Bedrock Casing Seat and Annular Seal
A homeowner needs a replacement well on a 0.5-acre lot in Sussex County. The driller proposes an 8-inch borehole to 300 ft with 6-inch steel casing set 8 ft into competent granite encountered at 40 ft. The annulus is to be filled with bentonite chips poured from the surface; the driller says the chips will bridge and seal against the rock. A neighboring well 250 ft away produces 12 µg/L arsenic and 0.6 mg/L iron. The homeowner needs at least 10 gpm for household and lawn use. During drilling, a fracture at 180 ft yields about 25 gpm, and the driller wants to stop at 220 ft. The local health department will require a pump test and water sample before occupancy. The driller has a tremie pipe and neat-cement grout on the truck but prefers chips because they are faster.
1. Is the proposed casing seat and grouting method acceptable? Explain what you would require before the well is completed.
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Reject the plan as proposed. Casing seated only 8 ft into rock is marginal; NJ well construction rules generally require the casing to be firmly seated in competent bedrock (commonly at least 10 ft) and the annular space sealed to the surface. More important, bentonite chips poured from the top are not a reliable annular seal: they bridge, hydrate unevenly, and leave voids, especially against an open fracture at 180 ft. Require a tremie grout from the bottom of the annulus upward using neat cement or a high-solids bentonite grout. Calculate grout volume from the annular capacity plus a modest excess, watch for returns, and do not stop at the first loss zone. If returns are lost, stage-grout or thicken the mix and document. The purpose is a continuous low-permeability barrier, not just filling a hole.
2. The driller proposes to stop at 220 ft and test at 25 gpm. What test and sampling sequence would you specify, and what would each step prove?
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Develop the well first - airlift or surge until discharge is clear - because drilling fines can mask yield and plug fractures. Disinfect, then run a 24-hour constant-rate test at a rate above the intended 10 gpm, say 15 gpm, measuring pumping level, drawdown, and recovery. A short test at 25 gpm only proves the fracture can store water for minutes; it does not prove a sustainable 10 gpm. Sample after disinfection and before any treatment for total coliform/E. coli, nitrate, arsenic, iron, manganese, pH, hardness, and the PWTA suite if a property transfer applies. The neighbor’s 12 µg/L arsenic means arsenic is a real risk, even if this well looks clear. Record all data.
3. Grout returns are lost at 60 ft. What is the correct response, and why not just pour chips?
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Lost returns mean the grout is entering the formation, not that the annulus is sealed. Keep the tremie below the grout surface, pump slowly, and thicken the mix or use a staged approach: grout the lower interval, let it set, then grout the upper interval. Do not switch to chips; they will bridge above the loss zone and leave the lower annulus open. If the loss is severe, pressure-grout with a controlled mix and verify with volume and, where required, a geophysical or camera check. Document the loss zone and the remedy on the well log.