Private well construction in granite formations
Building a new home in the country usually means you’ll also need a well drilled. In central and northern Wisconsin, drinking water wells must be drilled into a layer of hard, crystalline bedrock that underlies much of the upper Great Lakes region. Well drillers call this rock granite, even though it’s made up of at least several types of rock.
- What you should know before you hire a well driller
Learn all you can about granite well construction. Rotary drilling rigs often are used to construct granite wells. Blasting, in which dynamite is exploded inside the drillhole, or hydrofracturing, in which water is forced into bedrock to open up cracks, are sometimes used to increase the amount of water a granite well will yield.
Ask questions when you begin looking for a well driller so you understand these well drilling processes.
Hire a licensed driller. Only licensed well drillers may drill drinking water wells in Wisconsin. Because it takes a driller with a lot of expertise to drill a granite well, start your well-drilling project by hiring a competent, licensed well driller who is willing to explain granite well construction to you.
Check with neighbors to find a reputable driller. Encourage the driller you select to sign a clear, understandable and complete contract so you maintain a record of agreement on your well drilling project. Remember that drillers don’t create water — they search for it. If the well they drill produces a low yield of water or the water is high in iron, you may still have to pay for the drilling.
Be available while your well is being drilled. If a chosen well location isn’t working out the driller may have to stop and try another site or switch to a different well construction method. Be available during the drilling process so you can make decisions about these items with your driller. You'll avoid both inconvenience and needless expense.
- Site your well properly
The casing pipe in a completed well must rise above the ground at least 12 inches to protect the well from potential contamination. For that reason, the well should not be installed in a traffic area, such as a driveway Figure A and Figure B.
To protect drinking water quality, wells also must be sited up-slope and away from all sources of contamination. Wisconsin’s well code specifies that the well be sited at least:
8 feet away from any building sewer;
25 feet away from a septic tank and a minimum of 50 feet away from any sewage drainfield or mound system;
50 feet away from a street sewer;
100 feet away from a buried petroleum product tank (except only 25 feet away from a buried fuel oil tank serving a single family residence); and
1,200 feet from a landfill (or variance required).
Dozens of other minimum distances, or “setbacks,” also are required to prevent drinking water well contamination from silos, manure piles, barn gutters, cemeteries and other sources.
Make sure you tell your driller upfront about all possible contamination sources on your lot or on neighboring lots. Also mention any future development plans for these lots. This information will help your driller site your well away from potential contamination sources.
- Casing pipe installation
To protect a granite well from surface contamination, at least 40 feet of approved steel well casing pipe 6 inches or larger in diameter must be installed into the ground.
Often the well driller will set the pipe into an upper enlarged drillhole, then drive the pipe to a firm seat at the top of bedrock. If the granite lies deeper than 40 feet, the driller may drive the pipe from the start rather than set the pipe in an upper drillhole. This approach is used only if soil near the surface is caving sand and gravel. If soil near the surface is clayey and less likely to cave in, an upper enlarged drillhole must be drilled to 20 feet before the casing is placed and driven. (An exception now allows the well driller to mound approved clay granules around the top of the casing during driving instead of constructing an upper enlarged drillhole.)
- Drillhole requirements for the upper portion of the well
If the well enters granite less than 40 feet beneath the ground surface, the wellhole must meet several requirements.
- The upper portion of the drillhole, into which the casing pipe is placed, must extend to a depth of at least 40 feet.
- If the casing pipe is assembled with welded joints, the portion of drillhole holding the casing pipe must be at least 2 inches larger in diameter than the casing pipe.
- If the casing pipe is assembled with threaded couplings, the portion of the drillhole holding the casing pipe must be at least 4 inches larger in diameter than the casing pipe.
- Drillhole requirements for the lower portion of the well.
The lower, uncased portion of the drillhole, where water is collected, may be constructed either before or after the casing pipe is set and the space around the casing pipe is sealed.
A drillhole in hard bedrock will stand open without a casing pipe. If the lower drillhole is drilled into bedrock where a lot of water is present water will flow into the drillhole. Pumping — normally with a submersible pump — will then provide drinking water.
- Sealing the casing pipe
Properly sealing the space around the outside of the well casing prevents surface water or other contamination from running down the outside of the well casing and entering the lower bedrock drillhole where the water you want is stored.
To properly seal a granite well casing into bedrock, the space around the outside of the casing must be filled with cement grout from bottom to top. If the upper enlarged drillhole is only 2 inches larger in diameter than the casing, the grout must be pumped into the space around the casing using a conductor pipe to fill the entire space with grout from the bottom up. The grout may be allowed to flow down the conductor pipe under the force of gravity if the space around the casing pipe is at least 4 inches larger in diameter than the pipe.
Just pouring cement down along the outside of the casing without a conductor pipe won’t do the job because the cement may plug up alongside the well casing before reaching bottom, causing unsealed gaps to occur. Also, the space above the bedrock may also be full of water, which could dilute the cement and prevent proper sealing.
Wisconsin’s well code requires grout to be mixed in a ratio of five to 6 gallons of water to one 94-pound bag of Portland cement. It takes at least four bags of cement to fill an 8-inch diameter space around a 40-foot length of 6-inch-diameter well casing. If the drillhole is 10 inches in diameter, at least 11 bags of cement will be needed.
Cement grout must be allowed to set for at least 12 hours before any further well construction is started.
If granite lies deeper than 40 feet, cement grouting is not required unless the well driller extends the enlarged drillhole more than 5 feet into the granite. This helps protect the bedrock from contamination.
- Methods for getting enough water from a granite well
Most granite bedrock contains few cracks at depths normally reached by wells. That’s why it’s often difficult to get enough water from a granite well. The few water-bearing cracks present in granite usually occur in the first 200 feet.
If your driller has constructed a granite well that looks like it’s not going to produce water, the driller may ask to stop drilling and try another spot.
The driller may also recommend blasting the well with dynamite to open water-bearing rock fractures. Blasting for this purpose may only be done by a person licensed to blast wells and under the supervision of a licensed driller.
A third, more successful and less dangerous, method is hydrofracturing, in which large volumes of water are injected into a drillhole under thousands of pounds of pressure to open water-bearing cracks in bedrock.
If the only water available in the bedrock lies shallower than 40 feet, and the contractor drilled to at least 150 feet trying to obtain water, the driller, with your permission, may request a variance from the DNR to set less than 40 feet of casing. You or the well driller may call the nearest to request a variance. Sometimes it’s possible to get approval right away. The DNR will then send a follow-up letter of approval. In other cases, the DNR may place special conditions on variance approvals, so the process may take a little longer.
- What if the flow in my granite well is slow?
Most people with new homes would like to have water flowing through indoor taps at about 6 gallons a minute, but some granite wells will not produce that much.
Many drillers recommend extending the bedrock drillhole to great depths for low-yielding wells because the drillhole can act as a water storage reservoir.
A 6-inch diameter drillhole or casing can hold 1½ gallons of water for every foot of depth. If your new well is deep, it may store a large volume of water even if the well becomes recharged with groundwater very slowly. A 6-inch diameter well containing 200 feet of standing water could store 300 gallons of water.
Families who use water prudently can get by with a well yielding 1 gallon a minute or less if the well has enough depth to provide storage capacity.
- What are the driller’s responsibilities to the well owner?
After the well is completed, well drillers are required to disinfect your new well with chlorine, test pump it to determine its water yield (in gallons per minute), collect a water sample to check for bacteria and subsequently give you a copy of the bacteria test results. The driller must also give you a construction report for your well within 30 days following well completion. The report includes the casing diameter and depth, casing length and the well’s water yield.
Review this information and file it in a safe place. It may become critical if problems develop with your well or pump and repairs are needed.
- Maintaining your new well
To be on the safe side, you should get your well tested for bacteria at least once a year or any time you notice a change in the way your drinking water tastes or smells. If your well water becomes cloudy every time it rains, you should get a bacteria sample done immediately because you may have a serious contamination problem in your well.
You may send the sample to the nearest laboratory certified to do bacteria analyses or to the State Laboratory of Hygiene in Madison.
In some areas, the entire usable underground water supply is contaminated throughout its depth, so it may not be possible to obtain a safe water sample. However, this alone will not relieve you of your responsibility to pay the driller. In these areas, it may be necessary to apply to the DNR for approval to install a chlorination treatment unit.
- How do bacteria get into my well?
Most groundwater comes from rain and snow that falls near your home and filters into the ground. That’s why nearby land use can directly affect local water quality.
Geology can also affect drinking water quality. The granite bedrock in northern and central Wisconsin, for instance, lies close to the surface. Little soil, sand or gravel lies on top of it to filter out bacteria that can contaminate groundwater. Once bacteria or contaminants enter cracks in bedrock, they can move and spread rapidly.
Poorly constructed wells, those having short casings or inadequate cement seals, can allow surface water to move down into the lower bedrock drillhole and contaminate your well. This commonly occurs at old homesteads, which often have wells consisting of short lengths of casing set at shallow depths. The casings may also have holes caused by corrosion.
An experienced well driller can check casing depth by running a magnet and weight down the inside of the casing, then measure the length of cable lowered into the well.
If not properly abandoned, old wells and drillholes can act as drains, allowing contaminants to flow into the groundwater that supplies you with drinking water from your well. Old unused wells, poorly constructed wells, dug wells or open drillholes should be filled from the bottom up with cement or other approved material.