Drinking water for a Nagaur dhanii
A dhanii of 38 households and about 210 people in Jayal block, Nagaur district, needs a drinking-water plan for the next five years. The area gets roughly 340 mm of rain a year, almost all of it between July and September. The only local source is a 90 m borewell whose water tests at about 4,200 mg/L total dissolved solids, far above the 500–2,000 mg/L range treated as acceptable for drinking. The hamlet's single 40,000-litre underground tanka is dry by early February in most years. The PHED tanker filling station is 46 km away and tankers now come twice a week. The nearest Indira Gandhi Canal branch is 22 km off and the hamlet holds no canal outlet. The panchayat is weighing three proposals: (A) formalise and continue tanker supply, (B) build a rooftop rainwater tanka for every household, (C) install a community reverse-osmosis plant on the existing borewell. The average roof is 70 m² and the hamlet keeps about 640 cattle, goats and camels.
1. Which proposal should be the primary drinking-water source, and which supporting measure cannot be avoided? Justify with an arithmetic estimate of rooftop harvest and of household storage.
Read the worked answer
Rooftop arithmetic. 70 m² × 0.34 m rainfall × 0.8 runoff coefficient ≈ 19 m³, i.e. about 19,000 litres per household per year, or roughly 7.2 lakh litres for 38 households.
Demand. Drinking and cooking at 10 L per person per day: 210 × 10 × 365 ≈ 7.67 lakh litres a year. So even the theoretical harvest meets only about 94% of that, and after first-flush diversion, evaporation and overflow the realistic figure is 55–70%.
Storage kills option B as a stand-alone. A 5,000-litre household tanka holds barely a quarter of one year's yield; the rest spills in two or three August storms and is lost.
Option A is a 46 km lifeline with recurring cost and no quality control, weakest exactly in the pre-monsoon weeks when the tanka is dry.
Recommendation: C as the primary source (steady, treatable, quality-controlled), with B as a mandatory supplement and A retained only as an emergency backup. Neither B nor A alone is defensible.
2. The sarpanch wants to abandon the borewell once the RO plant runs. Is that wise? Where should the 4,200 mg/L water be used, and what must be done with the RO reject?
Read the worked answer
Do not abandon it. An RO plant needs feedwater; running it on tanker water defeats the whole design and multiplies cost.
Keep the borewell for non-drinking uses. Livestock tolerate markedly higher salinity than people; water around 4,200 mg/L sits at the upper end of what is normally considered usable for adult cattle, so it needs monitoring rather than prohibition. With roughly 640 animals needing 25–40 litres a day, livestock alone will absorb most of the output. Washing, toilets, kitchen gardens, and construction water should also come from the borewell, not from the RO line.
Reject water is 25–40% of the feed and is more saline than the source. It must not enter the tanka, a drinking pond or a recharge pit that feeds the same aquifer. Route it to toilet flushing and to salt-tolerant plantation (ber, cactus, khimp), with a lined evaporation pit as the last resort.
Only the drinking-and-cooking stream should pass through the membrane; treating washing water is money burnt.
3. What single operational failure would most likely wipe out the gains by year two, and which clauses should the panchayat write into its resolution?
Read the worked answer
Most likely failure: the RO plant stops working - fouled or exhausted membranes, an unpaid electricity bill, an absent operator, and no money for replacement. Village water schemes rarely fail for want of a source; they fail for want of maintenance.
Clauses to write in:
- A named, trained operator plus an annual maintenance contract, funded by a fixed levy (for example ₹30–50 per household per month) held in a dedicated bank account, not the general fund.
- A written reject-water disposal plan and scheduled testing of the treated water (TDS, nitrate, microbial) at a stated frequency, with results painted on the tank wall.
- A pre-monsoon routine for the tankas - cleaning, first-flush bypass, and a rule that tanker supply is emergency backup only, never a substitute.
Test of success: no household should be more than a short walk from a functional low-TDS point in May, and the treatment plant should be running on 300 days of the year. If either fails, the plan has failed regardless of what was built.