A water well is a hole, shaft, or excavation used for the purpose of extracting ground water from the below ground. Upon excavation of the well shaft the water may have to pressurize to get it to the surface or it may flow naturally. When this well flows naturally this is known as an artesian well. The wells purpose usually determines location of a well. The most important considerations for drinking and irrigation water-production wells are ground water quality and long-term groundwater supply.
The criteria for portable water quality for drinking water wells are based on local or state drinking water quality standards. The ground water production must be able to meet the pumping requirements of the well. For an average small to medium size community, water wells production systems may produce from 100 to 500 gpm.
Upon the determination of a well location, a preliminary well design is completed. For many large production wells, an observation well is drilled before to obtain more information about the depth of water-producing zones, confining beds, well production capabilities, water levels, and groundwater quality. The final design is subjected to observations made at site in the drilling of the observation well. The overall objective of the design is to create a structurally stable, long-lasting, efficient well that has enough space to house pumps or other extraction devices, allows ground water to move effortlessly and sediment-free from the aquifer into the well at the desired volume and quality, and prevents bacterial growth and material decay in the well.
A well consists of a bottom sump, well screen, and well casing (pipe) surrounded by a gravel pack and the appropriate surface and borehole seals. Water enters the well through perforations or openings in the well screen, which runs continuously along the bore or at specific depth intervals. The latter is necessary when a well taps multiple aquifer zones, to ensure that screened zones match the aquifer zones from which water will be drawn. In alluvial aquifers, which commonly contain alternating sequences of coarse material (sand and gravel) and fine material, the latter construction method is much more likely to provide clean, sediment-free water and is more energy efficient than the installation of a continuous screen.
In the well the screens purpose is to keep sand and gravel from the gravel pack out of the well while allowing ample water flow to enter the casing. The screen should also be designed to allow the well to be properly developed. The most common types of screens used are Slotted, louvered, bridge-slotted screens and continuous wire wrap. Slotted screens provide poor open area, which are not suited for proper well development and maintenance and is therefore not commonly recommended. Wire wrap screens or pipe-based wire wrap screens have the best performance but is more expensive, the additional cost of wire wrap screens can be offset by installing the screens sections in the most productive formations in the borehole.
The blank well casing between and above the well screens helps to prevent fine and very fine formation particles from entering the well, provide a route from the aquifer to the surface, provision of proper housing for the pump, and to protect the pumped ground water from interaction with shallower ground water that may be of lower quality. The annular space between the well screen, well casing, and borehole wall is filled with gravel or coarse sand known as a gravel pack or filter pack. The gravel pack prevents sand and fine sand particles from moving from the aquifer formation into the well. The gravel pack does not exclude fine silt and clay particles. The uppermost section of the annulus is normally sealed with a bentonite clay and cement grout to ensure that no water or contamination can enter the annulus from the surface. The depth to which grout must be placed varies by county.
Once the well is completed and developed, it is a good practice to conduct an aquifer test (or pump test). A pump test requires that the well is pumped at a constant rate or with stepwise increased rates over a period of 12 hours to 7 days, while the water levels in the well are checked and recorded frequently as they decline from their static water level to their pumping water level. A pump test determines the efficiency and capacity of the well. The information about the pumping rate and resulting pumping water levels plays a critical part in properly sizing pump sets