Determination of Quality aspects of Groundwater

SAR, RSC and TH

• Sodium adsorption ratio (SAR) is a measure of the suitability of water for use in agricultural irrigation, as determined by the concentrations of solids dissolved in the water. It is also a measure of the sodicity of soil, as determined from analysis of water extracted from the soil.

• The residual sodium carbonate (RSC) index of irrigation water or soil water is used to indicate the alkalinity hazard for soil. The RSC index is used to find the suitability of the water for irrigation in clay soils which have a high cation exchange capacity.

• When dissolved sodium in comparison with dissolved calcium and magnesium is high in water, clay soil swells or undergoes dispersion which drastically reduces its infiltration capacity

Bicarbonate hazard of irrigation waterHigh carbonate (CO3

=) and bicarbonate (HCO3-) increases SAR index (around >3-4mEq/L or >180-

240mg/L). Let's explain why:

Bicarbonate and carbonate ions combined with calcium or magnesium will precipitate as calcium carbonate(CaCO3) or magnesium carbonate (MgCO3) when the soil solution concentrates in drying conditions.

The concentration of Ca and Mg decreases relative to sodium and the SAR index will be bigger. This will cause an alkalizing effect and increase the PH. Therefore when a water analysis indicates high PH level, it may be a sign of a high content of carbonate and bicarbonates ions.

Residual Sodium Carbonate (RSC)The RSC has the following equation:

RSC=(CO3-+HCO3

-)-(Ca2++Mg+2)

It is another alternative measure of the sodium content in relation with Mg and Ca. This value may appear in some water quality reports although it is not frequently used.

If the RSC < 1.25 the water is considered safe

If the RSC > 2.5 the water is not appropriate for irrigation.

TH

• Temporary hardness is a type of water hardness caused by the presence of dissolved bicarbonate minerals (calcium bicarbonate and magnesium bicarbonate). When dissolved, these minerals yield calcium and magnesium cations (Ca2+, Mg2+) and carbonate and bicarbonate anions (CO3

2−, HCO3

−).