Low Specific Capacity Well Proposed for Higher Irrigation Rate
A 420-ft domestic well is completed with 6-inch casing to 300 ft and an open borehole from 300 to 420 ft. Static water level is 80 ft. A constant-rate pump test at 20 gpm for 8 hours stabilizes with a pumping water level of 230 ft. Recovery is to 85 ft within 30 minutes. A nearby well 250 ft away shows 40 ft of drawdown while the test well is pumping. The owner wants 30 gpm for a larger irrigation system and proposes setting the pump at 400 ft. The driller has not run a step-drawdown test. The well is in a confined sand and gravel aquifer; the nearby well is 160 ft deep and completed in the same unit.
1. Calculate drawdown and specific capacity. What do they indicate about the well's productivity?
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Drawdown is pumping water level minus static water level: 230 ft minus 80 ft equals 150 ft. Specific capacity is pumping rate divided by drawdown: 20 gpm divided by 150 ft equals about 0.133 gpm per foot of drawdown. That is a low specific capacity and indicates the well is not very efficient or the aquifer is tight near the borehole. Recovery to 85 ft after 30 minutes leaves 5 ft of residual drawdown, which suggests slow re-equilibration and possibly well loss or limited recharge. The driller should not assume the well can produce 30 gpm just because the pump can be set deeper.
2. Estimate the pumping water level at 30 gpm using the data, and state the assumptions and limitations.
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Using a constant specific capacity of 0.133 gpm/ft, drawdown at 30 gpm would be 30 divided by 0.133, or about 225 ft. Adding the 80-ft static level gives an estimated pumping water level of about 305 ft. That estimate assumes specific capacity stays constant, which is usually optimistic: at higher rates, turbulent well losses and aquifer losses increase, so actual drawdown is likely greater. If the nearby well also pumps, interference could add more drawdown. A pump set at 400 ft would leave only about 95 ft of submergence under the linear estimate and less under realistic conditions. The driller should run a step-drawdown test before sizing the pump and should set it with adequate submergence above the expected pumping level.
3. What concerns does the 40-ft drawdown in the nearby well raise, and what would you recommend before increasing pumping?
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The 40-ft drawdown in a well 250 ft away shows strong hydraulic connection and a real interference risk. The nearby well is only 160 ft deep, so a pumping level near 305 ft in the test well could reduce or dewater the neighbor's supply, especially if both wells run at the same time. Before increasing the rate, the driller should run a cooperative test with the neighbor's well monitored, measure both water levels, and evaluate whether the neighbor's pump intake would be exposed. The owner should consider a lower pumping rate, a storage tank, or a different pump setting. If the neighbor has a water right, impairment could also become a regulatory issue, so documenting the test and notifying the neighbor is prudent.