Soils as a Key Component of the Critical Zone 4

Soils and Water Quality

Guilhem Bourrié, Guilhem Bourrie

• Éditeur: Wiley-ISTE
• Année: 2018
• ISBN:
  9781119571858
• Édition: 1
• Sujet: Biological Sciences
• Sous-sujet: Environmental Science

This book provides the most up-to-date knowledge on water in soils and applications for the best use of our water resources. It first addresses the influence of soils on water quality, which is linked to rock weathering, soil formation, acidity and waterlogging. Here, the constituents of soils – such as clay minerals and iron oxides – play a major role. These modifications also have an impact on biogeochemical processes at the global scale, including the carbon cycle and the composition of the atmosphere. Secondly, this book discusses soil salinity, alkalinity and sodification in climates spanning from Mediterranean to arid. Here, water quality results from the concentration of solutes by evaporation and the transpiration of plants. The proper management of irrigation both protects soils against acidification and ensures sustainable agroecological development, while improper management leads to soil degradation and groundwater overexploitation. Lastly, the book describes how excess transfer of phosphorus in lakes results from a cascade of liberation and immobilization in the structure of the surrounding landscape. This leads to a general integrative method to limit eutrophication and restore the quality of water bodies.

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 ...