In the case of ionic strength, when it increases, electrical potential as a function of distance from the colloid decreases and colloid repUlsion also decreases.
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 MgCI2, 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.
394 WATER AND SOLUTE TRANSPORT PROCESSES
o
~
.E c
:u o
:::l
"t:l
a: Q)
-c
~ Q)
a.
3.16 X 10-3 M MgCI2
500~---5~
Hours
Figure 10.3. Influence of soil leaching by distilled water on saturated hydraulic conductivity (from Quirk and Schofield, 1955, with permission).
The presence of exchangeable Na+ could also significantly decrease soil permeabil- ity. 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
K= kg/n where
k = permeability of the soil (related to soil texture and structure)
g = gravitational constant
(10.2)
n = kinematic viscosity or the ratio of solution viscosity over the fluid density For soil systems contaminated with Na+, kinematic viscosity is not significantly affected, thus the components controlling water flow velocity are the hydraulic gradient (Ll<jl/L\X) 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, tortu- osity, and void shape.
The deterioration of soil physical properties influencing k is accelerated directly or indirectly by the presence of high Na+ on the soil's exchange complex and the electrolyte composition and concentration of the soil solution. To improve the physical properties of Na-affected soils, Ca2+ is usually added to replace Na+ on the exchange sites. Calcium reduces clay swelling and enhances clay flocculation. The data in Figure 10.4 show that as salt concentration increases, saturated hydraulic conductivity increases and reaches a maximum which is independent of Na+. However, as salt concentration decreases, the decrease in saturated hydraulic conductivity is related to the N a+ in relationship to Ca2+ (SAR, see Chapter 11). The higher the SAR is, the lower
10.2 SOIL DISPERSION-SATURATED HYDRAULIC CONDUCTIVITY 395
o~~~~~~~ __ ~~~~~~~ __ ~ __ ~~~~
2 10 800
Total Cation Concentration c (me/I)
Figure 10.4. Influence of pH and SAR on saturated hydraulic conductivity.
the saturated hydraulic conductivity. Additional components influencing the effect of Na+ on saturated hydraulic conductivity of soil include clay mineralogy, clay content, soil bulk density, Fe and Al oxide content, and organic matter content. The hydraulic properties of soils dominated by 1: 1 type clay mineralogy (i.e., kaolinite) and Fe or Al oxides are relatively insensitive to variation in soil-solution composition and concen- tration, in contrast to those dominated by 2: 1 type clay minerals (i.e., montmorillonite).
Organic matter increases soil sensitivity to Na+.
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 AI-OH polymers from the interlayer. The presence of AI-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 AI-hydroxy islands which block their interlayer spaces. It is well known that these AI-hydroxy components are removed at low or high pH through dissolution mechanisms. This interlayer removal
396 WATER AND SOLUTE TRANSPORT PROCESSES
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
pH 4.3 Cl
(mmol L-l) SARa ESP
5 2.6 5.5
50 49.6 59.1
200 - b
Source: Marsi and Evangelou, 1991 a aSAR in (mmol L-l)1/2.
pH 6.1
SAR ESP SAR
1.6 1.1 0.4
29.4 45.5 20.8
90.4
hrhreshold values are not reported because the reduction in SHC is less than 20%.
pH7.5 ESP
0.6 80.8 80.5
would be expected to increase the dispersion potential of the mineral by allowing free expansion. Similar phenomena of AI-hydroxy interlayer removal have been demon- strated to be the cause for failed septic systems. In addition to increased swelling, dispersion can also be enhanced in such systems. When removed from interlayer positions, these positively charged AI-hydroxy components would increase the effec- tive surface charge available for sodium adsorption, thus increasing the probability for soil structural destabilization.
1.5 1.4 1.3 1.2 \
1.1 \ dKI Ko
1.0
0 0.9
~ ....
~ 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
0 4.8 9.6 14.4 19.2
EC (mmhos em-I)
Figure 10.5. Influence of acid MgS04 solutions on saturated hydraulic conductivity (from Evangelou, 1997, unpublished data, with permission).