Soil Dispersion—Saturated Hydraulic Conductivity

The presence of exchangeable Na+ could also significantly decrease soil permeability. The mechanism(s) responsible for decreasing soil permeability in the presence of Na+ can be demonstrated by looking into the components controlling water or soil solution movement potential under saturated conditions. Soil-saturated hydraulic conductivity is described by 

For soil systems contaminated with Na+, kinematic viscosity is not significantly affected, thus the components controlling water flow velocity are the hydraulic gradient (A /AX) and soil permeability (k). The latter component (k) is influenced by clay dispersion, migration, and clay swelling. These processes may cause considerable alteration to such soil matrix characteristics as porosity, pore-size distribution, tortuosity, and void shape. 

Generally, the same total quantity of Na+ in a variably charged soil will reduce saturated hydraulic conductivity more effectively at a lower pH than at a higher pH. This is because in variably charged soils, as pH decreases, CEC decreases and soil-saturated hydraulic conductivity decreases because the same amount of Na+ represents a greater ESP at a lower soil pH. Note also that, generally, for the same ESP or SAR value, the saturated hydraulic conductivity decreases as pH increases. The data in Table 10.1 show that as pH increases, a smaller SAR is needed to reduce saturated hydraulic conductivity by 20%. Furthermore, as expected, as total salt concentration increases, the SAR value needed to decrease saturated hydraulic conductivity by 20% increases. 

The increase in soil pH could be implicated in increasing soil dispersion as well as in increasing clay-swelling potential. This is likely because of the removal of Al-OH polymers from the interlayer. The presence of Al-OH polymers at the lower pH values may limit interlayer swelling. Clays that have the basic 2 1 mineral structure may exhibit limited expansion because of the presence of Al-hydroxy islands which block their interlayer spaces. It is well known that these Al-hydroxy components are removed at low or high pH through dissolution mechanisms. This interlayer removal 

TABLE 10.1. Sodium Adsorption Ratio (SAR) and Exchangeable Sodium Percentage (ESP) Values Associated with 20% Reduction in Saturated Hydraulic Conductivity (SHC) for Pembroke Soil (10- to 30-cm Incremental Depth) at Three pH Values 

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Saturated hydraulic conductivity may also be influenced by dramatic changes in solution viscosity as well as the soil s dispersive potential. The data in Figure 10.5 represent saturated hydraulic conductivity as a function of EC. It appears that as EC increases, hydraulic conductivity decreases. The soil material in this study represents a Kentucky mine spoil. The predominant salt in the solution was an acid, MgS04. Suspension data showed that as MgS04 concentration increased, colloid dispersion increased. This could be due to an increase in solution viscosity, which also has a suppressing effect on saturated hydraulic conductivity (see Eq. 10.2). 

Figure 11,5, Relationships between relative saturated hydraulic conductivity and percent dispersion index of two Kentucky soils (dispersion index = percent of total clay remaining waterborne after 1 hr of settling in an Imhoff cone) (from Marsi and Evangelou, 1991c, with...

The dispersion phenomenon in the two humid soils (Pembroke and Uniontown) was evaluated through the use of an Imhoff cone test and a permeameter. The Imhoff cone is commonly used by engineers to determine settleable solids (see Chapter 9). The results of clay dispersion obtained by the Imhoff cone test are expressed as a dispersion index (percent of total clays in the soil sample dispersed), which is correlated with relative saturated hydraulic conductivity. This is shown in Figure 11.5. It demonstrates that each of the soils, depending on its clay content (Pembroke 59% Uniontown 20%), exhibits unique saturated hydraulic conductivity behavior with respect to the dispersion index. Also, in each of the soils, various mechanisms (different line slopes) appear to control saturated hydraulic conductivity. 

The phenomenon of soil dispersion with respect to Na+ loads (magnitude of ESP or SAR) appears to be unique to all soils on at least one particular point. As the total salt or Cl- concentration in the water increases, the dispersion index decreases and the saturated hydraulic conductivity increases (Fig. 11.6). When this occurs, the soil-water system becomes toxic to plants and organisms owing to high osmotic pressures. When chloride concentration in solution increases beyond 6000 mg L 1, Na ions near clay surfaces begin to dehydrate because of high osmotic pressure in the surrounding solution. This causes clay particles to flocculate (flocculation is the reverse of dispersion) and, consequently, the saturated hydraulic conductivity of the soil increases. 

Many processes in soil are controlled by colloid flocculation or dispersion. One such process is hydraulic conductivity. The data in Figure 10.3 show that for a Mg2+-saturated soil containing a solution of 3.16 x 10 2 M MgCl2, its hydraulic conductivity decreased by 35% after 5 hr of leaching with distilled water (Quirk and Schofield, 1955). This demonstrates that as solution ionic strength approaches zero, soil hydraulic conductivity decreases significantly owing to soil dispersion induced by a decompressed electric double layer. 

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