New Screen Well Won't Clean Up
Your company completes a 6-inch PVC-cased well in a Coastal Plain sand and gravel aquifer at 240 ft. The 40-ft screen, 0.020-inch slot, is set from 200 to 240 ft. Static water level is 45 ft. After mud-rotary drilling, the driller bailed for one hour and called it developed. A test pump at 5 gpm draws the level to 140 ft after three hours, and the water is visibly sandy and turbid. The owner’s design demand is 8 gpm for a 4-bedroom house with irrigation. The pump is a 1.5 hp submersible set at 190 ft. The driller proposes to 'run it hard for a week and let it clean itself.' Chlorination has not been done.
1. What is wrong with the development so far, and what development sequence should be used before any pump test is accepted?
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One hour of bailing is not development; it is only a limited removal of loose cuttings. Mud rotary drilling leaves a filter cake and mud solids in the screen and gravel pack. The high drawdown and sand show that the screen is still plugged and the aquifer is not yet producing through the full open area. Running the pump hard for a week can pack fines tighter against the screen and damage the pump.
Use a mechanical surge-and-bail sequence: lower a surge block inside the screen, work it slowly from top to bottom with short strokes, then bail the loosened fines. Repeat until the water clears and the sand content drops. Because the casing is PVC, use gentle surge forces and avoid a heavy block or rapid strokes that can damage the screen. If a bailer cannot keep up, use air-lift development with a controlled air line, but watch for sand production. Development is finished when the water is clear and the drawdown at a given pumping rate stabilizes, not when a fixed number of hours has passed.
2. Design the pump test and disinfection/sampling plan. Would you accept the well at 8 gpm as it now stands?
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No. At 5 gpm the drawdown is 95 ft, so specific capacity is about 0.05 gpm per foot of drawdown. That is very poor for a sand-and-gravel screen well and suggests plugging or an undersized screen/open area. After development, run a step test at three rates, for example 4, 8, and 10 gpm, for one hour each, then a constant-rate test at 8 gpm for at least 8 hours. Measure pumping level, recovery, and sand content. If the specific capacity remains low or sand persists, inspect the screen slot and gravel pack; the 0.020-inch slot may be too large for the formation, or the pack may be missing.
Disinfect with a chlorine solution at about 100 mg/L, with at least 12–24 hours contact time, then flush until the chlorine residual is gone. Sample for total coliform, E. coli, and nitrate through a certified laboratory. Do not accept the well for the house until it passes bacteria testing and the 8-gpm test shows stable drawdown without sand.