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Water Well Design and Construction

THOMAS HARTER is UC Cooperative Extension Hydrogeology Specialist at the University of California, Davis, and Kearney Agricultural Center.
WATER WELL BASICS
WATER WELL BASICS
A water well is a hole, shaft, or excavation used for the purpose of extracting
ground water from the subsurface. Water may flow to the surface naturally after excavation
of the hole or shaft. Such a well is known as a flowing artesian well. More commonly,
water must be pumped out of the well.
Most wells are vertical shafts, but they may also be horizontal or at an inclined
angle. Horizontal wells are commonly used in bank filtration, where surface water is
extracted via recharge through river bed sediments into horizontal wells located
underneath or next to a stream. The oldest known wells, Qanats, are hand-dug horizontal
shafts extending into the mountains of the old Persian empire in present-day
Iran.
Some wells are used for purposes other than obtaining ground water. Oil and gas
wells are examples of this. Monitoring wells for groundwater levels and groundwater
quality are other examples. Still other purposes include the investigation of subsurface
conditions, shallow drainage, artificial recharge, and waste disposal.
In this publication we focus on vertical water-production wells commonly used
to supply water for domestic, municipal, and agricultural uses in California. Our purpose
is to provide readers with some basic information about water wells to help
them understand principles of effective well construction when they work with a professional
driller, consultant, or well servicing agency for well drilling and maintenance.
DETERMINING A WELL LOCATION
The location of a well is mainly determined by the well’s purpose. For drinking and
irrigation water-production wells, groundwater quality and long-term groundwater
supply are the most important considerations. The hydrogeological assessment to determine
whether and where to locate a well should always be done by a knowledgeable
driller or professional consultant. The water quality criteria to use for drinking water
wells are the applicable local or state drinking water quality standards. For irrigation
wells, the primary chemical parameters of concern are salinity and boron and the
sodium-adsorption ratio.
Enough ground water must be available to meet the pumping requirements of
the wells. For large municipal and agricultural production wells, pumping rate
requirements range from about 500 to 4,000 gallons per minute (gpm). Small- and
medium-sized community water systems may depend on water wells that produce
from 100 to 500 gpm. Individual homes’ domestic wells may meet their needs with as
few as 1 to 5 gpm, depending on local regulations. To determine whether the desired
amount of ground water is available at a particular location and whether it is of
appropriate quality, drillers and groundwater consultants rely on their prior knowl- 8086 FWQP
edge of the local groundwater system, experience in similar areas, and a diverse array

of information such as land surface topography, local vegetation, rock fracturing

(where applicable), local geology, groundwater chemistry, information on thickness,
depth, and permeability of local aquifers from existing wells, groundwater levels,
satellite or aerial photographs, and geophysical measurements.
In most cases, the well location is further limited by property ownership, the
need to keep surface transportation of the pumped ground water to a minimum, and
access restrictions for the drilling equipment. When locating a well, one should also
consider the proximity of potential sources of contamination such as fuel or chemical
storage areas, nearby streams, sewer lines, and leach fields or septic tanks. The presence
of a significant barrier between such potential sources and the well itself is very
important for the protection of the well.
WATER WELL DESIGN AND INSTALLATION
Once the well location has been determined, a preliminary well design is completed.
For many large production wells, a test hole will be drilled before well drilling to
obtain more detailed information about the depth of water-producing zones, confining
beds, well production capabilities, water levels, and groundwater quality. The final
design is subject to site-specific observations made in the test hole or during the well
drilling.
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 (see sidebar, Well Design Objectives).
A well consists of a bottom sump, well screen, and well casing (pipe) surrounded
by a gravel pack and appropriate surface and borehole seals (Figure 1). Water
enters the well through perforations or openings in the well screen. 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 screen. Hardrock wells, on the

other hand, are constructed very differently. Often, the borehole of a hardrock well will stand open and will not need to be screened or cased unless the hard rock crumbles easily.

chart

The purpose of the screen is to keep sand and gravel
from the gravel pack (described below) out of the well while
providing ample water flow to enter the casing. The screen
should also be designed to allow the well to be properly
developed (see Well Development). Slotted, louvered, and
bridge-slotted screens and continuous wire wrap screens are
the most common types. Slotted screens provide poor open
area. They are not well suited for proper well development
and maintenance, and are therefore not recommended. Wire
wrap screens or pipe-based wire wrap screens give the best
performance. The additional cost of wire wrap screens can
be offset if you only install screen sections in the most productive
formations along the borehole.
The purposes of the blank well casing between and
above the well screens are to prevent fine and very fine formation
particles from entering the well, to provide an open
pathway from the aquifer to the surface, to provide a 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 (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. 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. Minimum requirements are defined
in the California Well Standards (Bulletin 74–90, California

Department of Water Resources [http://www.water.ca.gov]):
50 feet for community water supply wells and industrial
wells and 20 feet for all other wells. Local county ordinances
may have more stringent requirements depending on local
groundwater conditions.
At the surface of the well, a surface casing is commonly
installed to facilitate the installation of the well seal. The
surface casing and well seal protect the well against contamination
of the gravel pack and keep shallow materials from
caving into the well. Surface casing and well seals are particularly
important in hardrock wells to protect the otherwise
open, uncased borehole serving as a well.
WELL DRILLING
Wells can be constructed in a number of ways. The most
common drilling techniques in California are rotary, reverse
rotary, air rotary, and cable tool. Auger drilling is often
employed for shallow wells that are not used as supply

Drilling a Well: Overview
The process of designing and constructing a
water well begins when you make arrangements
with a licensed driller or with a professional
consultant who designs the well and
oversees the work of the licensed driller. We
strongly recommended against any reliance
on dowsers or well witchers to locate a well
site. Research shows no scientific or other
reliable basis to substantiate the use of water
dowsing as a means to locate a well site.
The driller or consultant finds a suitable
location to meet the specified purpose of the
well and a preliminary design is established.
Once the drilling rig is set up, the drilling
process itself may last from a few hours (for a
shallow, small-diameter well) to several weeks
(for a deep, large-diameter well). Sometimes,
particularly for large production wells and
where water quality is particularly important,
the driller will drill a small-diameter pilot
hole before drilling the well bore. From information
obtained from the pilot hole, a driller
or consultant can determine aquifer formations
and groundwater quality at various
depths and then optimize the final well
design for the specific hydrogeological conditions
at the site. Appropriate materials
(screen, casing, gravel) can then be ordered
in a timely fashion prior to the final drilling.
Once the well bore is drilled, the driller
installs well casing and well screens and fills
the annulus around the casing with a gravel
(filter) pack and the appropriate cement and
bentonite seal to prevent water from leakingbetween uncontaminated and contaminated
aquifers or from the land surface into the well
(bentonite is a special type of clay used to
seal against water leaks). Then the driller
develops the well (see Well Development),
implements an aquifer test, completes the
sanitary seal of the well head, and installs a
pump and power source. Proper design, construction,
development, and completion of
the well will result in a long life for the well
(as long as half a century or more) and efficient
well operation.

wells. In unconsolidated and semi-consolidated
materials, (reverse) rotary (Figure 2)
and cable tool methods are most commonly
employed. Hardrock wells generally are
drilled with air rotary drilling equipment.
Properly implemented, all of these
drilling methods will produce equally efficient
and productive wells where ground
water is available. Cable tool drilling generally
is less labor-intensive but takes more
time than (reverse) rotary drilling. Reverse
rotary and rotary drilling require large
amounts of circulation water and the construction
of a mud pit, something to be
considered if the well is to be drilled in a
remote location with no access to water.
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.
Because of timing issues, it is better—especially in remote areas—to drill a pilot
hole a good deal ahead of the well construction date and obtain all pertinent log
information early on from the pilot hole. The well design can then be completed and
the proper screen, casing, and gravel materials can be ordered for timely delivery prior
to the drilling of the well.

Note that a copy of all well log information should be given to the person who

pays for the drilling job. The Department of Water Resources keeps copies of all well
logs and has a large collection of past well logs. These can be requested by a well
owner if the original records are unavailable. The well log contains important information
about construction details and aquifer characteristics that can be used later for
troubleshooting well problems.

WELL DEVELOPMENT
After the well screen, well casing, and gravel pack have been installed, the well is
developed to clean the borehole and casing of drilling fluid and to properly settle the
gravel pack around the well screen. A typical method for well development is to surge
or jet water or air in and out of the well screen openings. This procedure may take
several days or perhaps longer, depending on the size and depth of the well. A properly
developed gravel pack keeps fine sediments out of the well and provides a clean
and unrestricted flow path for ground water.
Proper well design and good well development will result in lower pumping
costs, a longer pump life, and fewer biological problems such as iron-bacteria and
slime build-up. Poorly designed and underdeveloped wells are subject to more frequent
pump failures because sand and fines enter the well and cause significantly
more wear and tear on pump turbines.
Poorly designed and underdeveloped wells also exhibit greater water level drawdown
than do properly constructed wells, an effect referred to as poor well efficiency.
Poor well efficiency occurs when ground water cannot easily enter the well screen
because of a lack of open area in the screen, a clogged gravel pack, bacterial slime
build-up, or a borehole wall that is clogged from incomplete removal of drilling mud
deposits. The result is a significant increase in pumping costs. Note that well efficiency
should not be confused with pump efficiency. The latter is related to selection of a
properly sized pump, given the site-specific pump lift requirements and the desired
pumping rate.
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, typically for 12 hours to 7 days, while the water levels

in the well are checked and recorded frequently as they decline from their standing

water level to their pumping water level. Aquifer tests are used to determine the efficiency
and capacity of the well and to provide information about the permeability of
the aquifer. The information about the pumping rate and resulting pumping water
levels is also critical if you are to order a properly sized pump.
Once the well development and aquifer test pumping equipment is removed, it
may be useful to use a specialized video camera to check the inside of the well for
damage, to verify construction details, and to make sure that all the screen perforations
are open.
WELLHEAD PROTECTION
The construction of the final well seal is
intended to provide protection from leakage
and to keep runoff from entering the wellhead
(Figure 3). Minimum standards for surface
seals have been set by the California
Department of Water Resources (DWR Bulletin
74–90). It is also important to install backflow
prevention devices, especially if the well water
is mixed with chemicals such as fertilizer and
pesticides near the well. A backflow prevention
device is intended to keep contaminated water
from flowing back from the distribution system

into the well when the pump is shut off.

pumps

FOR MORE INFORMATION
For more online information on groundwater-related topics, visit
http://waterquality.ucanr.org and http://groundwater.ucdavis.edu.
You’ll find detailed information on many aspects of field crop production and
resource conservation in these titles and in other publications, slide sets,
CD-ROMs, and videos from UC ANR:
Nutrients and Water Quality, slide set 90/104
Protecting Groundwater Quality in Citrus Production, publication 21521
Sediments and Water Quality, slide set 91/102
To order these products, visit our online catalog at http://anrcatalog.ucdavis.edu.
You can also place orders by mail, phone, or FAX, or request a printed catalog of
publications, slide sets, CD-ROMs, and videos from
University of California
Agriculture and Natural Resources
Communication Services
6701 San Pablo Avenue, 2nd Floor
Oakland, California 94608-1239
Telephone: (800) 994-8849 or (510) 642-2431
FAX: (510) 643-5470
E-mail inquiries: danrcs@ucdavis.edu
An electronic version of this publication is available on the ANR Communication Services Web

site at http://anrcatalog.ucdavis.edu


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