Low-Yield Well With Iron, Manganese, and Arsenic
A 4-inch well is 420 ft deep, static water level 180 ft, yield 0.75 gpm. The pump is a 1/2-hp submersible set at 400 ft, with a 20-gallon bladder tank and a 30/50 pressure switch. The family uses about 250 gpd and complains of pressure surging and the pump short-cycling every couple of minutes. Water analysis: iron 3.0 mg/L, manganese 0.8 mg/L, arsenic 12 µg/L, hardness 120 mg/L as CaCO3, pH 6.5, and total coliform absent. The well casing is 6-inch to 60 ft. The homeowner wants a 'bigger pump' and asks whether a 1-hp pump will fix the pressure. The driller has a 120-gallon atmospheric storage tank, a booster pump, a float switch, and a low-level cutoff available on the truck.
1. Why is the pump short-cycling, and will a 1-hp pump fix it?
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The 20-gal bladder tank at 30/50 has only about 6-7 gal of drawdown. If the pump output exceeds household demand, the pump brings the system to 50 psi, the tank fills a few gallons, and the pump restarts. A bigger pump will make this worse because it adds pressure faster and cycles more sharply; it does not add storage or well yield. The real constraints are the 0.75-gpm yield and the small tank. Check the tank precharge, which should be 28 psi with a 30/50 switch, the switch setting, and whether the pump is sucking air or the well is being drawn down.
2. Design a storage and pump-protection scheme.
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Use the 120-gal atmospheric storage tank, but recognize that 250 gpd peak use means 120 gal is less than a day of storage; a 200-300 gal tank is better if the family wants reliability. Fill the storage tank from the well with a float switch and a low-level cutoff in the well so the pump cannot run dry. Set the submersible at 400 ft only if the drawdown will not expose it; a low-level probe or pressure transducer is safer. Add a booster pump after the storage tank to maintain 40/60 pressure and isolate the well pump from household peaks. A cycle stop valve on the booster can further reduce cycling. Do not solve a storage problem with a larger well pump.
3. Propose a treatment train for the water quality.
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The water has iron 3.0 mg/L, manganese 0.8 mg/L, arsenic 12 µg/L, which is above the 10 µg/L MCL, pH 6.5, and moderate hardness. Treat in this order: pH adjustment to about 7.0-7.5, oxidation by aeration or chlorine to convert iron and manganese to filterable solids, filtration for iron and manganese, then arsenic removal by adsorption media or co-precipitation and filtration. If chlorine is used, include a contact tank and possibly carbon to remove residual chlorine before the arsenic media if the media is chlorine-sensitive. A softener is optional for hardness 120 mg/L; it is not the primary arsenic or iron treatment. Re-test arsenic after treatment and confirm the effluent is below 10 µg/L.