1
Water Quality in Soils

Rain water is almost pure water, with an influence of aerosol and spray that dissipates when the distance to the ocean increases. Groundwater and spring water contain mineral salts, which they have acquired in the soils, superficial formations or rocks through which they have travelled. The lifetime in this phreatic critical zone of superficial groundwater ranges from a few months to a few years. This is enough for water to change its chemical composition and become adequate for plant, animal and human feeding. This also implies that the protection of the quality of water resources involves a good management of the critical zone. Correspondingly, water quality provides information about the direction and intensity of biogeochemical processes, which take place in the critical zone. While the study of soils gives us indications gathered over thousands of years, the study of water quality gives us information on current processes and on the influence of patterns of land use by humans.

Two main hydrological situations can be distinguished:

• - exorheic situations, where surface waters have an outlet to the ocean, either through hydrographic networks that drain them or through groundwater tables that communicate with other watersheds open to the ocean; evolution is mainly subtractive, soils appear as a transitional state of the Earth's surface between the weathering front or chemical erosion, and the mechanical erosion front;
• - endorheic situations, where surface waters have no outlet to the world ocean, concentrate by means of evaporation and deposit their salts; soils are an intermediate structure within sedimentation.

In both cases, the situation is controlled by tectonics, which opens or closes the outlet to the world ocean, and by climate, which regulates water intake and aridity.

As the climate changes, from wet to dry conditions, chemical elements, according to their inherent properties, are dissolved, transported and precipitated in landscapes. This is what Tardy [Tar69] called "ion chromatography in landscapes" (Figure 1.1): ions migrate in the critical zone in solution, bind to exchange sites or reprecipitate, and then regroup and migrate to groundwaters inside an ionic chromatography column.

Figure 1.1. Ion chromatography in landscapes

(source: [Tar69], modified). For a color version of this figure, see www.iste.co.uk/bourrie/soils4.zip

Upstream, under an equatorial or tropical humid climate, in crystalline massifs, the large excess of rain on evapotranspiration results in dissolving all the elements of groups I, II, IV and V (see Figure 1.8 of Chapter 1 of the book Soils as a Key Component of the Critical Zone 3), alkalis, alkaline earths, oxyanions (sulfate), as well as chloride and silica. Only the elements of group III relatively accumulate, because the others are exported: Ti(IV), Al(III), Fe(III), Cr(III), which yields ferricrete and bauxite individualization (molar ratio Si/Al = 0). This is the field of allitization and ferrallitization. Several hundreds of thousands of years are necessary to completely alter the initial rock, here typically a granite of the continental crust.

Further downstream, less humid tropical climate conditions slow down the evacuation of dissolved silica (group IV) and kaolinite is formed (molar ratio Si/Al = 1). This is the field of monosiallitization. Still further downstream, under conditions of a dry tropical climate with a pronounced dry season, the addition of silica and elements of group II leads to the formation of aluminous (beidellite) and ferrous (nontronite) dioctahedral smectites1; these two types of smectites have a molar ratio Si/Al larger than 1 and incorporate Ca and Mg; this is the field of bisiallitization, including vertisols and calcareous vertisols. Finally, in floodplains in semi-arid climate where flooding water spreads from allogeneous rivers2 smectites are also formed, but in magnesium trioctahedral form rather than ferrous or aluminous, given that Al(III) and Fe(III) have been immobilized further upstream. In these environments ranging from semi-arid to arid, evaporation takes precedence on rainfall and pH increases. This is the field of basic chemical sedimentation. The clays formed are fibrous clays of the palygorskite type, and salts such as gypsum or even more soluble salts are formed in subarid brown earth and salt-affected soils, trapping the elements of groups I and II, up to sodium sulfate, mirabilite Na2SO4 · 12 H2O and halite NaCl. Studies on the basin of the Chad Lake [Gac80] confirm the validity of this overall pattern.

This chapter essentially focuses on the case of exorheic systems dominated by drainage. The second case, dominated by evaporation, will be studied in the chapter devoted to irrigation in semi-arid and arid Mediterranean regions (Chapter 2).

1.1. Elementary weathering reactions

Predominant minerals in lithosphere rocks are aluminosilicates, feldspars and micas; in addition, ferrous or magnesium silicates (pyroxenes, amphiboles) can also be found therein as well as quartz in the presence of silica in excess, or conversely olivine in the presence of a deficit of silica. Eruptive rocks are partially glassy, and glass alteration is more rapid. The main elementary reactions are given in Table 1.1.

Table 1.1. Main weathering reactions of minerals of the continental crust. Albite and anorthite are the two end members of plagioclase feldspars. Phlogopite and annite are the two end members of biotite (black mica). Forsterite and fayalite are the two end members of olivine. These are the essential components of crystalline, crystallophyllian and eruptive rocks, granites, gneiss, micaschists, basalts, etc. For simplification, cation hydration water molecules have been omitted

Mineral

Dissolution or hydrolysis reaction

Quartz

Orthose

Albite

Anorthite

Muscovite

Phlogopite

Annite

Forsterite

Fayalite

1.2. Weathering as a CO2 sink

Since the elements of groups I and II are stable in cationic form, the balance of charges requires that reactions, with the exception of the dissolution of quartz, consume protons. In the absence of a renewed input of acids, the pH increases very quickly and the reaction stops. In soils, these reactions are maintained by dissociation of CO2 from the oxidation of organic carbon, following the reaction:

In the end, it is therefore biological activity, more specifically, the respiration of roots, of microflora and decomposers, which provides the necessary H+. The consequences of this are fundamental:

• - the positive electric charges of cations of groups I and II, alkalis, alkaline earth and Al(III) are balanced by the anion HCO3-, more generally by the alkalinity of the solution;
• - weathering acts as a CO2 sink; each molecule of CO2 which dissociates to provide a proton dissolves in the form of HCO3- instead of returning back to the atmosphere. This is therefore a "leak" in the photosynthesis-respiration cycle. To each of the halfreactions of Table 1.1, it is thus necessary to add the half-reaction [1.1].

To assess the overall effect of weathering as a CO2 sink, the fate of ions balanced by HCO3- should be taken into account downstream, in soils and especially in sedimentary basins. The composition of the ocean is constant over long periods of time. Na+, K+ and partly Mg2+ ions return to silicate state, and consequently HCO3- recombines with H+ to restore CO2 to the atmosphere following the reverse reaction (Table 1.1). The balance is thereby zero over a long period. A part of Mg2+ and Ca2+ precipitate in the form of calcite, pure or slightly magnesian, and dolomite CaMg(CO3)2. This traps CO2 for very long periods, and thus removes it from the atmosphere. It is this mechanism which is responsible for the reduction of pCO2 in the atmosphere by several tens of % at approximately 3 × 10-4 atm.

The transient effect of this mechanism with regard to an increase in pCO2, such as currently, is not fully quantified. Volcanic eruptions are involved by both releasing immediately large quantities of CO2 and by supplying large amounts of glass, more easily alterable than granular rocks, therefore slowly trapping even larger quantities of CO2, for example during the formation of "volcanic provinces" such as the Deccan trapps.

1.3. Neoformations

1.3.1. Neoformation reactions

Knowing the chemical composition of spring water and rain waters, by effecting the difference one obtains the amount of dissolved elements originating from the critical zone. The first fact is that the elements of group III, Al(III) and Fe(III) (see Figure 1.7 in Chapter 1 of the book Soils as a Key Component of the Critical Zone 3) are present only in very small quantities in...