Appendix 2

Acidifying irrigation water to prevent calcium carbonate scale formation…
When and how to do it

  • If necessary, irrigation water can be acidified to neutralize excess carbonate (CO32-) and bicarbonate (HCO3-) that occurs naturally in waters originating from limestone aquifers.
  • Excess carbonate is most likely to occur wherever the irrigation water source is a limestone aquifer.
  • How to determining if a potential problem exists:
    1. The best way to determine if irrigation water contains an excessive carbonate concentration is to have the water tested for liming potential by titrating it with an acid.
    2. If titration is not available, the next best way is to estimate the carbonate concentration from the calcium and magnesium concentrations.
      • This estimation assumes that most of the Ca and Mg in a water sample is a result of dissolved Ca and Mg carbonates.
      • To estimate total carbonates (bases) from a Ca/Mg water test, use this formula:
        meq/L of bases = (ppm Ca x 0.05) + (ppm Mg x 0.083).
      • If a water test does not show Ca/Mg concentrations but does show hardness (as in Table 1), dividing hardness by 50 provides a good estimate of bases in meq/L.
  • Determining how much acid to apply to irrigation water is a several step process. The amount depends on the bicarbonate concentration in the water and the strength of the acid used. Properties of common acids are shown in Table A-2-1.
  • Use the following steps to calculate the amount of acid to apply:
    1. Have your irrigation water sample analyzed for total carbonates.
    2. From Table A-2-1, determine the appropriate rate calculation factor for the acid to be used./li>
    3. Multiply the factor by the milliequivalents of base per liter (meq/L) that the water contains.
    4. The result is the mL of acid that should be applied per 100 gallons of irrigation water. To convert from mL to fluid ounces, divide mL by 29.6.
Example calculation
  1. An irrigation water sample contains 4 meq/L of base.
  2. The acid to be used is 93% sulfuric.
  3. How much acid needs to be injected for each 100 gallons of irrigation water applied?
  4. The factor for 93% sulfuric acid is 8.7.
  5. 4 meq/L x 8.7 = 34.8.
  6. Therefore, need to inject 34.8 mL (1.2 oz) acid per 100 gallons of irrigation water.
  • This acid addition will neutralize 80% of the bases in the water. It is not necessary to neutralize 100% to make the bicarbonate problem insignificant. Not trying to neutralize all of the bicarbonates allows some room for error. The risk of over acidification is not worth it.
  • Acids are highly toxic and corrosive chemicals, and should be handled with great care. Precautions when using acids include:
    • Use goggles and protective clothing when handling.
    • When mixing acids with water, always add the acid to the water, never vice-versa, in a well-ventilated area.
    • Dilute concentrated acid with water in a non-metal mixing tank prior to injecting into the irrigation system.
    • Avoid over-application, which can severely damage plants.