Pump Setting Depth and Yield in a Low-Yield Fractured Rock Well
A 400-foot bedrock well yields only about 2 gallons per minute on a short pump test, with static water at 120 feet and pumping water level dropping to 300 feet during the test. The owner wants a submersible pump set near the bottom to maximize storage. The driller has a 1/2-hp and a 3/4-hp pump available, a pitless adapter, and a torque arrestor. The well has 6-inch casing to 60 feet and open rock below.
1. How should the driller use the test data to choose pump size and setting depth?
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The test shows a low sustained yield (about 2 gpm) and a large drawdown (static 120 ft to pumping 300 ft), so the well has limited recharge and the pump must be matched to yield, not to total depth. Setting the pump near the bottom to 'maximize storage' risks pumping the well dry, cycling, and burning the motor.
Reasoning:
- Size the pump so its output does not exceed the sustainable yield (roughly 2 gpm) or the well will draw down below the pump intake. A 1/2-hp pump is more likely appropriate than 3/4-hp if it can meet the required head at low flow.
- Set the pump high enough above the pumping level to stay submerged during drawdown, accounting for seasonal decline, and install a low-water cutoff.
- Use a torque arrestor and proper cable support in the open-rock section.
The judgment is to protect the pump from dry-running rather than chase maximum submergence.
2. What completion details reduce the risk of the well being pumped dry or the pump failing?
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Install a low-water/run-dry cutoff set above the pump, a pressure or cycle control matched to the low yield, and a storage tank sized so the pump runs efficiently rather than short-cycling. Set the pump at a depth that keeps it submerged at the worst expected pumping level, not at the bottom. Use a torque arrestor and secure the drop pipe and cable. Consider a cistern or storage to buffer the low yield.
The reasoning: with about 2 gpm yield and 180 ft of drawdown, protecting the pump and managing storage is more important than maximizing setting depth.
3. What should the driller tell the owner about long-term yield and options if the well cannot meet demand?
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The owner should be told the well is low-yield (about 2 gpm) and that household demand may exceed it; storage and water-conservation measures help, but the well may not support high-demand uses. Options include additional storage, hydrofracturing or re-development of the bedrock fractures (with uncertain results), or a second well. The driller should set expectations in writing and document the test data, pump setting, and low-water protection so the owner understands the limits and the driller is protected if yield disappoints.