Bacteriological Failure After Shallow Casing Repair
A 6-inch PVC-cased bedrock well serving a four-bedroom house is 240 ft deep with 38 ft of casing through 32 ft of overburden. Static water is 22 ft below grade; the submersible pump intake sits at 220 ft. After a contractor replaced the pitless adapter and added a 5-ft casing extension, the family reported cloudy water and the lab found total coliform and E. coli. The contractor shock-chlorinated the well and the sample cleared, but coliform returned within 10 days. A site visit shows loose soil in the annular space around the casing at about 4 ft depth and no visible grout. A 2-inch conduit for a geothermal heat pump passes through the same borehole annulus and is also unsealed. The well is 60 ft from the septic tank and 90 ft from the leaching field. The owner wants to know whether another chlorination will suffice or whether the well must be abandoned and replaced.
1. What is the most likely contamination pathway, and why did shock chlorination fail?
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The most likely pathway is the unsealed annular space and the unsealed 2-inch conduit, not the water-bearing fractures alone. Shocking the water column disinfects the well bore and pump, but it does not disinfect a continuous soil or gravel channel that connects surface or septic-influenced water to the casing. E. coli is a fecal indicator, so the 60-ft separation from the septic tank and the unsealed annulus are a plausible route. Chlorination clears the sample temporarily because it kills organisms in the water column; the annulus re-inoculates the well after the chlorine dissipates. Before concluding, verify the casing is sound and the cap is tight, and rule out a contaminated sample tap. A second chlorination without sealing the annulus is likely to fail again.
2. Describe a grouting plan that would seal the annulus and conduit, including method and verification.
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Remove the loose soil, expose the casing, and seal the annulus and conduit by tremie grouting from the bottom upward with a neat cement or bentonite grout mixed to the manufacturer's density. Use a positive-displacement pump and a tremie pipe that reaches the bottom of the annular space; do not pour from the surface. Grout continuously until return at grade is the same material. For the conduit, install a mechanical seal or grout collar and pressure-test if accessible. Calculate the theoretical volume and compare it with actual take; a large excess suggests a void or loss zone. Allow the grout to set, then redevelop and chlorinate the well, and collect a confirmatory coliform sample after the chlorine has dissipated. Verification is both physical (return, volume, seal) and bacteriological (consecutive absent samples).
3. What regulatory and setback issues must be resolved before returning the well to service?
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The driller should notify the local health department and follow Connecticut well completion and reporting requirements; a bacteriological failure may require the well to remain out of service until clear. Check the applicable Connecticut private-well setbacks: a 60-ft separation from a septic tank may be below the required distance depending on local code, and 90 ft from a leaching field may also be marginal. If the existing well cannot meet setbacks, the local health department may require a variance, treatment, or replacement. Do not rely on chlorination alone as a fix. Document the grout seal, sampling, and any communication with the health department. If the annulus cannot be sealed because the conduit is damaged, the well may need to be abandoned by a licensed well driller and a replacement sited with proper setbacks.