic environment in which they live. The basic physicochemical properties of the internal environment of these animals—osmotic pressure, the composition of salts in the blood and lymph—are basically the same as those of marine water: between the external and internal environment of these animals there exists a close and deep connection. For an example, one can point to the composition of salts in marine water and in the body of the jellyfish Cyanea according to Macallum's data (per 100 parts): Salts, Marine Water, Jellyfish Cyanea: Cl 1.6543 / 1.6842; CaO 0.18931 / 0.11349; MgO 0.04943 / 0.04878; Na 0.18377 / 0.16946; Total 0.033503 / 0.068955; Total salt content 0.91898 / 0.89926; 2.98264 / 2.9279. The dependence of the osmotic pressure of the body fluids of marine invertebrates on the osmotic pressure of marine water is especially sharply expressed. The freezing point depression of marine water in the Mediterranean Sea, near Naples, Δ = -2.3°; the freezing point depression of the body fluids of Alcyonium palmatum (from coelenterates) Δ = -2.196°, Astropecten aurantiacus (from echinoderms) Δ = -2.312°, Sipunculus nudus (from worms) Δ = -2.31°, Homarus vulgaris (from crustaceans) Δ = -2.29°, Octopus macropus (from cephalopod mollusks) Δ = -2.24°. The osmotic pressure of the blood of these animals changes in accordance with the change in the salinity of the external environment: thus, according to studies by Rodier, near Arcachon (Atlantic Ocean), where the water has a lower salinity than in the Mediterranean Sea, the blood of invertebrates freezes at t° -1.89°, instead of -2.3°. Water constitutes by weight the main constituent part of animals and plants. Usually, the water content in an organism exceeds 50% of the total weight, and in some species reaches 95-98% (in some algae and aquatic animals—Cestus, Rhizostoma, Salpa). * Various tissues of the human body contain water from 70% (skin) to 83.5% (kidneys); only bone tissue contains 50% and adipose tissue 15% water. Loss of water leads to a slowing of metabolism and to the cessation of vital functions. A correlation has been established between the water content in plant tissues in a normal state and the amount at which the death of the organism begins. ** According to Pfeffer, plants sensitive to drying out die when the loss of water exceeds 40-50% of its normal content in the organism; others, less sensitive, such as, for example, Sedum elegans (a plant from the family Crassulaceae), can lose up to 80-90% of water and, nevertheless, retain their viability. In the frog, according to the data of various authors, a loss of water of up to 30-40% of the initial body weight is the limit; with further drying, the animals die. Dehydration of the organism in aquatic animals can be achieved by increasing the osmotic pressure in the external environment. Thus, by increasing the salinity of marine water, it is possible to cause a loss of water in the marine polychaete (from polychaete worms), Fabricia sabella, equal to 60% of the initial weight. With a decrease in the water content in the organism, animals pass into a so-called catabiotic state, physiological functions stop, oxidative processes cease or slow down to such an extent that they cannot be detected even by the most sensitive physiological methods. Isolated organs of higher animals after a loss of water of up to 77.14–91.25% (rabbit ear) or up to 25% of their weight (frog heart), upon rehydration, again restore their functions to a certain degree (the reaction of vessels to various stimuli, heart contractions are resumed). Some invertebrates * Cestus veneris (Venus's girdle) and Rhizostoma (jellyfish)—from coelenterates, Salpa—from tunicates.

The higher the water content in plant tissues, the more sensitive they are to water loss. Aquatic animals (some swamp species of rotifers, roundworms, and mites), the so-called tardigrades, can withstand drying out to an air-dry state. With an increase in the water content in an organism, the intensity of metabolism increases rapidly. Thus, barley grains containing 10-12% water in an air-dry state release 0.35 mg of CO₂ within 24 hours per 1 kg of weight. With an increase in water content to 14-15%, the release of CO₂ increases to 1.4 mg and reaches 3.59 mg at a water content of 19-20%. Thanks to a peculiar, exceptional combination of properties, water is a substance better adapted than others to serve as the primary substrate for vital processes. Important biological features of water include its high melting and boiling points compared to other bodies similar in their chemical structure. The maximum density at t° = 4° (3.98°), i.e., the melting point, and the rapid decrease in viscosity with a decrease in t° from 0 to -25°, can be explained only by the fact that water is not a simple substance, but a polymerized one, i.e., its simple molecules are capable of combining into more complex complexes; with an increase in t°, the complex particles of water break down into simpler ones. Among the thermal properties of water, which are of great importance for life, one should point out the high heat capacity of water and the high values of the latent heat of fusion and vaporization; thus, to raise the t° by a certain amount or to convert water from a solid state to a liquid or from a liquid to a gaseous one, it is necessary to expend a very significant amount of heat. This amount of heat is much higher than that of other substances, with rare exceptions (ammonia). Substance: Water, Ethyl alcohol, Benzene, Bromine, Mercury, Ammonia. Heat capacity (cal): 1, 0.5-0.7, 0.322, 0.1051, 0.0333, 1.23. Latent heat of fusion (cal): 80, 30.1, 16.2, 2.82, 2.8, 108. Latent heat of vaporization (cal): 536.0, 236.5, 109.0, 43.7, 62.0, 295.0. Thanks to these properties, temperature fluctuations in water bodies are mitigated, and the regulation of the t° of warm-blooded animals is facilitated, etc. Water is an energetic and universal solvent. Due to this, liquids contained in the bodies of animals and plants can simultaneously contain substances possessing different chemical and physico-chemical properties. But, at the same time, water possesses a certain chemical inertness, as a result of which substances dissolved in biological fluids can retain their individual properties during movements within the organism. * The water molecule consists of volatile elements. Therefore, by analogy with such substances as H₂S (melting point = -83°, boiling point = -61°), SO₂ (melting point = -73°, boiling point = -10°), ClO₂ (melting point = -76°, boiling point = +9°), one could expect that the corresponding points for water would be < -150° and -100°.

Thanks to its exceptionally high dielectric constant, water possesses great dissociating power. Ionization enhances the ability of substances to react with each other, and this circumstance is also very favorable for a living organism. Water itself, admittedly to a very small degree, is capable of dissociating into ions. Water ions, H and OH ions, belong to the number of the most active agents playing an exceptionally important role in biochemical processes; a change in the ratio between H and OH can have a profound influence on the physico-chemical state of biocolloids. The dissolution of a substance in water is connected with its hydration, i.e., with the emergence of unstable bonds between the molecules of the dissolved substance and the molecules of water. According to modern views, this bond is of an electrostatic nature and depends on the fact that water molecules are dipoles, i.e., bodies with two electric poles, capable of orienting themselves in the electric field arising around dissolved particles. Not only ions of inorganic compounds, but also colloids that are part of the organism, are hydrated: their particles are surrounded by shells of water molecules. The greater or lesser hydration of biocolloids that are part of the organism is of enormous importance for vital processes. It depends both on the physico-chemical properties of the colloids themselves and on the ionic composition of the medium and, especially, on its active reaction. The dynamics of vital phenomena are connected with constant movements of water, with its redistribution between individual parts of the organism. Between the blood and the lymph bathing the tissues of animals, as well as between the tissues of the organism, on the one hand, and the blood and lymph on the other, a constant exchange occurs: for example, a change in the osmotic pressure of the blood entails a redistribution of water in the lymph and tissues. These changes have an exceptionally important physiological significance, influencing the intensity and direction of the processes occurring in the organism. Water constantly enters the organism and is removed from it; part of the water is formed in the organism itself as a result of chemical reactions. In reversible reactions involving water, according to the law of mass action, an increase in the amount of water leads to hydrolysis, the splitting of a substance; a decrease, on the contrary, leads to its synthesis (for example, reversible reactions of saponification of complex esters by water). It is also known that the breakdown of proteins into amino acids is connected with the addition of water and, conversely, the synthesis of polypeptides from amino acids with the splitting off of water. In protoplasm, which is a complex heterogeneous and condensed system, water is found, mainly, in a bound state. Entering into diverse relationships with various parts of the organism, water takes part in the creation of complex structures of the organism. S. Skadovsky.

VI. Hydrology. Hydrology (from the Greek hydor—water and logos—word, science) is the science of the life of waters on the globe. It studies phenomena related to water, the laws governing these phenomena, the properties of water, their distribution and circulation on Earth, and the influence of their cycle on human life and human interests. The connection of hydrology with other disciplines. The cycle of water in the atmosphere and on Earth exerts a significant influence on the changing face of the globe, on the existence of organic life upon it, and on human activity. Dissolving to one degree or another all types of minerals included in the composition of geological strata, performing the work of erosion, transport, and deposition of crushed rock products, water is a powerful factor in the geological and topographical changes of the Earth. It represents one of the main constituent parts of organic matter; it performs the work of dissolving and transporting nutrients and removing the waste of organisms. Its excess or deficiency and its properties exert a significant influence on human health, on the improvement and sanitation of places of settlement, and on agriculture. Waterways have always been a cheap means of transport. Finally, water, which drives hydraulic engines, is a source of free energy. Hence, the connection of hydrology with meteorology, geology, physical geography, as well as with the cycle of agricultural sciences, with hydraulics, hydraulic engineering, hygiene, and sanitary engineering is clear. Hydrology, previously considered a part of geophysics, only in the 20th century emerged as an independent discipline taught in higher schools, where various branches of hydraulic engineering are studied—water transport, the use of hydraulic energy, sanitary and agricultural hydraulic engineering. Subdivision of hydrology. Studying waters, depending on their location, hydrology is divided into the following parts: 1) hydrology of atmospheric waters—hydrometeorology, 2) hydrology of surface waters, which includes departments on rivers (potamology), on lakes (limnology), on oceans (oceanology or oceanography), on glaciers (glaciology)—and 3) hydrology of underground waters—hydrogeology. In addition, hydrography and hydrometry are adjacent to hydrology, which studies the dynamics of water. The former deals with the description of the waters of the globe, and the latter with methods of accounting for water reserves. The region where free water is found on the globe—the hydrosphere—extends in the atmosphere above the surface of the Earth to a height of 10–12 km, above which water vapor is not encountered, and to a depth of 9–10 km below the Earth's surface, where, apparently, rocks become plastic, devoid of pores in which water could circulate. The main mass of chemically unbound water is located in the oceans, where its reserve amounts, according to rough estimates, to 1.30 billion cubic km. Land water and atmospheric moisture occupy a volume of about 4.5 million cubic km. There is no accurate data on the reserve of liquid water in the bowels of the Earth. It can only be said that it hardly exceeds 1/3 of the amount of oceanic water. The weight of the water shell does not exceed 0.0003 of the weight of the entire Earth. The region of the hydrosphere slightly exceeds the region of the biosphere. The amount of water participating in its annual cycle, i.e., evaporating from the surface of the oceans and land and falling on the entire surface of the globe (510 million sq. km) in the form of precipitation, is calculated at 465,300 cubic km per year. In this case, it is conventionally assumed that the same amount of precipitation falls on Earth per year as water evaporates, i.e., the annual income of moisture is equal to its expenditure. These 465,300 cubic km of water evaporating under the action of solar rays are equivalent to a layer of water of 910 mm covering the entire Earth. 82.5% of the water vapor formed during the year is obtained from the surface of the oceans and only 17.5% from the surface of the land. The greater part (76%) falls back in the form of precipitation on the surface of the oceans, and 24%, or 112,000 cubic km, falls on the surface of the land, equal to 149 million sq. km. Thus, 6.5% more precipitation falls on land than evaporates from it, i.e., this amount, equal to 30,000 cubic km, is brought from the ocean to the land, and the same amount must flow through rivers from the land into the ocean to restore the equilibrium of the cycle. From this, it is clear that water vapor brought from the sea plays far from the main role in precipitation falling on land, constituting only 27% of the latter. If we convert the values of the elements of the cycle to the thickness of a uniform layer, it turns out that, on average, 750 mm of atmospheric precipitation falls on land, 550 mm evaporates, and 200 mm is brought from the sea and flows back there. These average values in the water balance equation: precipitation = evaporation + runoff (surface and underground) experience significant deviations in different places on the globe. The actual amount of evaporation from the Earth's surface does not yet lend itself to direct accounting. All instruments used for its determination (evaporimeters) consist of a vessel with water and record the amount of possible evaporation with a constant abundance of moisture, or so-called evaporability. Actual evaporation from the Earth's surface covered with vegetation can differ noticeably from evaporability. Indirectly, the amount of evaporation from any basin can be judged by the difference between precipitation and runoff. The amount of evaporation, in general, decreases from the equator to the poles. In the tropics, on average, it reaches 2,300 mm per year, in the dry region of the trade winds—even 2,500 mm, while in the temperate zone it is, on average, approx. 400 mm. For the European part of the USSR, evaporation increases from the N.W. to the S.E. (Leningrad—320 mm, Moscow—417 mm, Lugansk—745 mm). For our Central Asian semi-deserts and deserts, the figures are even higher (Akmolinsk—926 mm, Nukus—1,798 mm, Sultan-Bend—2,764 mm). Evaporation increases with an increase in t°, saturation deficit, and wind speed. Evaporation also depends on the state of the soil cover. Thus, soil moisture, its capillarity, the proximity of groundwater to it, its dark color, and its location on a southern slope contribute to an increase in evaporation. Vegetation, especially forest, has a great influence on evaporation. Part of the fallen precipitation evaporates back into the atmosphere from the crowns of trees, but under the forest canopy, evaporation is moderated by protection from solar rays and wind; the forest dries out the lower layers of the soil, extracting water from them with its roots for transpiration, which does not cease even in air saturated with water vapor. In general, the forest insignificantly increases the amount of precipitation, slows down the melting of snow, and reduces the amount of runoff. Forest vegetation evaporates more than grassy vegetation, and the latter more than bare soil. Evaporation from the vegetative cover always exceeds evaporation from an open water surface, with the exception of arid periods when vegetation reduces its moisture consumption or dies altogether. Strong evaporation with a lack of precipitation is capable of turning a country into a desert (Turkestan); weak evaporation, even with a small amount of precipitation, contributes to the formation of swamps (northeast of the RSFSR). Staying in the tropics and deserts is burdensome for Europeans not so much due to high t° as due to the lack or excess of evaporation due to the saturation or dryness of the air. The amount of atmospheric precipitation of various types (rain, snow, sleet, hail, etc.) is accounted for with the help of rain gauges. Precipitation is formed as a result of the expansion and cooling of moist air when it rises, as well as during the cooling of moist air as a result of a decrease in the Earth's radiation or its contact with cooled objects. The amount of falling precipitation depends on the geographical location of the area. It is especially high where powerful ascending air currents are formed: at the equator, on the leeward slopes of mountains, on coasts, in areas of cyclone passage. The highest annual amount of precipitation was observed in India, in Cherrapunji (12,665 mm), the lowest—in the deserts of Chile (5 mm). In the USSR, the most precipitation falls on the Caucasian coast of the Black Sea (Batum, up to 2,500 mm). In the rest of the Union, there is significantly less precipitation; their value decreases from the N.W. to the S.E. Thus, in the W. and N.W., precipitation is from 500 to 650 mm, in the central part 400–500 mm, further to the S.E. their amount drops to 160 mm in Astrakhan, in Turkestan it is less than 100 mm. In Siberia, precipitation is, on average, from 300 to 400 mm; it noticeably increases on the Pacific coast. In most of the USSR, the maximum precipitation falls in the summer. Solid precipitation (snow) in the middle zone constitutes from 1/5 to 1/3 of their total annual amount. The intensity of precipitation decreases together with its duration and with an increase in the area on which it falls. The most intense precipitation is observed in our country in the S.W. Precipitation is a very variable meteorological element, and its value fluctuates strongly in different years. A 33–35-year periodicity of wet and dry years has been established.

The quantity and distribution of precipitation are important from a sanitary point of view: 1) the falling of precipitation contributes to the cleansing of the atmosphere from dust; 2) in arid regions, atmospheric precipitation water is collected in cisterns and reservoirs for water use; 3) the quantity of diverted storm water must be taken into account when installing sewage systems; 4) an excess of moisture on the surface of the earth leads to the waterlogging of the soil, the reproduction of midges and mosquitoes, and the spread of diseases characteristic of a humid climate. Part of the precipitation that has not evaporated into the atmosphere and has not seeped into the soil flows along the surface of the earth into rivers and seas. The quantity of water that passes through any cross-section of a river in 1 second is called the river discharge in this section and is usually expressed in cubic meters or liters per second. Determining the magnitude of runoff from a river basin reduces to finding the discharge at the mouth of its main river using hydrometric methods. By dividing the river discharge, expressed in liters per second, by the area of the basin in square kilometers, we obtain the so-called runoff modulus, expressing the magnitude of runoff in liters per second per 1 sq. km of the basin area. The runoff coefficient of a basin is also often determined, representing the percentage ratio of the magnitude of river runoff for a given time to the precipitation for the same period. Due to the dependence of runoff on many factors that change over time, and consequently, the need for precise and long-term observations of the magnitude of water discharge, the magnitude of runoff is known at the present time for a very limited number of rivers. In general, it can be said that river runoff, depending on a number of climatic factors that primarily determine the quantity and nature of precipitation and evaporation, increases with an increase in precipitation and decreases with an increase in evaporation. Snowmelt is of great importance for runoff, when from 1/3 to 2/3 of the annual amount of runoff passes through our rivers. For small basins, it is important to know the runoff of storm waters. From this, it is evident that the magnitude of runoff depends, in addition to the amount of precipitation, on the time of year. Due to high evaporation, the rivers of deserts often do not reach the sea, and their basins represent so-called endorheic regions, having their own closed cycle of water, distinct from the general one. All this illustrates the position stated by A. I. Voeikov that rivers are a product of climate. The topography of the basin also has significant importance for the magnitude of runoff; a slight slope, a large size, and an elongated or lobed shape of the catchment area, which contribute to losses from evaporation and seepage, reduce runoff. The state of the soil, its permeability, water capacity, the occurrence of subsoil layers, and the nature of the vegetation cover, by determining the nature of seepage, also exert their influence on runoff. As an example of the magnitude of runoff, the following data can be cited. The average annual runoff coefficient of the Volga at Yaroslavl is 41% of precipitation, the Oka at Orel is 21%, the Dnieper at Kiev is 35%. Fluctuations in the runoff coefficient are significant. Thus, for the Dnieper at Kiev, it varied over 26 years from 14% to 45%. In the same place, it is, on average, 88% of monthly precipitation for April, and only 7% for July, from which the role of snowmelt and evaporation is evident. The annual runoff modulus of the Volga at Yaroslavl is maximum - 62.6 l/sec, minimum - 2 l/sec; the Oka at Orel is maximum - 248 l/sec, minimum - 0.02 l/sec; the Dnieper at Kiev is maximum - 50.1 l/sec, minimum - 0.92 l/sec. The feeding of rivers at different times of the year and in different climatic conditions depends on rain, snow, or spring waters. In accordance with the nature of the feeding, horizons of flood, high, and low waters are observed on rivers. During the passage of high spring and summer or autumn flood waters, the velocities and discharges of rivers increase, and along with them, the quantity of transported and suspended sediments. It is at this very time that rivers, mainly, perform their geological work of erosion, transport, and accumulation of sediments, and therefore the turbidity of their waters increases. Conversely, at low horizons, one can expect the shallowing and even drying up of rivers. With a decrease in velocities and discharges, the quantity of suspended substances also falls, but the quantity of dissolved substances increases due to the groundwater feeding of the river. Rivers, being subject to pollution, possess the property of self-purification (which has great sanitary significance) due to dilution and then oxidation of polluting substances, especially with sufficient flow velocity. Thus, at a velocity of 1 m per second, a river is purified over a distance of 30--35 km. The shape of a river flow is determined by the equilibrium between its living force and the resistance of the banks. The processes of freezing and opening of rivers play an important role in the water and thermal regime of the basin. Reservoirs of standing water on land include lakes, ponds, and swamps. Lakes can be drainage lakes, having an outflow to the sea, and endorheic lakes. The latter are more often found in deserts and are mostly salty. Lakes smooth out river runoff, reducing floods, and regulate the thermal regime of the surrounding area. In hot countries, they increase evaporation; in cold ones, they contribute to the condensation of precipitation. Lakes are settling basins for suspended substances; therefore, the water of deep lakes is clean. Small ponds do not possess the properties of lakes; among all open water bodies, their water is of the worst quality, because, being easily polluted by wastewater, it is poorly purified due to the small size of the ponds. The peat soil of swamps, by absorbing water and spending it on evaporation, delays runoff. Swamp water, rich in dissolved humic substances and iron and poor in O₂, is generally unsuitable for drinking. An excess of surface and atmospheric moisture makes swampy terrain unhealthy for people and agricultural animals. Besides evaporation and runoff, part of the fallen precipitation seeps into the ground. Filtration depends on the physical and chemical properties of the soils, which determine the water properties of the soil (its water permeability, water capacity, and capillarity). Permeable rocks include fractured and granular ones: limestone, chalk, sand, loess; poorly permeable ones include dense, cohesive, and water-saturated ones: crystalline rocks, clay, peat, cemented sands, etc. A small slope of the terrain, loose soil cover, the presence of vegetation, and light and prolonged precipitation increase seepage. The greatest seepage is observed after snowmelt and in late autumn, while abundant summer precipitation mostly flows off and evaporates. In addition to filtration, some part of groundwater can originate through the condensation of water vapors that are lighter than air and penetrate into the soil. If water vapors condense upon contact with cooled soil, their pressure drops, due to which their inflow from the side of the air, where their pressure is higher, is strengthened, and condensation is maintained. Water that has seeped into the soil moves along the slope of the underlying impermeable layers at an insignificant velocity, forming streams and basins of groundwater. Depending on the alternation of permeable and water-resistant layers, several aquifers can exist underground. If the feeding area of an inclined aquifer, which coincides with the place where it emerges on the earth's surface, is sufficiently extensive and located high up, then the water in it can be under pressure, when it is covered from above by an impermeable layer, similar to how water is under pressure in a water pipe fed from a pressure tank. Depending on geological conditions, one can distinguish among underground waters: 1) soil waters, close to the earth's surface, not covered by an impermeable layer, 2) groundwater, deeper, covered, but not under pressure, 3) artesian, deep, covered, always under pressure, and 4) spring waters, emerging on the earth's surface. Underground waters close to the earth's surface show fluctuations in discharge depending on precipitation and evaporation, although with some delay, and often dry up during droughts. They are easily polluted by products of the decomposition of organic substances found in the soil through which they seep. Deeper waters, due to prolonged filtration, are freed from mechanical impurities, organic substances, and bacteria, but are enriched with mineral substances leached from neighboring rocks with the help of O₂ and CO₂ present in the water. Being in thick layers of bedrock, they are mostly abundant in water and possess a constant t°. Therefore, artesian waters serve as a reliable source of water supply. The quantity of water provided by various types of wells is determined by the size and nature of their feeding area, the properties of the aquifer, the intensity of water extraction, and the proximity of the wells to each other. Springs emerging at the foot of an aquifer exposed in a depression of the relief are called descending; in them, water enters under the action of gravity; most springs belong to this type. Springs that produce a jet beating upward are called ascending; in them, water moves by virtue of hydrostatic pressure or gas pressure (artesian wells, geysers). The closer the feeding area of a spring is to the earth's surface, the more easily it can be polluted, dry up, and change its t° with the seasons.

The deeper its recharge area, the more difficult it is for it to be contaminated from the earth's surface, the more mineralized it is, and the higher its temperature (warm and mineral springs). To protect underground water supply sources, special protection zones are designated, extending to their entire recharge area. The central hydrological organization in the USSR is the State Hydrological Institute under the All-Union Academy of Sciences in Leningrad. In addition, issues of hydrology are handled by the departments of communications, agriculture, the navy, the Supreme Council of the National Economy, sanitary supervision bodies, and geographical and regional studies organizations. Attached to the State Hydrological Institute is the Committee of All-Union Hydrological Congresses. Among the periodicals touching upon issues of hydrology, one should note: "Izvestiya Gosudarstvennogo Gidrologicheskogo Instituta" (Proceedings of the State Hydrological Institute), "Zapiski po Gidrografii" (Notes on Hydrography), "Vestnik Irrigatsii" (Irrigation Bulletin), "Vodny Transport" (Water Transport), "Sanitarnaya Tekhnika" (Sanitary Engineering), "Zhurnal Geofiziki i Meteorologii" (Journal of Geophysics and Meteorology), "Meteorologichesky Vestnik" (Meteorological Bulletin), "Priroda" (Nature), and others. (LIT. - SEE ART. 271). V. Troitsky.

VII. Aquifers. The surface strata of the earth's crust usually consist of alternating layers of permeable (e.g., sand) and impermeable (e.g., clay) rocks. From a sanitary perspective, the study of these layers is very important. Layers of permeable rocks, saturated with water and underlain by impermeable or water-resistant layers, are called aquifers. Aquifers enclosed between impermeable layers (strata) are called interstratal. If an aquifer is only underlain by an impermeable layer, but has free communication with the atmosphere and atmospheric precipitation from above, it is called a groundwater horizon. Aquifers are usually designated by the geological age of the water-bearing layers (e.g., Jurassic, Middle Carboniferous aquifer, etc.). If a thickness of layers of the same geological age contains several aquifers, then in such cases, as well as for regions with a firmly established number of aquifers, an arbitrarily established numbering is introduced to designate the latter. The main source from which the surface strata of the earth's crust receive water is the atmosphere. The water contained in it seeps deep down and penetrates into the layers of permeable rocks (infiltration). In addition, some part of atmospheric moisture enters aquifers through the condensation of water vapor from air penetrating into the surface layers of the earth's crust, which is possible only where the rock of the aquifers is not separated by impermeable layers from communication with the atmosphere. For groundwater horizons, this condition exists everywhere; their recharge area coincides with their area of distribution. For interstratal aquifers, the recharge area is located where the layers containing them, due to uneven bedding, emerge on the earth's surface, and specifically in the most highly situated areas of such outcrops. The height of the position of the recharge area is therefore the main factor determining the degree of hydrostatic pressure (head) of the waters of interstratal aquifers. Interstratal aquifers with pressurized water are called artesian. Since the predominant number of interstratal waters have one or another pressure, the names "interstratal" and "artesian" aquifer have in practice become almost synonymous. The rocks containing water do not always lie in even layers of constant thickness over large areas. Often they are interrupted in their bedding, or their character changes towards a decrease in the degree of permeability, or, finally, they lie in isolated lenses of greater or lesser size. This determines the area of distribution, configuration, and continuity or discontinuity of the aquifers. Closely related to these properties of aquifers is their productivity, which is determined by 1) the quantity of water in them, depending on their thickness and distribution, and 2) the degree of permeability of the rocks containing them, allowing for the possibility of a greater or lesser speed of water movement in them. The significance of the main types of aquifers for water supply is determined by the geological structure of the area and economic conditions. In the conditions of the Russian Plain, groundwater horizons play a leading role in this regard, since, being easily accessible for exploitation, they feed a huge number of dug wells, whereas interstratal aquifers (in particular, artesian ones) in most cases require the construction of artesian wells for their use. But in a sanitary regard, the water of groundwater horizons is inferior to interstratal ones, especially in densely populated areas, since the former are weakly or not at all protected from surface contamination by decay products of organic substances of animal origin, whereas the latter are protected from this by the impermeable layers covering them.

VIII. Sources of water and their sanitary assessment. Natural water, by its origin, can be conditionally divided into A) atmospheric waters (rain, snow, etc.), B) underground waters (ground, artesian waters), C) above-ground, or surface fresh waters (rivers, lakes, ponds), D) marine waters. A. Atmospheric (rain, snow, hail, frost, dew) water usually contains in solution, besides air gases, nitric acid, nitrous acid, NH₃, organic compounds, salts of Na, Ca, Mg, and mechanical impurities in the form of dust and bacteria floating in the air. In some cases, bromide, iodide compounds, sulfurous acid, hydrochloric acid, and traces of other compounds were found in rainwater. The quantity of various substances in atmospheric waters depends on the state of the atmosphere through which the rain or snow passes. The composition of meteoric waters will be different after a prolonged drought and during a period of prolonged rains. Their composition is significantly influenced by the composition of the soil and local sanitary conditions that pollute the air. In general, quantitatively, all these impurities are insignificant and only in industrial areas can they reach significant values, mainly due to sulfurous compounds. In industrial areas, where factories and plants release a large quantity of smoke and soot into the air, meteoric waters are polluted to a greater extent and contain a large amount of soot, dust, sulfurous and other compounds. In large cities, due to numerous house furnaces heated by coal, the air, especially in the winter period, is also polluted to a significant degree by soot and sulfurous compounds. The amount of solid residue found in meteoric waters is, on average, small; it ranged from 0.8 to 60 mg per 1 liter of water. The insignificant content of solid residue in meteoric water allows (in practical calculations) one to consider that it absorbs gases in the same proportions as chemically pure water. The average chemical composition of meteoric waters, obtained by J. König on the basis of numerous analyses of rainwater samples taken in industrial and non-industrial cities, is given in the table on page 263. Meteoric waters are used quite often for drinking and various household purposes in arid places or in places where ground or surface water sources are located at great distances from populated areas. As an example, one can point to a number of localities in the south-east of the USSR, in particular to the former Stavropol Governorate, where many villages widely use rainwater collected in specially constructed cisterns for household and drinking purposes. In some arid localities, the population uses only snow water in the winter time. In a sanitary regard, rainwater collected in cisterns must be treated with great suspicion and, in any case, with even greater precautions than is required in relation to open natural reservoirs. B. Underground waters (ground and artesian). 1. Ground waters are usually called underground waters that accumulate due to atmospheric precipitation absorbed by the soil and form the first water horizon from the surface, located on the first impermeable layer from the surface. Ground waters, being under the pressure of only the atmosphere, cannot rise upward on their own; therefore, the level of ground waters in wells and boreholes, unlike pressurized artesian waters, does not rise but remains at the depth at which it was encountered; upon pumping, the level of ground waters drops and, depending on hydrogeological conditions, more or less slowly recovers. Fluctuations in the level of ground waters reach significant values and depend on the unevenness of precipitation, the nature of vegetation, and the ratio between the amount of atmospheric precipitation and evaporation, which also explains the significant fluctuation in the level of ground waters by periods of the year. The influence of atmospheric precipitation on the level of ground waters is especially noticeably reflected in the layers closest to the surface; depending on hydrogeological conditions, this influence manifests itself in a greater or lesser interval of time from the moment of precipitation (from a few to 30 or more days). As a rule, in the cold season (autumn, spring), the rise in level occurs faster than in the warm season, because in the hot season a significant amount of precipitation is spent on evaporation and plant nutrition. Over the years, depending on fluctuations in the amount of falling atmospheric precipitation, similar fluctuations in the level of ground waters are observed. The depth of occurrence of ground waters is different in different places. In the USSR, the depth of occurrence of ground waters increases from north to south, being in polar regions almost at the surface of the earth, and in southern places (near the Black Sea) reaching a depth of up to 60 m. Ground waters, due to the difference in pressure at different points, are in constant motion. This pressure is most often caused by the difference in water level. The speed and direction of ground waters depend on complex hydrogeological conditions; the geological structure of the aquifer—the depth of occurrence and the relief of the impermeable bed—is of especially great importance here. In depressions formed by an impermeable water-bearing layer, significant underground basins can form, which, depending on the water level in them, feed the general flow or are completely separated from it. In loose aquifers, the direction of movement of ground waters strictly follows the slope of the layer. In fractured rocks, the movement of water is more complex, as it depends on a complex network of cracks. Where aquifers reach the surface of the earth, natural sources (springs) are formed. In flat areas, outcrops of aquifers are found in river valleys and ravines; sometimes outcrops of aquifers are observed on gentle slopes of hills (swamping of the area). Emerging on the surface of the earth in river valleys, ground waters have a significant influence on the regime of rivers, increasing their supply, changing the chemical composition and temperature of river water. Natural ground waters are widely used for water supply (wells of various types). In addition to natural ground waters, at the suggestion of the engineer Richert, the use of so-called "artificial ground and artesian waters" has been successfully applied in various places in Sweden. The latter are obtained by discharging river or lake waters into soil layers. This same method, slightly modified, is used in many other places in Europe (Dresden, Toulouse, Nancy, Budapest, etc.) and consists in the construction of wells and galleries along the banks of rivers and lakes, fed by both ground water and the water of rivers and lakes. According to Richert's method, the water supply in the Ruhr region and in Frankfurt am Main has been reorganized. In the USSR, the city of Arzamas is supplied with artificial waters. The main influence on the composition of ground waters is exerted by the composition of the soil through which atmospheric water passes, while the rocks in which further underground movement occurs have a comparatively weak influence on the composition of ground waters. Conditions of their movement can also have a significant influence on their composition. When water stagnates, it becomes more saturated than when it is in motion. This explains why the water of self-flowing springs of the same water horizon is usually less mineralized than the water of dug wells. When moving through fine-grained rocks, ground water is to a significant degree freed from previously acquired pollution. The finer the sand and the thicker the sandy layer, the better natural filtration occurs. Waters occurring in thick fine-grained sands are distinguished by greater purity. Coarse-grained rocks, and especially fractured ones, relatively easily allow pollution to pass to a significant depth. Depending on the depth of the water horizon and the structure of the overlying rocks, there is a fluctuation in the salt composition of ground waters and their temperature. With easy permeability of rocks (gravel, sands, fractured rocks) covering the horizon of ground waters, significant changes in the salt composition of ground waters are observed during periods of prolonged rains and snowmelt. Deeper ground waters and ground waters covered by layers of impermeable rocks (interstratal) are significantly less subject to seasonal fluctuations, often providing water of constant composition. Penetrating into the soil, atmospheric waters gradually lose dissolved O₂, which is consumed in various biochemical processes occurring in the soil. Instead of the lost O₂, the water receives from the soil layers free and bicarbonate CO₂, nitrates, sulfates, and salts of other oxygen acids. The conditions of the formation of ground waters and the dependence of their composition on soil conditions indicate the great sanitary significance of the state of the soil in the area of ground water recharge. Sanitary protection of ground waters should consist primarily in preventing soil pollution within the areas of recharge and accumulation of ground waters (see Sanitary protection zones).

Deep groundwater, covered from above by thick layers of impermeable rock with a large catchment area, is in better condition in terms of protection from pollution, but in each individual case, it is still necessary to study the extent, thickness, thinning out, and relief of the overlying and protective impermeable rock through geological surveys. 2. Artesian waters, see Artesian waters. B. Surface waters, or water bodies. All open water bodies located on the earth's surface can be divided into flowing and standing water bodies. Flowing water bodies usually include rivers, while standing water bodies include lakes, ponds, and swamps. 1. Rivers. Diverse natural and cultural conditions and seasonal phenomena significantly influence the fluctuations in the composition of river water, which is highly varied not only for different rivers but also for the same river in different places and at different times of the year. The water of rivers flowing through crystalline rocks poor in lime contains significantly fewer solid substances than the indicated average figures, sometimes approaching the composition of rainwater (Neva River). Rivers flowing through sedimentary and, especially, limestone rocks or fed by groundwater are distinguished by a significant content of lime and magnesia (hardness). Rivers fed by swamp water contain humic substances in their water. Rivers receiving significant amounts of wastewater from cities, factories, and plants change their composition sharply accordingly and have higher amounts of solid substances. The main part of the solid residue of river water consists of carbonate and sulfate salts of lime and magnesia. Calcium carbonate salts account for 35-94% of the total solid residue. The water of rivers flowing into seas, at their mouths, due to the admixture of seawater, contains more calcium sulfate than calcium carbonate. River waters, like atmospheric ones, contain a greater or lesser amount of gases: O₂, N, and CO₂. The total amount of gases in river waters is usually found in the range of 14.0-56.5 cubic cm per 1 liter of water. In addition to dissolved substances, river waters contain various suspended substances, the quality and quantity of which fluctuate within significant limits depending on local conditions. Suspended substances usually consist of fine particles of clay, sand, alkaline earth carbonates, organic substances, and microscopic organisms. During flood periods, rivers contain large amounts of turbidity and organic substances. The water of the Dniester River at the point of intake by the Odessa water supply system takes on the appearance of coffee grounds during flood periods. However, there are significantly fewer solid substances dissolved in the water in floodwater. In winter, when the feeding of rivers by groundwater increases and the inflow of atmospheric water decreases, river water concentrates and purifies. To provide an idea of the chemical composition of river waters, we present (see the two following tables) the results of chemical analysis of the waters of the Neva, Moscow, and Elbe rivers (according to Surin). In terms of bacteria, the composition of river waters fluctuates even more than the chemical composition. The Moscow River at the point of water intake for the Moscow water supply system contains from 300-1,000 to 5,000-12,000 bacteria per 1 cubic cm (during flood periods). The same river in the city of Moscow contains from 8,500 to 62,600 bacteria. The Neva River at a depth of 15 meters at the water intake point contains from 25 to 5,000 bacteria per one cubic centimeter. The physical-chemical and biological properties of river waters are subject to large changes depending on the conditions of the river's supply, climate, the rocks of the riverbed, the nature of the vegetation, and other conditions that determine the river's regime. A characteristic difference between river waters and groundwater lies in the scarcity of salt composition, significant amounts of suspended substances, and colloidal organic substances. This feature of river waters disappears in many places, as some rivers provide significantly mineralized water. Populated areas and industrial enterprises located in its basin have a significant influence on the river in a sanitary sense. The latter circumstance forces one to be very cautious in choosing the location for river water intake for water supply purposes, as even the best methods of water disinfection (filtration, chlorination, etc.) cannot guarantee sufficient disinfection of heavily polluted river waters. For river water supplies, the sanitary condition of the river basin above the water intake point is of great importance. The task of sanitary supervision includes preventing factory and household sewage from entering the river and monitoring the sanitary condition of all populated areas located in the river's region. Sanitary supervision must strictly monitor the movement of those epidemic diseases among the local population (typhoid fever, dysentery, cholera, etc.) that are dangerous in terms of transmission through water. It is for this purpose that the organization of a so-called sanitary protection zone serves (see Sanitary protection zones). 2. Lakes. The chemical composition of lake waters is very similar to the composition of river waters and depends, mainly, on the geological rocks surrounding the lake and the composition of the inflowing waters. Lakes located in low-lying areas contain more solid substances than mountain lakes, the water of which is very close in composition to atmospheric water. Drainage lakes are usually fresh, but if the inflow of water is highly saline due to soil conditions and the outflow is small compared to the inflow, the drainage lake can become salty. In closed lakes, continuous evaporation causes an increase in the concentration of salts in the lake water (salt lakes). The chemical composition of lake waters is illustrated by analytical data (according to Surin)—see the following table. The main element characterizing a lake is its area and depth. The most important physical-chemical and biological processes are related, mainly, to the depth of the lake. In a sanitary sense, populated areas, industrial production, and shipping also influence the quality of lake waters. In large lakes, at significant distances from the shore, lake waters are usually cleaner than in the coastal zone, and they are cleanest of all in the deep layers, from where water should be drawn for water supply purposes, at a depth of 8-12 m. Lake waters are usually cleaner than river waters because sedimentation processes take place in them under more favorable conditions. Furthermore, in deep lakes, there is a leveling of temperature, as only the upper layers are subject to strong heating (in summer) and freezing (in winter), while the temperature of the water changes little in the middle and deep layers. But even lake waters, except in cases where the lake is located in uninhabited areas, must undergo appropriate purification for water use purposes. 3. Swamp waters should not serve as sources of water supply (see Swamps). 4. Ponds. In addition to reservoirs for collecting large quantities of water (see Reservoirs), small reservoirs (ponds) are often built in small populated areas to collect atmospheric water. In areas located on watersheds, where groundwater is at a great depth, dug ponds very often serve as sources for drinking and household purposes. In other places, where there are wells for drinking purposes, ponds are built, mainly, for fire-fighting purposes. Among all sources, ponds are the most dangerous in a sanitary sense, as atmospheric waters, before entering the ponds, wash away all the dirt from the surrounding area and bring it into the pond. Non-flowing ponds are in particularly unfavorable conditions, as stagnant water with a large amount of pollution (organic substances) quickly rots.

By correctly choosing the location for a pond and implementing appropriate hydrotechnical measures, it is possible to place ponds in more favorable sanitary conditions, but even with the observance of these measures, pond water must be regarded as water that is polluted and dangerous in a sanitary respect. When constructing ponds and using them for drinking and household purposes, it is necessary to pay special attention to the choice of the location for the pond and to the hydrotechnical measures ensuring the correct collection and storage of water. Despite the unfavorable sanitary conditions, pond water supply is quite widely distributed in the USSR. A whole series of localities in the southeast, central, and northern provinces, located on watersheds, are supplied with water from ponds. As an example, one can cite the Moscow province, where pond water supply is encountered in settlements located on watersheds (Klinsky, Dmitrovsky, Moscow districts). In a monographic work ("On the Question of Drinking Waters of the Klinsky District", 1894), N. D. Sokolov comes to the following conclusion: "It turns out that as the percentage of the population using drinking water from ponds increases, both general and infant mortality among them increases, the percentage of those who died under 5 years of age in relation to the sum of all deaths also increases, the average life expectancy decreases, and the population growth (the ratio of deaths to births) decreases, although not in such a strict sequence; all this occurs despite the fact that the birth rate is significantly higher in parishes with pond water than in parishes whose population does not use such water for drinking."

G. Sea water in its natural form is unsuitable for water supply. Only in localities where it is impossible to obtain natural fresh water is artificially desalinated sea water used. Such water is also used on sea vessels when the reserves of natural fresh water obtained in ports are exhausted. Desalination of sea water is carried out with the help of specially constructed apparatuses for desalination. Desalination of water is very expensive and results in soft and tasteless water, which is reluctantly used for drinking. Desalination of sea water for water supply purposes in the USSR was used in Baku (before the construction of the Shollar water pipeline) and in Krasnovodsk. The sanitary evaluation of water sources very often presents a difficult task. When choosing a source, it is necessary to provide a correct evaluation of it and foresee its near future if long-term use of the source for water supply purposes is assumed. The diversity of natural and cultural-domestic conditions influencing the regime and composition of a water source does not allow for the application of template requirements to the composition and quality of water. In each individual case, when evaluating water, one must take into account the peculiarities of local conditions. The main task in the hygienic evaluation of a water source is the establishment of the origin of the source, the natural composition of the water, and those changes in the composition of the water that occur due to the cultural-domestic conditions of the population. If such an influence, in the sense of pollution, is significant and constant, then by means of laboratory (physico-chemical, bacteriological, biological) methods of investigation, it is not difficult to establish it and provide a sanitary evaluation of the water source. In other cases, when the occurring pollution is small and inconstant, the task of the hygienist is complicated, requiring long-term (periodically, by seasons) and thorough observations. Often, even through long-term studies, it is difficult to resolve the question of whether a particular change in the composition of water occurs due to the natural peculiarities of the regime or due to temporary and accidental pollution. When evaluating water sources, it is necessary to keep in mind that, depending on the character and regime of the water source, the evaluation of the obtained research results may also change. Thus, for example, for surface groundwater sources and open reservoirs with changing water composition (under the influence of atmospheric precipitation), the constant absence of chemical and biological-bacteriological indicators of pollution provides a basis for a favorable evaluation of the water at a given moment. The question of the future of such a source is decided by the geological-hydrological, sanitary, and other conditions of the given locality. For deep groundwater, possessing constancy of composition (artesian waters, deep groundwater) and temperature, any fluctuation in chemical composition and temperature, even in the absence of so-called indicators of pollution, should always arouse suspicion of the possibility of pollution of the given water horizon. The unusual appearance of fluctuations in the composition of such waters directly indicates an inflow of some waters of a different origin and composition; depending on the quality of the latter, the sanitary significance of the inflow for the given water horizon is determined. Such a case, quite frequent in practice, indicates the necessity of studying, if possible, all water horizons of the given locality. This same circumstance requires local medical-sanitary organizations to constantly study all local water sources, because otherwise the resolution of practical questions (e.g., when choosing sources for water pipelines) will be delayed by the necessity of conducting preliminary long-term and expensive observations. The task of sanitary investigation usually includes the study of all those local conditions which directly or indirectly can have a harmful influence on the composition of water. Since the sources of pollution are the products of human and animal vital activity, the hygienist must first of all thoroughly study the local conditions of the population's life, paying main attention both to the development of waterborne epidemics and to the possibility of their spread in the given local conditions (see also Water Supply). I. Khetsrov.

IX. Water as a source of infection by animal parasites. 1. With contaminated water, cysts of the dysentery amoeba (Entamoeba histolytica) and other amoebae, as well as flagellates (Trichomonas intestinalis), Giardia (Giardia intestinalis), etc., can be swallowed. Eggs of certain parasitic worms, which develop in the human intestine into the corresponding forms of parasites, such as: Ascaris (Ascaris lumbricoides), whipworm (Trichocephalus), pinworm (Enterobius vermicularis), dwarf tapeworm (Hymenolepis nana), etc., also enter the human intestine with water. Contamination of water with them occurs when fecal masses enter it directly or from open-type latrines; when soil is contaminated with excrement or garden fertilizers; when worm eggs and cysts are carried by rainwater into sources and water bodies; when wells are contaminated by seeping or flowing feces from nearby latrines; when water already brought into the house is contaminated through dirty hands or dishes, etc. Not only human fecal masses, but also the excrement of various animals can contain parasite eggs that also infect humans (for example, the whipworm and Ascaris of the pig). 2. With water, a person can swallow larvae of parasites for which water is a normal temporary habitat. This is how infection with the liver fluke (Fasciola hepatica) occurs, the cercariae of which encyst on the surface of water or on aquatic plants. Infection of livestock (respectively, humans) with flukes occurs when swallowing encysted cercariae with drinking water or when eating grass (respectively, plant food) onto which these cysts have landed from water. Similar routes of infection are also valid for the fluke (Fasciolopsis buski), the cercariae of which encyst on the leaves and fruits of the water caltrop, or water chestnut (Trapa natans), eaten in China in a raw state. Cases of swallowing larvae of Ankylostoma duodenale and Necator americanus with water are possible, but this route of infection plays no significant role in the epidemiology of ancylostomiasis. 3. With water, a person swallows an intermediate host infected with a parasite, of which he is the definitive host. This is the case with infection by the Guinea worm (Dracunculus medinensis; recently renamed Fullebornius). Infection with the Guinea worm occurs when swallowing a cyclops with larvae with drinking water (Bukhara). 4. The larvae of some parasites that live for a certain time in water actively burrow into human skin and penetrate through the vessels to the place of habitation in the host's body. Such is the method of infection with bilharzia (dioecious flukes living in the veins of the liver, bladder, and rectum—Schistosoma japonicum, Sch. haematobium, and Sch. Mansoni), the cercariae of which have the ability to pass through human skin (not yet discovered within the USSR). 5. Finally, water is an intermediate habitat for some stages of development of parasites with which a person is infected by eating a second intermediate host; e.g., the broad fish tapeworm (Diphyllobothrium latum): eggs from the human intestine enter the water, where an embryo in the form of an oncosphere emerges from the egg; the first intermediate host is the cyclops crustacean, which swallows the oncosphere, from which the procercoid stage develops; the second intermediate host is a fish (with the plerocercoid stage, forming from the procercoid of the swallowed cyclops); the definitive host is a human (the adult worm in the intestine, developing from the plerocercoid of the eaten fish); water plays a similar role as a habitat for some flukes whose second hosts are fish (e.g., Clonorchis sinensis, Opisthorchis felineus, etc.). To prevent infection with parasites through water, one should drink boiled water or, in extreme cases, filtered water.

E. Pavlovsky. Water as a source of the spread of infectious diseases—see Waterborne infections.