Typical Well Construction
Surface water poses problems for private systems
Surface water sources require consideration of additional factors not usually associated with ground water. The risk of contamination of surface water is great, and should be used only when ground water sources are not available. It should only be used for irrigation and similar uses, not for human or animal consumption.
Obviously then, a private household system should rely on ground water. Ground water is found in underground strata layers known as water bearing formations; water has gone through extensive filtering over time to reach the aquifer layers. The nature of these formations is shown below.
The surface layer in most areas is topsoil. Water passes through it easily. Next, there may be a layer of clay or hardpan. Either material is so tightly packed that it resists water flow. Beneath this stratum, or in place of it, there may be more porous or permeable material. Finally comes bedrock which is a water bearing formation that contains fractures and aquifer reservoirs.
Water bearing formations and the water table
There are two basic types of water bearing formations. One is sand, or a mixture of sand and gravel; the other is solid rock, interlaced with crevices or fissures. In the case of a sand & gravel formation, the space between individual particles is filled with water. In a rock formation, the water is found in fissures and cracks.
The water table is a more or less continuous surface, below which the formation is saturated with water. Depending upon the area, water table depth can vary from a few feet to hundreds of feet. It tends to follow the contours of the land, sloping downward from hilly areas to low points. This water table is usually referenced in supplying springs/dug wells.
Common Water Well System Design Cross-section
Basic types of wells
Water wells are classified according to the method used to build them. Springs (dug wells)are simply made by excavating a hole several feet in diameter to a fairly shallow depth. The circular hole is lined with concrete tiles to prevent cave-in. Because depth is limited to 25-30 ft, a dug well is practical only when the water table is near the surface of the earth. For this reason, there is a strong likelihood of contamination by seepage drainage and occasion to fail due to drought. In many areas, dug wells are prohibited by law -VT and NH do not have this law. Click here to view a brochure regarding proper spring well construction.
Drilled wells
Drilled wells are used when greater depth or volume are needed to supply the building needs. Wells are commonly drilled today by rotary well drilling; a rotating bit is fixed to the end of the drill rod and chews into the rock or other earth materials. Air or water are used to transport the cuttings to the surface.
Some wells reach copious surface water above bedrock, and are called "gravel" wells. They tend to have the same issues as springs and are not recommended.
The "bedrock" drilled hole is lined with steel pipe casing to keep the well from caving in and protects the ground water source from being contaminated by the surface water. The States of Vermont and New Hampshire require that the well casing and sanitary well seal (attached to the bottom piece of steel casing) must be hammered into at least 10 ft of good bedrock to be a legally viable bedrock well. The space between the borehole and outer walls of the casing are sealed with a cement or clay grout.
Once the casing has been "set", a smaller bit is used to continue drilling until adequate water is found via a fracture or access to an aquifer.