---
title: "Hardness of well water with strontium 25.26 mg/l"
subject: "Water Supply Engineering"
chapter: 4
key: "semester_5/water_supply_engineering:hardness-bk81"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/hardness-bk81
site: BCT Engineering
---

# Hardness of well water with strontium 25.26 mg/l

Water Supply Engineering · Chapter 4: Quality of Water

- Asked in: 2081 Baisakh, 6 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/hardness-bk81
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-4

## Question

The analysis of water from a well showed the following result in mg/l: Na$^+$ = 205, Mg$^{++}$ = 45, Ca$^{++}$ = 75, K$^+$ = 21.5, Cl$^-$ = 40, HCO$_3^-$ = 92, SO$_4^{--}$ = 22.8, NO$_3^-$ = 12, CO$_3^{--}$ = 112. The concentration of strontium (Sr) is equivalent to a hardness of 25.26 mg/l. Calculate the carbonate hardness, non-carbonate hardness and total hardness.

## Short answer

Using equivalent factors, total hardness = sum of Ca, Mg and Sr hardness = 397.57 mg/L as CaCO3. Alkalinity from HCO3- and CO3-- is 262.05 mg/L. Carbonate hardness = 262.05, non-carbonate = 135.52 mg/L.

## Answer

Hardness (or alkalinity) due to an ion $= \text{concentration (mg/l)}\times\dfrac{50}{\text{equivalent weight}}$ as CaCO$_3$. Equivalent weights: Ca = 20.04, Mg = 12.15, Sr = 43.81, HCO$_3$ = 61.02, CO$_3$ = 30. Strontium is given as 25.26 mg/l hardness. Na$^+$, K$^+$, Cl$^-$, SO$_4^{2-}$ and NO$_3^-$ do not cause hardness or alkalinity.

#### Total hardness
| Ion | mg/l | Hardness (mg/l as CaCO$_3$) |
|---|---|---|
| Ca$^{2+}$ | 75 | 187.13 |
| Mg$^{2+}$ | 45 | 185.19 |
| Sr | | 25.26 |
| **Total** | | **397.57** |

#### Alkalinity
$$
\begin{aligned}
\text{HCO}_3^-:&\ 92\times\frac{50}{61.02} = 75.39\\
\text{CO}_3^{2-}:&\ 112\times\frac{50}{30} = 186.67\\
\text{Total alkalinity} &= 262.05\ \text{mg/l as CaCO}_3
\end{aligned}
$$

#### Carbonate and non-carbonate hardness
Alkalinity (262.05) is less than total hardness (397.57), so CH = alkalinity and

$$
\text{NCH} = 397.57-262.05 = 135.52\ \text{mg/l}
$$

**Answer: Carbonate hardness = 262.05 mg/l; non-carbonate hardness = 135.52 mg/l; total hardness = 397.57 mg/l (as CaCO$_3$).**

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Total hardness is sum of:
   A. Ca, Mg and Sr hardness = 397.57 mg/l
   B. Ca, Mg, Na hardness
   C. only Ca
   D. Ca and K
   Answer: A. 187.13+185.19+25.26. (Key point: Total hardness)

2. Alkalinity includes:
   A. HCO3- only
   B. HCO3- (75.39) and CO3 (186.67) = 262.05 mg/l
   C. Cl- and SO4
   D. Na+ and K+
   Answer: B. CO3: 112 x 50/30. (Key point: Alkalinity)

3. Carbonate hardness is:
   A. 397.57 mg/l
   B. 135.52 mg/l
   C. equal to alkalinity, 262.05 mg/l
   D. 75.39 mg/l
   Answer: C. Alkalinity is less than TH. (Key point: CH)

4. NCH is:
   A. 262.05
   B. 25.26
   C. 660 mg/l
   D. 397.57 - 262.05 = 135.52 mg/l
   Answer: D. TH minus CH. (Key point: NCH)

## Related in Water Supply Engineering

- [Hardness from ion analysis (Ca 65, Mg 35)](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ahardness-c71) — Water Supply Engineering, chapter 4
- [Hardness from ion analysis with strontium](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ahardness-b81) — Water Supply Engineering, chapter 4
- [Ca and Mg concentration, alkalinity (300 mg/l hardness)](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ahardness-ca-mg-equal-300) — Water Supply Engineering, chapter 4, asked 2×
- [Hardness with Ca, Mg, Sr equal, TH 150](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ahardness-as70) — Water Supply Engineering, chapter 4
- [Alkalinity and Ca, Mg: TH 280, CH 75](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ahardness-s73) — Water Supply Engineering, chapter 4
- [Ca and Mg concentration, NCH, alkalinity: TH 400 ppm](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ahardness-bk79) — Water Supply Engineering, chapter 4

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