Rules of Thumb

FIGURE 6.11 Effects of SAR and specific conductivity on soil permeability. Zone A exhibits large reductions in soil permeability to water and will severely restrict the amount of water available to plant roots. Zone B has low to moderate reductions in soil permeability and may require care in irrigation practices and choice of crops. Zone C water has little or no reduction in soil permeability.

FIGURE 6.11 Effects of SAR and specific conductivity on soil permeability. Zone A exhibits large reductions in soil permeability to water and will severely restrict the amount of water available to plant roots. Zone B has low to moderate reductions in soil permeability and may require care in irrigation practices and choice of crops. Zone C water has little or no reduction in soil permeability.

Example 6.8: Determination of Sodium Adsorption Ratio

Determine the SAR for two different water supplies, A and B. Measured concentrations were

Supply A: Ca2+ = 4.6 mg/L Mg2+ = 1.1 mg/L Na+ = 144 mg/L TDS = 255mg/L Supply B: Ca2+ = 108 mg/L Mg2+ = 33 mg/L Na+ = 63 mg/L TDS = 1050 mg/L

Solution:

1. Determine the Ca2+, Mg2+, and Na+ concentrations in meq/L. (See discussion of equivalent weight in Chapter 1.)

Concentration

Constituent Equivalent Weight Water Supply A Water Supply B (g/eq or mg/meq) (mg/L) (meq/L) (mg/L) (meq/L)

Concentration

Constituent Equivalent Weight Water Supply A Water Supply B (g/eq or mg/meq) (mg/L) (meq/L) (mg/L) (meq/L)

Ca2+

20.0

4.6

0.23

108

5.40

Mg2+

12.15

1.1

0.090

33

2.72

Na+

23.0

255

11.1

63

2.74

2. Compute the SAR of the two waters.

[Mg2

11.1

This is a very high SAR value, and the water would be unacceptable for irrigation. Note that the TDS value for water supply A is moderate (255 mg/L) and that Supply A water is excellent for domestic use.

2 74

Water supply B has a low SAR value and would be an excellent source of irrigation water. The TDS value is high (1050 mg/L), and water supply B is an inferior source of domestic water compared with Supply A. Supply B will require more treatment to produce acceptable finished drinking water.

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