Flowing Artesian Well in Glacial Sand and Gravel
A new 6-inch PVC well is planned for a lakeside cottage in Cayuga County. Test boring shows 4 feet of topsoil, 28 feet of fine-to-medium sand, then 30 feet of gravel, then shale. A piezometer in the gravel reads 3 feet above ground surface when the borehole is open. The driller proposes an 8-inch borehole to 65 feet, 6-inch Schedule 40 PVC casing, a 15-foot factory slotted screen, and a bentonite seal above the screen. The owner wants the well to flow by gravity to a cistern and asks whether the flow can be used to avoid a pump. The site is 100 feet from the lake, and the local health department requires a sanitary seal. The driller has a bag of 3/8-inch bentonite chips and neat cement grout on site.
1. Select the screen, filter pack, and annular seal. Explain the reasoning and identify any problem with the proposed materials.
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Start with sieve analysis of the sand and gravel. Because the producing zone includes fine-to-medium sand, a factory slotted screen alone may clog or pass sand. Use a clean, washed filter pack selected so the formation bridges on the pack, and choose the screen slot to retain the pack, typically a slot that holds about 90% of the pack. The 3/8-inch bentonite chips are too coarse for a thin annular seal above a screen; they can bridge and leave voids. Use fine bentonite pellets or a bentonite/cement grout, placed carefully, with a seal of adequate thickness. Above the seal, grout the annulus to surface to meet the sanitary-seal requirement. For the artesian head, the casing must extend above grade, be sealed, and have a flow-control valve. Do not attempt to grout against uncontrolled flow; control the head with a packer or valve first, then pressure-grout.
2. Can the 3-foot artesian head replace a pump to the cistern? What design and operating problems must be addressed?
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A 3-foot head is only about 1.3 psi. It can move water to a cistern at the same grade through a large, properly sloped pipe, but it will not provide useful household pressure and may be unreliable in winter. More importantly, an open flowing well is a contamination and freeze risk. The design should include a sealed wellhead, a sample tap, a flow-control valve, a meter, and an insulated or pitless line to the cistern. Provide a safe overflow and an air gap so the cistern cannot back-siphon into the well. Keep a pump and pressure tank for reliability and pressure. Test the water for coliform, iron, manganese, and other site-specific constituents before use, and inspect the grout seal because a flowing annulus can wash out grout during construction.