Low-Yield Well, Short Cycling, and Failed Shock Chlorination
An existing 6-inch bedrock well is 250 ft deep with casing to 40 ft. It serves a four-bedroom house with a 1/2-hp submersible pump set at 220 ft, a 44-gallon bladder tank, and a 30/50 pressure switch. A 4-hour yield test found 1.5 gpm at 210 ft of drawdown; static water level is 25 ft. Peak household demand is 8 gpm for 15 minutes. The pump short-cycles every 45-60 seconds when two fixtures run, and the well produces fine sand after pumping. A water test is total coliform positive, E. coli negative, after the owner poured 1 gallon of 5.25% household bleach into the well and flushed the next day. The owner wants to 'upgrade to a 1-hp pump and a bigger tank' to fix low pressure.
1. Evaluate the proposed 1-hp pump and larger tank fix, and recommend a system that matches the well.
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A 1-hp pump is the wrong direction. The well test produced only 1.5 gpm at 210 ft of drawdown; static is 25 ft, so the pumping water level was about 235 ft. The existing pump is set at 220 ft-above the pumping level-so it can break suction, which explains short cycling and sand. A larger pump would draw the well down faster and worsen both problems. The real fix is to limit pumping below the well yield and add storage. Set a low-yield pump of about 1.0-1.5 gpm as deep as safely possible, with the intake at least 10 ft above the bottom and low-water protection. Add a 200-300 gallon atmospheric storage tank plus a booster pump to meet the 8-gpm peak. A 44-gallon bladder tank cannot supply 15 minutes at 8 gpm, about 120 gallons. If the well cannot sustain even 1 gpm, deepen or hydrofracture it, or use a cistern. Size the pressure tank for minimum pump runtime, not just nominal drawdown.
2. Why did the owner's shock chlorination fail, and what is a proper disinfection procedure?
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One gallon of 5.25% bleach in a 250-ft, 6-inch well is roughly 370 gallons of water, giving an initial dose near 140 mg/L, which is in the right range. But the owner flushed the next day, probably did not circulate water through the house, did not pre-clean the well, and did not address sand or biofilm. Coliform can hide in biofilm on the pump, drop pipe, and pressure tank. A proper procedure is: calculate water volume, about 1.5 gallons per foot for 6-inch casing, less if the water column is lower; add chlorine to give 100-200 mg/L; circulate with a clean hose until chlorine is detected at each fixture; leave 12-24 hours; then flush until chlorine is gone. Clean the wellhead and fix any casing or grout defect. Retest after 7-14 days with a certified laboratory. If coliform returns, investigate the source; continuous chlorination or UV may be needed, but do not rely on chemicals alone if the well is damaged.
3. What regulatory and operational steps should the driller and owner take before returning the well to service?
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First repair the pump setting and controls so the well is not pumped dry, then verify with a run test. Disinfect, retest for total coliform and E. coli, and check turbidity, pH, iron, and manganese because sand and biofilm can mask problems. If the well serves a public water system, notify MassDEP and follow 310 CMR 22.00 monitoring and public-notice rules; for a private residence, report to the local board of health as required. Keep records of the pump setting, tank size, disinfection dose and contact time, and laboratory results. Advise the owner that a bigger pump is not a substitute for yield and storage, and that persistent coliform may require a licensed well contractor to inspect the casing, grout, and well cap. Do not return the well to service until the bacteriological sample is clear and the pump is protected from low water.