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Water Well Design and Construction

In partnership Farm Water Quality PlanningA Water Quality and Technical Assistance Program for California REFERENCE SHEET is part ofthe Farm Water QualityPlanning (FWQP) series,developed for a short coursethat provides training for grow-ers of irrigated crops who areinterested in implementingwater quality protection prac-tices. The short course teachesthe basic concepts of Water -sheds, nonpoint source pollution(NPS), self-assessment tech-niques, and evaluation tech-niques. Management goals andpractices are presented for avariety of cropping : Water well Design andConstruction THOMAS HARTER is UC Cooperative Extension Hydrogeology Specialist at the University ofCalifornia, Davis, and Kearney Agricultural well BASICSA Water well is a hole, shaft, or excavation used for the purpose of extractingground Water from the subsurface.

cased unless the hard rock crum-bles easily. ANR Publication 8086 2 Well Design Objectives • Highest yield with minimum draw-down • Good quality water with proper protection from contamination • Sand-free water • Long lifetime (>50 years) • Reasonable short-term and long-term costs access tube for water level reading gravel feed pipe ...

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Transcription of Water Well Design and Construction

1 In partnership Farm Water Quality PlanningA Water Quality and Technical Assistance Program for California REFERENCE SHEET is part ofthe Farm Water QualityPlanning (FWQP) series,developed for a short coursethat provides training for grow-ers of irrigated crops who areinterested in implementingwater quality protection prac-tices. The short course teachesthe basic concepts of Water -sheds, nonpoint source pollution(NPS), self-assessment tech-niques, and evaluation tech-niques. Management goals andpractices are presented for avariety of cropping : Water well Design andConstruction THOMAS HARTER is UC Cooperative Extension Hydrogeology Specialist at the University ofCalifornia, Davis, and Kearney Agricultural well BASICSA Water well is a hole, shaft, or excavation used for the purpose of extractingground Water from the subsurface.

2 Water may flow to the surface naturally after exca-vation of the hole or shaft. Such a well is known as a flowing artesian well . More com-monly, Water must be pumped out of the wells are vertical shafts, but they may also be horizontal or at an inclinedangle. Horizontal wells are commonly used in bank filtration, where surface Water isextracted via recharge through river bed sediments into horizontal wells locatedunderneath or next to a stream. The oldest known wells, Qanats, are hand-dug hori-zontal shafts extending into the mountains of the old Persian empire in wells are used for purposes other than obtaining ground Water . Oil and gaswells are examples of this. Monitoring wells for groundwater levels and groundwaterquality are other examples. Still other purposes include the investigation of subsur-face conditions, shallow drainage, artificial recharge, and waste this publication we focus on vertical Water -production wells commonly usedto supply Water for domestic, municipal, and agricultural uses in California.

3 Our pur-pose 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 pro-fessional driller, consultant, or well servicing agency for well drilling and A well LOCATIONThe location of a well is mainly determined by the well s purpose. For drinking andirrigation Water -production wells, groundwater quality and long -term groundwatersupply are the most important considerations. The hydrogeological assessment to de-termine whether and where to locate a well should always be done by a knowledgeabledriller or professional consultant. The Water quality criteria to use for drinking waterwells are the applicable local or state drinking Water quality standards. For irrigationwells, the primary chemical parameters of concern are salinity and boron and thesodium-adsorption ground Water must be available to meet the pumping requirements ofthe wells.

4 For large municipal and agricultural production wells, pumping raterequirements range from about 500 to 4,000 gallons per minute (gpm). Small- andmedium-sized community Water systems may depend on Water wells that producefrom 100 to 500 gpm. Individual homes domestic wells may meet their needs with asfew as 1 to 5 gpm, depending on local regulations. To determine whether the desiredamount of ground Water is available at a particular location and whether it is ofappropriate quality, drillers and groundwater consultants rely on their prior knowl-PUBLICATION 8086 FWQP REFERENCE SHEET OF CALIFORNIAD ivision of Agriculture and Natural of the local groundwater system, experience in similar areas, and a diverse arrayof information such as land surface topography, local vegetation, rock fracturing(where applicable)

5 , local geology, groundwater chemistry, information on thickness,depth, and permeability of local aquifers from existing wells, groundwater levels,satellite or aerial photographs, and geophysical most cases, the well location is further limited by property ownership, theneed to keep surface transportation of the pumped ground Water to a minimum, andaccess restrictions for the drilling equipment. When locating a well , one should alsoconsider the proximity of potential sources of contamination such as fuel or chemicalstorage areas, nearby streams, sewer lines, and leach fields or septic tanks. The pres-ence of a significant barrier between such potential sources and the well itself is veryimportant for the protection of the well Design AND INSTALLATIONOnce the well location has been determined, a preliminary well Design is many large production wells, a test hole will be drilled before well drilling toobtain more detailed information about the depth of Water -producing zones, confiningbeds, well production capabilities, Water levels, and groundwater quality.

6 The finaldesign is subject to site-specific observations made in the test hole or during 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 thewell at the desired volume and quality, and prevents bacterial growth and materialdecay 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 (Figure 1). Waterenters the well through perforations or openings in the well screen. Wells can bescreened continuously along thebore or at specific depth latter is necessary when a welltaps multiple aquifer zones, toensure that screened zones matchthe aquifer zones from whichwater will be drawn.

7 In alluvialaquifers, which commonly containalternating sequences of coarsematerial (sand and gravel) and finematerial, the latter constructionmethod is much more likely toprovide clean, sediment-free waterand is more energy efficient thanthe installation of a continuousscreen. Hardrock wells, on theother hand, are constructed verydifferently. Often, the borehole of ahardrock well will stand open andwill not need to be screened orcased unless the hard rock crum-bles Publication 8086 well DesignObjectives Highest yield withminimum draw-down Good qualitywater with properprotection fromcontamination Sand-free Water long lifetime(>50 years) Reasonable short-term and long -term costsaccess tube for Water level readinggravel feed pipesandpump motor housingconductor casinggrout sealpump shaft(blank) casingpump bowlgravel packwell screensumpbottom platesilty claywater tablegravelly sandclay loamsandclayFigure 1.

8 Components of a purpose of the screen is to keep sand and gravelfrom the gravel pack (described below) out of the well whileproviding ample Water flow to enter the casing. The screenshould also be designed to allow the well to be properlydeveloped (see well Development). Slotted, louvered, andbridge-slotted screens and continuous wire wrap screens arethe most common types. Slotted screens provide poor openarea. They are not well suited for proper well developmentand maintenance, and are therefore not recommended. Wirewrap screens or pipe-based wire wrap screens give the bestperformance. The additional cost of wire wrap screens canbe offset if you only install screen sections in the most pro-ductive formations along the purposes of the blank well casing between andabove the well screens are to prevent fine and very fine for-mation particles from entering the well , to provide an openpathway from the aquifer to the surface, to provide a properhousing for the pump, and to protect the pumped groundwater from interaction with shallower ground Water thatmay be of lower annular space between the well screen, well casing,and borehole wall is filled with gravel or coarse sand (calledthe gravel pack or filter pack).

9 The gravel pack prevents sandand fine sand particles from moving from the aquifer forma-tion into the well . The gravel pack does not exclude fine siltand clay particles; where those occur in a formation it is bestto use blank casing sections. The uppermost section of theannulus is normally sealed with a bentonite clay and cementgrout to ensure that no Water or contamination can enter theannulus from the surface. The depth to which grout must beplaced varies by county. Minimum requirements are definedin the California well Standards (Bulletin 74 90, CaliforniaDepartment of Water Resources [ ]):50 feet for community Water supply wells and industrialwells and 20 feet for all other wells. Local county ordinancesmay have more stringent requirements depending on localgroundwater the surface of the well , a surface casing is commonlyinstalled to facilitate the installation of the well seal.

10 Thesurface casing and well seal protect the well against contami-nation of the gravel pack and keep shallow materials fromcaving into the well . Surface casing and well seals are partic-ularly important in hardrock wells to protect the otherwiseopen, uncased borehole serving as a DRILLINGW ells can be constructed in a number of ways. The mostcommon drilling techniques in California are rotary, reverserotary, air rotary, and cable tool. Auger drilling is oftenemployed for shallow wells that are not used as supply3 ANR Publication 8086 Drilling a well : OverviewThe process of designing and constructing awater well begins when you make arrange-ments with a licensed driller or with a profes-sional consultant who designs the well andoversees the work of the licensed driller. Westrongly recommended against any relianceon dowsers or well witchers to locate a wellsite.


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