Iron Bacteria and Shock Chlorination After Development
After drilling a 300-foot, 6-inch bedrock well in Dutchess County, the driller develops it by air-lifting for two hours. The yield is 12 gpm with a pumping level at 150 feet. Water is red-brown and turbid at 25 NTU; a slippery film develops on the pump cable, and neighbors report a similar sulfur odor in their wells. The driller plans to shock-chlorinate the well with 12.5% sodium hypochlorite at 200 mg/L using 500 gallons of standing water, wait 24 hours, then pump to waste. The pump is set at 250 feet; the pressure tank and household plumbing are isolated. The homeowner asks whether the well can be put into service the next day. The site has a ditch along the road and a trout stream 150 feet downhill. The driller has chlorine test papers, a clean tank, and a hose.
1. Estimate the volume of 12.5% sodium hypochlorite needed to reach 200 mg/L in 500 gallons of standing water. Show the calculation and state the assumptions.
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Convert volume: 500 gal × 3.785 L/gal = 1,893 L. Chlorine mass needed = 1,893 L × 200 mg/L = 378,600 mg = about 379 g as available chlorine. A 12.5% sodium hypochlorite solution has about 125 g available chlorine per 1,000 g of solution. At a density near 1.2 kg/L, that is about 150 g available chlorine per liter. Volume needed = 379 g ÷ 150 g/L = about 2.5 L, or roughly 0.7 gallon. Verify the label because percent strength and density vary. Add the hypochlorite slowly with circulation, wear PPE, and never mix it with acid. The estimate assumes the 500 gallons is the water volume contacted and that chlorine demand from the turbidity and biofilm is not yet satisfied.
2. Evaluate the disinfection plan. What must be done before and during chlorination to make it effective?
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The plan is incomplete because 25 NTU turbidity and a slimy film indicate iron bacteria and biofilm, which create high chlorine demand. Much of the dose may be consumed before it disinfects. First redevelop or surge the well and pump to waste until turbidity and sediment are reduced. Clean the pump cable and wellhead. Then chlorinate with the plumbing isolated, circulate to distribute the solution, and confirm a measurable free chlorine residual at the top and bottom of the water column. Maintain contact for 12 to 24 hours, keep pH below about 8, and re-dose if the residual falls too low. The pump manufacturer should be checked for chlorine exposure limits. Test papers are only a rough field check; use a DPD-type test if possible. Do not put the well into service until the chlorine has been removed and bacteriological samples are satisfactory.
3. How should the chlorinated water be disposed of, and what tests are required before the well is placed in service?
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The water must not be discharged to the trout stream or to a ditch that reaches it without dechlorination. Pump to an approved upland area or ditch, and neutralize with ascorbic acid or sodium thiosulfate while monitoring chlorine with a field test. Dispose according to local requirements and avoid flooding the septic system or wetland. Before service, flush until chlorine residual is at a safe household level, then collect a coliform/E. coli sample using proper sterile technique. Also measure turbidity, iron, manganese, and any site-specific constituents. If coliform is positive, do not simply repeat chlorination blindly; look for a source such as a defective cap, unsanitary pump, or annular connection, correct it, and retest. The homeowner should not use the water until the results are satisfactory.