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NHDES Technical Bulletin           WD-WS-1-4

Dug Well Design

Three sections of the Department of Environmental Services (DES) provide assistance to those citizens with private wells. They are the DES's Laboratory Services Unit, the Water Supply Engineering Bureau and the New Hampshire Water Well Board. Their addresses and telephone numbers appears at the end of this document.

For the most part, this document assumes you are evaluating the installation of a new well. Suggestions concerning inspection of existing dug wells can be found on the end of this factsheet. For well abandonment see the DES fact sheet WD-WSEB-1-10.

State: A person or firm in the well construction business must be licensed by the NH Water Well Board. The Board requires the submission of a "well completion report" describing the well's design, construction, soil conditions encountered and the well yield.

There are statewide design criteria rules for dug well construction and placement. These rules were originally adopted by the Board in 1983 and revised in 1990. The rules are numbered Env-We 100-900. There are no state requirements concerning minimum well quality or quantity.

Finally RSA 477:4-c requires, when selling a home with an on-site water system, disclosure of the water system's location, malfunctions, date of installation, date of the most recent water test and whether or not the seller has experienced a problem such as an unsatisfactory water test.

Local: Some towns may have local permit requirements relative to the placement, construction, water quantity or quality for private wells. Please contact your local health officer or code enforcement officer for particulars.

Determining How Much Water You Use
To determine needed well yield, you must first estimate your water demand. A typical household requires approximately 5 gallons per minute (gpm) to meet modest domestic water needs, however, as little as 2 or 3 gpm could be tolerated. Factors to be considered when determining your family's minimum demand on your water system include: the number of water uses that you have, their flow rates, how many of these uses could occur simultaneously and for what duration.

Determining How Much Well Yield You Need
What minimum well yield will satisfy your family's water needs is more difficult to identify. A low yield well (1-3 gpm) may be acceptable if one makes use of water stored in the well hole or in large basement storage tank(s). The storage in the well is not available if the water table drops in the summer and fall.

Such storage in the well may allow installation of a higher capacity pump if the duration of pumping will be short. In such cases, a low water cut-off device should be installed to prevent damage to the pump. The typical 2-1/2 foot diameter well casing has a storage volume of approximately 35 gallons per foot of water depth. Large non-pressurized storage tank(s) installed in your basement can serve this same function of accumulating water during periods of non-use, however, an additional pump will be necessary to pressurize this water.

The NHDES has published a document entitled "Well Yield, How Much Water is Enough?" which is available for a fee of $2.00 through the Water Well Board. The United States Department of Agriculture, Farmers Home Administration Guidelines require a minimum well yield of 1200 gallons for a four-hour period.

It should also be noted that a well's yield may change with time. This change can be a seasonal variation or long-term trend. Thus, one should try to develop the maximum well depth within economic reason when the well is first installed.

It is not uncommon to have at least a 5-10 foot variation between spring and fall groundwater levels. A one foot loss of water level in a well containing only two feet of water is critical, while a similar change in a well containing 10 feet of water is not too significant.

Dug wells are very affected by drought. This condition can be minimized by excavating the bottom of the well significantly below the seasonal low water table. The seasonal high water table can be determined by soil experts based on color change of the soil. It is difficult to determine the seasonal low water table. Consequently the depth of a dug well is normally governed by the capability of the construction equipment or when you encounter bedrock or till. The best time to construct a dug well is in the early fall when low water tables facilitate construction. This will allow a deep well while minimizing muddy conditions and excavation cave-in.

Water Quality Considerations
Dug wells experience iron, manganese, and taste and odor conditions approximately as often as bedrock wells. Dug wells generally do not experience arsenic, fluoride and radiological problems that are seen in bedrock wells. Dug wells often experience bacterial problems caused by poor construction and nonprecise building materials

Well Protection and Placement
Since dug wells take water from the highest water table, they are extremely sensitive to those activities that take place in the immediate vicinity of the well.

Examples of chemical hazards to your well include the application or inadvertent spillage of fertilizer, pesticides, and inappropriate disposal of old crankcase oil, anti-freeze or solvents, or salt brine from water softeners. Thus, the use of chemicals in your backyard or that of your uphill neighbors may negatively affect the quality of the water from which your well draws.

Tests for many of these chemicals involve complex and costly laboratory procedures. Thus the best and least costly approach, to protect water quality is prevention of pollution rather than treatment after the fact. Be careful with respect to the use and disposal of chemicals near and upstream of your well.

The following protective distances are required or recommended when locating a well for a private single family home:

  1. Surface water and drainage culverts should not pass within 25 feet of a well; 50 feet is recommended.
  2. Animals should not be penned or tied within a minimum of 20 feet of a well. DES recommends 75 foot separation.
  3. Leach fields and septic tanks shall not be located within 75 feet of a well.
  4. Well should not be located within 50 feet of the right-of-way line of roads, preferably more.
  5. Well shall not be placed within 75 feet of adjacent property which you do not control. (See RSA.485-A:30-b). If placement is made necessary within 75 feet, a "standard release form" is required to be signed by the well owner and given to DES, the town health officer and the registry of deeds. Since most zoning codes require a 10 foot setback, this distance is effectively 65 feet.
  6. A well should not be placed in locations subject to ANY flooding unless the immediate (25 foot radius) vicinity of the well is built up above the flood level.
Choosing The Well Type You Will Purchase
Based on the considerations above, such as soil depth, water needs, and existing pollution you should now be ready to choose the type of well (either bedrock or dug) that you believe is best for your lot and home. As previously stated, this fact sheet pertains to dug wells.

Contracting With a Dug Well Contractor
Prior to actual excavation you will need to provide the contractor with guidance as to what concept will govern the amount of work to be done. Normally, dug wells are installed on a lump sum contract basic. However this method does not insure the deepest well unless that issue is specified as part of the agreement. The nature of the contract is between the homeowner and the well contractor.

Excavation and Backfill
When beginning excavation, place different soil types in different piles so that they may be replaced in the same sequences as removed. Upper soil layers may be high in bacteria, organics and readily soluble iron and manganese. If these poor quality soils are placed below the water table during backfilling, water quality problems may occur.

The well should be as deep as possible to prevent drought affects. Excavate in the Fall. Try to prevent an accumulation of fine silt in the bottom of the excavation. An accumulation of silt may form an impervious layer reducing the entry of water into the completed well excavation. When placing the crushed stone and casing, break-up this fine sediment layer.

At least 5 feet of soil should cover the highest level of crushed stone.

Stone Bed
The well casing is normally set on a 2 + foot layer of 2"+ crushed stone. This allows convenient leveling of the well tile, and also allows the entry of water from the outside surrounding soil. Place multiple layers of graded pea stone above the larger crushed stone to act as a transition zone. This will prevent the backfill from settling into the crushed stone in the future. DES does not recommend the use of straw, tar paper and other degradable materials on top of crush stone as they may potentially cause bacterial and taste problems and also fail with time.

Pipe or Concrete Tile Sections
Pipe sections should be joined by bell and spigot or tongue and groove connections. This is critical since frost heaves or pressure form uphill soil, on a sloped site, will often displace tiles that are not locked into one another. Once the joints are sufficiently offset, soil will fall into the well. This in turn will cause settlement of the surrounding soils leading eventually to a cave-in of the backfill plus sand damage to the pump and cause bacteria. Joints should be oriented as shown in diagram (i.e. outside tongue facing down). The joints between the highest two well casings should be cement mortared to achieve a water tight condition. An alternative is the use of a safe sealant. These are identified in DES fact sheet WD-WSEB-18-1.

Water should enter the well at the bottom, either horizontally or up through the bottom. This will ensure maximum filtration of water through the soil. The lowest concrete casing may have perforations in its side wall. Wire reinforced concrete is suggested for well casing rings and top cover.

To insure filtration of all water entering the well, an impervious apron of clay or fine silt should be placed entirely around the well. This apron should be approximately 10 feet wide or as wide as the excavation that was made to install the well. The apron slope should be approximately 2 inches per foot. A greater slope should be considered if substantial settlement of the backfill is expected. Finally, the apron should be loamed and seeded to assure a stable condition. Without sloped backfill, puddles of contaminated water will form in contact with the well casing and potentially leak into the well. This condition often results in bacterial pollution.

A center observation hole is NOT recommended. If one exists on an existing well, it should be sealed tightly to prevent the leakage of contamination into the well from above. If the observation hole is subsequently opened, the seal must be remade. Acceptable substances for sealing the observation hole include cement mortar and those in fact sheet WD-WSEB-18-1.

The cover should overhang the well and have raindrip on the underside to prevent the leakage of contamination into the well.

The cover shape shown in the diagram is not regularly available in New Hampshire; however, we are requesting that concrete precasters adopt this shape in order to solve the underside leakage problem. The underside of the well cover may not be totally flat. It is suggested that reference marks be placed on the cover and casing to insure the same alignment each time the cover is moved. Mortar or sealants can then be placed on the vertical sidewall to match cover irregularities. DES suggests the cover overhang the casing by at least 3 inch. This may help keep the soils next to the well casing dry which in turn may reduce the incidents of frost heaving of the top casing.

If you feel there is need for an inspection port in the cover, the design shown in the sketch would address leakage. Another option to allow an observation viewport would be to frabricate a second light weight, easily removable cover to to fit over the entire concrete cover. This second cover would act like a rain shield. This shield should be lockable to prevent vandalism and could be made from welded steel or aluminum or sheet metal. Plastic sheeting is not recommended.

Water Supply Line and Pump
Provide at least one foot of clearance between the suction of the pipe and the bottom of the well. Provide a minimum of 5 feet of cover over the water line to the home for frost protection. Before backfill, first take field measurements and then draw an accurate sketch of the precise route of the water line from the well to your home. Duplicate this sketch, laminate and attach one copy to your pressure tank or fuse box.

If the pump is installed in the well, seal the inside of the electrical conduit to reduce that radon gas entry route into the home. A centrifugal pump is most often used in dug wells. Jet pumps allow easy repair but are energy inefficient.

Determining the Well's Safe Yield
You should know the well's safe yield. Once the well is dug, a pump test can be performed. The safe yield of a newly completed well can be determined (and the well can be flushed) by pumping water to waste continuously over a sustained period of 24 or more hours. The pumping rate should be measured by noting the number of minutes required to fill a known volume container (such as a 20/32 gal. trash can). The water level in the well should be measured as the pumping continues.

The intent of the test is to develop an equilibrium between the amount of water being pumped out of the well and the amount which is replenished naturally from the ground. The discharge should be piped at least 200 feet from the well, and downhill, if possible, to prevent recycling or "double counting." Do not run this dirty water through your plumbing.

If the drawdown in the well is at a relative maximum depth and has stabilized, this can be considered as the maximum safe yield of the well for that season of the year. This test should be run in the early fall when the groundwater table is at its lowest. Otherwise it must be recognized that a lower water table will reduce the well's yield. The use of a high capacity pump or fire truck to pump the well dry once then allowing it to refill, does not, in our opinion, provide useful information.

Disinfection - Chlorination
For new dug wells or where well pumps have been recently replaced, it is most important to clean the well before chlorinating or testing for bacteria. The well may have to be continuously pumped for days (or weeks, in a few new well cases) before this cleaning process is complete. Chlorine, regardless of its concentration is NOT able to reach bacteria trapped inside accumulations of mud. We strongly advise that a bacterial test NOT be taken until the well has been thoroughly flushed.

The well can be disinfected by adding chlorine. One gallon of 5.25 percent sodium hypochlorite (i.e. common store bleach, such as Clorox) in 1,000 gallons of water will provide a good disinfecting solution of 50 parts per million (ppm). The quantity of water inside the casing of dug wells is shown below:

Volume in Wells (Gallons)
Diameter 1' 2' 3' 4'
Water Depth
15 60 130 230
30 120 260 460
60 240 520 930
120 470 1060 1880

The volume inside the well casing does not include the volume of water readily available in the crushed stone that normally surrounds the well. As a rough rule of thumb, we suggest doubling the calculated volume.

The procedure outlined below should be followed when you disinfect your well. Add the chlorine directly to the well plus an extra amount for the water in the crushed stone beyond the casing. Mix by using a strong flow of water through a clean garden hose recirculated directly back into the well. Run each faucet in your home until a chlorine smell is detectable. Close the faucets and allow the chlorine to stay in the well and the plumbing 12-24 hours.

Chlorine will not work its way uphill against the general movement of the water table, downhill. If your well's construction is "perfect" and if you still have bacterial problems, try digging one or two small holes into the gravel "upstream" of the well and adding a dilute chlorine solution directly into the ground. Refill holes with fine grain soil. Then, flush that area with water from a hose to disperse the chlorine down to the water table. After 12-24 hours, flush chlorine from the system. Flush onto the ground but not into a stream or pond.

Testing New Wells for Water Quality
After ALL the chlorine is flushed from the well and plumbing system, a sample can be taken for bacterial and chemical testing.

Remember all chlorine must be flushed from the well. Water quality samples with any chlorine residual can NOT be tested for bacteria. The presence of chlorine can be checked by using chlorine test kit. Persons with larger swimming pools normally have such test kits.

For persons desiring to use the DES laboratory, the Legislature has established an $10 fee for a bacterial test and an $75 fee for "standard analysis." Only agency sampling containers can be used. The time to process the standard samples is approximately three weeks in the summer and two weeks otherwise. Different containers are required for sampling radon gas and industrial solvents.

Sampling containers can be obtained from the Laboratory Services Unit. (See DES fact sheet WSPCD-WSEB-1996-12 for a discussion of which water quality factors to test in private well.) We recommend that a bacterial sample be taken on an annual basis for dug wells in view of their shallow construction and casing materials.

When inspecting an existing dug well, look for any defects or openings in the casing which will allow foreign substances or small animals to enter the well. Also look for points where surface run-off can enter the well casing directly.

Existing dug wells, especially those built of field-stone, are frequently subject to construction deficiencies which allow surface run-off, carrying bacteria and virus, to enter the well unimpeded. To prevent this problem, all joints between the field stone should be mortared and then the entire outside surface of the well should be uniformly coated with cement mortar so as to provide a smooth, one piece exterior surface. This sealing should extend down below the ground surface to the extent that conditions permit.

Typically in older wells, the backfill around the well casing has settled. Where concrete or V.C. tile is used, the top two joints should be sealed if possible. Once completed, soil should be mounded up around outside of the well as shown previously.

Wood covers are another common dug well deficiency in that they are susceptible to rotting. Further, water and debris can fall between the slats and into the well. We recommend a solid one-piece concrete cover. The cover should extend beyond the well casing. The backfill next to the well casing should be mounded.

To achieve reliable bacterial water quality, the well construction MUST exclude ALL dust, dirt, surface water, and animals. If the well is not tight and if there is not effective filtration of every drop of water entering the well, the well is NOT safe.

Department of Environmental Services
Licensing Well Drillers, Well Design Code
Water Well Board
PO Box 2008, 64 N. Main St.
Concord, NH 03301

Water Quality Testing & Sample Containers
DES Laboratory Unit
PO Box 95, 6 Hazen Dr.
Concord, NH 03302-0095
271-3445 and 271-3446

Water Quality Analysis, Discussions of Treat- ment Options
Water Supply Engineering Bureau
PO Box 95, 6 Hazen Dr.
Concord, NH 03302-0095