Coastal Georgia Completion Design Above a Saltwater Zone
A coastal Georgia property near Kingsland needs a 6-inch domestic well. The log shows 0-25 ft sandy soil, 25-150 ft fine sand with shells, 150-250 ft limestone, and a known saltwater zone below 300 ft with chloride above 250 mg/L. Static water level is 18 ft. The driller plans an 8-inch borehole to 280 ft, 6-inch PVC casing to 160 ft, and open hole from 160 to 280 ft. The homeowner wants 'as much water as possible' and asks whether to go to 350 ft. A nearby well completed at 340 ft has 180 mg/L chloride and occasional salt taste. The area has a history of summer overpumping. Design and construction decisions must balance yield, water quality, and Georgia casing and grouting rules.
1. Recommend a completion depth and casing/grouting program, with reasoning. Address the homeowner's request to drill to 350 ft.
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Do not drill to 350 ft. The 300-ft saltwater zone and the nearby 340-ft well with 180 mg/L chloride show that deeper is not better here; additional depth would increase the chance of pulling saltwater into the completed interval. A total depth of 260-280 ft keeps the open interval in the limestone above the saltwater zone while still giving enough saturated thickness for a domestic supply. Set 6-inch PVC casing through the sand to about 160 ft, seating it at least 10 ft into the limestone so the casing shoe is in competent rock, then leave 160-280 ft as open hole. Use an 8-inch borehole so the annular space is at least 1 inch on each side; centralize the casing and grout the full annulus from shoe to land surface with neat cement or a cement-bentonite mix placed by tremie. A full-length grout seal is required to prevent the sand interval and surface water from communicating with the limestone. If the sand is a usable aquifer, it still should be isolated because the target is the limestone and the sand may be hydraulically connected to saltwater. Document the chloride baseline before the pump is installed.
2. Explain how you would decide between open hole and screen in the sand interval, and what slot size you would use if screening were required.
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The default should be to case off the sand and complete open hole in the limestone, because the sand is not the target and open hole maximizes yield from fractures. Screening the sand would be appropriate only if the limestone proved too tight and the sand had to be used as the producing interval. In that case, sieve the sand samples: choose a slot that retains about 90 percent of the formation sand, then design a filter pack with a grain size about 4 to 6 times the formation sand's 50th percentile. For fine uniform sand, a 0.010-0.020 inch slot with a graded filter pack is typical; a slot chosen only from a catalog without sieve data is a guess. A screen completion also requires a grout seal above the screen and a filter pack placed outside it, which adds cost and risk. If the limestone yields adequately, the simpler and more durable choice is open hole with casing seated into rock, not a screen in the sand.
3. What operational and monitoring steps would reduce the risk of saltwater intrusion after the well is placed in service?
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Saltwater intrusion in this setting is driven by vertical migration and by drawdown that pulls deeper water upward. Keep the pump intake high in the open interval, well above 280 ft, and avoid setting it near the bottom where the chloride front is closer. Measure chloride at startup and then quarterly for the first year, and annually thereafter; a rising trend is the early warning, not a salty taste. Record pumping water level and specific capacity at each service call, because declining specific capacity at the same rate indicates increasing drawdown and greater upconing risk. Do not oversize the pump: a 5-10 gpm domestic demand does not justify a 20 gpm pump that draws the water level far down. If chloride begins to rise, reduce the pumping rate, raise the pump, or consider a packer to isolate the lower part of the open hole. The nearby 340-ft well is a useful sentinel; its 180 mg/L chloride shows the area is already stressed in summer.