Pump Test Interpretation and Interference at a Valley Margin
A 72-hour constant-rate pumping test is run on a new 10-inch well at 300 gpm. Static water is 120 ft. Drawdown is 35 ft at 1 hour, 42 ft at 24 hours, 44 ft at 48 hours, and 45 ft at 72 hours. Recovery is 90 percent within 30 minutes. A nearby stock well 900 ft away shows a 3-ft water-level decline during the test. The pump intake is set at 220 ft. The owner wants a permanent pump sized for 400 gpm and asks whether the well can sustain it. The aquifer is described as confined fractured volcanic rock; the driller's log shows a 30-ft clay confining layer above the producing zone. There is no step-drawdown test. The driller must interpret the test and advise on pump setting, sustainable rate, and monitoring.
1. Calculate specific capacity and interpret whether the test reached stabilized conditions.
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Specific capacity = 300 gpm / 45 ft = 6.7 gpm/ft. The last 24 hours added only 1 ft of drawdown (44 to 45 ft), so the test is close to stabilized for practical purposes. Recovery of 90 percent in 30 minutes is also favorable. However, a single-rate test cannot separate aquifer loss from well loss, so the specific capacity may not be constant at 400 gpm. The 3-ft decline at 900 ft shows the cone of depression reaches the neighbor, so the well is not isolated.
2. Estimate drawdown at 400 gpm and decide whether the pump intake at 220 ft is adequate.
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A linear estimate at 400 gpm is 400/6.7 ≈ 60 ft of drawdown. With static water at 120 ft, the pumping level would be about 180 ft, leaving about 40 ft of submergence above a 220-ft pump intake. That looks adequate on paper, but well loss usually increases faster than rate, so actual drawdown could be greater. Recommend a step-drawdown test or at least a 400-gpm trial before setting the pump. Set the intake with a safety margin, install a low-water cutoff, and do not promise 400 gpm until the test is run.
3. What does the 3-ft decline at 900 ft indicate, and what monitoring or follow-up is needed?
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The 3-ft decline at 900 ft is interference, not necessarily damage. In a confined fractured-rock aquifer, drawdown can travel far along fractures. The driller should record the observation, notify the owner, and recommend monitoring the neighbor's well before and during pumping. If the neighbor's water level or yield is affected, the parties may need a mitigation plan or a reduced rate. The driller should also check the well permit and basin conditions for any monitoring or spacing requirement. The test data should be reported with the well record so the interference is documented.