Sand, Chlorine Dose, and Pump Placement
A new 6-inch PVC well is 220 ft deep with casing to 140 ft and a 20-slot screen from 140 to 220 ft. Static water is 45 ft; pumping level is 110 ft at 15 gpm. After development by surging, the water still carries fine sand. The pump contractor proposes to set a 10-gpm submersible at 210 ft, just above the bottom, and to disinfect the well with 1 gallon of 5.25% household bleach. The well has about 175 ft of water above the screen and within the casing. The owner wants the well put into service quickly because the old well has failed.
1. Calculate the free chlorine concentration from 1 gallon of 5.25% bleach in the water column. Is it adequate, excessive, or correct for a new well?
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A 6-inch casing holds about 1.47 gallons per foot. The water column is 220 ft minus 45 ft static depth, or 175 ft, so the volume is about 175 x 1.47 = 257 gallons. One gallon of 5.25% bleach contains about 52,500 mg/L available chlorine as a concentrated stock. Diluted into 257 gallons, the dose is about 52,500 / 257 = 204 mg/L, or roughly 200 ppm. For new-well disinfection, a common target is about 100 ppm free chlorine with 12-24 hours contact time. Therefore 1 gallon is excessive for this water volume; about 0.5 gallon of 5.25% bleach would give approximately 100 ppm. Excessive chlorine can be corrosive and harder to flush, and it may not fix the sand problem. The driller should develop the well until sand is controlled, then dose to the target, circulate, contact, and flush until chlorine is gone before sampling.
2. Evaluate the proposed pump setting at 210 ft.
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Setting the pump at 210 ft places it inside the screened interval, only 10 ft above the bottom. That is poor practice: the intake is in the zone most likely to draw fine sand, and the pump may stir up sediment and cavitate. The pump should normally be set above the top of the screen, with enough submergence below the pumping water level. With the screen top at 140 ft and pumping level at 110 ft, a setting near 130 ft would keep the intake above the screen while maintaining about 20 ft of submergence. The exact setting should also account for seasonal drawdown and pump manufacturer requirements. If the well still produces sand after proper development, the screen slot and gravel pack should be re-evaluated rather than lowering the pump deeper.
3. What development steps should precede disinfection, and how would you judge whether development is complete?
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Development should remove drilling fluid and fines from the formation and pack. Use surging, overpumping, or jetting appropriate to the screen and formation, starting gently and increasing as the well clears. Monitor sand content, turbidity, and specific capacity. Development is complete when the water is substantially clear of sand and turbidity has stabilized at a low level, not merely after a fixed number of hours. If sand persists, check whether the screen slot is too coarse, the gravel pack is missing or misplaced, or the pump is too deep. Disinfect only after development, because continued surging after chlorination will disturb the chlorine contact and may reintroduce fines. Then chlorinate, contact, flush, and sample for coliform bacteria before placing the well in service.