Granite Well: Casing Seat, Yield Test, and Arsenic Concern
An 8-inch down-the-hole hammer hole is drilled to 400 ft for a four-bedroom home with lawn irrigation. The 6-inch steel casing is set to 40 ft and grouted to surface. Water is struck at 120 ft (0.5 gpm), 210 ft (2 gpm), and 330 ft (8 gpm). Static water level is 55 ft. A 2-hour yield test at 12 gpm produces 80 ft of drawdown, and the 330-ft fracture is clearly the main producer. Lost circulation occurred at 300 ft, so the driller finished with foam. A neighboring well 250 ft away reports arsenic at 12 µg/L. The client insists on the “best producing well” and wants the pump set as deep as possible. The state requires casing into competent rock and a minimum 50-ft annular seal. The hole is straight and the owner has not yet tested the water.
1. Determine the final casing seat and grouting program. Explain how the 330-ft producing fracture and the 50-ft seal requirement drive your decision.
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The 40-ft casing is too shallow for two reasons: it does not meet the 50-ft minimum seal, and it leaves the first water strike at 120 ft unsealed. I would extend the 6-inch steel casing to 60–70 ft, confirm at least 10 ft of rock socket in competent rock, and grout the annulus to surface with a positive-displacement pump using neat cement or a bentonite-cement grout. I would not case through the 330-ft fracture; that is the main producer, and casing it would cut yield. The 300-ft lost-circulation zone is below the casing seat, so it affects drilling and development, not the annular seal. Before grouting, verify hole diameter and use a tremie or grout shoe. Check grout return and volume; lost returns in the open hole do not excuse a short seal. Document the seat, volume, and pressure.
2. Using the yield-test data, recommend a pumping rate and pump-setting depth. Show the calculation and state the main assumption.
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Specific capacity = 12 gpm / 80 ft = 0.15 gpm/ft. At the requested 8 gpm, drawdown = 8 / 0.15 = 53 ft, so pumping level = 55 + 53 = 108 ft. The 2-hour test is short, so I would call 8 gpm the preliminary safe yield and set the pump at about 160 ft, giving roughly 50 ft of submergence and room for seasonal decline. I would not set the pump at 330 ft just because that is the producing fracture; that risks pulling fine material and air and makes future service harder. Confirm with a longer constant-rate test and a recovery measurement. If the client wants more than 8 gpm, test 10–12 gpm for 4–8 hours and watch for interference, sediment, and drawdown approaching the pump inlet.
3. The client asks whether the neighbor’s arsenic result means their water is unsafe and whether the deep fracture should be sealed. Give a testing and decision plan.
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Arsenic is not automatically a shared aquifer problem at 250 ft, but it is a plausible concern in granitic terrain because arsenic can come from sulfide minerals or fracture fill. Test the new well for arsenic, iron, manganese, pH, alkalinity, sulfate, and conductivity after development and disinfection. If arsenic is below 10 µg/L, document and retest annually. If it exceeds 10 µg/L, do not immediately seal the only good fracture; treatment (adsorption, anion exchange, or oxidation/filtration depending on arsenic species) or blending is usually preferable. Sealing the 330-ft fracture would sacrifice the 8-gpm yield and may not solve the source. Explain the options, the need for a second confirmation sample, and any state reporting requirement.