Water Well Design and Construction

Transcript

1 REFERENCE SHEET 11.3 FWQP PUBLICATION 8086 Reference: Water Well Design and Construction is UC Cooperative Extension Hydrogeology Specialist University of at the THOMAS HARTER and Kearney Agricultural Center. Davis, California, UNIVERSITY OF TER WELL BASICS WA CALIFORNIA water well is a hole, shaft, or excavation used for the purpose of extracting A Division of Agriculture ground water from the subsurface. Water may flow to the surface naturally after exca- and Natural Resources vation of the hole or shaft. Such a well is known as a flowing artesian well. More com- http://anrcatalog.ucdavis.edu monly, water must be pumped out of the well. Most wells are vertical shafts, but they may also be horizontal or at an inclined In partnership with where surface water is bank filtration, angle. Horizontal wells are commonly used in extracted via recharge through river bed sediments into horizontal wells located are hand-dug hori- Qanats, underneath or next to a stream. The oldest known wells, zontal 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 http://www.nrcs.usda.gov wells are examples of this. Monitoring wells for groundwater levels and groundwater quality are other examples. Still other purposes include the investigation of subsur- face conditions, shallow drainage, artificial recharge, and waste disposal. Farm Water Quality Planning In this publication we focus on vertical water-production wells commonly used to supply water for domestic, municipal, and agricultural uses in California. Our pur- A Water Quality and Technical Assistance Program pose is to provide readers with some basic information about water wells to help for California Agriculture them understand principles of effective well construction when they work with a pro- http://waterquality.ucanr.org fessional driller, consultant, or well servicing agency for well drilling and mainte- nance. EFERENCE S HEET is part of This R the Farm Water Quality DETERMINING A WELL LOCATION Planning (FWQP) series, developed for a short course The location of a well is mainly determined by the well’s purpose. For drinking and that provides training for grow- irrigation water-production wells, groundwater quality and long-term groundwater ers of irrigated crops who are supply are the most important considerations. The hydrogeological assessment to de- interested in implementing termine whether and where to locate a well should always be done by a knowledgeable water quality protection prac- driller or professional consultant. The water quality criteria to use for drinking water The short course teaches tices. wells are the applicable local or state drinking water quality standards. For irrigation the basic concepts of water- nonpoint source pollution sheds, wells, the primary chemical parameters of concern are salinity and boron and the (NPS), self-assessment tech- sodium-adsorption ratio. and evaluation tech- niques, Management goals and niques. Enough ground water must be available to meet the pumping requirements of practices are presented for a the wells. For large municipal and agricultural production wells, pumping rate variety of cropping systems. 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-

2 2 ANR Publication 8086 edge of the local groundwater system, experience in similar areas, and a diverse array We ll Design of information such as land surface topography, local vegetation, rock fracturing Objectives (where applicable), local geology, groundwater chemistry, information on thickness, • Highest yield with depth, and permeability of local aquifers from existing wells, groundwater levels, minimum draw- satellite or aerial photographs, and geophysical measurements. down In most cases, the well location is further limited by property ownership, the Good quality • need to keep surface transportation of the pumped ground water to a minimum, and water with proper access restrictions for the drilling equipment. When locating a well, one should also protection from consider the proximity of potential sources of contamination such as fuel or chemical contamination storage areas, nearby streams, sewer lines, and leach fields or septic tanks. The pres- • Sand-free water ence of a significant barrier between such potential sources and the well itself is very important for the protection of the well. Long lifetime • (>50 years) TER WELL DESIGN AND INSTALLATION WA • Reasonable short- term and long- Once the well location has been determined, a preliminary well design is completed. term costs 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) surround- ed by a gravel pack and appropriate surface and borehole seals . Water (Figure 1) enters the well through perforations or openings in the well screen. Wells can be screened continuously along the bore or at specific depth intervals. access tube for water level reading The latter is necessary when a well taps multiple aquifer zones, to gravel feed pipe pump motor housing ensure that screened zones match the aquifer zones from which conductor casing sand water will be drawn. In alluvial grout seal aquifers, which commonly contain silty clay water table alternating sequences of coarse pump shaft material (sand and gravel) and fine material, the latter construction (blank) casing method is much more likely to gravelly sand pump bowl provide clean, sediment-free water and is more energy efficient than gravel pack clay loam the installation of a continuous screen. Hardrock wells, on the well screen sand other hand, are constructed very sump differently. Often, the borehole of a hardrock well will stand open and clay bottom plate will not need to be screened or cased unless the hard rock crum- bles easily. Figure 1. Components of a well.

3 3 ANR Publication 8086 The purpose of the screen is to keep sand and gravel Drilling a Well: Overview from the gravel pack (described below) out of the well while The process of designing and constructing a providing ample water flow to enter the casing. The screen water well begins when you make arrange- should also be designed to allow the well to be properly ments with a licensed driller or with a profes- developed (see Well Development). Slotted, louvered, and sional consultant who designs the well and bridge-slotted screens and continuous wire wrap screens are oversees the work of the licensed driller. We the most common types. Slotted screens provide poor open strongly recommended against any reliance area. They are not well suited for proper well development on dowsers or well witchers to locate a well and maintenance, and are therefore not recommended. Wire site. Research shows no scientific or other wrap screens or pipe-based wire wrap screens give the best reliable basis to substantiate the use of water performance. The additional cost of wire wrap screens can dowsing as a means to locate a well site. be offset if you only install screen sections in the most pro- ductive formations along the borehole. The driller or consultant finds a suitable location to meet the specified purpose of the The purposes of the blank well casing between and well and a preliminary design is established. above the well screens are to prevent fine and very fine for- Once the drilling rig is set up, the drilling mation particles from entering the well, to provide an open process itself may last from a few hours (for a pathway from the aquifer to the surface, to provide a proper shallow, small-diameter well) to several weeks housing for the pump, and to protect the pumped ground (for a deep, large-diameter well). Sometimes, water from interaction with shallower ground water that particularly for large production wells and may be of lower quality. where water quality is particularly important, The annular space between the well screen, well casing, the driller will drill a small-diameter pilot and borehole wall is filled with gravel or coarse sand (called hole before drilling the well bore. From infor- ). The gravel pack prevents sand the gravel pack or filter pack mation obtained from the pilot hole, a driller and fine sand particles from moving from the aquifer forma- or consultant can determine aquifer forma- tion into the well. The gravel pack does not exclude fine silt tions and groundwater quality at various and clay particles; where those occur in a formation it is best depths and then optimize the final well to use blank casing sections. The uppermost section of the design for the specific hydrogeological con- annulus is normally sealed with a bentonite clay and cement ditions at the site. Appropriate materials grout to ensure that no water or contamination can enter the (screen, casing, gravel) can then be ordered annulus from the surface. The depth to which grout must be in a timely fashion prior to the final drilling. placed varies by county. Minimum requirements are defined Once the well bore is drilled, the driller in the (Bulletin 74–90, California California Well Standards installs well casing and well screens and fills Department of Water Resources [ .water.ca.gov]): http://www the annulus around the casing with a gravel 50 feet for community water supply wells and industrial (filter) pack and the appropriate cement and wells and 20 feet for all other wells. Local county ordinances bentonite seal to prevent water from leaking may have more stringent requirements depending on local between uncontaminated and contaminated groundwater conditions. aquifers or from the land surface into the well At the surface of the well, a surface casing is commonly ( is a special type of clay used to bentonite installed to facilitate the installation of the well seal. The seal against water leaks). Then the driller surface casing and well seal protect the well against contami- develops the well (see Well Development), nation of the gravel pack and keep shallow materials from implements an aquifer test, completes the caving into the well. Surface casing and well seals are partic- sanitary seal of the well head, and installs a ularly important in hardrock wells to protect the otherwise pump and power source. Proper design, con- open, uncased borehole serving as a well. struction, development, and completion of the well will result in a long life for the well WELL DRILLING (as long as half a century or more) and effi- cient well operation. We lls 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

4 4 ANR Publication 8086 wells. In unconsolidated and semi-consoli- dated materials, (reverse) rotary (Figure 2) and cable tool methods are most common- ly employed. Hardrock wells generally are drilled with air rotary drilling equipment. Properly implemented, all of these drilling methods will produce equally effi- Reverse rotary rig cient and productive wells where ground Pump water is available. Cable tool drilling gen- Mud pit erally is less labor-intensive but takes more (connected to borehole) time than (reverse) rotary drilling. Reverse Rotating drill rod rotary and rotary drilling require large Drill rods amounts of circulation water and the con- Drilling mud (flowing down by gravity) struction of a mud pit, something to be considered if the well is to be drilled in a Borehole wall remote location with no access to water. Drilling mud with drill cuttings (flowing up by pumping) During drilling, drillers must keep a detailed log of the drill cuttings obtained Rotating drill bit 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 geophysi- cal measurements in the borehole. Special- ized equipment is used to measure the pted from Driscoll, a Johnson Principles of reverse rotary drilling. 1996. Figure 2. (ad resistance electric self-potential or al and the Screens/A Wetherford Company is publisher and copyright holder.) spontaneous potential of the geological mate- rial 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 resis- tance 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 open- ings, 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 infor- mation about construction details and aquifer characteristics that can be used later for troubleshooting well problems.

5 5 ANR Publication 8086 WELL DEVELOPMENT After the well screen, well casing, and gravel pack have been installed, the well is to clean the borehole and casing of drilling fluid and to properly settle the developed 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 proper- ly 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 fre- quent 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 draw- poor well efficiency. down than do properly constructed wells, an effect referred to as 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 efficien- cy 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 ). For an aquifer test, the well is pumped at a constant rate or pump test 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 effi- ciency 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 perfora- tions 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 but with leak- Figure 3. Properly completed well with elevated concrete seal ( into the well when the pump is shut off. ing lubricant ) .

6 6 ANR Publication 8086 FOR MORE INFORMATION For more online information on groundwater-related topics, visit quality.ucanr.org oundwater.ucdavis.edu. and http://water http://gr 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: [email protected] An electronic version of this publication is available on the ANR Communication Services Web http://anrcatalog.ucdavis.edu. site at Publication 8086 © 2003 by the Regents of the University of California, Division of Agriculture and Natural Resources. All rights reserved. The University of California prohibits discrimination against or harassment of any person employed by or seeking employment with the University on the basis of race, color, national ori- gin, religion, sex, physical or mental disability, medical condition (cancer-related or genetic char- acteristics), ancestry, marital status, age, sexual orientation, citizenship, or status as a covered veteran (special disabled veteran, Vietnam-era veteran or any other veteran who served on active duty during a war or in a campaign or expedition for which a campaign badge has been autho- rized). University Policy is intended to be consistent with the provisions of applicable State and Federal laws. Inquiries regarding the University’s nondiscrimination policies may be directed to the Affirmative Action/Staff Personnel Services Director, University of California, Agriculture and Natural Resources, 300 Lakeside Drive, 6th Floor, Oakland, CA 94612-3550, (510) 987-0096. For infor- mation about obtaining this publication, call (800) 994-8849. For information about down- loading, call (530) 754-5112. pr-3/03-WJC/CR This publication has been anonymously peer reviewed for technical accuracy by University of California scientists and other qualified professionals. This review process was managed by the ANR Associate Editor for Natural Resources.

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