[p.T] Water Well Manual

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A project of Volunteers in Asiaer Well Manual

by : Ulric P. Gibson and Rexford D. SingerPublished by:United States Agency for InternationalDevelopmentWashington, DC 20532 USAPaper copies are $ 9-00.Available from:Premier PressP.G. Box 4428Berkeley, CA 94704 USA

Reproduction of this microfiche document in anyform is subject to the same restrictions as thoseof the original document.


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A PRAGTICAL GUIDE FOR LOCATICONSTRUCTING WELLS FOR INDIVIDUALAND SMALL COMMUNITY WATER SUPPLIES


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WATER IS ONE OF MANS MJST IMPORTANTRESOURCES. ITS PROPER UEVELOPMENTMZ:ANS OF WELLS IS A MATTER OF INCREASINGTHIS BOOK DISCUSSE 5 THE LOCATION,CONSTRUCTION, OPERATI~JN, AND MAINTE-SMALL W ELLS USED PI:,MARILY FORAND SMALL COMMUl:ITY WATER

AUTvlORS, WRITING IN A Cy,EAR AND EASY-TO-MANNER, PRESENT THE f:lJNDAMENTALS OFWELLS SO AS TO BE USEFUL TO INDIVIDUALOWNERS, FARMERS, ANI.:) STUDENTS AS WELLTO THOSE PROFESSIONALL Y INVOLVED SUCH ASDRILLING CONTRACTD!ZS, ENGINEERS, AND

TECHNIQUES FOR I.,EVELOPING GROUNDARE COMPREHENSIVELY DESCRIBED -THE WATER SUPPkIES ARE FOR AGRI-INDUSTRIAL, Of,! HUMAN NEEDS. TOUNDERSTANDING THE riUTHORS HAVE IN-MORE THAN 100 ILLUSTRATIONS THROUGH-

BOOK WAS ORIGINALLY PUBLISHED BYAGENCY FOR INTERNATIONAL DEVELOPMENTTHE UNITED STATES GOVERNMENT TO ASSISTPEPPLE LIVING IN THE DEVELOPING COUNTRIESTHE WORLD WHO ARE WITHOUT ADEQUATEOF GOOD QUALITY WATER. THIS NEWHAS SEEN PREPARED SO THAT ALL PERSONSIN WATER RESOURCES MAY BENEFITTHIS VALWABLE BOOK.


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hmier PwssEditorial Advisory Boanifor Water Resources

Harvey 0. Bat&TCharles E MeyerDavid K. Todd


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A PRACTICAL GUlDE FOR LOCATING AND CONSTRUCTING WELLSFOR lNDh/lDUAL AND SMALL COMMUNITY WATER SUPPLIES

Uhic P. GibsonExecutive Engineer, Water Supply, Rural Areas

Ministry of Works & Hydraulics, Guyana

Rexford D. SingerAssociate Professor of Environmental HealthSchool of Public HealthUniversity of Minnesota

PREMIER PRESSBerkeley, California


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WATEREL1

MANUAL

Covers Copyright @ by Premier Press 197 !

Published by theAgency for InternationalDevelopment of the U.S.Department of State underthe title Small Wells Mamai,1969.

Text reprinted i971 byPREMiER PRESSP. 0. Box 4428Berkeley, California 94704

Library of CongressCatalog Card Number: 71-l 53696

Printed in the United States of America

Foul-t11 Printing, May 1977

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The authors wkh to express their appreciation to the Health Service,Oftlce of Wt?ron Hunger. United States :lgtfi~q tor International Develop-ment for making the publkation of this manual poss\lble.We art particularlyindebted to the UOP-Johnson Division, Universal Oil Products Company, St.Paul. %Iinnesota or their advice and assistance n preparing the tnanuscriptand for their contribution of valuable inft)rmstion and illustrations and to Mr.Arpad Rumy for the preparation of man of the illustrations. We also wish toexpress sincere gratitude to ail pcrs:ns who have offered comments,suggestions nd assistance r who havegiven their time to critically review themanuscript.In preparing this manual. an attempt h;ts been made to bring togetherinformation and material from a variety of sources. We have endeavored togive proper credit for the direct use of material from these sources, and anyomission of such credit is unintentional.

It has been estimated that nearly two-thirds of the one and a half billionpeople living in the developing countries are without adequate supplies ofsafe water. The consequencesof this deficiency are innumerable episodes ofthe debilitating and incapacitating enteric diseaseswhrch annually affect anestimated 500 million people and result in the deaths of as many as 10million about half of whom are children.Although there are many factors limiting the installation of small watersystems, the lack of know!edge in repdd to the availability of ground waterand effective means of extracting it fc-11se by rural communities is a majorelement. It is anticipated that this manual will make a major contributiontoward fiiling this need by providing the man in the field, not necessarilyanengineer or hydrologist, with the information needed to locate, construct andoperate a small well which can provide good quality water in adequate quan-tities for small communities.The Agency for International Development takes great pride in cooper-ating with the University of Minnesota in making this manual available.

Arthur H. HollowaySanitary Engineer,Health Service, Office of War on HungerAgency for International Development

. . .111


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PageACKNOWLEDGEMENTS iiFOREWORD . . .111

1. INTRODUCTION 1PURPOSE 1SCOPE 1PUBLIC HEALTH AND RELATED FACTORS 1Importance of Water Supplies. Ground-Waters Impor-tance. Need for Proper Development and Management ofGround Watt-bResources.

*L.ORIGiN, OCCllRRENCE AND MOVEMENT OF GROUNDWATER 4THE HYDROLOGIC CYCLE 4SUBSURFACE DISTRIBUTION OF WATER 4Zone of Aeration. Zone of Saturation.GEOLOGIC FORMATIONS AS AQUIFERS 7Rock Classification. Role nf Geologic Processes nAquifer Formation.GROUND-WATER FLOW AND ELEMENTARY WELL HY-DRAULICS 10Types of Aquifers. A,quifer Functions. Factors AffectingPermeability. Flow Toward Wells.QUALITY OF GROUND WATER ;72Physical Quality. Microbiological Quality. Chemical Qual-ity.

3. GROUND-WATEREXPLORATIONEEOLOGIC DATAGeologic Maps. Geologic Cross-Sections. Aerial Photo-graphs.INYENTORY OF EXISTING WELLSSURFACE EVIDENCE

2828

3031

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4. WATER WELL DESIGNCASED SECTIONBUTARE SECTlONType and Construction of Screen. Screen Length, Size ofOpenings and Diameter.SELECTION OF CASING AND SCREEN MATERIALSWater Quality. Strer&h Requirements. Cost. Miscel-laneous.GRAVELPACKING AND FORMATION STABILIZATIONGravel Packing. Formation Stabilization.SANITARY PROTECTIONUpper Terminal. Lower Terminal of the Casing.Grouting

and Sealing Casing.5. -#ELL CONSTRUCTION

WELL DRILLING METHODSBoring. Driving. Jetting. Hydraulic Percussion. Sludger.Hydraulic Rotary. Cable-Tool Percussion.INSTALLING WELL CASINGGROUTING AND SEALING CASINGWELL ALIGNMENTConditions Affecting Well Alignment. Measurement ofWell Alignment.INSTALLATION OF WELL SCXEENSPull-back Method. Open Hole Methr?d. Wash-downMeth-od. Well Points. Artificially Gravel-Packed Wells. Re-covering Well Screens.FISHING OPERATIONSPreventive Measures.Preparations for Fishing. CommonFishing Jobs and Tools.

6. WELL COMPLETIONWELL DEVELOPMENTMe&an&l Surging. Backwashing. Development of Grav-el-Packed Wells. Dispersing Agents.WELL DISINFECTION

Pi;ge333434

47

5052

5555

69707375

86

9696

104

V


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S. PUMPING EQUIPMENT 114CONSTANT DISPLACE>lE?ZT PLhlPS I17Reciprocating Piston Pumps. Rotary Pumps. HelicalRotor Pumps.VARIABLE DISPl.A(EMENl- PUMPS 120Cen rlfugal Ptuqx. Jet Pumps.DEEP WELL PUMPS 11-lLineshaft Pumps. Subrnersibk P!lrnps.PRIMING OF PUMPS I 10PUMP SELECTION 137SELECTION OF POWER SOLrRCE 11xMan Power. Wind. Electricity. Internal CombustionEngine.

4. SANITARY PROTECTION OF GROUND-WATER SUPPLIES 134POLLUTION TRAVEL IN SOILSWELL LOCATiONSEALING ABANDONED WELLS

REFERENCESCWEDIi FOR ILLUSTRATIONSAPPENDICES

APPENDIX A. MEASUREMENT OF PERMEABILITY

13813914(!

APPENDIX B. USEFUL TABLES AND FORMULAS 111INDEX 151

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C PTER I

I CTI

PURPOSEThis manual is intended to serve as a basic introductory text book and toprovide instruction and guidance to field personnel engaged n the construc-tion, operation and maintenance of small diameter, relatively shallow wellsused primarily for individual and small community water supplies.it is aimed particularly at those persons who have had little or noexperience in the subject. An attempt has been made to treat the subjectmatter as simply as possible in order that this manual may bF:of benefit notonly to the engineer or other technically trained individual (inexperienced inthis field) but also the individual home owner, farmer or non-technicallytrained community development officer. This manual should also proveuseful in the training of water well drillers, providing the complementarybackground material for their field experience. The reader who is interestedin pursuing the subject further, and with reference to larger and deeper wells,is referred to the list of references o be found at the end of this manual.SCOPE

This. manual covers the exploration and development of ground-watersources in unconsolidated formations, primarily for the provision of smallpotabie water supplies. Its scope has been limited to the consideration ofsmall tube wells up to 4 inches in diameter, a maximum of approximately100 feet in depth and with yields of up to about 50 U.S. gallons per minute(All references are to U.S. units. Conversion tables are to be found inAppendix B). The location, design, construction, maintenance and rehabilita-tion of such wells are among the various aspects discussed. The abovelimitation on well size (diameter) rules out the zonslderation of dug we!ls infavor of the much more efficient and easier o protect bored, driven, jetted ordrilled tube wells. However, a method of converting existing dug wells to tubewells is discussed.PUBLIC HEALTH AND RELATED FACTORShportance of Water SuppliesWater is, with the exception of ai?, the most important single substance omans survival. Man, like all other forms of biological life, is extremelydependent upon water and can survive much longer without food than he canwithout water. The quantities of water directly required for the properfunctioning of the body processes re relatively small but essential.

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While man has always recognized ihe importance of water for his internalbodily needs. his recognition of its importance to health is a more recentdevelopment. dating back oniy a century or so. Since that time, much hasbeen learned about the role of inadequate and contaminated water supplies nthe spread of water-borne diseases.Among the first diseases ecognized to bewater borne were cholera and typhoid fever. Later, dysentery, gastroenteritisand other diarrhea1 diseaseswere added to the list. More recently, water hasalso been shown to play an important role in the spread of certain viraldiseases uch as nfectious hepatitis.Water is involved in the spreadof conznunicable diseasesn essentially twoways. The first is the well known direct ingestion of the infectious agentwhen drinking contaminated water (e.g. dysentery, typhoid and othergastrointestinal diseases).The second is due to a lack of sufficient water forpersonal hygiene purposes. Inadequate quantities of water for the mainte-nance of personal hygiene and environmental sanitation have been shown tobe major contributing factors in the spread of such diseasesas yaws andtyphus. Adequate supplies of water for personal hygiene also diminish theprobability of transmitting some of the gastrointestinal diseasesmentionedabove. The latter type of interaction between water and the spreadof diseasehas been recognized by various public health organizations in developingcountries which have been trying to provide adequate quantities of water ofreasonable, hough not entirely satisfactory, quality.Health problems related to the inadequacy of water supplies are universalbut, generally, of greater magnitude and significance in the underdevelopedand developing nations. It has been estimated that about two-thirds of thepopulation of the developing countries obtain their water from contaminated?111!3rees.he World Health Organization estimates that each year 500 millionpeople suffer from diseases ssociatedwith unsafe water supplies. Due largelyto poor water supplies, an estimated S,OOO,OOOnfants die each year fromdiarrheal diseases.In addition to the human consumption and health requirements, water isa!so needed for agricultural, industrial and other purposes. Though all ofthese needs are important, water for human consumption and sanitation isconsidered to be of greater social and economic importance since the healthof the population influences all other activities.Ground-Waters ImportanceIt can generally be said that ground water has played a much lessimporatnt role in the solution of the worlds water supply problems than itsrelative availability would indicate. Its outaf-sight location and the associatedlack of knowledge with respect to its occurrence, movement and developmenthave no doubt contributed greatly to this situation. The increasing acquisitionand dissemination of knowledge pertaining to ground-water development willgradually allow the use of this source of water to approach its rightfuldegreeof importance and usefulness.More than 97 percent of the fresh wn:lteron our planet (excluding that inthe polar ice-caps and glaciers) is to be found underground. While it is not

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practicable to extract all of this water becauseof economic tendother re;lsons.the recoverab le quantities would. no doubt, esceed the available supplies offresh surface water found in rivers and lakes.Ground-water sources also represent water that is essentially irl ;ioragewhile the water in rivers and lakes is generally i.n transit, being replacedseveral times a year. The available quantity of surface water at any givenlocation is also more subject to seasonal luctuations than is ground water. Inmany areas, the extraction of ground water can be continued long afterdroughts have completely depleteil rivers. Ground-water sources are, there-fore, more rehable sourcesof water in many instances.As will be seen in Chapter 2. ground waters are usually of much betterquality than surface waters. due to the benefits of percolation through theground. Oftener than not, ground water is also mctre readily available whereneeded, requiring less transportation and, generally, costir?.gess to develop.Greater emphasis should, therefore, be placed on the development and use ofthe very extensive ground-water sources o be found throughout the world.Need for Proper Development and Management of Ground-Water ResourcesWhile some ground-water reservoirs are being repienished year after yearby infiltration from precipitation, rivers, canals and so on, others are beingreplenished to much lesser degrees or not at all. Extraction of water fromthese latter reservoirs results in the continued depletion or mining of thewater.Ground water aiso often seeps nto streams, thus providing the low flow(base flow) that is sustained through the driest period of the year. Conversely,if the surface water levels in streams are higher than those in ground-waterreservoirs, then seepage takes place in the opposite direction, from thestreams into the ground-water reservoirs. Uncontrolled use of ground watercan, therefore, affect the levels of streams and Iakes and consequen.tly heuses o which they are normally put.Ground-water development presents special problems. The lack of solu-tions to these problems have, in the past, contributed to the mystery thatsurrounded ground-water development and the limited use to which groundwater has been put. The proper development and management of ground-water resources requires a knowiedge of the extent of storage, the rates ofdischarge from and recharge to underground reservoirs, and the use ofeconomical means of extraction. It may be necessary to devise artificialmeans of recharging these reservoirs where no natural sources exist or tosupplement the natural recharge. Researchhas, in recent years, considerablyincreased our knowledge of the processes involved in the origin andmovement of ground water and has provided us with better methods ofdevelopment and conservation of ground-water supplies. Evidence of thisincreased knowledge is to be found in the greater emphasis being placed onground-water development.


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An understanding of the processes and fxtors affecting the origin,occurrence, and movement of ground water is essential to the properdevelopment and use of ground-water resources.Of importance in determin-ing 2 satisfactory rate of extraction and suitable uses of the water are aknowledge of the quantity of water present, its origin. the direction and rateof movement to its poi:rt of discharge, the discharge rate and the rate atwhich it is being replenished, and the quality of the water. These points areconsidered in this chapter in as simplified and limited a form as the aims andscope of this manual permit.THE HYDROLQGIC CYCLEThe hydrologic cycle is the name given to the circulation of water in itsliquid, vapor, or solid state from the oceans to the air, air to land, over theland surface or undergrountl, and back to the oceans Fig. 2.1).Evaporation, taking place at the water stirface of oceans and other openbodies of water, results in the transfer of water vapor to the atmosphere.Under certain conditions, this water vapor condenses o form clouds whichsubsequently release their moisture as precipitation in the form of rain, hail.sleet, or snow. Precipitation may xcur over the oceans etuning some of thewater directly to them or over land to which winds have previouslytransported the moisture-laden air and clouds. Part of the rain falling to theearth evaporates with immediate return of moisture to the atmosphere. Ofthe remainder, some, upon reaching the ground surface, wets it and runs offinto surface streams finally discharging in the ocean while another partinfiltrates into the ground and then percolates to the ground-water flowthrough which it later reaches the ocean. Evaporation returns some of thewater from the wet land surface to the atmosphere while plants extract someof that portion in the soil through their roots and, by a process known astranspiration, return it through their leaves o the atmosphere.SUBSURFACE DISTRIBUTION OF WATERSubsurface water found in the interstices or pores of rocks may be dividedinto two main zones (Fig . 2.2). These are the zvtle of aeration and the .zo/reof saturation.Zone of AerationThe zone of aeration extends from the land surface to the level at whichall of the pores or open spaces n the earths materials are completely fiiled or

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I \I /\ n /- Sun -

. .t-%rcolarlon- -V Fresh ground water ~-1.w 1,.-1 nrnnn

-----_- _

_ - -; .-- -- -. formotionr --- -.- --7 -- -c-- _--- ..-_ IF ---- - - .--- TJ__ __ - -: tmpermeable- -_._- z -- --- -__ -z -..- ---- ---- - -.---_ --___ -a. - -~ - - ------ --- -~

Fig. 2.1 THE HYDROLOGIC CYCLE.


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.sit of Soil Water

tntermediate Belt

Capillary FringeWater. Table t-- -

Ground Water

Fig. 2.2 DIVISIONS OF SUBSUR-FACE WATER.

saturated with water. A mixture ofair and water is to be found in thepores in this zone and hence itsname. It may be subdivided intothree belts. These are (1) the belt ofsoil water, (3) the intermediate beltand (3) the capillary fringe.The be/r of soil wafer lizs im-mediately below the surface and isthat region from which plants ex-tract, by their roots, the moisturenecessary or growth. The thicknessof the belt differs greatly with thetype of soil and vegetation, rangingfrom a few feet in grass-landsandfield crop areas to several feet inforests and lands supporting deep-rooted plants.The qdlary ftitlge occupies thebottom portion of the zone ofaerh:ion and lies immediately abovethe zone of saturation. Its namecomes from the fact that the waterin this belt is suspendedby capil-iary forces similar to those whichcause water to rise in a narrow orcapillary tube above the level of thewater in a larger vessel nto which the tube has been placed upright. The

narrower the tube or the pores, the higher the water rises. Hence, thethickness of the belt depends upon the texture of the rock or soil and may bepractically zero where the pores are large.The intemrediute belt lies between the belt of soil water and the capillaryfringe. Most of its water reaches t by gravity drainage downward through thebelt of soil water. The wster in this belt is called intermediate (vadose)water.

Zone of SaturationImmediately below the zone of aeration lies the zone of saturation inwhich the pores are completely fdled or saturated with water. The water inthe zone of saturation is known as ground water and is the only form ofsubsurface water that will flow readiiy into 3 well. The object of wellconstruction is to penetrate the earth into this zone with a tube, the bottornsection of which has openings which are sized such as to permit the inflow ofwater from the zone of saturation but to exclude its rock particles.Formations which contain ground water and will readily yield it to wells arecalled aquifers.

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IC FQRXA~NMIIS AS AQUIFERSFor convenience, . :)iogists describe all earth materials as roclis. Rocksmay be of the ~~onti.J ,-refl type (held firntiy together by compaction,cementation and otLe:, K A~.,Psut!~ ts granite. sandr,>r-* e;\d imestone orrtrncomoiidated type (IL, .-:aterials) such as clay, sand Jnd gravel. The termshml and soft are also usr,~: .odescribe consolidated and unconsolidated rockyrespectively.Aquifers may be composed of consolidated or unconsolidated rocks. Therock materiais must be sufficiently porous (contain a reasonably highproportion of pores or other openings to solid material) and be sufficientlypermeable (the openings must be interconnected to permit the travei of waterthrough them).Rock ClassificationRocks may be classified with respect to their or+;? into the three maincategories cf sedimentary rocks, igneous rocks, and lxidmorphic rocks.Sedimentary rocks are the deposits of material derived from theweathering and erosion of other rocks. Though constituti;.; only about 5percent of the earths crust they contain an estimated 95 percent of theavailable ground water.Sedimentary rocks may be consolidated or unconsolidated dependingupon a number of ?zctors such as the type of parent rock, mode ofweathering, means of transport, mode of deposition, and the extent to whichpacking, compactiorl, and cementation have taken place. Harder rocksgenerally produce sediments of coarser texture than softer ones. Web;>erin;by mechanical disintegration (e.g. rock fracture due to temperature varlutions) produces coarser sediments than those produced by chemical decom-position. Deposition in water provides more sorting and better packing ofmaterials than does deposition directly onto land. Chemical constituents inthe parent rocks and the environment are responsible or the cemerltatioll ofunconsolidated rocks into hard, consolidated ones. These factors aisoinfluence the water-bearing capacity of sedimentary rocks. Disintegrated shalesediments are usually fine-grained and make poor aquifers while sedimentsderived from granite or other crystalline rocks usually form good sand andgravel aquifers, particularly when considerable water transportation hasresulted in well-rounded and sorted particles.Sand, gravel, and mixtures of sand and gravel are among the unconsoli-dated sedimentary rocks that form aquifers. Granular and unconsolidated,they va.ry in particle size and in the degree of sorting and rounding of theparticies. Consequently, their water-yielding capabilities vary considerably.However, they consitute the best water-bearing formations. They are widelydistributed throughout the world and produce very significant proportions ofthe water used n many countries.Other unconsolidated sedimentary aquifers include marine deposits,alluvial or stream deposits (including deltaic deposits and alluviai fans), glacialdrifts and wind-blown deposits such as dune sand and loess [very fine siltydeposits). Great variations in the water-yielding capabilities of these forma-tions can also be expected. For example, the yield from wells in sand dunes

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and loess may be limited by both the finenessof the material and the limitedareal extent and thickness of the deposits.Limestone, essentially calcium carbonate. and dolomite or calcium-magnesium carbonate are examples of consolidated sedimentary rocks knownto function as aquifers. Fractures and crevices caused by earth movement,and later enlarged into solution channels by ground-water flow through them,form the connected o:,enings through which flow takes place (Fig. 2.3).Flows may be considerable where solution channelshave developed.

A BFig.2.3 A.FRACXJRESlNDENSELIMESTONETHROUGHWHICHFLOWMAYOCCUR.B. SOLUTION CHANNELS IN LIMESTONE CAUSED BY GROUND-WATERFLOWTHROUGHFRACTURES.

Sandstone, usually formed by compactron of sand deposited by rivers nearexisting sea shores, is another form of consolidated sedimentary rock thatperforms as an aquifer. The cementing agents are responsible for the widerange of colors seen in sandstones. The water-yielding capabilities ofsandstones ary with the degreeof cementation and fracturing.Shales and other similar compacted and cemented clays, such as mudstoneor siltstone, are usually not considered to be aquifers but have been known toyield small quantities of water to wells in localized areas where earthmovements have substantially fractured such formations.Igneous rocks are those resulting from the cooling and solidification ofhot, molten materials called magma which originate at great depths within theearth. When solidification takes place at considerable depth, the rocks arereferred to as intrusive or plutonic while those solidifying at or near theground surface are called extrusive or volcanic.Plutonic rocks such as granite are usually coarse-textured and non-porousand are not considered to be aquifer;. However, water has occasionally beenfound in crevices and fractures ol the upper, weathered portions of suchrocks.Volcanic rocks, because of the relatively rapid cooling taking place at thesurface, are usually fine-textured and glassy in appearance. Basalt or traprock, one of tlze chief rocks of this type, can be highly porous andpermeable as a result of interconnected openings called vesicles ormed by thedevelopment of gasbubbles as the lava (magma flowing at or near the surface)cools. Basaltic aquifers may also contain water in crevices and broken up orbrecciated tops and bottoms of successiveayers.

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Fragmental materials discharged by volcanos. such as ash and cinders, havebeen known to form excellent aquifers where particle sizes are sufficientlylarge. Their water-yielding capabilities vary considerably, depending on thecomplexity of stratification, the range of particle sizes, and shape of theparticles. Examples of excellent aquifers of this type are to be found inCentral America.Metanwrphic rock is the name given to rocks of all types, igneous orsedimentary, which have been altered by beat and pressure. Examples ofthese are quart&e or metamorphosed sandstone, slate and mica schist fromshale, and gneiss rom granite. Generally, these form poor aquifers with waterobtained only from cracks and fractures. Marble, a metamorphosed lime-stone, can be a good aquifer when fractured and containing solution channels.With the above description of the three main rock types, it should now beeasier to understand why an estimated 95 percent of the available groundwater is to be found in sedimentary rocks which constitute only about 5percent of the earths crust. The wells described in this manual will be thoseconstructed in unconsolidated sedimentary rocks which are undoubtedly themost important sources of water for small community water supply systems.Role of Geologic Processes in Aquifer FormationGeologic processesare continually, though slowly, altering rocks and rockformations. So slowly are these changes taking place that they are hardlyperceptible to the human eye and only barely measurable by the mostsensitive instruments now available. Undoubtedly, however, mountains arebeing up-lifted and lowered, valleys filled or deepened and new ones created,sea shores advancing and retreating, and aquifers created and destroyed.These changes are more obvious when referred to a geologic timetable withunits measured n thousands and ,millions of years and to which reference canbe made .in almost any book on geology.

Geologically old as well as young rocks may form aquifers but generallythe younger ones which have been subjected to less compression andcementation are the better producers. Geologic processes determine theshape, extent, and hydraulic or flow characteristics of aquifers. Aquifers insedimentary rock formations for example vary considerably depending uponwhether the sediments are terrestrial or marine in nature.Terrestrial sediments, or materials deposited on land, include stream, ake,glacial, and wind-blown deposits. With but few exceptions they are usually oflimited extent and discontinuous, much more so than are marine deposits.Texture variations both laterally and vertically are characteristic of theseformations.AZZMaI or stream deposits are generally long and narrow. UsuallySUbSUrface,or below the valley floor, they may also be in the form of terracesindicating the existence of higher stream beds in the geologic past. Thematerial in such aquifers may range in size from fine sand to gravel andboulders. Abandoned stream courses and their deposits are sometimes buriedunder wind-home or glacial depos,ts with no visible evidence of theirexistence. Where a rapidly flowing stream such as a mountain streamencounters a rapid reduction of slope, the decrease in velocity causes a

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deposition of large aprons of material known as alluvial fans. These sedimentsrange from coarser to finer material as one proceeds away from themountains.Glacial deposits found in North Central U.S.A., Southern Canada, andNorthern Europe and Asia may bc extensive where they result fromcontinental glaciers as compared to the more localized deposits of mountainglaciers. These deposits vary in shape and thickness and exhibit a lack ofinterconnection because of the clay and silt accumulations within the sand,gravel and boulders. Outwash deposits swept out of the melting glacier bymelt-water streams are granular in nature and similar to alluvial sands.Theswifter melt-water streams produce the best glacial drift aquifers.Lake deposits are generally fine-textured, granular material deposited inquiet water. They vary considerably in thickness, extent, and shapeand makegood aquifers only when they are of substantial thickness.

GROUND-WATER FLOW AND ELEMENTARY WELL HYDRAULICSTypes of AquifersGround-water aquifers may be classified as either water-table or artesianaquifers.A water-table aquifer is one which is not confined by an upperimpermeable layer. Hence, it is also called an unconfined aquifer. Water inthese aquifers is virtually at atmospheric pressureand the upper surface of thezone of saturation is called the water table (Fig. 2.2). The water table marksthe highest level to which water will rise in a well constructed in a water-tableaquifer. The upper aquifer in Fig. 2.4 is an example of a water-table aquifer.An artesian aquifer is one in which the water is confined under a pressuregreater than atmospheric by an overlying, relatively impermeable layer.Hence, such aquifers are also called confined or pressure aquifers. The nameartesian owes ts origin to Artois, the northernmost province of France, wherethe first deep wells to tap confined aquifers were known to have been drilled.Unlike water-table aquifers, water in artesian aquifers will rise in wells tolevels above the bottom of the upper confining layer. This is becauseof thepressure created by that confining layer and is the distinguishing featurebetween the two types of aquifers.The imaginary surface to which water will rise in wells located throughoutan artesian aquifer is called the piezomettic surface. This surface may beeither above or below the ground surface at different parts of the sameaquifer as is shown in Fig. 2.4. Where the piezometric surface lies above heground surface, a well tapping the aquifer will flow at ground level and isreferred to as a flowing artesian well. Where the piezometric surface liesbelow the ground surface, a non$owing artesiarl well results and some meansof lifting water, such as a pump, must be provided to obtain water from thewell. It is worthy of note here that the earlier usageof the term artesian wellreferred only to the flowing type while current usage ncludes both flowingand non-flowing wells, provided the water level in the well rises above thebottom of the confming layer or the top of the aquifer.

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the pc>:ti,sity.This quautity is related tu the property known 9s t+e ::pec*ijkyield and defined as the volume of water released ront ;I unit volu:;~eof theaquifer material when allowed to drain freely by gravity (Fig. 7.5). TIK

,,A!+-- ft.Static woter ,f, ., ~::.~~:.~~~:~:tese, - j ... ._.(.:C .,.,.,:>:.:.

Water drotned bygrowtty from 1.0cu ft of sand

Fig. 2.5 VISUAL REPRESENTATIONOF SPECIFIC YIELD. ITSVALUE HERE IS 0.10 CULIXXCAFT OF AQUIFER.

PL-meability is a measure of thecapacity of an aquifer to transmitwater. It is related to the pressuredifference and velocity of flow be-tween two points under laminar ornon-turbulent canditions by the following equation known as Darcys Law(after iierrry Darcy, the French engineer who developed it).

rennaining vulunte uf water not re-moved by gravity drainage is held bycupiktry forces sucl~ as found inthe capillary fringe and by otherforces of attraction. It is calledthe specificreterttiott and. likethe specific yield. may be ex-pressed as a decimal fraction orpercentage. As defined, porosity istherefore equal to the sum of thespecific yield and the specific reten-tion. An aquifer with a porosity of0.25 or 25 percent and a specificyield of 0.10 or 10 percent would,therefore, have a specific retcnt ionof 0.1 S or IS percent. One millioncubic feet of such an aquifel wouldcontain 250,000 cubic feet ofwate~of which 100,000 cubic feet wouldbe yielded by gravity drainage.Conduit jim.ticm: The propertyof an aquifer related to its conduitfunction is known as the perttw-ubility.

where Vh,hzPP

(2. I)is the velocity of flow in feet per day,is the pressure at the point of entrance to the section ofconduit unlder consideration in feet of water,is the pressure at the point of exit of the same section in feetof water,is the length of the section of conduit in feet, andis a constant known as the coefficient of permeability butoften referred to simply as the permeability.


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Equation (2.1) may be modified to readv = PI

h1 - hz. where I = -, P and is called the hydraulic gradient.

Slope equals hydmulic. . . . gradient.---. -7,

Direction of flowfrom I to 2

- .. ,, ,:.,, _, ,. ..,, .. . :: .. ;. . ..,.,. .c ,...;..~ :.-. :.. _ ....... ,: . ) .;. ., :::

Fig 2.6 SECTiON THROUGHWATER-BEARING SANDSHOWING THE PRESSUREDIFFERENCE thl- hdCAUSING FLOW BETWEENPOINTS 1 AND 2. THE HY-DRAULIC GRADIEhTISEQUAL TO TME PRESSUREDIFFERENCE DIVIDED BYTHE DISTANCE, i!, BE-TWEEN THE POINTS.

(2.2)

The quantity of flow per unit oftime through a given cross-sectionalarea may be obtained from equation(2.2) by multiplying the velocity offlow by that area. Thus,Q=AV=PIA (2.3)

where Q is the quanity of flowper unit of timeand A is the cross-sectional

area.Based on equation (2.3) the co-efficient of perrneubility may,therefore, be defined as the quan-tity of hater that will flow througha unit cross-sectionalarea of porousmaterial in unit time under a hy-draulic gradient of unity (or I = 1 O)at a specified temperature, usuallytaken as 60F. In ground-water

problems, Q is usually expressed ngallons per day (gpd), A in squarefeet (sq ft) and P, therefore, ingallons per day per square foot(gpd/sq ft). The coefficient of per-meability can also be expressed inthe metric system using units of liters per day per square meter under ahydraulic gradient of unity and at a temperature of 15SC.It is important to note that Darcys Law in the form shown in equation(2.3) states hat the quantity of water flowing under iaminar or non-turbulentconditions varies in direct proportion to the hydraulic gradient and,

therefore, the pressure difference (hI - h2) causing he flow. This means thatdoubling the pressure difference will result in doubling the flow through thesame cross-sectional area. By definition, the hydraulic gradient is seen to beequivalent to the slope of the water table for a water-table aquifer or of thepiezometric surface for an artesian aquifer.Considering a vertical cross-section of an aquifer of unit width and havinga total thickness, m, a hydraulic gradient, I, and an average coefficient of

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permeability, P, we see from equation (2.3) that the rate of flow, q, throughthis crosssection is given byq=PmI (2.4)

The product Pm of equation (2.4) is termed the coejficient of transntissi-bility or transmissivity, T, OI the aquifer. By further considering that the totalwidth of the aquifer is W, then the rate of flow, Q, through a verticalcross-sectionof the aquifer is given by

Q=qW=TIW (2.5)The coej@ient of trunsmissibility is, therefore, defined as the rate of flowthrough a vertical cross-sectionof an aquifer of unit width and whose heightis the total thickness of the aquifer when the hydraulic gradient is unity. It isexpressed n gallons per day per foot (gpd/ft) and is equivalent to the productof the coefficient of permeability and the thickness of the aquifer.

Factors Affecting PermeabilityPorosity is an important factor affecting the permeability and, therefore,the capacity of an aquifer for yielding water. This is clearly evident since anaquifer can yield only a portion of the water that it contains and the higherthe porosity, the greater is the volume of water that can be stored. Porositymust, however, be considered together with other related factors such asparticle size, arrangement and distribution, continuity of pores, and forma-t on stratification.The volume of voids or pores associated with the closest packing ofuniformly&zed spheres (Fig. 2.7) will represent the same percentage of thetotal volume (solids and voids) whether the sphereswere all of tennis ball sizeor all l/l000 inch in diameter. However, the smaller pores between the lattersphereswould offer greater resistance o flow and, therefore, causea decrease

Fii. 2.7 UNIFORMLY SIZEDSPHERES PACKED INRHOMBOHEDRAL ARRAY.Fig. 2.8 UNIFORMLY SIZEDSPHERES PACKED IN CU-BICAL ARRAY.

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in permeability even though the porosity is the same.The packing of tile spheres displajred in Fig. 2.7 is referred to as therhombohedral packing. The porosity for such a packing can be shown to be0.26 or 26 percent. The spheres may also assumea cubical array as shown inFig. 2.8 for which the porosity is 0.476 or 47.6 percent. These porosities

apply only to perfectly spherical particles and are included here to give theorder of magnitude of the porosities that naturally occurring uniform sandsand gravels may approach. A loose uniform sand may, for example. have aporosity of 46 percent. Clays, on the other hand, exhibit much fligherporosities ranging from about 37 percent for stiff glacial clays to as high as 84percent for soft bentonite clays.Consideration of the effects of particle size and arrangement onpermeability would be incomplete without simultaneously considering theeffect of particle distibution or grading. A uniformly graded sand, that is,one in which all the particles are about the same size, wilt have a higherporosity and permeability than a

less uniform sand and gravel mix-ture. This is so because the finersand fills the openings between thegravel particles resulting in a morecompact arrangement and fess porevolume (Fig. 2.9). Here, then, is anexample of a finer material having ahigher permeability than a coarserone due to the modifying effect ofparticle distribution.Flow cannot take place through

Fig. 2.9 NON-UNIFORM MI XT URE porous material unless the passagesOF SAND AND GRAVEL in the material are interconnected,WITH LOW POROSITY ANDPERh%tBII.ITY. that is to say, there is continuity ofthe pores. Since permeability is ameasure of the rate of flow under stated conditions through porous material,then a reduction in the continuity of the pores would result in a reduction inthe permeability of the material. Such a reduction could be causedby silt,clay, or other cementing materials partially or completely filiing the pores ina sand, thus making it almost impervious.An aquifer is said to be stratiflied when it consists of different layers offine sand, coarse sand, or sand and gravel. Most aquifers are stratified. Whilesome strata contain silt and clay, others are relatively free from thesecementing materials and are said to be clean. Where stratification is such thateven a thin layer of clay separates wo layers of clean sand, this results in thecutting off of the vertical movement of water between the sands. Perme-ability may also vary from layer to layer in a stratified aquifer.A brief discussion on the measurement of permeability is to be found inAppendix A.

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Flow Toward WellsConverging j&w: When a well is at rest, that is, when there is no flowtaking place from it, the water pressure within the well is the sameas that inthe formation outside the well. The level at which water stands within thewell is known as the static water level. This level coincides with the watertable for a water-table aquifer or the piezometric surface for an artesianaquifer. Should the pressure be lowered within the well, by a pump forexample, then tire greater pressure n the aquifer on the outside of the wellwould force water into the well and flak thereby results. This lowering of thepressure within the well is also acz$ *nanied by a lowering of the water levelin and around the well. Water fl,j~ Through the aquifer to the well from alldirections in what is known as ~0 ergingflow. This flow may be consideredto take place through successit lindrical sections which become smallerand smaller as the we!1 is apy ied (Fig. 2.10). This means that the areaacross which t&e flow takes r also becomes successivelysmaller as the

R, -2R, A, = 2A2v, 2V,

Fig. 2.10 F L 0 W CONVERGES TO-WARD A WELL, PASSINGTHROUGH IMAGINARYCYLINDRKAL SURFACESTHAT ARE SUCCESSIVELYSMALLER AS THE WELL ISAPPROACHED.

Darcys Law, equation (2.2),tells us that the hydraulic gradientvaries in direct proportion to thevelocity. The increasingvelocity to-wards the well is, therefore, ac-companied by an increasing hy-draulic gradient. Stated in otherterms, the water surface or thepiezometric surface develops an in-creasingly steeper slope toward thewell. In an aquifer of uniform shapeand texture, the depression of thewater table or piezonetric surfacein the vicinity of a pumped orfreely flowing well takes the formof an inverted cone. This cone,known as the cone of depression(Fig. 2.1 l), has its apex at the water level in the well during pumping, and itsbase at the static water level. The water level in the well during pumping isknown as the pumping water level. The difference in levels between the staticwater level and the surface of the cone of depression is known as thedrawdown. Drawdown, therefore, increases rom zero at the outer limits ofthe cone of depression to a maximum in the pumped weft. The radius ofinfluence is the distance from the center of the well to the outer limit of thecone of depression.

Fig. 2.12 shows how the transmissibility of an aquifer affects the shape ofthe cone of depression. The cone is deep, with steep sides,a large drawdown,

well is approached. With the samequantity of water flowing acrossthese sections, t follows from equa-tion (2.3) that the velocity in-creasesas the area becomes smaller.

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-- Rodlus of Influence --+Static water level_------ --.--

TDrowdownin we8

C--Well Screen

Fig. 2.11 CONE OF DEPRESSION INVICINITY OF PUMPEDWELL.

and a small radius ot influencewhen the aquifer transmissibility islow. With a high transmissibility,the cone is wide and shallow, rhedrawdown being small, and theradius of influence large.Rechg~ md bortndar-y ejfects:When pumping commences at Iwell. the initial quantity of waterdischarged comes from the aquiferstorage immediately surroundingthe well. The cbne of depression isthen small. As pumping continues,the cone expands to meet the in-creasing demand for water from theaquifer storage. The radius of in-fluence increases and, with it, thedrawdown in the well in order toprovide the additional pressurehead required to move the waterthrough correspondingly greaterdistances. If the rate of pumping iskept constant, then the rate of

FRadius =IS,OCO ft--

Transmissibility - IO.OCO gpd/ft

Radius = 40,000 ft

Transmissibi!ity - IOO.000 gpd/ft

- -Fig. 2.12 EFFECT OF DIFFERING COEFFICIENTS OF TRANSMISSIRILITY UPONTHE SHAPE, DEPTH AND EXTENT OF THE CONE c)F DEPRESSION,PUMPING RATE AND OTHER FACTORS BEING THC SAME IN BOTHCASES.

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r Discharging well

Fig. 2.14 SYMMETRY OF CONE OF DEPRESSION AFFECJ.ED BY RECHARGEFROM STREAM.

r Discharging well

Fig. 2.15 CONE OF DEPRESSION IN VICINITY OF IMPERMEABLE BOUNDARY.

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yield of the individual wells (Fig . 2.16). The drawdowrl at any point on tilecomposite cone of depression is equal to the sum of ihe drawdowns ai thatpoint due to each of the wells being pumped separately. In particular, thedrawdown for ;I specific disclltlrge illStatic water level -_ a well affected by interference is

greater fllA11lit unaffected value b?the amount of drawdown ait thatwell contributed by the interferingwells. In other words, the dischargeper unit of drawdown commonlycalled the specific capacit), of thewell is reduced. This means thatpumping must take place from agreater depth in the well, at a greatercost, to produce the same qaan:ityof water from the well if it were notFig. 2.16 INTERFERENCE BETWEENADJACENT WELLS TAi- subject to interference.PING THE SAME AQUIFER. Ideally, the solution would be tospace the wells far enough apart toavoid the mutual interference of one ok the other. Very often this is notpractic,tJ for economic reasonsand the wells are spaced ar enough apart. notto eliminate interference. but to reduce it to acceptable proportions. Forwells use3 for water supply purposes, spacingsof 115 o 50 feet between wellshave bee11 ound to be satisfactory. Spacings may be less in fine sandformations, in thin aquifers or when the drawdowrt is not likely to exceed 5feet. Greater spacings may be used where the depth and thickness of theaquifer are such as to permit the use of screen engths in excessof IO feet.There arc many patterns which may be used when grouping weils (Fig.2.17). Where the aquifer extends considerable distances n all directions fromthe site of a proposed wetI field, the most desirable arrangement is one inwhich the wells are locaf!:d at equal distances on the circumference of acircle. This pattern equalizes the amount of interference suffered by eachwell. It should be obvious that a well placed in the center of such a ring ofwells would suffer greater interference than any of the others when all arepumped simultaneously. Such centrally placed wells should be avoided in wellfield layouts.

Where a known source of recharge exists near a proposed site the wellsmay be located m a semi-circle or along a line roughly parallel to the source.The latter arrangement is the one often used to induce recharge o an aquiferfrom an adjacent stream with which it is connected. This is a very usefultechnique in providing an adequate water supply to a small community longafter the stream level becomes so low that only an inadequate quantity ofpoor quality water can be obtained directly from the stream. This is possiblesince the use of wells perrnirs the withdrawal of water from the permeableriver bed and the quality is enhanced by the filtering action of the aquifermaterials.

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ID)

Fig. 2.17 LAIOUT PATTERNS FOR MULTIPLE WELL SYSTEMSUSED AS WATERSUPPLY SOURCES.CENTRALLY LOCATED PUhlP EQUAL.iZES SUCTIONLIFT.

Well-point System

Saturated sand ASub-soil

I

Water levelwhile pumping\ \ \

Fig. 2.18 WELL-POINT DEWATERING SYSTEM.

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Multiple well or well-point systems are also used OJI engineering construo-tion sites for de-watering purposes, i.e. to extract water from an area toprovide dry working conditions (Fig. 2.18). The significant differencebetween this use and that for water supplies is the fact that it is JIOWimportant to create interference in order to lower water levels as much aspossible. Closer well spacings than those recommended for water supplypurposes are, therefore, necessary. Well spacings for de-watering systemsusually range from 2 to 5 feet depending upon the permeability of thesaturated sand, the depth to which the water table is to be lowered and thedepth to which the well points can be installed in the formation. It isimportant to note that the de-watering processmay require as much as a dayof pumping before excavation can begin and must be continued throughoutthe excavation. Nevertheless,de-watering has often proved more economicalthan pumping from within a sheet pile surrounded working area.QUALITY OF GROUND WATER

Generally, the openings through which water flows in the ground are verysmall. This considerably restricts the rate of flow while at the same timeproviding a filtering action against particles originally in suspension n thewater. These properties, it will be seen, considerably affect the physical,chemical, and microbiological qualities of ground water.Physical QualityPhysically, ground water is generally clear, colorless, with little or nosuspended matter, and has a relatively constant temperature. This isattributable to its history of slow percolation through the ground and theresulting effects earlier mentioned. In direct contrast, surface waters are veryoften turbid and contain considerable quantiiies of suspended matter,particularly when these waters are found near populous areas.Surface walersare also subject to wide variations of iemperature. From the physical point ofview, ground water is, therefore, more readily usable than surface water,seldom requiring treatment before use. The exceptions are those groundwaters which are hydraulically connected to nearby surface waters throughlarge openings such as fissures and solution chamlels and the interstices ofsome gravels. These openings may permit suspendedmatter to enter into theaquifer. In such cases, astes and odors from decaying vegetation may also benoticeable.Microbiological QualityGround waters are generally free from the very minute organisms(microbes) which cause disease and which are normally present in largenumbers in surface waters. This is another of the benefits that result from theslow filtering action provided as the water flows through the ground. Also,the lack of oxygen and nutrients in ground water makes it an unfavorableenvironment for disease-producing organisms to grow and multiply. Theexceptions to this rule are again provided by the fissures and solutionchannels found in some consolidated rocks and in those shallow sand and

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gravel aquifers where water is extracted in close proximity to pollutionsources, such as privies and cesspools.This latter problem has been dealt within more detail in Chapter 9, where the sanitary protection of ground-watersupplies is discussed. Poor well construction can also result in thecontamination of ground waters. The reader is referred to the section inChapter 4 dealing with the sanitary protection of wells.The solution of the potable water supply problems of Nebraska City,Nebraska, U.S.A. in 1957 bears striking testimony to the benefits derivedfrom percolation of water through the ground and the general advantages faground-water supply over one from a surface source. For more than 100 yearsprior to 1957, Nebraska City depended upon the Missouri River for itsdomestic water supply. The quality of the water in the river deteriorated asthe years went by due to the use of the river for sewageand other forms ofwaste disposal. To the old problems of high concentrations of suspendedmatter, dark coloration from decayed vegetation and highly variabletemperatures (too warn in summer and too cold in winter) was addedbacterial pollution. So bad was this sittration that the Missouri River, in thisregion, soon beczrre recognized as a virtual open sewer and the water nolonger met the requirements of the United States Public Health ServiceDrinking Watei- Standards for waters suitable to be treated for municipal use.The search for a new source of supply for Nebraska City led to the use ofwells drilled into the sands hat underlie the flood plain of the Missouri Riverat depths up to 100 feet. Wells drilled a mere 75 feet from the rivers edgeand drawing a considerable percentage of their water from the river yielded avery high quality, clear water that showed no evidence of bacterial pollutionor noticeable temperature variation. The lessonsof Nebraska City can be putto beneficiai use n many other areasof the world.Chemical QualityThe chemical quality of ground water is also considerably influenced by itsrelatively slow rate of travel through the ground. Water has always been oneof the best solvents known to man. Its relatively slow rate of percolationthrough the earth provides more than am


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10 ppm of iron means that in every million pounds (or kilograms) c,i thewater examined there will be found 10 pounds (or kilograms) of iron.Another very common form of expression is that of milligrams per liter (mg/lor mg per 1) which is the number of milligrams of the mineral found in oneliter of water. This latter unit d iffers so little from the former that ihey are,for all practical purposes, considered equal and are ~on~n~only usedinterchangeably.The following are among the more important chemical substancesandproperties of ground waters which are of interest to the owners of smallwells: iron, manganese, chloride, fluoride, nitrate, sulfate, hardness. totaldissolved solids, pH, and dissolved gasessuch as oxygen, hydrogen sulfide,and carbon dioxide.Zrorz and nlarzgarleseare usually considered together because of theirresemblances n chemical behavior and occurrence in ground water. It isimportant to note that iron and manganese, n the quantities usually found inground water, are objectionable because of their nuisance values rather than

as a threat to mans health. They both cause staining (reddish brown in thecase of iron and black in the case of manganese)of plumbing fixtures andclothes during laundering. Iron deposits may accumulate in well screensandpipes, restricting the flow oi water through them. Iron-containing waters alsohave a characteristic taste which some people find unpleasant. Such waters,when first drawn from a tap or pump, may be clear and colorless, but uponallowing the water to stand, the iron settles out of solution giving a cloudyappearance to the water and later accumulating in the bottom as arust-colored deposit.Chlorides occur in very high concentrations in sea water, usually of theorder of 20,000 mg/l. Rainwater, however, contains much less han I mg/l of

chloride. Aquifers containing large chioride concentrations are usually coastalones directly connected to the sea or which were so connected some time inthe past. Excessivepumping of wells in aquifers directly connected to the seaor to brackish-water rivers will cause hese high chloride-containing waters tomove into the otherwise fresh water zones of the aquifers. Expert technicaladvice should be sought on the possibility of such an occurrence.Water with a high chloride content usually has an unpleasant taste andmay be objectionable for some agricultural purposes. The level at which thetaste is noticeable varies from person to person but is generally of the orderof 350 mg/l. A great deal depends, however, on the extent to which peoplehave been accustomed to using such waters. Animals usuaily can drink waterwith much more chloride than humans can tolerate. Cattle have, reportedly,been known to consume water with a chloride content ranging from 3000mg/l to 4000 mg/l.

FZunride concentrations in ground water are usually small and mainlyderived from the leaching of igneous rocks. Notable among the few casesofhigh concentrations is the reported 32 mg/l from a flowing well near SanSimon, Arizona, U.S.A. High concentrations have also been reported in someparts of India, Pakistan and Africa.

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dissolved solids content would therefore be expected to present rhe taste,laxative and other problems associated with the individual minerals. Suchwaters are usually corrosive to well screens and other parts of the wellstructure.pH is a measure of the hydrogen ion ccncentration in water and indicateswhether the water is acid or alkaline. It ranges n value from 0 to 14 with avalue of 7 indicating a neutral water, valuesbetween 7 and 0 increasingly acidand between 7 and 14 increasingly alkaline waters. Most ground waters in theUnited States have pH values ranging fr0.m about 5.5 to 8. Determination ofthe pH value is important in the control of corrosion and many processesnwater treatment.

The dissolved ox-vgen content of ground wsters is usually low particularlyin waters found at great depths. Oxygen speeds up the corrosive attack ofwater upon iron, steel, galvanized ron, and brass.The corrosive process s aLc,more rapid when the pH is low.Hydrogen sulfide is recognizable by its characteristic odor of rotten eggs.

It is very often found in ground waters which also contain iron. In addition tothe odor, which is noticeable at as ow a concentration as 0.5 mg/l, hydrogen;r sulfide combines with oxygen to produce a corrosive condition in wells andalso combines with iron to form a scaledeposit of iron sulfide in pipes. Mostof the hydrogen sulfide can be removed from ground water by spraying itinto the air or allowing it to cascade n thin layers over a seriesof trays.Carbon dioxide enters water in appreciable quantities as the waterpercolates through soil in which plants are growing. Dissolved in water, itforms carbonic acid which, together with the carbonates and bicarbonates,controls the pH value of most ground waters. A reduction of pressure,such ascaused by the pumping of a well, results in the escapeof carbon dioxide and

an increase n the pH value of the water. Testing of ground-water samples orcarbon dioxide content and pH, therefore, requires the use of specialtechniques and should be done at the well site. The escapeof carbon dioxidefrom a water may also he accompanied by the settling out of calciumcarbonate deposits.While the above list includes those chemical substances hat are likely tobe of greatest general concern to owners of small wells, it is by no means anexhaustive one nor intended to be such. Conditions peculiar to specific areasmay require analyses of ground waters for other substances.The group ofelements often referred to as the trace elements because of the very lowconcentrations in which they are usually found in water are here worthmentioning. Among these are arsenic, barium, cadmium, chromium, lead andselenium, all of which are considered toxic to man at very low levels of intake(the order of a fraction of 1 mg/l). Since the rate of passage f some of theseelements through the body is very slow, the effects of repeated doses areadditive and chronic poisoning occurs.Trace elements generally are not present in objectionable concentrations inground waters but may be so in a few specific areas. t has been reported forexample, that arsenic has been found in sufficiently high concentrations in

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ground waters in some parts of Argentina and Mexico to be consideredinjurious to health. Problems are most likely to arise in areaswhere wastedischarges from industries, such as electro-plating, and overland runoffcontaining high concentrations of pesticides (insecticides and herbicides)enter aquifers.The presence of these trace elements in drinking water are generally notdetectable by taste or smell or physical appearance of the water. Properchemical analyses are required for their detection. Health departments,laboratories, geological survey departments, and other competent agenciesshould be consulted in areas where waste disposal is likely to increase thenatural content of these elements in ground water or where the natural levelsare likely to be high becauseof the local geology.

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Water can be found almost anywhere under the earths surface. There is,however, much more to ground-water exploration than the mere location ofsubsurface water. The water must be in large quantities, capable of sustainedflow to wells over long periods 7-t easonable ates, and of good quality. To bereliable, ground-water explorat!on must combine scientific knowledge withexperience and common sense. t cannot be achieved by the mere waving of amagic forked stick as may be claimed by those who practrce what is variouslyreferred to as water witching, water dowsing, or water divining.Finding the right location for a well that produces a good, steady watersupply all the year round is usually the job of scientists trained in hydrology.These scientists are called hydrologists. Their help may be sought fromgeological survey departments, governmental and private engineering organi-zations. and universities if and when available. These experts should always beconsulted for large scale ground-water development schemesbecauseof thegreat capital expenditures usually involved. However, it should be apparentfrom the remaining sections of this chapter that a sufficient number of thetools of the hydrologist is based upon. the application of common sense,intelligence and good judgement to permit their reasonably successfuluse bythe average individual interested in the location of small wells. Theinterpretation of geologic data may present problems though, with some help,these need not be totally insurmountable to some of our readers.The use ofwell inventories and surface evidence of ground water location should bemuch less difficult and find greater general application.The following sections describe the simpler tools of the hydrologist and hisuse of them. The more sophisticated methods of exploration involving the useof geophysics are considered beyond the scope of this manual and, therefore,have been excluded. It is sufficient to note that they are available to thehydrologist to provide him with additional information on which to base hisselection.

GEOLOGIC DATABefore visiting the area to be investigated, the hydrologist seeks out andstudies all available geologic data relating to it. These would include geologicmaps, cross-sectionsand aerial photographs.


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Geologic MapsGeologic maps, of which Fig. 3.1 is an example, show where the differentrock formations, consolidated or unconsolidated, come to the land surface oroutcrop, their strike or the direction in which they lie, and their dip or theangle at which they are inclined to the horizontal. Other useful informationLegend

Alluvwm, Includesunconsolidated sand,grovel, slit and cloy

Char formotlonhmestonej-q

Looml;bremotwx

ISontos formotlonshaleIOsoge hmest0neIhternom sandstone

Strike and Dip.Test hole

Fig. 3.1 EXAMPLE OF A GEOLOGIC MAP SHOWING TEST HOLE LOCATIONS.shown would include the location of faults and contour lines indicating depthto bedrock throughout the area. Faults are lines of fracture about which therock formations are relatively dislocated. They are the result of forces actingin the earth to cause lateral thrust, slippage or uplift. The hydrologist candetermine the location and area1 extent of aquifers from the type andlocation of rock outcrops and the location of faults. Faults are also likelysites for the occurrence of springs. The width of the outcrop and angle of dipindicate to him the approximate thickness of an aquifer and the depths towhich it can be found. The combination of strike and dip tell him in whichdirection he should locate a well to obtain the maximum thickness of theaquifer. The surface outcrops also indicate the possible areasof recharge o anaquifer and, by deduction, the direction of flow in the aquifer. The bedrockcontours indicate the maximum depth to which d well should be drilled insearchof water.Geologic Cross-SectionsGeologic cross-sections provide some of the main clues to the ground-water conditions of a locality. They indicate the character, thickness, andsuccession of underlying formations and, therefore, the depths and thick-nesses of existing aquifers. The main sources of information for thepreparation of these sections are well records and natural exposureswhere therock faceshave not been greatly altered by weathering. Examples of the latter


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depth. thickness. and description of rock fornrarions penetrated: water levelvariations as successive strata are penetrated; yields from water-bearingformations penetrated and the corresponding drawdowns; the form of wellconstruction; and the vield and drawdown of the well upon completion.Many drilling orgsnizaiions also keep samples of rocks from the variousformations penetrated. Associated with the well log should be a record of thewater quality (physical and chemical characteristics) of water-bearing strataencountered. Of further interest to the hydrologist would be records of anytests making use of the well or materials from the well to determine thehydraulic characteristics such as permeability and transmissibility of theaquifer. To compiete the picture. he would be interested in records of thev&ations in yield and water quality and a history of any problems associatedwith the well since its completion. The hydrologist may wish to have newchecks made on some aspecrs of ihese records sush as rhe water quality andyield.A!1 these records may not be available f rom any single source. In additionto the various agencies already mentioned. the hydrologist may have toconsult well owners and drilling organizations.With records from a sufficient number of wells, the hydrologist would nowbe in 3 position to make a contour map of the water-table or upper surface ofthe zone of saturation. To do this. he uses the measured depths from Iihndsurface to the water table at wells and the height of the land surface relativeto sea level which he obtains front topographic maps or a site survey. He thenconnects ail points of equa! elevation of the water table on a map. Thiscontour map shows the shape of the water surface. It is a very important mapin tltat it shows not only the depth below which ground water is stored butalso, from the slope of the water table, the direction in which the watermoves.SURFACE EVIDENCEThe hydrologist is now ready to visit the area and take a closer look at anysurface evidence of ground-water occurrence. He will exantine in greaterdetail the important superficial features hc had noted on the topographicmaps and aerial photographs. Among the features that would provide valuableclues would be land forms, stream patterns. springs, lakes, and vegetation.Ground water is likely to occur in larger quantities under valleys thanunder hills. Valley fills containing rock waste washed down from mountainsides are often found to be very productive aquifers. The material may havebeen deposited by streams or sheet floods with some of the finer materialgetting into lakes to form stratified lake beds. Some of these deposits mayafso be found to have been transported by wind and redeposited as sanddunes. All these and other factors intluence the rate at which the valley fillwill yield water. Coastal terraces, formed by the sinking and raising of coastalareas relative to sea level in the geologic past. and coastal and river plains areother land forms that wouid indicaie the presence of good aquifers.Any evidence of surface water such as streams. springs. seeps, swamps, orlakes is a good indication of the presence of some ground water, though not

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necessarily in usable quantity. The sand and gravel deposits found in riverbeds may very often extend laterally into the river bdankswhich may bepenetrated by shallow. highly product&e wells.Vegetation, particularly water-loving types when found in arid regions,provides good clues to shallow ground-water occurrence. Evidence ofunusually thick overgrowth is generally a sure pointer to the presence ofstreams and other surface waters, the vicinities of which would be likely sitesfor ground-water investigations. Fig. 3.3 demonstrates the application ofsome of the principles outlined above n the selection of possible well s ites.

Fig. 3.3 SURFACE EVIDENCE OF GROUND-WATER OCCURRENCE. (Adaptedfrom Fig. 4, Wafer Supply For Rural Areas And Small Communities. WHOMonograph Series No. 4.., 1959.)I - Dense vegetation indicating possible shallow water table and proximityto surface stream.2 - River plains: possible sites for wells in water-table aquifer.3 - Flowing spring where ground water outcrops. Springs may also befound at the foot of hills and river banks.4 - River beds cut into water-bearing sand formation . Indicate possibilityof river banks as good well sites.

In many areas, some of the records, maps, and other relevant informationso far discussedwill not be available to the hydrologist. Where the size of theproject warrants it, he will arrange to fill in the information as far as ispracticabie. in other situations, very likely the case or the size of weiis hereinconsidered, he will simply use his best judgement based on the readilyavailable information.

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development), thus making a much smaller well practicable. In addition toensuring the relatively free entry of water into the well at low velocity, thescreen must provide structural support against the collapse of the unconsoli-dated formation material and prevent the entry of this material with waterinto the well.CASED SECTIONThe selection of the well casing dimerer is usually controlled by the typeand size of the pump that is expected to be required for the desired ofpotential yield of the well. The well casing must be large enough toaccommodate the pump with s)lfficient clearance for easy installation andefficient opera?ion. For larger wells, such as those used for municipal andindustrial suppiies, the casing diameter should be chosen as wo nominal sizes(never :ess than one nominal size) larger than that of the pump bowls. Forwells of 4 inches and less in diameter it is satisfactory to select a casingdiameter which is one nominal size arger than that of the pump bowls, pumpcylinder or pump body. The above assumes he use of a deep-well type ofpump which is usually suspendedby pipe column and/or shaft within the wellcasing. A pump having a bowl diameter (see Fig. 8.11) greater than 3 inchesshould not, according to this rule, be installed in a $-inch diameter casing.In small wells where pumping water levelsbelow ground surface are knownto be within the practical suction limits (15 feet or less)of most surface-typepumps, such pumps are either directly connected to the top of the well casingor connected to a suction pipe suspended nside the well casing. The wellcasing diameter may then be selected in relation to the diameter of thesuction or inlet of the pump, bearing in mind that it is not good practice torestrict the suction capacity of the pump by using pipe of a smaller diameterthan that of the suction side of the pump.In larger and deeper wells than those being considered, it is sometimesadvantageous or economic and other reasons o reduce the casingdiameter atlevels below the lowest anticipated pumping depth. This is done bytelescoping one or more smaller sized casing sections through the uppermostone. This saves he extra cost of extending the large diameter casing all theway down to the aquifer when a smaller size of pipe would be sufficient toaccommodate the anticipated flow with reasonablehead oss. However, thereis little justification for this type of design in wells of 4 inches and less ndiameter and not more than 100 feet deep.INTAKE SECTIONType and Construction of Screen

The single factor with greatest influence on the efficient performance of awell is the design and construction of the well screen. A properly designedscreen combines a high percentage of open area for relatively unobstructedflow into the well w ith sufficient strength to resist the forces to which thescreen may be subjected both during and after installation in the well. Thescreen openings should preferably be shaped so as to facilitate flow into thewell while making it difficult for small particles to become permanently

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lodged in them and thus restrict flow. A discussion of various types of WPIIscreensand their uses s presented n the following paragraphs.The corrtimmts-slot type of well screen shown in Fig. 4. I is made withcold-drawn wire, appro,ximately triangular in section, wound spirally around acircular array oflongitcldinal rods. The wire is welded to the rods at all points

Fig.4.1 FABRICATION OF A CONTINUOUS-SLOT TYPE OF WELL SCREEN.at which they cross.The resulting cylindrical well screenbecomes a one-peice.rigid unit.The stronger the material used in construction, the smaller would be thedimensions of the wire rods and hence the greater the ratio of open area tosolid area of the screen surface. These screensare being made of metals suchas galvanized iron, steel, stainless steel and various types of brass. Experi-.mentsare also in progresswith the use of plastic materials.The percentageof open area s the factor exerting the greatest nfluence onthe efficiency of a screen. As will be shown later, the size of the well screenopening is determined from the size of the particles of the materialcomposing the aquifer. With this size fixed, the aim of screen design is toobtain the maximum possible total open area in a given length of screen,Thegreater the total open area, the lesser s the resistance o flow into the well.The entrance velocity through the larger intake area s also lower and so is theresulting head loss for flow through the screen. Hence we have a moreefficient well screen. The greater the percentage of open area in a screen, hegreater is the total open area n a given length of screen.Looking at it in another way, the greater the percentage open area of ascreen, the shorter is the length of screen equired for a given rate of flow at agiven velocity. This means that a saving in construction costs can be madethrotigh the use of a shorter length of screen. The continuous-slot type ofscreen provides more intake area per square foot of screensurface or per unitlength of screen than any other known type and, therefore, can result insavingswhen used.


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Fig. 4.3 THE Y-SHAPED OPENINGSOFTHECONTINUOUSSLOTTYPE OF SCREEN (RIGHT)ALLOWS SAN D GRAINSBARELY SMALLER THANTHE WIDTH OF THE OPEN-INGS TO PASS FREELYWITHOUT CLOGGING.OPENINGS WITHOUT THETAPER TEND TO HOLDPARTICLES JUST SMALLENOUGH TO ENTER THEM.

-

The pipe-base well screen isanother type of screen in use. Itconsists of a jacket around a perfo-rated metal pipe. The jacket may bein the form of a trapezoidal-shapedwire wound directly onto andaround the pipe (called a wrapped-on-pipe screen). Alternatively thewire may be wound over a seriesoflongitudinal rods spaced at fixedintervals around the circumferenceof the pipe. The latter is a moreefficient type of screen as the rodshold the wire away from the pipesurface to reduce the blocking ofthe screen openings. A strongerscreen can be obtained by using aslip-on jacket made of an integralunit of welded well screen.The perforations or holes in thepipe and the spacesbetween adja-cent turns of the wrapping wire form two sets of openings in this type ofscreen. Usuahy the total open area of the holes in the pipe is less than thatbetween the wrapping wire. It is, therefore, the holes in the pipe that controlthe performance of the screen.Tire percentageopen area n the pipe is usuallylow and hence this type of screen s relatively inefficient.Very often this type of construction is used in order to avoid making ascreen entirely of the costly noncorrosive alloys such as stainless steel,

bronze or brass. Such alloys are then used only in the jacket while the pipe isof steel. A screen so constructed with two or more metals would be subject tofailure from galvanic corrosion. Construction of the screenentirely of one ofthe noncorrosive alloys, while being more costly, will solve this problem andresult in a more durable screen.Drive points or well points, as they are commonly known, are shortlengths of well screen which are attached to successiveengths of pipe anddriven by repeated blows to the desired position in an aquifer or in aformation to be dewatered. A forged steel point is usually attached to thelower end to facilitate penetration into the ground.Well points are made in a variety of types and sizes. Most commonly, theyare designed for direct attachment to either l%inch or L-inchpipe. They canbe made of the continuous-slot type of well screen Fig. 4.9, thus benefittingfrom all the desirable features of that type of screen. Such screens willwithstand hard driving, but care should be taken to avoid twisting them whiledriving.A common type of well point is the brass acket type. It consists of aperforated pipe covered with bronze wire mesh which is, in turn, coveredwith a perforated brass sheet to protect it from damage. The pointed lower

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Fig.4.4 LOUVER- OR SHUTTER-TYPE WELL SCREEN, BESTUSED IN ARTIFIClALLYGRAVEL-PACKED WELLS.(From Layne and Bowler, inc.,Memphis, Tennessee.)

Slotted pipe is sometimes usedas a substitute for well screensparticularly in the smaller sizedwells under consideration in thismanual. The openings or slots inthe pipe are usually cut with asharp saw, electrically operated ifpossible, to maintain accuracy andregularity in size. Several othermethods have been used, however,such as cutting with an oxyacetylene torch and punching with a chisel and dieor casing perforator.

The method of construction immediately suggestsa number of importantlimitations to the use of slotted pipe as well screens.These are: (1) structuralstrength requires wide spacing of slots, resulting in a low percentageof openarea; (2) openings may be inaccurate, varying in size throughout the length ofeach slot; (3) openings narrow enough to control fine sandsare difficult, ifnot impossible, to produce; (4) the lack of continuity of the openings reducesthe efficiency of the process of well development; and (S) the slotting andperforation of steel pipe makes it more readily subject to corrosion, particu-larly at the jagged edgesand surfaces.Slotted plastic pipe has been finding increasing use n small diameter wellsin recent years. Its light weight and easeof handling make it suitable for usein remote areas not easily reached by motor driven vehicles. It isnoncorrosive and less costly than steel pipe in sizes4 inches n diameter and

end, made of forged steel, carries awider shoulder to protect thescreen from damage by gravel orstones while being driven. The lim-itations of pipe-base screens alsoapply to this type of well point.Another type of well-point con-struction is the brass tube typeconsisting of a slotted brass tubeslipped over perforated pipe. It hasan advantage over the wire-meshjacket type in that it is not as easilyripped or damaged.The sizes of openings for thecontinuous-slot type of well pointsare designated as described for thecontinuous-slot well screens.Mesh-covered well point openings aredesignated by the mesh size interms of the number of openingsper linear inch. The common sizesare 40,50,60,70 ;rnd 80 mesh.

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Fig. 45 CONTBNUOUSSLOT T Y P EOF WELL POINT AND EX-TENSION SECTION.

The most convenient type ofjoint for use with small diameterplastic pipe in well construct on isthe spigotted joint. For these oints,the manufacturers supply a quick-setting cement which provides morethan adequate and lasting strength.The slotted plastic-pipe screen canbe lowered into a previously drilledhole on the end of casing of thesame material. Steel clamps areused to suspend the string of pipeswhile adding new lengths. It mayalso be washed, open ended, with d jet of water into a previously drilledhole. Suitable drilling mud should be used during rotary drilling op-erations to prevent the open hole from collapsing while the string ofplastic pipe is being placed in position. Cart should be taken to wash thehole clear of all cuttings before placing the pipe. Plastic pipe generally requiresthe use of greater care during handling and installation operations than dometal pipes.

It cannot be contended that slotted plastic pipe will be as efficient a wellscreen as the continuous-slot type. However, when only small quantities ofwater are required from relatively thick (20 feet and greater) sand and gravelor gravel aquifers, efficiency loses some of its importance to economy andease of construction. Under these conditions, together with the ones alreadymentioned, slotted plastic pipe is an attractive alternative to the continuous-dot or other manufactured type of well screen. t is particularly suited to theprovision of individual water supplies n remote and inaccessibleareas.

smaller. In addition, the slots canbe easily made on location with asharp saw within reasonable imitsof accuracy. Slots cut spirallyaround the circumference of thepipe in the manner shown in Fig.4.6 will result in less weakening ofthe pipe and closer spacing of theslots than if they were made atright angles to the axis. Conse-quently, the percentage of openarea is greater. Slots made at rightangles to the a-xisof plastic pipe aresubject to tearing at both ends ifthe slotted pipe is bent when han-dling it during installation, Thistendency is reduced by the use ofthe spiral design.

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Manufacturers make well screens n two series of sizes, the telescope-sizeand the pipe-size or ID-size. Telescope-size screens are designed to betelescoped or lowered through the well casing to the final position. Thediameter of each screen s ust sufficiently smaller than the inside diameter ofthe corresponding size of standard pipe to permit the screen to be freelylowered through the pipe.The pipe-size or ID-size seriesof well screenshave the same nside diameteras the corresponding size of standard pipe. This type of screen s used when itis desired to maintain the same diameter throughout the full depth of thewell. They are provided, in the small sizes under consideration, with eitherwelded or threaded end connections.

Screen length: The screen ength selection can be influenced by the thicknessof the aquifer. While definite rules may be set, based on this relationship, forlarge wells it would be unwise to do so for small ones. A farmer or home-owner should not be burdened with a long and costly well screen n a thickaquifer when his requirements are so small as not to warrant it. The screenlength should be sufficient to meet his needs with a reasonabledrawdown inthe well. As already stated, a compromise must be made between well costand well efficiency. The other ex.treme must also be avoided. Economizationshould not be taken to the point where the length of screenprovided is suchthat the yield barely meets the owners present needs. A reasonableallowanceshould be made for his future ne:eds.Failure to do so may, in the long run,prove to be far more costly to the: owner.It is important to note that in a thick aquifer, well yield is much moreeffectively increased by increasing the screen ength than by proportionatelyincreasing the screen diameter. Doubling the screen diameter, for instance,will only result in an increase of 10 to 15 percent in the yield. In most cases,however, doubling the screen length will result in the yield being almostdoubled. It is, therefore, much better to use screen length as a controllingfactor on well yield rather than screendiameter in thick aquifers.The role played by aquifer characteristics in screen ength selection is bestdemonstrated with the use of a few examples. Where a thick layer of coarsesand or gravel underlies a layer of fine sand as shown in Fig. 4.714, he screenlength should be at least one-third the thickness of the coarsesand ayer. Forthe sitatuions shown in Fig. 4.7B and Fig. 4.7C, almost the entire thickness of .,the lower layer of coarse sand should be screened. Should this proveinadequate for the desired yield, then it would be necessary o extend thescreen a short distance into the overlying finer sand. Where a coarse sandoverlies a fine sand as in Fig. 4.7D, it should normally be sufficient to placethe screen in the coarse sand layer with the length being equal to aboutone-half the thickness of that layer.In thin aquifers confined by clays, particularly clays that tend to be easilyeroded when exposed to water, screen engths should be chosen so as o avoidthe possibility of placing screen openings opposite these clays. Screening ofclay layers could result in their collapse during the well development processwith the well forever producing a muddy water.

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(A) Coarse port of form&ion is th ick (8) Coarse pwt of formotion is thin

(Cl Alternate kyere of coaree and fine s4md (0) Coarse material obova fine sondFig. 4.7 RECOMMENDED POSITIONING OF WELL SCREENS IN VARIOUS STRA-TIFIED, WATER-BEARING SAND FORMATIONS.Screen slot operzirzg:An understanding of the method of selecting the size ofscreen slot openings first of all requires an understanding of the processandobjectives of well development. As previously stated, fine material occupiespart of the otherwise larger pore spacesof water-bearing formations, thus in-creasing the head losses due to friction and reducing the quantity of wateryielded per unit of drawdown in a well (specific capacity). The object of welldevehpment is to remove asmuch of this finer material as possible from a zonearound the well to improve the specific capacity and efficiency of the well.There are a variety of methods that are used for inducing the flow of this finematerial through the well screenand then extracting it by pumping or bailing.Some of these methods are described in Chapter 6. It is sufficient to note atthis point that well development involves the removal of

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