Driving a wellpoint
A wellpoint or drive point is a pipe with openings large enough to allow water to enter and small enough to keep the water-bearing formation in place. Well points suitable for hand driving are available in sizes from 1¼ to 2 inches in diameter and from 18 to 60 inches long. The size of openings in the wellpoint is determined by the relative grain size of the material in the water-bearing formation. Some of the finest grains adjacent to the wellpoint should be removed by pumping to make the well more productive. (figure 1). However, a wellpoint should not be expected to yield large quantities of water; 5 gallons per minute would be a reasonable yield. For larger flows of water, obtain a sample of the water-bearing formation by using an auger or other sampling device. A sieve analysis will determine the size of the well-point openings.
Figure 1. Installed wellpoint.
Why wellpoints are used
It may be desirable to develop a water supply for sprinkling lawns, gardens, etc. Persons may construct wells on their own property for domestic or irrigation use but all wells must be located and constructed in accordance with the requirements of the Minnesota Water Well Construction Code. A copy of the code can be obtained from the Document Section, Department of Administration, 117 University Avenue, St. Paul, MN 55155.
An economical supply of water can often be obtained from a shallow aquifer (water-bearing formation) through a wellpoint. The water table should be high, preferably within 10 feet and no deeper than 15 feet underground. The wellpoint must be driven deep enough to penetrate a water-bearing formation below the water table but the point usually does not exceed 25 feet in depth.
Shallow water tables are susceptible to pollution. Drainfields, dry wells, animal wastes, heavy fertilizer applications, etc. can contaminate a shallow water table. Recharge is usually from rainfall, falling directly above and percolating downward to the water table. As the water moves downward it may carry contaminants. Thus, extreme care and periodic testing are necessary if the water is to be used for drinking.
The following materials are needed: riser pipe (in 5- or 6-foot lengths with 6-inch nipple), drive couplings, drive cap, pipe thread compound, and wellpoint.
Several types of wellpoints are constructed of different materials. Figure 2 shows four common types. For mesh-covered wellpoints, the size of openings is designated by the mesh size. Common sizes, from larger to smaller openings, are 40-, 50-, 60-, 70-, and 80-mesh. For wellpoints with slot-type openings, common slot sizes are 18, 12, 10, 8, and 7 slot. Slot sizes are the opening width in thousandths of an inch. No. 18 slot is 0.018 inch wide; No. 12 is 0.012 inch wide, etc.
Figure 2. Types of wellpoints.
For continuous use, consider a stainless steel wellpoint. Experience has shown that less incrustation of minerals occurs in the screen openings and that the screen can be treated in place to remove incrustation.
Openings in the wellpoint should be large enough to permit the finer particles of the water-bearing formation to enter while keeping the coarser particles out. With proper sized openings, development of the well removes the finer particles and forms an envelope of porous and permeable material around the screen (figure 1).
A local well contractor or hardware dealer may have valuable suggestions on the most suitable opening size for the well point. It is usually advisable to select a fairly small opening size to avoid pumping sand. If a formation sample can be obtained, a sieve analysis will allow determination of the proper opening size.
The riser pipe should be galvanized pipe in 5- or 6-foot lengths for convenient hand driving. A standard 21-foot length of pipe cut into four pieces normally is adequate for a driven well.
Special drive pipe couplings, allowing the pipe pieces to butt together, are desirable. The impact of driving is transmitted through the pipe and not the pipe threads. Under severe driving conditions, standard pipe couplings may cause thread cracking or failure.
The drive cap is placed on the top of the nipple which threads into the drive coupling of the pipe section being driven. The cap transmits the blow to the pipe and protects the threads.
All pipe joints should be screwed tightly after threads are carefully cleaned. Pipe thread compound should be used to improve airtightness.
A post hole digger or soil auger, pipe driver, carpenter's level or plumb bob, and pipe wrenches are needed for hand driving a wellpoint. A pipe driver can be rented from a local hardware or building materials store. Portable electric drivers also are available for rental in some areas.
Pay careful attention to isolation distances from pollution or contamination sources. Since wellpoints usually penetrate only shallow aquifers the isolation distances must be 100 feet from a drainfield or animal yard and 150 feet from a cesspool or drywell.
Dig or drill a vertical hole 1–2 inches larger than the wellpoint and as deep as possible with the posthole digger or auger (figure 3). The hole usually cannot be drilled more than a foot below the water table.
Figure 3. Driving the wellpoint.
Rub a bar of soap overall the openings of the wellpoint to help prevent clay and sand from entering and to reduce friction during driving.
Attach a length of riser pipe to the open end of the wellpoint. Clean the threads, add the pipe thread compound to the outside threads, and make a tight connection with the pipe wrenches. Insert the wellpoint into the hole. Attach the nipple and drive cap to the top of the riser pipe. Do not use pipe thread compound on the drive cap.
Make sure that the pipe and wellpoint assembly are vertical by using the carpenter's level or the plumb bob. Use the driver to strike the drive cap with square, solid blows (figure 3).
When the drive cap is about 4 inches above the ground, unscrew the drive cap and nipple and place the unit on another section of pipe.
Clean the threads on the new section of pipe and on the pipe in the ground and add pipe thread compound to the outside threads. Tighten the joint using two pipe wrenches working in opposite directions to avoid twisting the assembly in the ground, and resume driving.
Continue the above procedure, adding sections of riser pipe as needed. To keep the threaded joints tight during driving, give the riser pipe an occasional half turn with a wrench. Use the wrench only to take up slack in the thread joints and take care not to twist the pipe severely.
Pour water into the top of the pipe at regular intervals. This makes driving easier and will determine when a water-bearing formation is reached. When a gallon of water disappears into the formation within 2 minutes after being poured into the well, it is unnecessary to drive any deeper.
Enough of the riser pipes should extend above ground so the desired pump can be attached conveniently. For example, a hand pitcher pump should be about 3 feet above the ground. For an electric pump, the top of the riser pipe should be a foot or more above ground to minimize the chance of well contamination in case of local flooding conditions.
A well, except where an approved pitless adapter or pitless unit is used, must be protected by a durable watertight concrete slab, platform, or floor, at least four inches thick, extending horizontally at least one foot in each direction from the well casing and sloped to direct water away from the casing casing (figure 1). A watertight seal, which may be asphalt or similar material to provide resiliency, must be provided between the casing and the platform.
Test holes, unsuccessful wells, and wells no longer used must be sealed and abandoned as prescribed in the Well Code.
In testing for bacteriological quality, the local community health service agency should be contacted for existing wells and the Groundwater Quality Control Unit of the Minnesota Department of Health for new wells.
There are several ways to develop a new well by surging and pumping. One simple method is to take a wooden rod or closed end pipe and–simulating piston action–rapidly work it up and down for about 5 minutes just below the water level in the well. This surging effect will draw fine, loose sand and silt into the well, leaving the coarser and more permeable material outside the wellpoint. Remove the fine sand from the well with a pitcher pump or other pump capable of handling sand.
Another means of cleaning out the sand is to jet water into the well with a garden hose inserted to the bottom. The sand and silt particles will wash out around the hose. Repeat until no more sand is obtained by pumping.
Follow the manufacturer's instructions when attaching the pump. If the well water is for domestic use, the pump should be connected to a drop pipe installed in the well installed in the well. A drop pipe is used so that the well casing pipe in contact with the soil is not under suction. The well casing will be under suction if the pump is connected directly to the well. Because of size limitations, a drop pipe often is not used with a 1¼ inch wellpoint. If the well is used for a drinking water supply, however, a standard-weight outer casing should be installed to a depth of 10 feet and the annular space between the casings grouted with cement. If a hand pump is used, it should have a closed spout directed downward and have a stuffing box for the pump rod. If the water is to be used for drinking, the well should be disinfected by adding one cup of 5.25% hypochlorite laundry bleach mixed in three quarts of water. Twelve hours after pouring this solution into the well, the well should be pumped until the chlorine odor is no longer distinguishable. Forty-eight hours after pumping, the well can be sampled for bacteriological examination. Contact your local community health service agency for sampling instructions and a sterile container. If a shallow well is used for domestic purposes, the water should be tested semi-annually or annually.
It is usually a sound policy to have a reliable licensed water well driller develop a sanitary water supply for a home. Water purity and personal health should be the major considerations in developing a domestic water supply.
University of Minnesota, U.S. Department of Agriculture, and Minnesota Counties cooperating.