Disinfection Dose and Invalid Bacteria Sample After Construction
A 6-inch PVC well is 180 ft deep with casing to 160 ft and static water level at 60 ft; the open hole below casing is about 6 inches in diameter. After construction, the crew estimates the well holds 120 gallons and adds 1 gallon of household bleach containing 6% sodium hypochlorite, claiming this gives 50 ppm. They circulate the chlorinated water for 10 minutes, pump to waste until the chlorine smell is weak, and collect a bacteria sample the same day from a tap after the pressure tank and carbon filter. The lab reports total coliform present. The homeowner uses a water softener and carbon filter. The driller wants to shock the well with 200 ppm chlorine and sample again from the same kitchen tap.
1. Calculate the actual water volume in the well and the chlorine concentration produced by 1 gallon of 6% bleach. Show why the crew’s 50-ppm claim is wrong.
Read the worked answer
A 6-inch diameter is 0.5 ft. Area = π/4 × (0.5)² = 0.196 ft². The water column is 180 − 60 = 120 ft, so volume = 0.196 × 120 = 23.6 ft³. In gallons: 23.6 × 7.48 = 176 gal. One gallon of 6% bleach contains about 3.785 L × 60 g/L = 227 g of chlorine. In 176 gal, or 666 L, that is 227,000 mg ÷ 666 L = about 341 mg/L, or 341 ppm. The crew’s 50-ppm claim is wrong because they underestimated the water volume. To make 100 ppm, they need about 66.6 g of chlorine, or 1.11 L of 6% bleach, about 0.29 gal. For 50 ppm, use about half that.
2. Evaluate the contact time and the sample point. Why is the coliform result not a valid measure of the well’s sanitary condition?
Read the worked answer
Ten minutes is far too short for disinfection; the well code generally requires a much longer contact time, commonly 24 hours at an approved concentration such as 100 ppm. Sampling the same day is also premature because chlorine residual can suppress bacteria and give a false negative, while the short contact time may leave viable bacteria. The sample was collected after the pressure tank and carbon filter, so it represents treated household water, not the raw well. Carbon filters can harbor bacteria and alter chlorine. The driller should sample at a raw well tap before treatment, after the chlorine has dissipated, using proper technique and holding time.
3. Outline the corrective disinfection and resampling procedure. What should be done if coliform returns again?
Read the worked answer
Calculate the water volume, add enough chlorine to reach the required concentration, circulate so chlorinated water contacts the casing and pump, and let it stand for the required contact time, typically 24 hours. Then pump to waste until chlorine is gone. Wait until no chlorine residual remains, then collect a raw untreated sample for total coliform and E. coli from a tap before the pressure tank, softener, and carbon filter. If coliform returns, do not just repeat the shock treatment. Inspect the well cap, casing, and grout for defects; check for a direct surface-water pathway; consider camera inspection and re-grouting. The driller should also correct the 200-ppm plan, which may be excessive and still fail if contact time and sample point are wrong.