Subtopic: Irrigation and drainage water quality and reuse
Question Statement:
According to the US Salinity Laboratory classification, an irrigation water having a Residual Sodium Carbonate ($RSC = [\text{CO}_3^{2-} + \text{HCO}_3^-] - [\text{Ca}^{2+} + \text{Mg}^{2+}]$ in $\text{meq/L}$) exceeding $2.5\text{ meq/L}$ is categorized as:
Options:
(A) Safe water
(B) Marginal water
(C) Unsuitable for irrigation
(D) Excellent water
Answer Key:
C
Step-by-Step Solution & Explanation
RSC criteria for irrigation water suitability:
• $RSC < 1.25\text{ meq/L}$: Safe
• $1.25 \le RSC \le 2.50\text{ meq/L}$: Marginal
• $RSC > 2.50\text{ meq/L}$: Unsuitable / hazardous due to severe sodium hazard.