Residual Sodium Carbonate Index
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Residual Sodium Carbonate Index

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.[1]

In the dispersed[clarification needed]soil structure, the plant roots are unable to spread deeper into the soil due to lack of moisture. However, high RSC index water does not enhance the osmotic pressure to impede the off take of water by the plant roots unlike high salinity water. Clay soils irrigation with high RSC index water leads to fallow alkali soils formation.[2][3][4]

Cross section of a mango tree with shallow roots in partially formed alkali soil

RSC index formula

RSC is expressed in meq/l units. RSC should not be higher than 1 and preferably less than +0.5 for considering the water use for irrigation.[5] The formula for calculating RSC index is:

  • RSC index = [HCO3 + CO3] - [Ca + Mg]
  • RSC index = HCO3/61 + CO3/30 - Ca/20 - Mg/12 (in case the ionic concentrations are measured in mg/l or ppm as salts)

While calculating RSC index, the water quality present at the root zone of the crop should be considered which would take into account the leaching factor in the field.[6] Calcium present in dissolved form is also influenced by the partial pressure of dissolved CO2 at the plants root zone in the field water.[7]

Natural water contamination

Global variation in soil pH. Red = acidic soil. Yellow = neutral soil. Blue = alkaline soil. Black = no data.

Soda ash [Na2CO3] can be present in natural water from the weathering of basalt which is an igneous rock. Lime [Ca(OH)2] can be present in natural water when rain water comes in contact with calcined minerals such as ash produced from the burning of calcareous coal or lignite in boilers. Anthropogenic use of soda ash also finally adds to the RSC of the river water.

Where the river water and ground water are repeatedly used in the extensively irrigated river basins, the river water available in lower reaches is often rendered not useful in agriculture due to high RSC index or alkalinity.[8] The salinity of water need not be high.

Soft water

Water quality with high RSC index is synonymous with the soft water quality in industrial water treatment terminology.[9] When calcium and magnesium salts are present in dissolved form in water, these salts precipitate on the heat transfer surfaces forming insulating hard scaling / coating which reduces the heat transfer efficiency of the heat exchangers. To avoid scaling in water cooled heat exchangers, water is treated by lime and or soda ash to remove the water hardness.

The following chemical reactions take place in lime soda softening process which precipitates the calcium and magnesium salts as calcium carbonate and magnesium hydroxide which have very low solubility in water.

  • CaSO4 + Na2CO3 ---> CaCO3? + Na2SO4
  • CaCl2 + Na2CO3 ---> CaCO3? + 2NaCl
  • MgSO4 + Ca(OH)2 + Na2CO3 ---> Mg(OH)2? + CaCO3? + Na2SO4
  • MgCl2 + Ca(OH)2 + Na2CO3 ---> Mg(OH)2? + CaCO3? + 2NaCl
  • 2NaHCO3 + Ca(OH)2 ---> CaCO3? + Na2CO3 + 2H2O
  • Na2CO3 + Ca(OH)2 ---> CaCO3? + 2NaOH
  • Ca(HCO3)2 + Ca(OH)2 ---> 2CaCO3? + 2H2O
  • Mg(HCO3)2 + 2Ca(OH)2 ---> Mg(OH)2? + 2CaCO3? + 2H2O
  • MgCO3 + Ca(OH)2 ---> Mg(OH)2? + CaCO3?

The excess soda ash after precipitating the calcium and magnesium salts is in carbonates & bicarbonates of sodium which imparts high pH or alkalinity to soil water.

Domestic use

The slippery feeling experienced when using soap with soft water occurs because soaps tend to bind to fats in the surface layers of skin, making soap molecules difficult to remove by simple dilution. In contrast, hard water contains calcium and/or magnesium ions which form insoluble salts, effectively removing the residual soap from the skin but potentially leaving a coating of insoluble stearates on tub and shower surfaces, commonly called soap scum.[10]

For people on a low sodium diet, the increase in sodium levels in the drinking water can be significant in case of high SAR index water. The American Heart Association (AHA) suggests that the 3 percent of the population who must follow a severe, salt-restricted diet should not consume more than 400 mg of sodium a day. AHA suggests that no more than 10 percent of this sodium intake should come from water. The EPA's draft guideline of 20 mg/L sodium in drinking water protects people who are most susceptible.[11]

Soda lakes

The endorheic basin lakes are called soda or alkaline lakes when the water inflows contain high concentrations of Na2CO3. The pH of the soda lake water is generally above 9 and sometimes the salinity is close to brackish water due to depletion of pure water by solar evaporation.

Soda lakes are rich with algal growth due to enhanced availability of dissolved CO2 in the lake water compared to fresh water or saline water lakes. Sodium carbonate and sodium hydroxide are in equilibrium with availability of dissolved carbon dioxide as given below in the chemical reaction

  • Na2CO3 + H2O <---> 2NaOH + CO2
  • NaHCO3 <---> NaOH + CO2

During day time when sun light is available, Algae undergoes photosynthesis process which absorbs CO2 to shift the reaction towards NaOH formation and vice versa takes place during night time with the release of CO2 from the respiration process of Algae towards Na2CO3 and NaHCO3 formation. In soda lake waters, carbonates of sodium act as catalyst for the algae growth by providing favourable higher concentration of dissolved CO2 during the day time. Due to fluctuation in dissolved CO2, the pH and alkalinity of the water also keep varying.[12]

Excessive algal blooms lead to faster eutrophication of the lake which is the reason for poor aquatic fauna in soda lakes.

See also

V.P.S. Bhadauria, Varsha Gupta, Rahul Raj and F.M. Prasad "Oil content and growth characters of lemongrass with respect to iron pyrite and RSC rich irrigation water" International Conference on Environment, Energy and Biotechnology IPCBEE. © (2012) IACSIT Press, Singapore 2012 vol.33, 205-208

V.P.S. Bhadauria,F.M. Prasad and Varsha Gupta "Effect of iron pyrites on the quality attributes of lemongrass (Cymbopogon flexuosus)," irrigated with high RSC water." Res.J.Chem.Envrion.2011 15 (2), 586-588


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