Flowing Well With a Bridged Annular Seal
An 8-inch steel-cased well on a sloping lawn flows at about 3 gpm at the casing top. The casing is 40 ft, bedrock is at 38 ft, and casing is set 2 ft into rock. The annular space was 'sealed' with bentonite chips dumped from the surface; a tag line finds a bridge at 20 ft. No grout returns were recorded. The well is 60 ft from a seasonally wet swale and 120 ft upslope from a pond. The owner wants to stop the flow, cut the casing below grade, install a pitless adapter, and use the well for a new house. A neighbor's well 300 ft away has had turbidity and iron bacteria since the flowing well was drilled.
1. What is the likely cause of the flow and the annular seal problem, and why is it a contamination risk?
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The casing is only 2 ft into bedrock, and the annulus was filled with dumped bentonite chips. The tag-line bridge at 20 ft shows the seal is not continuous. The well flows because the borehole intersects a confined or artesian fracture; the unsealed annulus gives that pressure a path upward along the casing. That is a hydraulic short circuit. It can transmit turbid water, iron bacteria, or surface contamination from the swale and pond into the well and adjacent fractures. The neighbor's turbidity and iron bacteria may be related if the flowing well is pressurizing a shared fracture network, although that requires investigation. A flowing well with a bad seal is also a safety and property-damage issue. The repair must create a continuous, low-permeability annular seal from the bottom of the casing to surface, not just a surface cap.
2. Design a grouting repair. Include how to handle the flow and the bridge.
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First control the flow without sealing the borehole: use a packer or temporary cap with a bleed valve and route flow away from the work area. Remove the bentonite bridge with a bit or wash pipe, then run a tremie to the bottom of the annulus. Use a quick-setting or accelerated neat-cement or cement-bentonite mix if flow is washing grout away; otherwise a standard cement-bentonite grout is acceptable. Grout from the bottom up in stages, keeping the tremie tip submerged, and vent the annulus. If flow is strong, pressure-grout in stages and let each stage set. Do not dump material from the top. After the grout sets, verify with a tag line or camera and check for continued flow at the surface. If flow persists, re-grout or install a packer and pressure-grout the remaining void. Notify the local board of health and follow 310 CMR 46.00; a licensed driller should supervise.
3. What records, notifications, and verification should follow the repair?
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Record the original construction, bridge depth, grout mix, quantities, pressures, stages, and any loss of circulation. Record final verification: tag-line or camera inspection, flow rate, water level, and whether flow at the casing stopped. Submit the required well completion or repair report to MassDEP and the local board of health under 310 CMR 46.00; verify the current deadline and local form. If the well will be used, retest for coliform and turbidity after the grout cures, and inspect the wellhead, pitless adapter, and surface drainage. The owner should understand that a grout repair does not guarantee the aquifer is uncontaminated. If the neighbor's iron bacteria is confirmed, disinfection and possibly treatment evaluation are needed. Keep a copy of all records on site for the water-system operator.