Wells can vary a great deal in depth required, flow rate, and water quality. Before buying a lot with a well in place, you should get the reported flow rate in writing and an up-to-date water quality report, readily available from most municipal health departments. Ask the local health inspector about their water testing procedures, which are usually inexpensive and well worth the cost and effort. If problems are detected with the water quality, these need to be identified and resolved before proceeding.
Also pay attention to the flow rate. While a good flow rate today does not guarantee a good flow rate in other seasons, an experienced local well driller should be able to give you a good idea about the flow characteristics of wells in your vicinity and alert you to any potential problems.
If there is no well in place, you should talk to at least two well drillers in the area, as well as neighbors, about well depths, flow rates, and water quality in the area. The local water management office may also be able to assist you with information about local groundwater. This will provide a good indication of whether and how you will be able to meet your expected water needs.
For example, if you need a lot more water than your nearest neighbors, you may need to drill deeper or drill more than one well. If a very deep well is needed, you will need to budget accordingly. However, well drilling is based on an educated guess of underground conditions — there are no guarantees.
Aquifers are natural areas of water storage under the ground. While many people think of aquifers as underground pools, they typically consist of areas of permeable sand and gravel, and cracks in solid rock where water is able to slowly flow. How much water the rock and granular soil can hold depends on how much open space exists between the particles, called its porosity. This ranges from 0 in solid rock to about 30 or 40 percent in gravel and sand.
Aquifers have a bottom, where the soil or rock is impermeable, and a top, called the water table. The water table may rise to the surface of the earth at a natural spring, or at a pond or lake, but usually it is well below the surface. Water tables can be a few feet thick or hundreds of feet from top to bottom and can range in area from the size of a pond to hundreds of miles across. Aquifers are replenished primarily by rainwater that percolates though the soil.
Unconfined vs. Confined Aquifers. In an unconfined aquifer, the top of the aquifer is the water table and the water must be pumped upward (see Illustration). If an aquifer is confined between bedrock and another layer of impermeable material, such as clay, the water may be under pressure and rise above the aquifer when drilled into. In some cases, the water will rise all the way to surface without a pump, creating an artesian well.
To reach the aquifer, you may need to drill 30 feet or 500 feet. Aquifers tend to follow the slope of the land above and water tends to move slowly through the aquifer following the slope. The rate of flow depends largely on the permeability of the rock and other material in the aquifer. When water is pumped from the aquifer by a well, it is drawn down in a area around the well. The more permeable the material in the aquifer around the well, the faster the well area will be replenished and the greater the well’s flow rate.
Depending on the geology, there may be and adequate supply of pure water relatively near the surface, or you may need to drill into deep water-bearing rock to get adequate flow and flow. Based on their knowledge of local conditions, an experienced well driller or hydrologist can help determine the best strategy.
SITING A WELL
A knowledgeable well driller, or if necessary, a hydrological engineer or consultant, should be consulted to help site your well. Your goal is to determine what location, depth, and well diameter is most likely to provide the best flow rate and water quality for the least money. In some cases, the building code will specify a minimum depth, setback from the property line, and distance from septic systems and other sources of contamination. Some require the well to be uphill from the septic system, a commonsense idea, but not always feasible on small lots where you will inevitably be downhill from someone else’s septic system if not your own. Regardless of code requirements, you will want to follow the a professional’s recommendations for a safe and dependable water source.
There are also practical concerns about distance from the house and access of drilling equipment and for maintenance. In addition, you may have preferences, such as not wanting the well in your front yard or in the middle of your future garden – so make sure you approve of the well site before drilling begins.
In some rural areas, homeowners still get their water from shallow, dug wells. Historically, these were dug by hand to just below the water table and lined with fieldstone, brick, or interlocking concrete tile (in newer wells), and capped with wood, stone, or concrete cover. They were typically 3 or 4 feet in diameter and typically from 10 to 30 or so feet deep, but in some cases were over 50 ft. deep. In the old days, a bucket or hand pump was commonly used, but most now use an electric pump.
Dug wells typically penetrate into just the top layer of the aquifer or into a small perched water table, which is separated from the larger aquifer below by an impermeable layer of soil. Water levels in these wells tend to fluctuate with seasonal variations in the water table and may dry up during extended dry spells. Modern shallow wells are typically excavated or bored with power equipment and lined with concrete tile. Because these can go deeper into the water table, they can produce a more reliable water supply than hand-dug wells.
Because traditional dug wells typically draw on shallow groundwater and lack a continuous casing, they are highly susceptible to contamination from nearby septic systems and from surface sources such as agricultural runoff, lawn chemicals, and other pollutants. However, dug wells can produce high-quality water they are designed and located properly. They should have a sealed casing and be located away from ponds or streams, and uphill from septic systems, livestock, and fuel tanks (see illustration).
For safety and good performance, dug wells should
• be dug during the dry season to reach deep enough into the water table
• have a watertight casing, such as large-diameter precast-concrete pipe, which sits on a sand or gravel base at the well bottom
• be located on high ground, not subject to flooding, and sloped away in all directions from the well
• have a tight-fitting cover at the top of the casing, about one foot above the ground
• have a pump in the inside your house or in a pump house to protect equipment, storage tank, and piping
• use a pitless adapter in cold regions to provide a frost-proof, sanitary seal to the water line
• be 100 feet away from farm animals, cesspools, or leach fields
• be 75 feet away from property you do not control
• be 25 to 50 feet away from drainage ditches or culverts
Where conditions are right, driven wells are an economical alternative. These are often used for cabins or vacation cottages where there is groundwater within about 15 feet of the surface. They are made by driving a small-diameter pipe, by hand or power tools, into water-bearing sand or gravel that lies above the bedrock. A screened well point, designed to filter out sediment, is typically attached to the bottom of the pipe before driving. Under favorable conditions, the pipe can be driven up to about 30 feet by hand, and 50 or more feet with a power driver.
There are a variety of ways to complete a driven well. Often the connection to the discharge pipe is made underground using a pitless adaptor. The access pit must be dug around the well to below the frost line in cold climates. Depending on local codes the pit can later be backfilled or can be lined with concrete tile and cap, similar to a dug well. The pump may be located inside the home or on top of the well within the concrete tile or a pump house.
Because these wells are simple and inexpensive, some people drill two and plumb them together for a better yield. However, like dug wells, they reach only shallow water, making them susceptible to contamination from surface sources.
Over 15 million homes in the US draw their water from private wells and millions more use water drawn from municipal wells. Nearly all of these are drilled wells (see illustration). Most modern wells are drilled by rotary drill machines, which can easily drill though solid rock. In some cases, a cable-tool drilling rig is used, also referred to as a percussion or “pounder” machine. In this technique, a heavy bit attached to the bottom of a wire cable is repeatedly raised and dropped, pounding its way down through the earth. Most private wells range from 100 to 500 feet deep, but in rare cases can exceed 1,000 feet.
The well design will depend on the specific geological conditions at the site. Most wells are drilled an inch or two wider than the casing diameter, which is commonly 6 inches. In the upper portion of the well, unless it is solid rock, the space between the casing and the drilled hole is later “grouted” with cement or bentonite (a special expansive clay). This seals the drilling hole to prevent contaminated surface water from migrating down the well casing to the aquifer. Depending on conditions, the bottom of the well may be drilled into solid rock, drawing its water from cracks in the rock.