Pump Losing Capacity in a Declining Water Table
A submersible pump set at 90 ft in a 150 ft well has served a farm for eight years. The pump is rated 10 gpm at 200 ft of head. Recently, output has fallen to about 5 gpm and the pressure switch trips on low pressure during irrigation. The driller measures a static water level of 75 ft, up from 45 ft when the well was new, and a pumping level of 110 ft at 5 gpm. The pump intake is at 88 ft. The driller suspects the water table has declined and the pump is now operating near or above its intake. The client wants more water without drilling a new well.
1. Explain why output has fallen and assess the risk to the pump.
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The static level has dropped 30 ft since the well was new, and the pumping level at only 5 gpm is already 110 ft - below the pump intake at 88 ft. The pump is running with little or no submergence, so it is cavitating and losing capacity rather than failing electrically. That explains the drop to 5 gpm and the low-pressure trips. Continuing to run it this way risks overheating, bearing damage, and a burned motor. The driller should not simply raise the pressure switch setting; the pump must be lowered or replaced. The immediate priority is to stop running the pump dry and confirm the actual pumping level with a proper drawdown test before deciding on a new setting.
2. Propose a remedy that restores supply without a new well, and state the setting and pump-selection reasoning.
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Lower the pump so the intake sits at least 10–20 ft below the pumping level. With a pumping level of 110 ft at 5 gpm, the intake should be near 125–130 ft, still above the well bottom at 150 ft and clear of any sediment. Because the water table has declined, the driller should re-test at the target rate to find the true pumping level at, say, 10 gpm, then set the intake below that. The replacement pump must be selected from its curve for the higher lift - a pump rated 10 gpm at 200 ft may no longer deliver at the new depth, so a higher-head model is needed. The driller should also advise the client that the aquifer is declining and monitor the level.