Wells

views updated

Wells

A well is a hydraulic structure for withdrawal of ground-water from aquifers. A well field is an area containing two or more wells. Most wells are constructed to supply water for municipal, industrial, or agricultural use. However, wells are also used for remediation of the subsurface (extraction wells), recording water levels and pressure changes (observation wells), water-quality monitoring and protection (monitoring wells), artificial recharge of aquifers (injection wells), and the disposal of liquid waste (deep-well injection ). Vacuum extraction system is a new technology for removing volatile contaminant from the unsaturated zone, in which vapor transport is induced by withdrawing or injecting air through wells screened in the vadose zone .

Well construction consists of several operations: (1) drilling; (2) installing the casing; (3) installing a well screen and filter pack; (4) grouting; and (5) well development.

Various well-drilling technologies have been developed because geologic conditions can range from hard rock such as granite to soft, unconsolidated geologic formation such as alluvial sediments.

Selection of a drilling method also depends on the type of the well that will be installed in the borehole, such as a water-supply well or a monitoring well. The two most widely used drilling methods are cable-tool and rotary drilling. The cable-tool percussion method is a relatively simple drilling method developed in China more than 4,000 years ago. Drilling rigs operate by lifting and dropping a string of drilling tools into the borehole. The drill bit crushes and loosens rock into small fragments that form a slurry when mixed with water. When the slurry has accumulated so the drilling process is significantly slowed down, it is removed from the borehole with a bailer. In rotary drilling, the bore-hole is drilled by a rotating bit, and the cuttings are removed from the borehole by continuous circulation of drilling fluid. Boreholes are drilled much faster with this method, and at a greater depth than with the cable-tool method. Other drilling methods include air drilling systems, jet drilling, earth augers, and driven wells.

Though well design depends on hydrogeologic conditions and the purpose of the well, every well has two main elements: the casing and the intake portion or screen. A filter pack of gravel is often placed around the screen to assure good porosity and hydraulic conductivity. After placing the screen and the gravel filter pack, the annular space between the casing and the borehole wall is filled with a slurry of cement or clay. The last phase in well construction is well development. The objective is to remove fine particles around the screen so hydraulic efficiency is improved.

A well is fully penetrating if it is drilled to the bottom of an aquifer and is constructed in such a way that it withdraws water from the entire thickness of the aquifer.

Wells are also used for conducting tests to determine aquifer and well characteristics. During an aquifer test, a well is pumped at a constant discharge rate for a period of time, and observation wells are used to record the changes in hydraulic head, also known as drawdown. The radius of influence of a pumping well is the radial distance from the center of a well to the point where there is no lowering of the water table or potentiometric surface (the edge of its cone of depression). The collected data are then analyzed to determine hydraulic characteristics. A pumping test with a variable discharge is often used to determine the capacity and the efficiency of the well. A slug test is a simple method for estimating the hydraulic conductivity of an aquifer, a rapid water level change is produced in a piezometer or monitoring well, usually by introducing or withdrawing a "slug" of water. The rise or decline in the water level with time is monitored. The data can be analyzed to estimate hydraulic conductivity of the aquifer.

The predominant tool for extracting vapor or contaminated groundwater from the subsurface is a vertical well. Howerver, in many situations where environmental remediation is necessary, a horizontal well offers a better choice, considering aquifer geometry, groundwater flow patterns, and the geometry of contaminant plumes. Extraction of contaminated groundwater is often more efficient with horizontal wells; a horizontal well placed through the core of a plume can recover higher concentrations of contaminants at a given flow rate than a vertical well. In other cases, horizontal wells may be the only option, as contaminants are often found directly beneath buildings, landfills, and other obstacles to remedial operations.

[Milovan S. Beljin ]