Dan H. Yaa lan I Department of Geology

Hebrew University of Jerusalem I S a h i ~ a t i ~ n and Sahity Israel

Soluble salts are present, usually in small amounts, in all rocks, soils, and waters. As a re- sult of certain conditions of climate, topography, or the nature of the rock and the soil, they accumulate in amounts many times higher than the average low con- centration. The combination of processes leading to such an accumulation is called salinization. Salinity is a measure of the total quantity of dissolved or soluble solids in water or soils, and is expressed in a variety of weight-per-~l-eight or weight-per-volume units. (For sea water it is conventionally expressed in g/kg.) The use of equivalent combining weights, especially for classification purposes, is now common.

Most of the topics outlined below fall within the gen- eral field of geochemistry, and more specifically to the geochemical circulation of elements and the laws govern- ing it (1-3). Applied aspects of the subject are of great concern to soil science and to hydrogeology (4-5).

The problem of salioization afflicts especially the dry regions of the world. Where surface water or ground water is used for irrigation or for other purposes its suit- ability is determined largely by its salt conceutration. Excessive accumulation of salts in the soil is harmful, making it unsuitable for plant growth.

In man's quest to control the harmful effects of salini- zation, much has already been learned about the occur- rence, distribution, and migration of the soluble salts in nature. The major components of natural waters are those of sea water: Na+, Mg2+, Ca2+, I<+, C1-, SOa2-, and HCOa-. The minor constituents of interest are compounds of B, Br, I, N, and P, and occasionally vari- ous metallic cations. The soluble salts precipitating and accumulating in soils and sediments are mostly chloride, sulfate, or carbonate compounds, usually asso- ciated with the sodium, calcium, or magnesium cations.

sively. Their average conceutration in the major units of the earth is listed in Table 1. They rank among the first 15 elements in total abundance in the earth's crust.

The primary sources of chloride and sulfate salts are the dissolved products of rock weathering, emanations from volcanic eruptions, and discharge from deep ther- mal energy sources. The solutes (soluble compounds) released and dispersed by weathering agencies pass through the pedosphere (soil) and often through the biological cycle of plants and animals. They are leached out from the soil by rain and are transported by surface and ground waters to reach ultimately the sea. During this migration their conceutration and composi- tion undergo many important changes as a result of their different mobility and affinity to form or interact with compounds met in their path. The migration and redistribution of the salts is almost exclusively through the agency of water, which thus acts both as solvent and as transporting vehicle. The rate of migration is governed by climatic and geomorphic factors governing the hydrological cycle. More or less temporary ac- cumulations of soluble salts occur in undrained soils, in salebearing sedimentary deposits, or as highly saline brines. Such salt reservoirs may be returned to the global circulation after a change in the local topo- graphical or climatic conditions, or through the action of man.

By far the largest reservoir of soluble salts is the ocean. Out of a total of 50 X 1016 tons of salts in the ocean, 28 X 1016 tons is chloride. Assuming that in the course of geochemical evolution a steady state system has been established, with equal amounts of salts enter- ing and leaving the major reservoirs, a global circula- tion pattern can be constructed, as exemplified by chlorine (Fig. 1).

The Geochemical Cycle of M ig ra t i on Sea Solts in the Atmosphere ond i n the Rain

Because chlorine and sulfur are the most characteris- Winds blowing over the ocean pick up a substantial tic elements of the solusphere (the soluble salts of the load of salt particles, which originate at the sea surface hydrosphere), t'hey have been investigated most inten- as spray or as a result of gaseous escape. Ordinary sea

winds carry 1-10 kg of sea salts per kma of air (about Table 1. The Average Abundance o f Chlorine 1&100 lb/cu mi). Storm winds mav carrv as much as

and Sulfur in Nature 100 kg of Balt particles per kma (1000ih/cu"ki). Winds C l P P ~ S P P ~ carry a great deal of the salts onto the land. Various

Igneous rocks 180 300 estimates indicate that as much as half of the total is Sedimentary rocks precipitated on the land as dry fallout. Some salt

Shales 100 2400 Sandstones 20 240 particles act as rain generating nuclei and are washed Carbonates 150 1200 out from the air by rainfall. Sails 100 850 Evidence from measurements of the chemical com- Deep sea sediments 21,000 1300

Waters position of rain indicates that annual precipitation of sea Ocesn 19,500 880 salts near the shore is high and reaches up to 100 g/m2. Rivers 7 . 8 3 .7 It decreases exponentially with distance from the coast Precipitation 2.5 0 . 6 to a few g/m2 annually about 50 km inland. During

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The geochemiw circulation of chlorine in nature. Circler indicate etti- mated quontitier in the respective spheres and reservoirs, in million tons chlorine. Arrows and orrociated figure, show direction of migration and onnvol involved, in million tons chlorine. (Modified after E. Erib- son (perronol comrnunicotion) with new doto added).

the formation of the sea spray and during its transport inland it undergoes changes in composition. Over the continent the airborne sea salts are mixed with salts of terrestrial origin, so that the average composition of the precipitation is quite dierent from that of sea water (Table 2). Precipitation is thus usually relatively richer in sulfate, bicarbonate, calcium, potassium, and nitrogen compounds, and also bromine. The average concentration is only about 5-10 ppm, yet practically all of the chloride and most of the sulfate salts present in lake and river water are of airborne cyclic origin.

River Water

Airborne salts precipitated over the land are found in the soil, and in streams, rivers, and lakes, where they are mixed with weathering products. During the time be- tween their deposition on the land and eventual runoff to the sea, the chemical composition changes by various processes. Evaporation and transpiration of plants in-

crease the concentration; reduction of sulfate and es- cape of hydrogen sulfide take place where anaerobic con- ditions prevail; cation exchange processes with the clay result in important changes in the relative proportion of the cations and in pH. By the time the solutes reach the downstream section of the major rivers, calcium and bicarbonate become the dominant ions, and both mag- nesium and sulfate usually exceed the airborne sodium chloride (Table 2). The mean weighted concentration of river water of the world is 120 ppm. Variations in composition are mainly related to the lithological char- acter of the drainage basin and to the chemistry of the rain water.

Climatic conditions have a great effect on the seasonal variation in runoff and on its composition in time and space. Lowest concentrations are generally obtained during seasons of high discharge. During floods the composition may equal that of the rainfall.

Water in large river systems rarely reaches concentra- tions higher than 1000 ppm. It is therefore generally suitable for irrigation, except where pollution or certain trace elements make its use undesirable.

Lake Water

The concentration of lake water varies from dilute (which closely resembles rain water in composition, e.g., small lakes on resistant rocks in humid climates) to highly concentrated brines (in closed basin lakes of arid regions). A concentration gradient and a distinct stratification with depth is usually established as a re- sult of unequal seasonal heating. Bottom reduction of sulfate is common, with attendant changes in pH and possible precipitation of calcium and magnesium car- bonate. In terminal drainage marshes the concentra- tion of many salts reaches saturation and the least solu- ble ones crystallize out. Some concentrated brines of saline lakes contain valuable accumulations of certain compounds which are being exploited, e.g., the Dead Sea for potash and bromine, Lake AIaggadi and Lake Natron for soda.

Subsurface Water

Soil water. Rain water percolating through the soil and through the zone of aeration dilutes the soil solu- tion, but a t the same time adds to it appreciable amounts of ions. It is the major agent of chemical weathering, and during its detention in the pedosphere it undergoes many significant transformations. Nutri- ent salts are taken up by plants, circulated, and stored in the biomass, and the water becomes enriched in bio- genic GO2. Oxidation and reduction processes affect

Table 2. Average Composition of Rainwater and River Water in Different Continents, Compared to Sea Water

Na K Ce Mg C1 SO, HCOa NOs ??HI Total parts per million

Rain water Northern Europe 2 .1 0 .4 1 .4 0 . 4 3 .5 2 .2 4 . 6 0 . 3 0 .4 15 SE Australia 2 .5 0 . 4 1 . 2 0 .5 4 . 4 Trace . . . . . . . . . 10

River water& North America 9 1 . 4 21 5 8 20 68 1 . . . 142 Europe 5 . 4 1 . 7 31.1 5 . 6 6 . 9 24 95 3 . 7 . . . 182 Austrdia 2 .9 1 . 4 3 . 9 2 .7 10 2 .6 32 0.1 . . . 59

Sea water diluted 5 X 10' times 2 .1 0.08 0.08 0.26 3 .9 0.54 0.03 ... . . . 7.1

After D. Carroll, Geol. Survey Water Supply Paper 1535+, 1962. LAfter D. A. Livingstone, Geol. Survey Professional Paper 44C-6, 1963.

592 / Journal of Chemical Education

chiefly the sulfur, nitrogen, iron, and manganese com- pounds. Ion exchange reactions take place with the clay minerals, releasing considerable quantities of cal- cium and magnesium where leaching is intensive. In arid regions at least some of the sodium from the precip- itation is adsorbed by the clay minerals. The soil solu- tion of arid soils repeatedly reaches saturation due to evaporation and precipitates its salts. I n general it is more concentrated than the soil solution of humid soils.

Ground water of the zone of saturation generally re- flects the lithological composition of the recharge area and of its climatic regime. Mineralization increases with distance from the intake area and is especially de- pendent on t,he hydraulic gradient and the effective porosity of the aquifer, i.e., the major factors controlling the rate of underground flow. Locally high concentra- tions are due to mixing with fossil salt water pools, pollution by man, or due to infiltration of oceanic water which invades the aquifer.

During the growing mineralization the relative pro- portions of anions change in a rather systematic way. A continuous increase in bicarbonate concentration is usually constrained by the solubility product of calcium and magnesium carbonates. The supply of sulfate is often restricted or checked by the precipitation of cal- cium sulfate. Chloride usually increases proportionally with the mineralization and in the more mineralized waters it generally predominates.

Artesian water in deep synclinal basins is recharged by atmospheric precipitation often in distant mountain areas. Salinity increases with distance from the intake area, and where there are several water-bearing layers separated by impermeable clays it generally increases with depth. Mineralization may reach concentrations exceeding several times that of sea water. It has been suggested that such high concentrations are due to in- clusion of fossil brines or subsurface evaporation. More recent investigations indicate that the confining shales act as selective semipermeable membranes, let- ting the water slowly seep through but retaining its ions. Oil field brines, which are usually highly concen- trated and characterized by the enrichment of biophile elements such as I<, B, Br, I, may have been concen- trated in this way.

Soil Solinizotion

The major saline regions of the world are found in poorly drained piedmont alluvial plains under semiarid and arid conditions, where large quantities of salts, leached from the mountains, accumulate in the slowly flowing ground water. When the water table is high they ascend into the soil due to the high evapo-tran- spiration rate.

In arid areas often only a shallow soil layer is wetted by the limited precipitation. Moisture is again rapidly extracted by evaporation, leaving all the salts behind in the soil. The salts, both airborne and those released by weathering, averaging up to 20 kg/m2 in the root zone of slightly salt-affected soils and often above 100 kg/m2 in saline soils, accumulate chiefly at the depth of the sea- sonal rainfall penetration. They are redistributed dur- ing floods or temporary wetter periods forming a charac- teristic sequence of layers or belts according to their relative geochemical mobility; C1 > SOr > COa. The differentiation is essentially a chromatographic separa-

tion process which is evident wherever salt movement takes place.

Close relation between the depth to the water table, ground water salinity, and salt accumulation in soils is established in semiarid regions. When the ground water table is above a so-called critical depth, about 2-3 m depending on soil texture, salt accumulation in the soil profile takes place. If the ground water reaches a level less than 1 m, and the loss of moisture is mainly by direct evaporation from the soil surface, the salts ac- cumulate in the top layer of the soil. A characteristic salt crust and salt efflorescence develops.

The relative tolerance of plants to salinity varies widely. Highly saline soils are completely bare of vegetation; on moderately salty soils only halophytic plants may be able to survive. Cultivated plants are usually retarded in their growth even at low salinities. The Role of M a n in Salinization

The impact of man on the circulation of soluble salts has been profound. As a consequence of irrigation the ground water table has risen in many areas and exceeded the critical depth for salt accumulation in the soil. As a result large areas in the ancient irrigated lands of the Near East, Central Asia, and also America have been abandoned. Where irrigation water is distributed in open ditches, zones of high salinity develop along the canals and in the low lying areas on the periphery of the irrigated area.

In more recent times man has learned to fight and control salinization, both natural and man-made, by lowering the water table through drainage and by leach- ing of the saline soils following the addition of suitable amendments. Large areas have been desalinized in this way in thelowlands of Europe, Central Asia, Russia, and California, and previously barren lands have been brought under cultivation. In a similar way large sectors of land along the Atlantic coast of Europe, inun- dated by sea water, have been leached of their soluble salts and reclaimed for cultivation.

In semiarid areas excessive pumping of ground water has resulted in the salinization and abandoning of many wells. In near coastal areas this may be due to the iu- vasion of sea water into the fresh water aquifer, in other cases it may be due to the indraft of lower lying, more salty water.

In all irrigated areas an increase of ground water salinity with time has been observed, which is a conse- quence of the increased evapo-transpiration, leaving a more concentrated solution to reach the aquifer. In- dustrial and town effluent is also invariably more saline than the source and often causes pollution of streams and aquifers. The discharge of highly saline oil field brines has been a troublesome source of salinization. There is no doubt that the control of soil salinization, and eventually the desalinization of saline water on a large scale, will become one of man's important tasks in the future.

Literature Cited

(1) HEM, F. D., U. S. Geol. Sumw Water Supplu Paper. 1473, . . - . . 269 pp., 1959.

(2) ERIKSSON, E., Tellus, 11, 375403 (1959); 12,63-109 (1960). (3) GORHAM, E., Geol. Sac. Amer. Bull., 72 , 795-840 (1961). (4) "Diagnosis and Improvement of Saline and Alkali Soils,"

U . S. Dept. Agr. Agricultural Handbook, 60, 160 pp., 1954. (5) "Salinity Problems in Arid Zones," P ~ c . of the Teheran

Symposium, UNESCO, Paris, 395 pp., 1961.

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