Weathering reactions soils

The final composition of stream water is the product of the weathering reactions and related processes outlined above. However, the chemical processes are influenced and controlled by an intricate combination of environmental factors that are characteristic for each drainage system. Therefore, the composition of the bedrock in an area and the residual material left at the surface as soil and subsoil exert a strong influence on the chemical composition of mnoff from the area. The reactions of water with this material are the ultimate geological control and are the source of soluble weathering products. 

Acidolysis is a similar weathering reaction to hydrolysis in that is used to weather minerals, but in this case the source of is not water but organic or inorganic acids. Humic and fulvic acids (discussed in Section 8.3.2), carbonic acid, nitric or sulfuric acid, and low-molecular-weight organic acids such as oxalic acid can all provide H to weather minerals. All of these acids occur naturally in soils in addition nitric and sulfuric acid can be added to soil by acid pollution. The organic acids are prevalent in the... 

Overall, weathering controls the chemistry of material that is transported into the sediment and that which stays behind in the soil. As an example, consider a general weathering reaction for an aluminosilicate (Stumm and Morgan, 1995) ... 

The weathering reactions given above show the key effects of weathering the breakdown of the original rock minerals, the consumption of H, and the release of cations and silica into solution which can then be used to make new minerals or be lost from the soil into the ground-water and rivers. 

In the next chapter (Chapter 27) we show calculations of this type can be integrated into mass transport models to produce models of weathering in soils and sediments open to groundwater flow. In later chapters, we consider redox kinetics in geochemical systems in which a mineral surface or enzyme acts as a catalyst (Chapter 28), and those in which the reactions are catalyzed by microbial populations (Chapter 33). 

The terrestrial weathering of organic matter derived from shales and soils results in the oxidation of carbon, which generates CO2. Dissolution of this CO2 in water produces carbonic acid. This weak acid serves to enhance chemical weathering reactions... 

If weathering reactions prevail (see Section 5.3), the ions present in water depend on both local soil and mineral composition. The concentration of total dissolved salts (and thus the ionic strength) will be moderate in surface waters, and the main ions will be Ca2+, HCO, and silicates, due to the presence of silica (as silicic acid) and calcite or limestone (i.e., crystalline CaCOj), one of the most common and abundant minerals in soil. 

The more fundamental approach to addressing the physical dimensions involved in weathering is to characterize the surface areas of the individual minerals, i.e., the specific mineral surface area S (m g )- The extent to which this specific surface area scales directly with the reactive surface areas in natural environments is a matter of considerable debate, particularly in regard to the accessibility of water. For unsaturated environments, such as those in most soils, the wetted surface area may be considerably less than the physical surface area of contained mineral grains (Drever and Clow, 1995). In addition, surface areas of microscopic features such as external pits and internal pores may be associated with stagnant water that is thermodynamically saturated and not actively involved in weathering reactions (Oelkers, 2001). 

Katz B. G. (1989) Influence of mineral weathering reactions on the chemical composition of soil water, springs, and groundwater, Catoctin mountains, Maryland. Hydrol. Process. 3, 185-202. 

Chemical weathering in soils provides base cations relatively slowly, but the total long-term neutralization capacity of soils is typically large. In contrast, ion-exchange reactions (cation and anion exchange) in soil are relatively rapid, and tend to buffer H , S04, and ANC concentrations. Solids in soils and sediments have surface charges that permit exchange of ions. For example... 

If the weathering rate equals or exceeds the rate of H release by the biota, such as would be the case in a calcareous soil, the soil will maintain a buffer in base cations and residual alkalinity. On the other hand, in noncalcareous acid soils, the rate of H release by the biomass may exceed the rate of H" consumption by weathering and cause a progressive acidification of the soil. In some instances, the acidic atmospheric deposition may be sufficient to disturb an existing H" balance between aggrading vegetation and weathering reactions. 

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