Chlorine Dose for a New Well With a Pressure Tank
A new 6-inch PVC well is 180 feet deep with static water level at 45 feet. The pump is set at 150 feet, and the drop pipe and pitless adapter connect to a 40-gallon pressure tank. The well produces 15 gpm and clears after 30 minutes of pumping. The installer plans to disinfect with household bleach at 100 ppm for 24 hours, then flush and collect a total-coliform sample. The nearest hardware store sells 5.25 percent sodium hypochlorite bleach. The well is for a farm market that opens in three days. The owner asks how much bleach to use, whether the pressure tank and drop pipe are included, and what to do if the first sample comes back positive. The crew has a chlorine test kit and a five-gallon bucket.
1. Calculate the water volume and the amount of 5.25 percent bleach needed for a 100 ppm dose.
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The water column is from the static level at 45 feet to the bottom at 180 feet, or 135 feet. A 6-inch inside diameter has an area of about 0.196 square feet, so the well volume is 0.196 times 135, or about 26.5 cubic feet. That is about 198 gallons. Add the 40-gallon pressure tank and allowance for piping, giving about 238 gallons. At 100 mg/L, the chlorine demand is 238 gallons times 3.785 liters per gallon times 100 mg/L, or about 90,100 mg of chlorine. Household bleach at 5.25 percent contains about 52,500 mg/L, so the required volume is 90,100 divided by 52,500, or about 1.72 liters, roughly 0.45 gallon or 58 fluid ounces. The crew should measure with a marked container rather than guess.
2. How should the crew disinfect the drop pipe, pitless adapter, and pressure tank, and confirm contact time?
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The crew should add the bleach as a dilute solution and circulate it through the well, drop pipe, pitless adapter, pressure tank, and distribution lines. Each fixture should be opened until chlorine odor or a test-kit reading confirms that the chlorinated water has reached it. The tank should not be bypassed unless the plan accounts for its volume separately. The system should then be left undisturbed for the required contact time, commonly 24 hours for a new well, with no domestic use that would dilute or flush the chlorine. The crew should test residual chlorine at the end of contact time to confirm the dose was present, then flush until the chlorine is gone before collecting the bacteriological sample.
3. The first total-coliform sample is positive. What should the installer do before the farm market opens?
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A positive total-coliform result means the well is not ready for service. The installer should not simply resample and hope for a better result. First, check for obvious contamination pathways: well cap, pitless adapter, casing seal, grout, plumbing, and sample technique. Then re-disinfect the well and distribution system, flush, and collect a new sample with proper handling. If the repeat sample is also positive, the installer should investigate the source, including the annular seal and any cross-connection, and may need to shock-chlorinate the plumbing or repair the well. The farm market should use an approved alternative water supply or remain closed until the well passes. The installer should document the positive result and corrective actions.