Siting One Borewell in an Over-Exploited Fluoride-Affected Granite Block
A gram panchayat near Chikkaballapur must site one new borewell for a fluoride-affected village. Two candidates survive a geophysical survey. Site A lies in a broad valley under 22–25 m of sandy saprolite over fractured granite-gneiss; the nearest mapped lineament is 600 m away; three existing wells within 300 m yield 1.0–1.8 L/s, stand at 18–24 m depth, and one of them shows fluoride of 2.4 mg/L against a permissible 1.0 mg/L. Site B sits on a lineament intersection seen on satellite imagery; soil cover is only 4 m, two wells within 150 m were abandoned dry at 95 m, and a third yields 0.6 L/s from a fracture at 78 m with fluoride of 0.6 mg/L. The block is categorised 'over-exploited', water levels have fallen about 1.1 m per year for a decade, and the monsoon delivers roughly 800 mm in about 45 rainy days. The panchayat wants a decision, a testing programme and a recharge plan on one page.
1. Which site would you recommend, and what conditions do you attach to the choice?
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Site A, with a quality condition.
In crystalline rock the saprolite is the reservoir and the fractures are the plumbing. Site A has 22–25 m of saturated weathered mantle - a large storage volume that carries a well through the dry months - plus three producing wells proving a connected fracture system nearby. Site B has a lineament, which is a structure, not a source: two dry holes within 150 m show the fracture is not productive at that spot, and 4 m of soil means almost no storage, so a well there will flashy in the monsoon and dry by March.
The honest counter-argument is that a lineament intersection sometimes gives 3–5 L/s where it is a genuine open fracture zone. Given the block's falling water levels and the fact that only one borewell can be funded, gambling on an unproven intersection is poor risk management.
Condition: the 2.4 mg/L fluoride at A dictates staged construction - drill to 60–70 m, and sample and test each water strike separately (saprolite, then each fracture). Complete the well in the horizon that gives acceptable fluoride; if all exceed 1.0 mg/L, provide defluoridation or dilute with low-fluoride water from B.
2. Using the given rainfall, recharge fraction and a proposed draft, check whether the village demand can be sustained.
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Recharge available. Catchment 0.5 km² = 500,000 m²; rainfall 0.8 m; gross rainfall volume = 400,000 m³/yr. At a conservative 10% recharge to groundwater:
0.10 × 400,000 = 40,000 m³/yr.
Proposed draft. One well at 1.5 L/s for 6 h/day, 350 days: 1.5 × 3,600 × 6 = 32,400 L/day = 32.4 m³/day 32.4 × 350 = 11,340 m³/yr.
Recharge is about 3.5 times the draft, so the resource is theoretically sufficient - but that arithmetic does not authorise the pumping. Three cautions:
- Recharge is diffuse over 0.5 km²; the well draws from a much smaller capture zone, so most of that 40,000 m³ never reaches it.
- Levels are already falling 1.1 m/yr, meaning current draft exceeds capture already; adding a well without recharge worsens it.
- The rain arrives in ~45 days, so it must be stored, not just infiltrated slowly.
Therefore: build check dams/nallah bunds and recharge trenches in the saprolite at Site A first, and set the pump duty at no more than about 1.0–1.2 L/s.
3. Design the pumping test and the water-quality programme, and state how you would interpret the results.
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Test. First a short step-drawdown test (three or four steps) to choose a safe discharge, then a 24-hour constant-rate test at about 1.5 L/s, with water levels logged in the pumping well and in one observation well 20–30 m away, followed by at least 2 hours of recovery.
Interpretation. Plot drawdown against log time (Jacob straight-line): T = 2.303 Q / (4π Δs), where Δs is drawdown per log cycle. With Q = 0.0015 m³/s and, say, Δs = 2 m: T = (2.303 × 0.0015) / (4π × 2) ≈ 1.4 × 10⁻⁴ m²/s ≈ 12 m²/day - a modest but usable fractured-granite value. Compute specific capacity (Q/s) as a direct siting index; check that recovery returns to within 0.2 m of the original level, and that the s-vs-log-t plot is a straight line (a flattening curve means a recharge boundary or leakage; a steepening curve means a barrier/fault).
Quality. Sample after pumping clears: fluoride, TDS, hardness, nitrate, iron, chloride. Because fluoride is the deciding parameter, sample each contributing fracture separately during drilling - the deep fracture water may be the high-fluoride one.