GROUND WATER SOURCES & TREATMENT, PART II

Well classification is usually based on:
a) type of aquifer from which they derive the water
b) the type of well construction
c) or on a "combination" of type of construction and aquifer

WELL COMPONENTS: Well structure, the pump, and discharge piping.
Well Head Base and Seal (pump pedestal) is the concrete that is formed at the ground-level to:
a) support the motor and/or pump (depending on the type of installation)
b) and to prevent surface water from entering the well
It is to be a minimum of 18 inches above the finished grade around the well bore.
The well casing is the piping-type component that lines the "borehole" to
a) prevent contamination from surface waters, etc
b) prevent the surrounding soils/gravels from collapsing into the hole.

Grout Seal is a concrete grout that is pumped between the soil/gravels of the surrounding borehole and the outer casing. (Occasionally, an outer conductor casing also has to be installed for well protection from surface waters that are less than reliable in water quality, or due to material structures that are unstable.) It is an additional measure taken to prevent the introduction of surface waters into the well and aquifer. It is usually specified as a seal to be installed from the surface to a depth of at least 50 feet.

The Column Pipe is placed into the well casing to convey the water to the surface.

Gravel packed wells feature an area of gravel placed between the well casing and the inner wall of the drilled hole. This gravel reduces the velocity of the approaching water through the perforations or well screen which in turn reduces the potential for fine sands to be drawn into the well. The gravel is most always 6 times to diameter of the sands around the well hole, and graded for uniform size.

The Sounding Tube is used to determine the depth of water. It is placed into the space between the column pipe and the well casing (or between the column pipe and inner casing for some gravel packed wells) to provide access for a sounding device which allows for the determination of the water level.

Casing Perforations allow the passage of water from the aquifer into the well borehole casing.

The Well Screen allows the passage of water into the tailpipe and then up into the pump.

TYPES OF WELL CONSTRUCTION:
a) Shallow wells are commonly dug or driven wells and deep wells are usually drilled.

b) Dug well consists of a vertical hole, usually four to six feet in diameter, which is excavated through top soil until the water bearing stratum is encountered. Such wells may be lined with concrete, brick, rough stone, or vitrified tile. That portion of the lining ex-tending from about one foot above the ground surface to at least ten feet below the surface should be watertight to exclude any surface drainage. Concrete is the best material for the upper part of the well; brick, rough stone or vitrified tile is usually used for the pervious section which extends down into the water bearing stratum.

c) Driven wells may be used when ground water is within about twenty-five feet of the ground surface and when there are no boulders or intervening rock formations. Such wells are easily constructed and can be protected against superficial pollution, although they are more subject to pollution than deep wells penetrating impervious material. The simplest type of driven well consists of a perforated brass strainer which is cone shaped at the end and which is attached to the bottom of an iron pipe. The pipe is driven through the upper soil layers and into the water-bearing material.

d) Cable Tool (percussion) is a type of well construction characterized by the use of a reciprocating steel bit that crushes the rock and soils by its pounding action. The pieces of rock are removed from the borehole periodically by bailing them out. The bit is suspended by a cable from the drilling rig. Best for coarse gravel aquifers, as the finer materials fall into the borehole.

e) Deep Wells are usually employed when the top soil strata are too impervious or do not contain ground water, wells must be either drilled into rock to obtain water from crevices or through the soil to deeper water-bearing strata (aquifers). Although the elimination of nearby sources of pollution is important, it should be remembered that polluting material may pass through fissured rock for great distances without the improvement in the quality of water which would result from filtration through the subsoil.

f) Drilled Wells are four to 12 inches in diameter, although in some instances they may be larger.
There are several methods, but the two basic ones are:
1) Direct Rotary Drilling Method: The rotary bit is sent down a pilot hole to make the 4 inch pilot hole the size of the future hole. Water and drillers mud are pumped down the drilling shaft into the bit, where the water and driller’s mud carry the material being removed to the surface. During the drilling operation "electric logging or e-logs" and samples of the materials being drilled out are taken. This information helps determine the placement of the screens or perforations which will allow the water into the well casing. If the surrounding aquifer material is of a finer size, then gravel is placed between the casing and the surrounding earth.
2) In Reverse Circulation the driller’s mud (the use of the additives) is not used. Instead existing water in the aquifer moves down the outside of the drill shaft, and is then brought up through the bit and then to the surface via the drill shaft. The well casing is usually set as in the direct method.

Metal casings can provide effective protection against the entrance of surface water and polluted ground water, if the casing is sealed to prevent the entrance of all sources of pollution. When the water-bearing material is sand or gravel, a well screen is usually attached to the bottom of the casing. Since water can only be lifted a maximum of about 20 feet by suction, the pump must be located below the ground surface near, or below, the water level in the well.

There are obviously design factors of the water production flow rate, and it’s required column pipe diameter, minimum casing diameter, casing perforations type/sizes and perforation surface area in each water-bearing strata, well screen type/size, entrance velocities into the well screen, maintenance activities and such that must be taken into consideration.

Gravel Packed Wells:
Gravel packed wells are often used when the water bearing stratum contains fine sand. Such a well is similar to a standard well except that for several inches around the well screen the sand has been removed and replaced with gravel. This type of construction increases the area of contact with the water bearing stratum. Resistance to the flow of water into the well area is decreased and the capacity is increased. The gravel outside the screen also helps to prevent sand from being drawn into the well during pumping. The overall operating results of a gravel packed well have been very satisfactory and some authorities advocate gravel packing as standard practice when tapping any water bearing stratum of un-consolidated material. The actual placing of the gravel may be done in a number of ways but requires skill and experience on the part of the well driller.

Springs occur where a water-bearing stratum reaches the surface of the ground or where fissures in rock "outcrop" at the surface under conditions so that ground water is forced through the fissures. The first type of spring is usually of local origin and great care must be taken to eliminate nearby sources of pollution. The origin of the water flowing from a rock spring is difficult to ascertain without detailed knowledge of the geological formations in the area.

Many methods have been developed to collect spring water. When the water does not emerge from the ground at a well defined point, it may be collected and carried to a well or basin by tile lines with open joints laid in ditches at right angles to the direction of underground flow. The tile is surrounded with crushed stone or gravel and the ditch is filled with clay to keep out any surface drainage.

Regardless of the type of construction, all spring basins should be covered, and the water source should be permanently protected by a structure of concrete or other impervious material so all water except that issuing from the spring will be excluded.

Surplus water should be piped from the covering structure in such a manner that surface water cannot enter the spring even during floods. It is not necessary to ventilate spring structures; all openings should be covered. Inspection manholes should be fitted with tight locked covers.

Local Factors:
The local factors of the aquifer type, depth, aquifer material type and material size, hardness of the rock formations that contain or exist between the surface and the aquifer desired to tap, quantity of water required and that which the aquifer can deliver, types and sizes of drilling equipment available and other local factors play an important part in deciding the type of construction and well installation that you will have.

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