CHANGES IN CHEMICAL COMPOSITION .1 Changes at the surface

Components dissolved in precipitation are in general already enriched at the soil surface because of evaporation of part of the precipitation. Other processes occur at the soil surface; for example, absorption of gaseous inorganic compounds

such as sulphur dioxide and ammonia. The sulphur dioxide is in general oxidized to sulphuric acid thus increasing’the sulphate concentration. Ammonia may also escape if slightly acid precipitation, containing ammonium, is turned alkaline at the soil surface due to the presence of calcium carbonate. Decomposing litter on the soil surface releases cations and anions but these may also be taken up by plant roots already near the surface. The more important processes seem to take place at deeper levels.

2.3.2.2 Changes in the root zone

It is generally recognized that three processes occurring in the root zone affect chemical composition, The first process is loss of soil water due to evaporation which increases concentration of dissolved components in the soil water. This is balance is maintained, as long as no human intervention takes place.

The third process is that characterized as weathering, although it is probably more appropriate to speak about mineral transformation, since new minerals may appear in the process (see Section 2.1.3.2). Mineral tansformation equilibria are treated quite extensively in the literature and the reader is referred to textbooks such as “Aquatic Chemistry” by Stumm and Morgan (1970) and specialized papers by Garrels (1967); Paces (1972a and b, 1973); Jacks ( 1973a and b); Tardy and Garrels ( 1974).

2.3.2.3 Chemical reactions and equilibria in the unsaturated zone

The unsaturated zone is in direct contact with the oxygen and carbon dioxide of the air. Reactions, hence, proceed under practically constant oxygen and carbon dioxide pressures. A suitable example demonstrating the course of chemical weathering is the breakdown of anorthite. It is found that this mineral is absolutely unstable under present environmental conditions. This means that it can never reach an equilibrium with any secondary mineral before it is completely transformed. The same seems to be true also for the biotite under conditions prevailing in the unsaturated zone. It is likely to be true also for primary minerals like hornblende and other ferromagnesian minerals, whereas soda and potash feldspars and muscovite seem to be able to attain chemical equilibria with secondary minerals.

Anorthite, for example, essentially breaks down according to the reaction:

CaA12Si208 (c) + 2C02 (9) + 2H20 (l)eCa 2+ puted. When equilibrium is attained, the following results:

p[H4SiOJ = 4.07 where p = -log pH

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which means that the concentration of dissolved SiOZ in the water is 5.1 mg/ 1. If the silica concentration is maintained below this value, Kaolinite will dissolve. Hence, under strong leaching conditions, such as those which can occur in the uppermost part of the soil under humid conditions, silica concentrations may become so low that the only thing left will be aluminum oxide in the form AlO.OH, the major ingredient in bauxite. Under drier conditions the silica concentration in water will be higher, preventing a breakdown of kaolinite.

The latter may then be transformed into montmorillonite through a reaction like:

0.8A12Si205(OH)4 (c) + 0.2Ca2+ (aq) + 0.4Mg2+ (aq) + 1.2HC03 (aq) + 2.4Si02 (amorphous)fJ 1.2C02 (g) + 1.2H20 (1)

Ca o.2("g o~4A11~6)Si4010(OH)2 +

This requires, of course, more silica than can be supplied by anorthite. However, minerals like albite and microcline contain excess silica for this purpose. Some magnesium is also required and this can be supplied from the weathering of minerals such as biotite, hornblende or olivine. It can be seen from the reaction above that the carbon dioxide pressure becomes important. If calcium is released from anorthite the carbon dioxide pressure will also determine if precipitation of calcite will take place. There are also other reactions to consider, such as the equilibrium between feldspars, kaolinite and montmorillonite.

The process of chemical weathering in the unsaturated zone can be summarized by considering two different climatological environments:

(a) In humid climates, with high percolation rates, primary minerals break down more or less completely leaving a residue of quartz, hydrous ferric oxide, and hydrous aluminum oxide. The hydrous ferric oxide may be dehydrated to Fe,Os giving the soil a deep red c,olour. Because of strong leaching practically no other secondary minerals are formed, and a laterization of the uppermost part of the weathered layer results. In cooler climates enough silica could be retained to allow for the formation of kaolinite except near the surface. The weathering type is podzolization.

(b) Under semi-arid conditions leaching is low and, hence, concentrations of soluble constituents are high. Primary minerals are transformed into ferric oxide, kaolinite and montmorillonite. Also calcium carbonate may be formed.

The stability conditions for kaolinite-montmorillonite are illustrated in Figure 2.3.2.1, which shows a so-called phase

Figure 2.3.2.1 Phase diagram of calcium carbonate potential with respect to the partial pressure of carbon dioxide.

diagram constructed on the assumption that the solution is saturated with respect to amorphous silica (Tardy and Garrels, 1974). The ordinate is plCa*+I + 2plHC03-I which can be called the calcium bicarbonate potential. On the abscissa the negative logarithm at the partial pressure of carbon dioxide is used as a variable. The phase boundaries are outlined.

Of particular interest is the phase boundary between kaolinite and calcium montmorillonite which depends on the plMg2+j + 2plHC03-1, that is, the magnesium bicarbonate potential. It is, however, seen that high values of the partial pressure of carbon dioxide favour kaolinite formation. In the figure some analyses are entered admittedly from deep ground water showing that both calcite saturation and equilibrium with montmorillonite are obtained for some of the samples.

The concentration level which can be reached in the unsaturated zone can be illustrated with anorthite. The reaction shown earlier produces two bicarbonate ions for each calcium ion. At equilibrium with calcite, the equation of the

from which the following concentrations (not corrected for activity coefficients) are obtained (Table 2.3.2.1):

Table 2.3.2.1. Concentration of calcium ions and bicarbonate ions in equilibrium in calcite in the unsaturated zone, at different carbon dioxide pressures (when weathering has taken place). tained along with other secondary minerals.

Where the water table in fractured hard-rock aquifers is at some depth, physico-chemical conditions in the un- saturated zone with regard to carbon dioxide and oxygen do not differ from those occurring near the surface. Reactions occur on the walls of fractures and fissures.

In thick unsaturated zones, gradients of weathering appear. As water moves downward it comes closer and closer to some mineral equilibrium. At some point secondary minerals start to form, arresting the weathering of primary minerals.

Since pH also increases, reaction rates slow down. Some reactions will, however, continue at a reduced rate. Anorthite would still tend to be transformed into, particularly, montmorillonite and the calcium released will be precipitated as calcite.

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2.3.2.4 Reactions in the saturated zone

In the saturated zone the water is cut off from fresh supplies of carbon dioxide and oxygen. This sets a limit on how far weathering can proceed. If weathering in the unsaturated zone is relatively intense not much will happen in the saturated zone. This can be demonstrated by considering a rather extreme case.

Assume that no weathering has taken place in the unsaturated zone because of a ground-water level which is close to the surface. The amount of carbon dioxide dissolved will depend on the partial pressure in the saturated zone. Assume a partial pressure of 1 Om2 atm of carbon dioxide. This will give a concentration of carbon dioxide in water of 0.33 mmol/l or 14.5 mg/l CO, at 25” C. It is apparent that this can match about 6 mg/l of Ca2+ as bicarbonate in solution. This is the maximum concentration of calcium, through weathering of anorthite, which conceivably can be found under the con- ditions outlined, and is about 10 per cent of that which could conceivably be found if weathering had taken place in the unsaturated zone. Hence, not much can happen in the unsaturated zone. In fact, because of some further silicate weathering precipitation of calcium carbonate may occur, partly because the pH must inevitably rise unless the system is buffered by bicarbonate obtained in the unsaturated zone.

As for weathering of ferromagnesian minerals, oxygen is of essential interest since it will enhance the weathering process by oxidizing the ferrous iron inside the minerals. This will effectively remove all oxygen from solution. The ferrous iron and manganese can appear in solution, provided the pH is not too high.

From the previous discussions there are strong indications that the greatest change in the chemical composition of percolating water takes place in the unsaturated zone, particularly where it is thick and well developed. Where the un- in the isotope ratios in precipitation with season, latitude, distance to the oceans, and altitude. The altitude variation is particularly valuable since it is of such magnitude that differences of 200 m in altitude can easily be detected using modern mass-spectometric equipment. Through such studies it has been possible, in a number of cases, to establish the altitude within a basin at which the ground water occurring lower in the basin originated. This may be of great value particularly for testing hypotheses on long-distance ground-water transfer due to specific tectonic patterns.

The use of stable isotopes in ground-water exploration programmes must be planned at an early stage in the programme so that the background variation in rainfall is well established. Stable isotope ratios have also been used to identify bodies of “fossil” ground water, dating back to some earlier pluvial period. This application can, of course, be of great interest in ground-water development schemes in hard rock areas.

Turnover rates of ground water can be assessed if storage and age of the water are known. Age determinations of

The possibility cannot be excluded that long-term fluctuations in climate cause long-term fluctuations in precipi- tation and dissolution of carbonate in the fractures of hard rocks. If so, there does not seem to be any really reliable way to correct apparent carbon-14 ages. Only carbon-14 assays of samples taken in the direction of ground-water move- ment can give reliable age estimates.

Fritz et al. (1979) and Back and Hanshaw (1965) provide detailed information on the use of isotopes in ground- water investigations.