--- title: "Water sources: quality and quantity" subject: "Water Supply Engineering" chapter: 2 key: "semester_5/water_supply_engineering:sources-quality-quantity" source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/sources-quality-quantity site: BCT Engineering --- # Water sources: quality and quantity Water Supply Engineering · Chapter 2: Sources of Water - Asked in: 2079 Baisakh, 6 marks; 2069 Asar, 4 marks; 2067 Asar (old course) (asked 3 times) - Video: [Sources of Water , Water Supply & Quality of Water |Basic Civil Made Easy](https://www.youtube.com/watch?v=iOSoMg7Ubgo) — Basic Civil Made Easy - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/sources-quality-quantity - Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-2 ## Question Describe the various sources of water (surface water in particular) used in water supply schemes with respect to their quantity and quality (suitability, merits and demerits). ## Short answer Surface sources are rivers, lakes, ponds, and impounded reservoirs; rivers are plentiful but turbid and polluted, lakes are steady but may have algae, ponds are poor and seasonal, reservoirs are reliable. Ground sources like springs, wells, and infiltration galleries are clear and need less treatment. Surface water generally requires sedimentation, filtration, and disinfection. ## Answer Sources of water are of two kinds: **surface sources** (rivers, streams, lakes, ponds, impounded reservoirs) and **ground sources** (springs, wells, infiltration galleries). Rain water is a minor source. #### Surface sources | Source | Quantity | Quality | Merits | Demerits | |---|---|---|---|---| | Rivers/streams | Large, varies with season; may fall to low flow in dry season | Turbid in monsoon, soft, polluted by sewage/industrial waste downstream | Plentiful, easy to tap | Needs full treatment; flood and low-flow problems | | Lakes | Large and steady | Usually clear, soft; may have algae, taste, odour | Large storage, less turbid | Pollution from bank activities; algae growth | | Ponds | Small, dries in summer | Poor, polluted by washing, cattle, vegetation | Local and cheap | Unsafe; seasonal | | Impounded reservoirs | Large and controlled by dam | Settled water, less turbidity; algae possible | Reliable all year, can serve flood control and power | High cost; evaporation, silting; submergence | Surface water is generally soft and has more turbidity, bacteria and organic matter than ground water, so it requires sedimentation, filtration and disinfection. #### Ground sources - **Springs:** small to moderate yield, clear and bacteriologically good; the best choice in the hills. - **Wells and tube wells:** steady yield, hard water, free from turbidity; may contain iron, manganese or arsenic. - **Infiltration galleries:** naturally filtered river-side water. #### Choice Surface water suits large urban supplies where big quantity is required, whereas ground sources suit small towns and rural areas where quality is good and treatment is not easy. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Rivers and streams generally have: A. Clear water all year B. Turbid water in monsoon and sewage pollution downstream, needing full treatment C. Constant small flow D. No treatment need Answer: B. Rivers are turbid in monsoon and polluted, requiring treatment. (Key point: River quality) 2. Why are ponds unsuitable as a source? A. They are huge B. They are always clean C. Small, seasonal and polluted by washing and cattle D. They are costly impounded Answer: C. Ponds are small, dry in summer and polluted. (Key point: Pond) 3. Compared with ground water, surface water has: A. Less turbidity B. More turbidity, bacteria and organic matter C. More hardness D. No need for filtration Answer: B. Surface water is softer but more turbid and contaminated, so sedimentation, filtration and disinfection are needed. (Key point: Surface vs ground) 4. A demerit of impounded reservoirs is: A. Unreliable supply B. High turbidity always C. High cost, evaporation, silting and submergence D. Small quantity Answer: C. Reservoirs are reliable but costly with evaporation, silting and submergence. 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