How deep should a well be?When is a well considered ÒdeepÓ?Do deeper wells always providebetter quality water?

This publication looks at thesequestions to help you understandimportant differences in waterquality that can occur betweenshallow and deep wells. YouÕll learnwhere the water in your well origi-nates. YouÕll discover that aÒshallow wellÓ in one locationmight be a Òdeep wellÓ in anotherpart of the county or state. AndyouÕll learn why, in some cases, ashallow well is actually a betterchoice than a deep well.

The information presented herefocuses on domestic wells that drawwater from sand and gravel, but itgenerally applies to bedrockaquifers as well.

How deep should mywell be?

To answer this question, con-sider the following points.

Wisconsin’s groundwatercomes from the sky.

The water pumped from wells iscalled groundwater. Contrary to

popular myth, the stateÕs ground-water does not come from under-ground rivers, Lake Superior orCanada. Rather, groundwater startsas precipitation that soaks into theground, usually within a few milesof your well. Groundwater is part ofnatureÕs water cycle.

Figure 1 helps you visualizeunderground water in the sand andgravel material found in much ofWisconsin. The upper layer of soil isknown as the unsaturated zone.Pulled downward by gravity, thewater in this zone moves slowlythrough pores in the soil, some ofwhich also contain air. Below theunsaturated zone, the saturated zone(zone of saturation) exists, where allthe pores between the sand grainsand gravel particles are filled withwater. The boundary between thesaturated and unsaturated zones iscalled the water table.

Water in the saturated zone,below the water table, is calledgroundwater. The saturated sand andgravel is referred to as an aquifer. Anaquifer holds water in the spacesbetween grains of sand or cracks inrocks, and transmits that water towells.

1

impervious material

water table

evaporation

precipitation

runoffinfiltration

saturated zone

wate

r cycle

unsaturated zone

transpiration

wate

r cyc

le

G3652

Do Deeper Wells Mean Better Water?Christine Mechenich and Byron Shaw

Figure 1. In the water cycle, precipitation which falls may run off into the stream or soak into the ground (infiltration). Some of the water that falls returns to the air by evaporation or passing through plants (transpiration). Water pulled through the soil by gravity into the saturated zone becomes groundwater.

The shallower the well, the nearer to the well yourwater originates.

Figure 2 provides more detailabout the possible pathways

underground water may follow. Italso shows several potential sourcesof groundwater contamination.

Various land uses and humanactivities, such as the septic system,barnyard or fertilized field in figure2, may contribute to groundwatercontamination. Gravity pulls waterthat soaks into the ground underthese areas almost straight down tothe water table. Contaminants maybe dissolved and carried along aswell. The soil filters out some ofthese materials, but may not be ableto remove them all.

When water reaches the watertable, it often changes direction.Below the water table, all the spacebetween sand grains is alreadyfilled with water. As a result,

incoming water moves laterally inthe direction of regional ground-water flow, and slowly makes itsway deeper into the aquifer as morewater descends. That process isshown by the black arrows belowthe water table in figure 2.Groundwater moves quite slowly,even in sand and gravelÑa fewinches to a few feet per day.

Notice in figure 2 that the waterflow to the shallowest well origi-nates in the barnyard, while waterin the deepest well comes from thewoodland farther away. Figure 2also illustrates how the normaldirection of groundwater flow canbe temporarily changed by wellpumping. Pumping causes a tempo-rary lowering of the water tablearound the well called a cone ofdepression.

Groundwater flows in a given direction.

Another observation you mightmake about figure 2 is that all

the groundwater moves from theright side of the drawing to the left.Besides flowing downward,groundwater also moves across aregion from higher to lower eleva-tion. Groundwater is recharged, orreplenished, in upland areas. It gen-erally moves toward low areas suchas lakes, streams and wetlands,where it discharges as it reemergesfrom the ground as seepage orspring water. The distance thatgroundwater can flow in Wisconsinis limited, however. Most ground-water from home wellsÑeven thedeepest onesÑcomes from within afew miles away. And as weÕvealready seen, the shallowest wellsgenerally pump up water that origi-nates in your own backyard.

2

Figure 2. Direction of groundwater movementand potential sources of contamination

barnyard septic system

groundwater table

grassland/woodland(no contamination)

fertilized fields

Cone of depressionfrom household wellor nearby irrigationwell—can reverse

direction of ground-water movement.

Knowing the direction ofgroundwater flow helps you deter-mine the most desirable locationand the appropriate depth for yourwell. For example, in figure 2, a wellowner might look at the land usesupgradient, or uphill in the ground-water flow system. Knowing thatseveral potentially polluting landuse activities were conducted nearthe well site, and that a natural areawas located farther upgradientmight help a well owner decide thata deeper well would provide betterquality water. Failing to considergroundwater flow direction andwell depth might lead to uninten-tionally ÒrecyclingÓ water from thebarnyard.

But determining the direction ofgroundwater flow in the hilly,glaciated areas of Wisconsin can bea complicated matter. If the landaround you slopes toward a majorstream, such as the Wisconsin River,you can assume that the ground-water underneath your land flowstoward the river. But when you lookat groundwater flow around agroundwater seepage lake, you mayactually find that groundwater

flows into the lake on one side andback into the ground on the other!Some counties have groundwaterelevation maps that can help youdetermine the direction of ground-water flow in your area. Check atyour county Extension office or theWisconsin Geological and NaturalHistory Survey at the address listedon the back page.

“Shallow well” and “deepwell” are relative terms.

When is a well consideredÒdeepÓ? The most important

aspect of depth is not the wellÕsabsolute depthÑ30 or 40 or 100feetÑbut rather how far its casingextends below the water table.

Well depth can affect both thequality and quantity of waterpumped from a well. As shown infigure 2, the quality of water in awell is influenced by the land useactivities that take place in itsrecharge area. As for quantity, a wellthat is not deep enough to reach thewater table will yield no water atall. Figure 3 shows two examples.

1. In location A, the depth from theland surface to the water table(the depth to water) is 20 feet. A25-foot deep well would be ashallow well. It would be mostsusceptible to contaminationfrom activities in the local area.Some part of the year it might bedry because of seasonal fluctua-tions in the water table. A deepwell in this case might be 60 feetdeep. Such a well would bebetter protected from local conta-mination and would not likelygo dry even during droughtyears.

2. In location B, the depth to wateris 50 feet. Our 25-foot deep wellfrom the previous example is adry hole. The 60-foot well is nowa shallow well. It draws waterfrom the top ten feet of theaquifer. It is susceptible to conta-mination from local activities,and might even be dry at times.If the water table is at 50 feet, adeep well might be 100 feet deep.

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water table

A

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Figure 3. ÒShallowÓ wells and ÒdeepÓ wells

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The amount of casing in a well is usually more important than the well’s total depth

For many wells drilled into the sand and gravel aquifer,the amount of casing—the pipe that lines the well hole—isnearly the same as the depth of the well itself. For example,a well that is 100 feet deep may possess 97 feet of casingand a 3-foot section of screen at the bottom that allowswater to enter. Without casing, sand and gravel wouldquickly collapse back into the hole, closing it (see figure 4).

Holes drilled into bedrock, such as granite or sandstone,stand small risk of collapsing. Therefore, it is not necessaryto install casing all the way to the bottom. Rather, aminimum depth of casing is often installed while theremainder of the hole is left unlined, allowing water to flowinto the hole from surrounding bedrock. For example, a wellthat is 100 feet deep in a sandstone formation may have 40feet of casing and 60 feet of open hole below the casing.Groundwater may enter this well at any point below the endof the casing—in this case, at any depth below 40 feet.

In figure 4, both wells are drilled to the same depth intoan aquifer. However, the well drilled into bedrock is able todraw water from a much shallower depth than the screenedwell drilled into sand and gravel. The depth of well casingcan therefore be a very important factor in the quality ofwater taken from a well.

When applying the information in this publication tobedrock wells, it is important to substitute the words“shallow-cased” or “deep-cased” for “shallow” or “deep.”

water table

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• • • • • • • • •

casing

rock

Figure 4. Casing depth in a bedrock anda sand-and-gravel well

Sand-and-gravel well Bedrock well

Well depth and constructionare regulated by theWisconsin Department ofNatural Resources (DNR).

Since 1936, well location and con-struction have been regulated

by the Wisconsin DNR underchapter NR 112 (now NR 812) of theWisconsin Administrative Code. Allwater-producing wells, whetherconstructed by licensed drillers orhomeowners, must meet theminimum standards outlined by thecode. The Well and PumpInstallation Code describes properwell location, including minimumseparation distances from potentialsources of contamination. It alsoprescribes proper well constructiontechniques and materials. Minimumwell and casing depths are given forspecific geologic environments.

It is critical that you consult theWisconsin well code for guidanceon construction materials, tech-niques, and proper well location. (Inaddition, you should considergroundwater flow direction, whichis not specifically covered in thecode.) And remember that the codeprovides minimum standards, soyou can take more precautions ifyou choose.

The DNR provides a basicsummary of the well code require-ments in its brochure titled You andYour Well (publication WS-002, 95rev). For more detailed informationabout the well code, contact yourDNR district private water supplyspecialist.

There are good reasons whyyou might want a deep well.

By now, you may be getting theidea that choosing the right

depth for a well isnÕt always easy. Ifyou install a shallow well, yourwater quality will be most influ-enced by your activities and thoseof your neighbors. If you install adeeper well, your water quality willbe influenced by land uses fartheruphill from you in the groundwaterflow system.

There is one other element toconsider in this problem, and thatÕsthe fourth dimensionÑtime. Thewater that you draw from a deeperwell is likely to be older, havingbeen in the ground for a longer timethan water from a shallow well.That fact gives rise to some impor-tant consequences.1. The water quality you see in a

shallow well today is probablythe result of land uses in the pastyear or two. The water quality ina deep well may reflect land usesof ten or more years ago. If yourneighbor spills five gallons ofgasoline, it might show up inyour shallow well fairly quickly.If the landfill a mile uphill leaks,it could take many years for thecontaminants to show up in yourdeep well. Groundwater in thesand and gravel aquifer movesvery slowly compared to waterin streamsÑapproximately afoot per day, or a mile in 16years.

2. Water quality in a deep wellusually changes more slowlythan in a shallow well. ThatÕsbecause groundwater does somemixing as it moves through theaquifer. A spill a mile away fromyour deep well in the sand andgravel aquifer might be substan-tially diluted with clean rechargewater before it gets to your well.Shallow wells are quite sensitiveto whatÕs going on immediatelyaround them, and their qualitymay vary season by season.

3. Some chemicals, such as pesti-cides or gasoline residues, breakdown or change over time. If apesticide soaked into the grounda mile away from you, it mightbe substantially changed in thetime it would take to reach yourdeep well. In a shallow well,where your water may only havebeen in the ground for a year,there is much less time for suchchanges to occur.

4. In the sand and gravel aquifer,the shallowest groundwater maybe the most corrosive. Shallowgroundwater is often corrosivebecause the natural mineralswhich could protect against cor-rosion are depleted in theshallow parts of the aquifer.Corrosive water can damagemetal, causing copper pipes toleak. When water is corrosive,lead solder (not allowed after1984) that joins copper pipes candissolve and enter drinkingwater at unsafe levels.

5

There are also good reasonswhy you might want ashallow well.

The fact that the water in adeeper well is older than the

water in a shallow well has somebenefits, as we saw earlier.However, that same fact can lead toseveral significant problems.1. Deeper wells may possess higher

levels of a number of naturallyoccuring nuisance and health-related contaminants. Mineralsthat dissolve in water such ascalcium and magnesium (whichcause hardness) are more plen-tiful deeper in the aquifer. Also,as the aquifer gets deeper, theamount of oxygen decreases,making it easier for certain min-erals like iron and manganese todissolve .

2. Deeper wells generally possesshigher levels of naturally occur-ring radioactivity than shallowwells, especially when they comein contact with the crystallinebedrock aquifer underneathsand and gravel. Preliminarystudies in northern Wisconsinhave shown elevated concentra-tions of radium, uranium andradon in some deeper wells.However, some shallow wellshave also been affected, espe-cially where the local sand andgravel is composed of crystallinerock particles.

3. If you live in a sparsely popu-lated non-agricultural area, andhave control over land usesupgradient, or uphill, in thegroundwater flow system,installing a shallow well allowsyou to maximize control of yourown drinking water quality. Forexample, if you have 40 acres ofnatural forest land, you might beable to install a well with waterthat is mainly recharged fromthat non-polluting land use.Remember, however, that youand your well driller must con-struct your well to certainminimum depths, using theWisconsin well constructioncode as your guide.

Driven-point wells canproduce high qualitydrinking water

This publication focuses on welldepth and not on well construc-

tion. However, people often think ofdriven-point wells as ÒshallowÓ anddrilled wells as Òdeep.Ó Figure 5shows two wells at the samedepthÑone a driven-point orÒsandpointÓ well; the other a drilledwell. Are there important differ-ences between the two? ■ A driven well is smaller in diam-

eter (11Ú4 to 2 inches) than adrilled well (4 to 8 inches).Because of the driven wellÕssmall diameter, the pump isinstalled outside the well, eitherin a basement or well house. Theexternal pump adds a potentialsafety hazard if the undergroundwater line is not installed so thatit is under pressure. A buriedsuction line could allow contam-inants to enter the water supply.A drilled well may have a sub-mersible pumpÑone with thepumping mechanism completelysubmerged inside the wellcasing, eliminating this hazard.However, a malfunctioning sub-mersible pump in a drilled wellcould leak oil used as a lubri-cant, causing water quality problems.

6drilled

screen

driven

screen with drive point

4–8 "1–2 "

Figure 5. Construction differences between driven and drilled wells

■ All driven wells and most drilledwells in sand and gravel have awell screen, the lower slottedsection of the casing whichallows water to enter. Becausedriven wells have a smallerdiameter (and in some cases,because of differing screentypes) their screens may besomewhat more susceptible toplugging from naturally occur-ring minerals and bacteria.

■ Studies conducted at theUniversity of Wisconsin-StevensPoint and the Centers forDisease Control show that col-iform bacteria contamination ismore common in drilled wellsthan in driven wells. All thereasons for this difference are notyet clear. However, the caps onmany older drilled wells haveopenings which allow insects toenter, which may introduce bac-teria into the wells. SinceFebruary 1991, Òvermin-proofÓwell caps have been required fornew drilled wells.

■ A well drilling license is notrequired to install a driven well.A person inexperienced in theinstallation of such wells couldmake errors that would con-tribute to future water qualityproblems.

Although there are differences,itÕs not true that a drilled wellalways produces better qualitywater. A driven well can serve as asafe and adequate water suply.

This information is not intended asa comprehensive evaluation of all thedifferences between drilled and drivenpoint wells. For a more detailedanalysis, refer to the publications listedat the end of this publication, or contactyour DNR private water supply spe-cialist.

The well constructionreport

Your wellÕs construction reportcontains important information.

If itÕs not currently in your posses-sion, make a point to locate it. Ifthere is a problem with your wellwater, or if you are concerned aboutwater quality, this information iscritical. The well construction reportincludes information about the:■ Date of well drilling and the

property owner at the time

■ Distance of structures such asthe septic tank from the well atthe time of drilling

■ Diameter and depth of the hole

■ Type of casing and other mate-rials used

■ Type and depth of soil and rockformations

■ Depth of water table

■ Water yield

Since 1936, licensed well drillershave been required to file well con-struction reports for drilled wellswith state agencies.

The Wisconsin Geological andNatural History Survey (address onback page) will attempt to locate thereport for you for a small fee.Provide a legal description of yourproperty (town, range, section,quarter-section and quarter-quartersection), and the year the well wasinstalled or the home built, alongwith the ownerÕs name.

Conclusion

Unfortunately, it is nearly impos-sible to know the ideal depth of

a well before you install it. If youhave neighbors, learning the depthof their wells and the correspondingwater quality might provide youwith some clues. A good rule ofthumb for well construction is Òasshallow as possible, as deep as nec-essary,Ó keeping in mind that youmust observe the requirements ofthe well code.

Choosing the ÒbestÓ well depthis not an easy matter. And, if landuses change, a well that once pro-duced good quality water couldbecome contaminated. However,looking at the direction of ground-water flow, the surrounding landuses, any existing groundwater con-tamination, and the natural qualityof groundwater in the area will allhelp you to choose a well depth thatwill provide the best quality waterfor you and your family.

7

For more informationPublications from the

University of Wisconsin-Extension are available from yourcounty Extension office or from:

Extension PublicationsRoom 170640 West Mifflin StreetMadison, WI 53703(608) 262-3346

Improving Your Drinking WaterQuality (G3378). 1986.

Maintaining Your Home Well WaterSystem (G3399). 1987.

Keeping Your Home Water Supply Safe(G3558-1). 1993.

Evaluating the Condition of YourPrivate Water Supply (G3558-2).1993.

Interpreting Drinking Water TestResults (G3558-4). 1993.

Choosing a Water Treatment Device(G3558-5). 1993.

Publications from theWisconsin Department ofNatural Resources are availablefrom DNR district offices or at thisaddress:

WDNRBox 7921Madison, WI 53707

Well Construction and PumpInstallation NR 812 (WisconsinAdministrative CodeÑState WellCode)

You and Your Well

Driven-Point (Sand-Point) Wells

Well Abandonment

Private Well Construction in GraniteFormations

Bacteriological Contamination ofDrinking Water

Nitrate in Drinking Water

Pesticides in Drinking Water

Radium in Drinking Water

Lead in Drinking Water

Iron Bacteria Problems in Wells

Sulfur Bacteria Problems in Wells

Volatile Organic Chemicals inDrinking Water

Answers to Your Questions aboutGroundwater

Radon in Private Well Water

Iron in Drinking Water

The Wisconsin Geologicaland Natural History Survey islocated at:

3817 Mineral Point RoadMadison, WI 53705(608) 262-1705

Authors: Christine Mechenich is an Extension groundwater education specialist with the Central Wisconsin GroundwaterCenter, College of Natural Resources, University of Wisconsin-Stevens Point. Byron Shaw is a professor of soil and waterscience with the University of Wisconsin-Stevens Point and a water quality specialist with the University of Wisconsin-Extension, Cooperative Extension.

Copyright © 1996 University of Wisconsin System Board of Regents.

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S.Department of Agriculture, University of WisconsinÐExtension, Cooperative Extension. University of WisconsinÐExtensionprovides equal opportunities in employment and programming, including Title IX and ADA requirements. If you need thisinformation in an alternative format, contact the Office of Equal Opportunity and Diversity Programs or call ExtensionPublications at (608)262-2655.

This publication is available from your Wisconsin county Extension office or from Cooperative Extension Publications, Room170, 630 W. Mifflin Street, Madison, WI 53703. Phone: (608)262-3346.

G3652 Do Deeper Wells Mean Better Water? I-04-96-5M-100-E