Step-rate test on a low-yield screened well
A 6-inch PVC-cased well is 260 ft deep with screen from 140 to 240 ft; static water level sits at 34 ft. The owner wants to replace a worn 10 gpm submersible and add lawn irrigation. A 2-hour test at 10 gpm holds the pumping level at 58 ft. A second 2-hour test at 20 gpm pulls the level to 128 ft and it is still falling about 1 ft every 10 minutes when you shut down; recovery to static takes 55 minutes. Water stays clear, no sand. The existing drop pipe is 1 1/4-inch poly set at 150 ft, and the pressure tank is in the basement, 90 ft from the wellhead. The owner's irrigation design calls for 18 gpm at the heads with about 20 psi of system loss.
1. Calculate specific capacity at both test rates and explain what the two numbers tell you about the well.
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At 10 gpm the drawdown is 58 minus 34, or 24 ft, so specific capacity is 10 / 24 = 0.42 gpm per foot of drawdown. At 20 gpm the drawdown is 128 minus 34, or 94 ft, giving 20 / 94 = 0.21 gpm per foot, and that figure is still dropping because the level had not stabilized.
If the well were behaving in a simple, linear way, doubling the rate should double the drawdown, roughly 48 ft. Instead it more than tripled, and the trend was still downward at shutoff. That disproportionate loss is the signature of well inefficiency: turbulent flow near the screen and in the gravel pack, plus the cone of depression reaching a boundary or a tighter part of the aquifer. The practical conclusion is that this well can support about 10 gpm continuously but not 18 to 20 gpm. The 55-minute recovery is another warning sign that the aquifer is slow to recharge. Report the data and both calculations rather than just the pumped rate.
2. Recommend a maximum continuous pumping rate and a pump setting depth, and defend both.
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Recommend 10 gpm as the continuous design rate. That is the highest rate at which you actually demonstrated a stabilized pumping level over 2 hours; the 20 gpm data never stabilized and should be treated as a stress test, not a usable yield.
For setting depth, use the stabilized pumping level at the design rate, 58 ft, then add margin. A common rule is to keep the intake at least 10 to 20 ft below the worst expected pumping level, so account for seasonal decline, drought, nearby pumping, and gauge error. That puts the setting around 80 to 85 ft, giving roughly 25 ft of submergence at 58 ft. Do not simply reuse the old 150 ft setting without thinking: it is below the top of the screen, which changes motor cooling and can put the motor in the turbulent entry zone. Confirm the manufacturer's minimum submergence and cooling-flow requirements, and make sure the setting stays well above the top of the screen as your state's rules require.
3. The owner insists on 18 gpm for irrigation and asks why you will not just install a bigger pump. Give your explanation and the options you would price.
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Explain plainly that a bigger pump does not make more water. The well already showed it cannot sustain 20 gpm: the level dropped 94 ft and was still falling. A pump sized for 18 gpm would pull the water below its intake, lose cooling flow, and run dry or trip on overload; it can also pull fines into the screen and permanently damage the well. The pump would be the first casualty and the aquifer the second.
Price three real options. First, a storage-tank system: fill a 1,500 to 2,500 gallon tank at 8 to 10 gpm through a float switch and feed the irrigation from a booster pump, which converts a low-yield well into a high-peak-rate system. Second, redesign the irrigation into more zones so peak demand stays at or below 10 gpm. Third, drill a second well or deepen this one, if the area supports it. Give the customer the test data behind each recommendation in writing, since the discussion is really about expectations.