Final Adjustments - SAR
Technical Profile
The objective of this process is to balance the concentration of sodium with calcium, magnesium and TDS in water prior to beneficial irrigation use.
The concentrations of sodium, calcium and magnesium must be balanced to avoid any reduction of water infiltration rates. Water that is intended for irrigation must meet requirements for salinity (TDS) and sodicity (Sodium Absorption Ratio - SAR).
Salinity is the measure of dissolved solids in water. It is generally measured as conductivity in micro Siemens or micro mho per centimeter. Electric conductivity above 3000 µS/cm is considered saline for domestic use. High salinity improves soil structure relative to hydraulic conductivity, but it reduces water take-up by a plant's root system.
Sodicity is the measure of relative sodium concentration as measured by Sodium Absorption Ratio (SAR). SAR is calculated as follows - Sodium Concentration divided by the Square Root of One-half the Sum of Calcium and Magnesium Concentrations:
SAR = [Na+]/(SQRT([Ca++]+[Mg++])/2)
where [ ] represents the ion concentration in milliequivalents per liter. Water with a SAR greater than 12 is considered sodic for domestic use. High sodicity is harmful to soil structure and reduces its hydraulic conductivity.
Pretreatment for RO or ED tends to replace calcium and magnesium with sodium. This increases SAR considerably. Once total salinity is reduced across the process, calcium or magnesium needs to be added to adjust water sodicity to a useful range. Generally, this is accomplished with the addition of calcium such as calcium chloride. The relationship between SAR and Salinity for beneficial irrigation use is shown below:
