Water Wells

More than half the world’s population depends on groundwater for daily water supply requirements. Virtually all groundwater is obtained from wells, specifically, production water wells.

Overview

A water well is constructed for the sole purpose of obtaining water from an aquifer. Other wells that are not water sources include observation, monitoring, and piezometer (for pressure measurement) wells. Some aquifers can yield groundwater in only minimum quantities per day, perhaps meeting the requirements of a single household. Other aquifers can support a cluster of production wells—for example, ten generously spaced wells, each with a safe capacity of 16,000 cubic meters per day (about 15.9 million liters, or about 4.2 million U.S. gallons). Such a cluster, or well field, of ten wells would supply the needs of a city of some 200,000 residents.

However, the safe yield of water wells is not solely a function of the capability of the aquifers they tap. The suitability of well design and the quality of well construction are also critical. In the science of geohydrology, the quality of a production water well, without respect to the character of its aquifer, is known as well efficiency. Some wells have an efficiency of 100 percent, and some have an efficiency of only 40 percent. A well at 40 percent efficiency would have a safe yield two and one-half times greater if both design and construction were executed as a perfect match with the geologic character of the aquifer being tapped. That is to say, a well is 100 percent efficient when all aspects of design and construction are matched to the particular aquifer, and efficiency decreases as the extent of mismatching increases.

Aquifer Performance

However, perfection in well design and construction will not make a poor aquifer into a good one. The performance of an aquifer is totally dependent upon its hydrogeologic setting and its physical character. An aquifer’s physical character is judged scientifically by its hydraulic conductivity, thickness, and areal dimensions. For example, a water-bearing sandstone with a consistent thickness of 50 meters throughout a county-sized area might support twice the quantity that it would if its thickness were 25 meters. Other aspects of an aquifer’s physical character are also involved.

How a good production well operates in its relation to an aquifer is quite straightforward. Take as an example the case of a well that penetrates a fully confined aquifer, say, 1,000 meters deep. At the time the construction is completed and before any pumping has commenced, the well’s static (nonpumping) water level is entirely determined by the natural water pressure within the aquifer. Last year’s rainfall has no bearing on that confined fluid pressure. This scenario is an artesian situation. The well might actually flow without any pumping due to the pressure exerted by the aquifer structure on top of the water it contains; however, it is assumed that the water in this well stands steady at a depth of 30 meters. This assumption means that the fluid pressure in the aquifer must be sufficient to boost the water up the well to within 30 meters of ground surface. In this example, the water in the aquifer must have a confined fluid pressure of nearly 100 kilograms per square centimeter.

At the moment pumping commences, whatever the rate of withdrawal, the well enters a nonequilibrium condition, whereas, prior to pumping, the condition was at equilibrium (also called steady state). The pumping water level starts to decline, rather quickly at first and then ever more slowly. At the 1,000-meter depth, groundwater in the aquifer immediately reacts to the loss of pressure in the well and begins to migrate within the aquifer toward the well’s intake area. That incoming water replaces the water that has been withdrawn to the ground surface. The groundwater migration toward the well proceeds radially inward, like spokes on a wheel. The pumping water level in the well continues to fall as long as nonequilibrium conditions prevail, perhaps for hours or even weeks. Eventually, the pumping water level stabilizes, and at that point in time hydraulic equilibrium has been reached. The moment that pumping of this hypothetical well is stopped, nonequilibrium conditions again prevail. Now, however, the situation reverses, and the well’s water level rises, rapidly at first and then ever more slowly, until the well has fully recovered to its original state. Hydraulic equilibrium has then been reached again.

The scenario is very different in the case of wells that tap unconfined aquifers, such as alluvial sands in river valleys or in deep basins. In unconfined aquifers, the loss of water level in a well upon start-up of pumping is usually at a slower pace due to the relative ease of movement of water through the saturated zone. So, too, is the rate of recovery when pumping ceases, due to the lack of excess pressure inherent in an unconfined aquifer. In unconfined aquifers, withdrawals of groundwater result in the draining of a cone-shaped portion of the aquifer itself, known as the cone of depression; that is, the water table actually declines in a cone-shaped area around the production well. It is this draining that causes slower responses because the act of dewatering requires time. Confined aquifers, in contrast, react to pumping with pressure changes only. Such aquifers remain full of water continuously.

Well Drilling

The initial major task in well construction is the drilling operation. The drilling plan often depends upon the area’s geological configuration. For example, where there is no prior record of drilling in an area, there may be no known aquifer at any depth. In that case, it would be folly to drill a full-diameter borehole initially. A slim hole, perhaps 10 centimeters in diameter, is sufficient to make a determination of the geology and of the presence or absence of an aquifer. The lower-cost slim hole, then, can be reamed to full well diameter once an aquifer has been identified, sometimes even evaluated as to hydrologic performance. If the initial drilling reveals no aquifer, the site is wisely abandoned with manageable loss from the cost of slim-hole drilling. This case is analogous to a wildcat oil test drilled far from any existing production.

When a specific aquifer is either known or has been discovered by means of the slim-hole drilling, the diameter of the final borehole is then largely determined by the anticipated pump size, because the hole must accommodate the casing and the casing must accommodate the flow of water to be extracted. High-capacity wells, especially where the pumping water level is deep, require large-diameter pump turbines, and correspondingly large well casings. Domestic wells for household water need pumps of only minimum diameters. Such wells can be designed and constructed with casings of only fifteen centimeters or less, seldom more than 7.5 centimeters in diameter, whereas wells for large industrial or public water supplies may need to have inside diameters of thirty or even forty centimeters to ensure adequate water flow for high-performance pumps.

It is ordinarily impractical simply to increase the diameter of a well in the expectation of greatly increasing the maximum yield of the well. Assume, for example, a well ten centimeters in diameter is tested and found to have a safe capacity of two liters per second. The water supply requirement, however, may be four liters per second. For that same well to have a safe capacity of four liters per second, solely by enlarging the diameter, the resulting well would require a diameter of about one meter. Therefore, it is more economical to have multiple wells of smaller diameter than to attempt to achieve the same total water supply with one huge well. The multiple-well plan is also advantageous for geohydrologic reasons, so long as the wells have adequate spacing between them so as not to interfere with each other. Optimal spacing is a calculation performed professionally by geohydrologists.

In some geologic situations, production water wells can be designed and constructed with neither casing nor well screen in the lower portion of the well. For this less costly design to be feasible, the rock left with open drill holes must be physically competent, such as limestone or granite. In all situations, however, the upper portion of the borehole requires a casing, primarily to protect the well against surface contaminants and also to provide for a stable wellhead. Surface casing requires positive sealing between the outside of the casing and the drilled borehole, as by cement grout.

In geologic situations where the borehole penetrates any soft sediment or rock lacking the strength to sustain itself with open-hole construction, a full-length casing is essential. A well screen is set in that portion of the borehole that penetrates the aquifer. Commercial well screens are fabricated in various designs and different metals, typically bronze or stainless steel. Below a well screen is a short section of blank casing known as the “rat hole,” which is set in the geologic material under the aquifer. The rat hole serves as a settling and collecting zone for fine-grained material that may enter the well during its lifetime.

Well screens are not filters and cannot serve as filters. Such a function would cause the well to cease its normal yield of water after a few weeks of service, due to flow-inhibiting deposits of sediments accumulating on the screen, and the well would require expensive rehabilitation at frequent intervals. Screens are for the sole purpose of giving the well physical integrity at aquifer depth, allowing water to pass readily from the aquifer into the well point while preventing other materials from doing so, and preventing weak aquifer material, such as sand, from collapsing into the well bore. The too-common practice of using casing with sawed slots or bored holes invariably results in poor well efficiency and usually dooms the well to a short life.

Well Development

The final stage of construction for most wells with screens is known as well development. The purpose of well development is to enhance the well’s safe yield by increasing the permeability of the aquifer immediately outside the well. Fine-grained aquifer material is stimulated to enter and pass through the screen, thereupon to be removed from the well. This has the effect of clearing debris from the pores and interstices of the aquifer material, and effectively maximizing the pore size and, thus, the permeability of the aquifer material. The reasoning behind the maneuver is easily understood. While a pump is extracting water from the well, groundwater in the aquifer is moving radially inward to replace the water that is being withdrawn. As the specific quantity of groundwater moves closer and closer to the intake area of the well, the space it occupies within the aquifer becomes ever more confined. Therefore, the water’s velocity must increase. (An analogy would be water flowing in a pipe of fixed diameter. Assume the pipe has a dent in it that reduces its diameter. When the water passes into the restricted place, its velocity increases until the restriction has been passed.) The removal of silt and fine sand from the aquifer just outside the well has the effect of locally increasing the permeability of the aquifer, which, in turn, helps to promote the increased flow velocity. Performance of wells can sometimes be improved 100 percent by the standard practice of applying well-development techniques, used by all good well contractors where the geologic conditions favor such treatment.

Because wells that flow from natural pressure are generally rare, most wells are useless until fitted with an appropriate pump. Where the standing water level in a well is no deeper than about seven meters, the well can be pumped by suction for supplying small domestic needs. Suction pumps remove a portion of the prevailing atmospheric pressure within the pump pipe, thus causing the water in the pump pipe to rise to ground surface. They are sometimes referred to as shallow-well pumps, even though the depth of the well has nothing to do with the feasibility of using suction pumps. Shallow-well pumps, such as for domestic use, are typically either suction pumps that lift water by generating a pressure differential within the system, or “jet” pumps that utilize the movement of a small stream of moving water to induce the movement of a larger amount of water.

By far, most well pumps are of the type where the suction principle cannot be applied. These are crudely referred to as deep-well pumps, also a misnomer. Pumps of many varieties and ratings as to the quantities of water they can move are on the market. A few of the most common varieties need mention, including the shaft turbine, submersible, and deep-well cylinder pumps. The shaft turbine pump is commonly used in high-capacity wells for industrial and public water supplies. The energy used to run it is usually supplied by electricity or diesel fuel. A rotating stainless steel shaft extends down the pump pipe to the turbine bowls set at some carefully predetermined depth below the well’s pumping water level. These pumps are capable of lifting water 100 meters or more.

Submersible pumps are in worldwide use for pumping both small domestic wells and high-capacity industrial and public water supply wells. They are always powered by electricity. The submersible pump operates essentially like a shaft turbine, except that there is no shaft. The electric motor drive is arranged in vertical orientation under the pump bowls and, therefore, submersed in the well water. All turbine pumps are sensitive to any fine sand or silt that some wells tend to produce along with the water. Turbine blades have close tolerances and suffer loss of efficiency when worn by grit. Deep-well cylinder pumps are widely used for small to modest pumping needs, often powered by wind energy, especially for livestock water supplies in remote areas. These pumps are hung deep in the well, suspended by the pump pipe. A stiff rod extends from ground surface inside the pump pipe and acts with reciprocating action. The cylinder contains a valved piston that is open on the downstroke and closed on the upstroke. Operation is virtually the same as many oil well pumps, driven by a working-head jack and an electric motor.

Principal Terms

aquifer: a permeable geologic structure that is saturated and capable of yielding groundwater on demand

casing: conventionally, a tubular material—usually a large metal, concrete, or plastic pipe—that is inserted into a raw borehole for the purpose of maintaining the integrity of the hole

cement grouting: the injection of a sand-cement mix by means of pressure, resulting in an envelope of concrete lining between the outside of a casing and the undisturbed geologic material of a well borehole

fully confined aquifers: those that lie sandwiched between impermeable formations and are therefore termed artesian aquifers

production water wells: human-made subsurface structures designed and constructed to provide access to a water supply from an aquifer

pump pipe: a tube that extends downward from the wellhead to a level below the top of the water in the well, and that serves as the conduit for water being pumped to the surface

rat hole: that portion of a well that is deliberately made to extend below the base of an aquifer

semiconfined aquifers: those that perform as if fully confined but receive replenishment from overlying or underlying water-bearing formations of lesser permeability than the aquifer itself

unconfined aquifers: those that have no covering material, whereby the depth to water in the well is the level of the water table; also known as water-table aquifers

wellhead: that portion of the casing that extends above the ground surface; its strength and stability permit the wellhead to be used as an anchor for pumping devices

well screen: a tubular device containing openings that permit groundwater to flow into the well from the aquifer, commonly part of a well point; some aquifers need no screen

Bibliography

Ahmed, Nazeer, and Environmental and Water Resources Institute (U.S.). Task Committee on Hydraulics of Wells. Hydraulics of Wells: Design, Construction, Testing, and Maintenance of Water Well Systems. American Society of Civil Engineers, 2014.

Akhtar, Naseem, et al. "Various Natural and Anthropogenic Factors Responsible for Water Quality Degradation: A Review." Water, vol. 13, no. 19, 2021, p. 2660. doi.org/10.3390/w13192660.

American Water Works Association, and American National Standards Institute. Water Wells. B American Water Works Association, 2020.

Detay, Michel. Water Wells: Implementation, Maintenance, and Restoration. Translated by M. N. S. Carpenter. Wiley, 1997.

Fetter, C. W. Contaminant Hydrogeology. 3rd ed., Waveland Publishing, 2018.

Jasechko, Scott, and Debra Perrone. "Global Groundwater Wells at Risk of Running Dry." Science, vol. 372, no. 6540, 2021, pp. 418-21. doi:10.1126/science.abc2755.

Kasenow, Michael. Aquifer Test Data: Analysis and Evaluation. Water Resources Publications, 2006.

Mahajan, Gautam. Evaluation and Development of Ground Water. APH Publishing Corporation, 2020.

Misstear, B. D. R., et al. Water Wells and Boreholes. 2nd ed., John Wiley, 2017.

Palmer, Christopher M. Principles of Contaminant Hydrogeology. 2nd ed., CRC Press, 2019.

Schön, Jürgen. Physical Properties of Rocks: Fundamentals and Principles of Petrophysics. 2nd ed., Elsevier, 2015.