Low-Yield Well With Short Cycling and Sediment
A 6-inch well is 300 ft deep with static water at 80 ft. A 2-hour yield test produced 1.2 gpm and drew the water level to 240 ft; it had not stabilized. The contractor installs a 1-hp submersible pump at 285 ft, a 20-gallon bladder tank, and a 40/60 pressure switch. The homeowner reports the pump cycles about every 45 seconds, pressure fluctuates, and the water becomes cloudy after pumping. The contractor says the well just needs a bigger tank. The pump intake is 15 ft above the bottom, and there is no low-water cutoff. The well has not been surged, bailed, or airlifted since drilling.
1. Diagnose the cycling and cloudiness. Is a bigger tank the primary fix?
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A bigger tank is not the primary fix. The well produces only about 1.2 gpm, but a 1-hp pump can produce far more than that at this head. The pump fills the 20-gallon bladder tank quickly, hits 60 psi, shuts off, and the house drawdown brings it back on in about 45 seconds. That short cycling is a pump-control and pump-sizing problem, not a storage-volume problem alone. The cloudiness suggests the pump is pulling the water level down into the pump intake or disturbing sediment and fracture fines because the well was never developed. The pump is set at 285 ft with the water at 240 ft during the test, leaving only about 45 ft of submergence; continued pumping or a lower water level can draw the intake near the sediment. The driller should reduce the pumping rate to match the well yield, add low-water protection, and develop the well before deciding on final storage.
2. Recommend pump setting, pump size, and storage. Show calculations.
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The pump should be sized for the sustainable yield, not for the maximum the well might briefly produce. At 1.2 gpm and roughly 240 ft of lift, the water horsepower is only about 0.07 hp (1.2 × 240 ÷ 3960), so a 1-hp pump is greatly oversized. A 0.5-hp pump is more appropriate and will reduce cycling and drawdown. Set the pump high enough above the bottom to avoid sediment but low enough to stay submerged during long pumping; a setting around 260 to 270 ft gives 20 to 30 ft of submergence above the 240-ft test level and keeps the intake well off the bottom. Install a low-water cutoff and a cycle stop valve or a larger bladder tank. For a 1.2-gpm well, a 2-hour storage target is about 144 gallons, so a 120- to 200-gallon tank or equivalent storage is reasonable. The driller should verify the final setting with a longer yield test.
3. How would you develop the well, and what indicators show it is successful?
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Development should begin after the grout has set and before the final pump is installed. Suitable methods for a fractured bedrock well include surging with a surge block, bailing, airlifting, or controlled overpumping, provided the driller does not pull the water level so low that the formation collapses or fine sediment is pulled into the borehole. The driller should work in stages, alternating agitation and removal of turbid water, and should not use a high-capacity pump that exceeds the well's safe yield for a prolonged period. Success is shown by improved specific capacity, a stabilized drawdown at a lower pumping rate, and decreasing turbidity. If turbidity increases or the well begins to produce sand, development should stop and the well should be re-evaluated. A post-development yield test should confirm the sustainable rate used to size the pump and storage.