Case Study 3: Siting a Borehole on Crystalline Basement
Three villages of 900 people need a borehole. Site A is on a valley floor with 8 m of clay-rich alluvium over weathered gneiss; the water table is 3 m deep, but a nearby hand-dug well yields only 0.1 L/s and becomes turbid after rain. Site B is on a midslope with 3 m of saprolite over 12 m of weathered gneiss; a fracture zone strikes 040° and two springs 300 m away yield 0.5 L/s together. Site C is on a ridge of fresh granite with thin soil; a 40 m dry hole lies 200 m away, but a 030° lineament is visible on satellite imagery. The drilling budget allows one hole to 60 m, and the village needs at least 0.3 L/s. The district water officer asks you to rank the sites, specify the target depth and testing programme, and state a contingency.
1. Rank A, B and C for a single borehole. Explain the hydrogeological reasoning, including why high clay content and a nearby dry hole are not automatically decisive.
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Rank: B > A > C.
- B: the two springs (0.5 L/s total) prove a connected fracture system and recharge; 12 m of weathered gneiss gives storage. The 040° fracture zone is a target, but drill across it, not parallel, to intersect several fractures.
- A: shallow water is attractive, but 8 m of clay-rich alluvium has low permeability and the well yield is only 0.1 L/s, below the 0.3 L/s need; turbidity suggests a poor sanitary seal or direct connection. Clay is not automatically fatal: it can confine a deeper aquifer, and sandy lenses may exist. But the evidence is weak.
- C: fresh granite has little primary porosity; the lineament may be a water-bearing fault, but the 40 m dry hole shows high risk. A 60 m hole may still miss the fracture. High clay at A reduces recharge but can protect water quality; the dry hole at C is not decisive because fracture permeability is heterogeneous.
2. Specify the target depth, drilling method and a 24-hour pumping test. What results would lead you to abandon the hole?
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Target: 45–55 m at B, below the 12 m weathered zone, to intersect the 040° fracture zone. Method: mud rotary through saprolite, casing to 15 m, then air-hammer or diamond drilling; log fracture strikes and inflow zones. Test: step test (1, 2, 3, 4 L/s or lower) then a 24-hour constant-rate test at 0.5 L/s, measuring drawdown and recovery. Calculate transmissivity and specific capacity. Abandon if: sustainable yield is below 0.3 L/s at drawdown greater than 70% of saturated thickness; or water has chloride above 250 mg/L, iron or manganese above drinking limits, or turbidity that persists after development. Also abandon if no water-bearing fracture is intersected by 60 m.
3. The first hole at B yields 0.15 L/s after development. What are your options, and which do you recommend? Explain why drilling at C is not automatically the best use of remaining funds.
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Options: deepen the hole; hydrofracture or acidise the fracture; develop the hole further; or drill a second hole 30–50 m along the 040° fracture. Recommend a second hole at B, because fracture zones are heterogeneous and a nearby offset hole is more likely to hit a parallel conductive fracture than a speculative lineament at C. Deepening may also work if the log shows a fracture at depth. C is high-risk: the 40 m dry hole and fresh granite mean the lineament may be barren; spending the remaining funds there could leave the village with no supply. If the second B hole fails, combine the low-yield borehole with rainwater harvesting and storage; do not promise 0.3 L/s from an untested site.