Wells and Pumps for Greenhouses
Wells can be constructed in a number of ways. The most common drilling techniques are rotary, reverse rotary, air rotary, and cable tool. Auger drilling is often employed for shallow wells that are not used as supply wells. In unconsolidated and semi-consolidated materials, (reverse) rotary and cable tool methods are most commonly employed. Hard rock wells generally are drilled with air rotary drilling equipment. During drilling, drillers must keep a detailed log of the drill cuttings obtained from the advancing borehole. In addition, after the drilling has been completed but before the well is installed, it is often desirable to obtain more detailed data on the subsurface geology by taking geophysical measurements in the borehole. Specialized equipment is used to measure the electrical resistance and the self-potential or spontaneous potential of the geological material along the open borehole wall. The two most important factors that influence these specialized logs are the texture of the formation and the salinity of the ground water. Sand has a higher resistance than clay, while high salinity reduces the electrical resistance of the geological formation. Careful, professional interpretation of the resistance and spontaneous potential log and the drill cuttings’ description provides important information about water salinity and the location and thickness of the aquifer layers. The information obtained is extremely useful when finalizing the well design, which includes a determination of the depth of the well screens, the size of the screen openings, and the size of the gravel pack material.
Well System Components
A water well is a hole, shaft, or excavation used for the purpose of extracting ground water from the subsurface. A well consists of a bottom sump, well screen, and well casing (pipe) surrounded by a gravel pack and appropriate surface and borehole seals (See Figure 13.2).
At the surface of the well, a surface casing is commonly installed to facilitate the installation of the well seal. The casing (or well pipe) is a very critical element in well construction. The casing serves as both a housing for the pump and as a vertical conduit through which water flows upward from the intake portion of the well to the level where it enters the pump. The well casing maintains the borehole at a specified size, to prevent collapsing of the borehole and prevents fine and very fine formation particles from entering the well.
The bottom of the casing must be fitted with a well screen which allows water to enter the well freely but prevents the entrance of coarse sand. Wells can be screened 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. Hard rock wells, on the other hand, are constructed very differently. Often, the borehole of a hard rock well will stand open and will not need to be screened or cased unless the hard rock crumbles easily.
The annular space between the well screen, well casing, and borehole wall is filled with gravel or coarse sand (called the 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; where those occur in a formation it is best to use blank casing sections.
Before the well can be put into operation, it must be developed. The purpose of well development is to provide sand-free water at maximum capacity. The drilling operation alters the hydraulic characteristics of a water-bearing formation in the immediate vicinity of the well caused by compaction, relocation of natural fine materials and/or migration of drilling fluids into the formation. As a result local permeability and hydraulic conductivity may be severely reduced, restricting water flow into the well. Development of a well reduces compaction and removes fine material from the pore space near the well.
Once the well is completed and developed, it is a good practice to conduct an aquifer test (or pump test). For an aquifer test, the well is pumped at a constant rate or with stepwise increased rates, while the water levels in the well are checked and recorded. The groundwater level is recorded, and the equilibrium point has been reached when the water level has become constant. This level is the dynamic water level.
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