Siting a Community Borewell in Weathered Khondalite
At a village 12 km west of Banki, Cuttack district, a three-member team must site a community borewell after two failures. The area has a 4–6 m laterite cap over weathered khondalite. A dug well 250 m away stands at 8 m depth in summer and yields 600 L/h with only 1.5 m drawdown. BW-1, drilled to 58 m on a gentle slope, passed 18 m of clay-rich saprolite and ended in fresh khondalite; its yield was 0.2 L/s, falling to 0.05 L/s after 40 minutes. BW-2, 90 m deep, was sited on a prominent N–S lineament on a satellite image. It struck a 12 m wide clay-filled shear at 70 m, and the water has 1.8 mg/L iron and a faint H2S smell. Three sites remain: S1 at a stream junction, S2 30 m east of the lineament where a shallow fracture trace meets a dyke, and S3 on a laterite plateau. The team has one resistivity meter, two days of field time, and drilling budget for only one more hole.
1. Which failed borehole is more diagnostic, and what does each failure indicate about the weathered-fractured aquifer?
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BW-2 is the more diagnostic failure. A lineament on a satellite image is a structural trace, not a permeability measurement. The 12 m clay-filled shear at 70 m explains both the low yield and the 1.8 mg/L iron and H2S: the water is sitting in a reducing, low-flow compartment, not an active fracture conduit. Siting directly on the trace therefore tested the wrong part of the structure. BW-1 shows a second failure mode: 18 m of clay-rich saprolite and fresh khondalite at 58 m mean the hole stopped in a low-storage, poorly connected zone; the fall to 0.05 L/s after 40 minutes is a storage-limited decline, not proof that the whole area is dry. Together they show that neither deep weathering nor a mapped lineament alone predicts yield; the target is a permeable fracture zone, preferably at a dyke contact or fracture intersection, with clay content low enough to transmit water.
2. Rank S1, S2 and S3, and state which field measurement could change your ranking.
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Rank S2 > S3 > S1. S2 is best because a fracture trace meeting a dyke creates a contact zone that can be fractured and less clay-rich than the shear; it is also 30 m off the lineament, avoiding the clay-filled core. S3 is second: a laterite plateau can recharge, but the underlying saprolite may be clay-rich and there is no structural target, so it is a low-risk but low-yield option. S1 is last: a stream junction gives shallow, seasonal water and a thin saturated zone. The dug well’s 600 L/h and 1.5 m drawdown show local storage, but a borewell there may fail in summer and is more vulnerable to contamination. Resistivity could change the ranking: a low-resistivity clay layer beneath S2 would downgrade it, while a fractured zone at 40–80 m would confirm it. A dry-season water-level measurement at S3 would also help.
3. Recommend a two-day verification plan using the one remaining borehole.
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Map and compass-measure fractures at S2 and S3, then run 1D resistivity at both with a Schlumberger array, extending AB/2 to 100 m, to distinguish clay from fractured rock. Drill one 100 m hole at the better target, not 60 m, because BW-1 failed at 58 m. Record fracture strikes and inflow zones while drilling. Case the hole and carry out a 72-hour constant-discharge pumping test with drawdown and recovery measurements; sample for Fe, Mn, H2S, hardness and coliforms. Set the pump intake above the main fracture and test at 1.0–1.5 L/s. If yield is below 0.3 L/s or iron exceeds 1.0 mg/L, stop and move to the second-ranked site rather than deepening blindly. This uses the single hole to test permeability, water quality and sustainable yield.