Fractured Bedrock Well That Will Not Clear
A driller finishes a 350 ft, 6-inch bedrock well in fractured granite. The well intersects a producing fracture near 310 ft, and initial air-lift development yields 25 gpm with heavy sediment for the first hour, then gradually clears. The driller switches to a test pump at 20 gpm; after three hours the water is still turbid with fine rock flour, and the pumping level has stabilized at 130 ft with a static level of 40 ft. The client needs 10 gpm for a household and is pressing for completion. The driller has a surge block, air compressor, and the test pump, and can return the next day. Nearby wells in the same formation typically clear within a day of development.
1. Decide whether the well is ready for pump installation or needs more development, and justify the call.
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The well is not ready. Fine rock flour persisting after three hours at 20 gpm, despite a stable pumping level, points to incomplete removal of drilling fines rather than a formation that cannot produce. The stable 130 ft pumping level at 20 gpm shows the fracture can supply water, so the problem is cleanliness, not yield. The driller should return with the surge block and compressor, surge the fracture zone around 310 ft, and air-lift until the discharge runs clear, then re-test. Installing a pump now would send abrasive fines through the pump and could plug the screen or intake. Completion should wait for a clear test at the design rate.
2. The client needs 10 gpm. Using the test data, assess whether the well can meet that demand and what pump setting is appropriate.
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The well produced 20 gpm with the pumping level at 130 ft, which is twice the client's 10 gpm need, so yield is adequate. At 10 gpm the drawdown would be less than at 20 gpm, so the pumping level should be shallower than 130 ft. A pump set around 150–160 ft, well below the 130 ft pumping level and above the 310 ft fracture, gives adequate submergence while keeping the intake away from the sediment-producing zone. The driller should confirm with a stepped test at 10 gpm that the pumping level stays above the intake, then select a pump whose curve delivers 10 gpm at the measured lift. The limiting factor is sediment, not capacity.