Question

If irrigation water contains carbonate, bicarbonates, calcium and magnesium ions as 4.0, 2.0, 2.5 and 1.5 me/L^-1, respectively, then the residual sodium carbonate content of irrigation water is:

• A. 0.5 me/l
• B. 1.0 me/l
• C. 1.5 me/l
• D. 2.0 me/l

Hint:

An RSC value $> 2.5\text{ me/l}$ makes irrigation water unsuitable, values between $1.25$ and $2.5\text{ me/l}$ are marginal, and values $< 1.25\text{ me/l}$ are safe. Here, $2.0\text{ me/l}$ represents marginal water quality.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Residual Sodium Carbonate (RSC) is a parameter used to evaluate the sodicity hazard of irrigation water.
It measures the excess of carbonate and bicarbonate ions relative to calcium and magnesium ions.
Key Formula or Approach:
The formula to calculate RSC is: \[ \text{RSC} = \left( \text{CO}_3^{2-} + \text{HCO}_3^- \right) - \left( \text{Ca}^{2+} + \text{Mg}^{2+} \right) \] Where all ion concentrations are expressed in milliequivalents per liter ($\text{me/l}$).

Step 2: Detailed Explanation:
From the given data:
- Carbonate ($\text{CO}_3^{2-}$) = $4.0\text{ me/l}$
- Bicarbonate ($\text{HCO}_3^-$) = $2.0\text{ me/l}$
- Calcium ($\text{Ca}^{2+}$) = $2.5\text{ me/l}$
- Magnesium ($\text{Mg}^{2+}$) = $1.5\text{ me/l}$
Substitute these values into the RSC formula: \[ \text{RSC} = (4.0 + 2.0) - (2.5 + 1.5) \] \[ \text{RSC} = 6.0 - 4.0 = 2.0\text{ me/l} \] Thus, the residual sodium carbonate content of the irrigation water is $2.0\text{ me/l}$.

Step 3: Final Answer:
The RSC is $2.0\text{ me/l}$, which corresponds to Option (D).