Road Salt, Fractured Bedrock, and a Replacement Well
A homeowner in Sullivan County has a 40-foot dug well that tests positive for total coliform and shows chloride of 120 mg/L. The house sits on a 1.5-acre lot; the town road is 80 feet north, the septic absorption field is 60 feet south, and the property line is 25 feet east. A replacement bedrock well is proposed at the only convenient spot, between the road and the house. Overburden is 12 feet of till; bedrock is fractured shale. A nearby 300-foot bedrock well has chloride 180 mg/L and sodium 95 mg/L; a 10-gpm pumping test at the proposed site shows turbidity rising from 2 NTU to 18 NTU within hours after rain. The driller wants to set 20 feet of casing into rock, grout the annulus, and stop at 180 feet. The homeowner asks whether the location is acceptable and whether the water will be safe. The local health department has not yet reviewed the plan.
1. Is the proposed location and completion acceptable? Explain the hydrogeologic and regulatory reasoning, and state what you would do before drilling.
Read the worked answer
Conclusion: do not drill at that spot until the source is better defined. The site is squeezed between two contamination sources. The existing well already shows chloride at 120 mg/L and the neighbor at 180 mg/L, which is strong evidence of road-salt impact in the bedrock. The 12 feet of till is thin cover, and the turbidity jump after rain suggests that shallow water or surface runoff can enter fractures quickly. Minimum setbacks may be met, but a setback is not a guarantee of protection. A better siting approach is to move the well cross-gradient to both the road and the septic field, maximize separation, and use a pilot hole to identify producing fractures. Before drilling, collect baseline samples from the existing and nearby wells for chloride, sodium, nitrate, total coliform/E. coli, turbidity, iron, and manganese; map septic components, road drainage, and bedrock fracture direction. During drilling, log water strikes and turbidity, and packer-test any significant fracture zone. If a clean zone cannot be isolated and sealed, relocate the well or plan for treatment.
2. The driller proposes 20 feet of casing into rock and 180 feet total depth. What completion details and testing would you require, and what would trigger abandoning the hole?
Read the worked answer
The casing seat should be in competent, low-permeability rock, not simply 20 feet below the top of rock. Verify competence from drilling rate, core, or packer testing. Grout the annulus continuously from the casing shoe to land surface with neat cement or an approved bentonite grout placed by tremie; do not use drill cuttings as fill. Extend the casing above grade and install a sanitary well cap. The 180-foot total depth is arbitrary; stop only after the significant water-bearing fractures have been evaluated. Require a stepped test that records yield, drawdown, turbidity, chloride, and coliform for each zone. Abandon or reseal the hole if the annulus cannot be grouted, if chloride continues to rise, if turbidity increases after rain, or if the water cannot be made potable with reasonable treatment. A single post-development sample is not enough to prove the well is protected.