G3652

Do Deeper Wells Mean Better Water?

Christine Mechenich and Byron Shaw

How deep should a well be? The information presented here Figure 1 helps you visualize When is a well considered ÒdeepÓ? focuses on domestic wells that draw underground water in the sand and Do deeper wells always provide water from sand and gravel, but it gravel material found in much of better quality water? generally applies to bedrock Wisconsin. The upper layer of soil is This publication looks at these aquifers as well. known as the unsaturated zone. questions to help you understand How deep should my Pulled downward by gravity, the important differences in water well be? water in this zone moves slowly quality that can occur between through pores in the soil, some of To answer this question, con- shallow and deep wells. YouÕll learn which also contain air. Below the sider the following points. where the water in your well origi- unsaturated zone, the saturated zone nates. YouÕll discover that a Wisconsin’s groundwater (zone of saturation) exists, where all Òshallow wellÓ in one location comes from the sky. the pores between the sand grains might be a Òdeep wellÓ in another he water pumped from wells is and gravel particles are filled with part of the county or state. And Tcalled groundwater. Contrary to water. The boundary between the youÕll learn why, in some cases, a popular myth, the stateÕs ground- saturated and unsaturated zones is shallow well is actually a better water does not come from under- called the water table. choice than a deep well. ground rivers, Lake Superior or Water in the saturated zone, Canada. Rather, groundwater starts below the water table, is called as precipitation that soaks into the groundwater. The saturated sand and ground, usually within a few miles gravel is referred to as an aquifer. An of your well. Groundwater is part of aquifer holds water in the spaces natureÕs water cycle. between grains of sand or cracks in rocks, and transmits that water to wells. 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.

precipitation

transpiration w

a w

t e a r t e c r

y c runoff c

le y

evaporation c l unsaturated zone infiltration e water table

saturated zone

impervious material

1 The shallower the well, incoming water moves laterally in Groundwater flows in a the nearer to the well your the direction of regional ground- given direction. water originates. water flow, and slowly makes its nother observation you might igure 2 provides more detail way deeper into the aquifer as more Amake about figure 2 is that all Fabout the possible pathways water descends. That process is the groundwater moves from the underground water may follow. It shown by the black arrows below right side of the drawing to the left. also shows several potential sources the water table in figure 2. Besides flowing downward, of groundwater contamination. Groundwater moves quite slowly, groundwater also moves across a Various land uses and human even in sand and gravelÑa few region from higher to lower eleva- activities, such as the septic system, inches to a few feet per day. tion. Groundwater is recharged, or barnyard or fertilized field in figure Notice in figure 2 that the water replenished, in upland areas. It gen- 2, may contribute to groundwater flow to the shallowest well origi- erally moves toward low areas such contamination. Gravity pulls water nates in the barnyard, while water as lakes, streams and wetlands, that soaks into the ground under in the deepest well comes from the where it discharges as it reemerges these areas almost straight down to woodland farther away. Figure 2 from the ground as seepage or the water table. Contaminants may also illustrates how the normal spring water. The distance that be dissolved and carried along as direction of groundwater flow can groundwater can flow in Wisconsin well. The soil filters out some of be temporarily changed by well is limited, however. Most ground- these materials, but may not be able pumping. Pumping causes a tempo- water from home wellsÑeven the to remove them all. rary lowering of the water table deepest onesÑcomes from within a When water reaches the water around the well called a cone of few miles away. And as weÕve table, it often changes direction. depression. already seen, the shallowest wells Below the water table, all the space generally pump up water that origi- between sand grains is already nates in your own backyard. filled with water. As a result,

Cone of depression from household well or nearby irrigation well—can reverse direction of ground- Figure 2. Direction of groundwater movement water movement. and potential sources of contamination

septic system barnyard fertilized fields grassland/woodland (no contamination) groundwater table

2 Knowing the direction of flows into the lake on one side and 1. In location A, the depth from the groundwater flow helps you deter- back into the ground on the other! land surface to the water table mine the most desirable location Some counties have groundwater (the depth to water) is 20 feet. A and the appropriate depth for your elevation maps that can help you 25-foot deep well would be a well. For example, in figure 2, a well determine the direction of ground- shallow well. It would be most owner might look at the land uses water flow in your area. Check at susceptible to contamination upgradient, or uphill in the ground- your county Extension office or the from activities in the local area. water flow system. Knowing that Wisconsin Geological and Natural Some part of the year it might be several potentially polluting land History Survey at the address listed dry because of seasonal fluctua- use activities were conducted near on the back page. tions in the water table. A deep the well site, and that a natural area well in this case might be 60 feet “Shallow well” and “deep was located farther upgradient well” are relative terms. deep. Such a well would be might help a well owner decide that better protected from local conta- hen is a well considered a deeper well would provide better mination and would not likely WÒdeepÓ? The most important quality water. Failing to consider go dry even during drought aspect of depth is not the wellÕs groundwater flow direction and years. absolute depthÑ30 or 40 or 100 well depth might lead to uninten- 2. In location B, the depth to water feetÑbut rather how far its casing tionally ÒrecyclingÓ water from the is 50 feet. Our 25-foot deep well extends below the water table. barnyard. from the previous example is a Well depth can affect both the But determining the direction of dry hole. The 60-foot well is now quality and quantity of water groundwater flow in the hilly, a shallow well. It draws water pumped from a well. As shown in glaciated areas of Wisconsin can be from the top ten feet of the figure 2, the quality of water in a a complicated matter. If the land aquifer. It is susceptible to conta- well is influenced by the land use around you slopes toward a major mination from local activities, activities that take place in its stream, such as the Wisconsin River, and might even be dry at times. recharge area. As for quantity, a well you can assume that the ground- If the water table is at 50 feet, a that is not deep enough to reach the water underneath your land flows deep well might be 100 feet deep. water table will yield no water at toward the river. But when you look all. Figure 3 shows two examples. at groundwater flow around a groundwater seepage lake, you may actually find that groundwater

Figure 3. ÒShallowÓ wells and ÒdeepÓ wells

A B

0 0

20 water 20 table 40 40 water table

feet 60 feet 60

80 80

100 100

3 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—is nearly the same as the depth of the well itself. For example, a well that is 100 feet deep may possess 97 feet of casing and a 3-foot section of screen at the bottom that allows water to enter. Without casing, sand and gravel would quickly 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 necessary to install casing all the way to the bottom. Rather, a minimum depth of casing is often installed while the remainder of the hole is left unlined, allowing water to flow into the hole from surrounding bedrock. For example, a well that is 100 feet deep in a sandstone formation may have 40 feet of casing and 60 feet of open hole below the casing. Groundwater may enter this well at any point below the end of the casing—in this case, at any depth below 40 feet. In figure 4, both wells are drilled to the same depth into an aquifer. However, the well drilled into bedrock is able to draw water from a much shallower depth than the screened well drilled into sand and gravel. The depth of well casing can therefore be a very important factor in the quality of water taken from a well. When applying the information in this publication to bedrock wells, it is important to substitute the words Figure 4. Casing depth in a bedrock and “shallow-cased” or “deep-cased” for “shallow” or “deep.” a sand-and-gravel well

Sand-and-gravel well Bedrock well

casing casing 0 0 face face water table water table 25 25 sand

50 50 sand open hole water enters 75 75 feet below land sur

feet below land sur rock

• • • 100 • • • 100 • • • screen

water enters

4 Well depth and construction There are good reasons why 2. Water quality in a deep well are regulated by the you might want a deep well. usually changes more slowly Wisconsin Department of y now, you may be getting the than in a shallow well. ThatÕs Natural Resources (DNR). Bidea that choosing the right because groundwater does some ince 1936, well location and con- depth for a well isnÕt always easy. If mixing as it moves through the Sstruction have been regulated you install a shallow well, your aquifer. A spill a mile away from by the Wisconsin DNR under water quality will be most influ- your deep well in the sand and chapter NR 112 (now NR 812) of the enced by your activities and those gravel aquifer might be substan- Wisconsin Administrative Code. All of your neighbors. If you install a tially diluted with clean recharge water-producing wells, whether deeper well, your water quality will water before it gets to your well. constructed by licensed drillers or be influenced by land uses farther Shallow wells are quite sensitive homeowners, must meet the uphill from you in the groundwater to whatÕs going on immediately minimum standards outlined by the flow system. around them, and their quality code. The Well and Pump There is one other element to may vary season by season. Installation Code describes proper consider in this problem, and thatÕs 3. Some chemicals, such as pesti- well location, including minimum the fourth dimensionÑtime. The cides or gasoline residues, break separation distances from potential water that you draw from a deeper down or change over time. If a sources of contamination. It also well is likely to be older, having pesticide soaked into the ground prescribes proper well construction been in the ground for a longer time a mile away from you, it might techniques and materials. Minimum than water from a shallow well. be substantially changed in the well and casing depths are given for That fact gives rise to some impor- time it would take to reach your specific geologic environments. tant consequences. deep well. In a shallow well, It is critical that you consult the 1. The water quality you see in a where your water may only have Wisconsin well code for guidance shallow well today is probably been in the ground for a year, on construction materials, tech- the result of land uses in the past there is much less time for such niques, and proper well location. (In year or two. The water quality in changes to occur. addition, you should consider a deep well may reflect land uses 4. groundwater flow direction, which of ten or more years ago. If your In the sand and gravel aquifer, is not specifically covered in the neighbor spills five gallons of the shallowest groundwater may code.) And remember that the code gasoline, it might show up in be the most corrosive. Shallow provides minimum standards, so your shallow well fairly quickly. groundwater is often corrosive you can take more precautions if If the landfill a mile uphill leaks, because the natural minerals you choose. it could take many years for the which could protect against cor- The DNR provides a basic contaminants to show up in your rosion are depleted in the summary of the well code require- deep well. Groundwater in the shallow parts of the aquifer. ments in its brochure titled You and sand and gravel aquifer moves Corrosive water can damage Your Well (publication WS-002, 95 very slowly compared to water metal, causing copper pipes to rev). For more detailed information in streamsÑapproximately a leak. When water is corrosive, about the well code, contact your foot per day, or a mile in 16 lead solder (not allowed after DNR district private water supply years. 1984) that joins copper pipes can specialist. dissolve and enter drinking water at unsafe levels.

5 There are also good reasons 2. Deeper wells generally possess Driven-point wells can why you might want a higher levels of naturally occur- produce high quality shallow well. ring radioactivity than shallow drinking water he fact that the water in a wells, especially when they come his publication focuses on well Tdeeper well is older than the in contact with the crystalline Tdepth and not on well construc- water in a shallow well has some bedrock aquifer underneath tion. However, people often think of benefits, as we saw earlier. sand and gravel. Preliminary driven-point wells as ÒshallowÓ and However, that same fact can lead to studies in northern Wisconsin drilled wells as Òdeep.Ó Figure 5 several significant problems. have shown elevated concentra- shows two wells at the same 1. Deeper wells may possess higher tions of radium, uranium and depthÑone a driven-point or levels of a number of naturally radon in some deeper wells. ÒsandpointÓ well; the other a drilled occuring nuisance and health- However, some shallow wells well. Are there important differ- related contaminants. Minerals have also been affected, espe- ences between the two? that dissolve in water such as cially where the local sand and ■ A driven well is smaller in diam- calcium and magnesium (which gravel is composed of crystalline eter (11Ú4 to 2 inches) than a cause hardness) are more plen- rock particles. drilled well (4 to 8 inches). tiful deeper in the aquifer. Also, 3. If you live in a sparsely popu- Because of the driven wellÕs as the aquifer gets deeper, the lated non-agricultural area, and small diameter, the pump is amount of oxygen decreases, have control over land uses installed outside the well, either making it easier for certain min- upgradient, or uphill, in the in a basement or well house. The erals like iron and manganese to groundwater flow system, external pump adds a potential dissolve . installing a shallow well allows safety hazard if the underground you to maximize control of your water line is not installed so that own drinking water quality. For it is under pressure. A buried example, if you have 40 acres of suction line could allow contam- natural forest land, you might be inants to enter the water supply. able to install a well with water A drilled well may have a sub- that is mainly recharged from mersible pumpÑone with the that non-polluting land use. pumping mechanism completely Remember, however, that you submerged inside the well and your well driller must con- casing, eliminating this hazard. struct your well to certain However, a malfunctioning sub- minimum depths, using the mersible pump in a drilled well Wisconsin well construction could leak oil used as a lubri- code as your guide. cant, causing water quality problems.

Figure 5. Construction differences between driven and drilled wells

1Ð2 " 4Ð8 "

screen with screen drive point

driven drilled 6 ■ All driven wells and most drilled The well construction Conclusion wells in sand and gravel have a report nfortunately, it is nearly impos- well screen, the lower slotted our wellÕs construction report Usible to know the ideal depth of section of the casing which Ycontains important information. a well before you install it. If you allows water to enter. Because If itÕs not currently in your posses- have neighbors, learning the depth driven wells have a smaller sion, make a point to locate it. If of their wells and the corresponding diameter (and in some cases, there is a problem with your well water quality might provide you because of differing screen water, or if you are concerned about with some clues. A good rule of types) their screens may be water quality, this information is thumb for well construction is Òas somewhat more susceptible to critical. The well construction report shallow as possible, as deep as nec- plugging from naturally occur- includes information about the: essary,Ó keeping in mind that you ring minerals and bacteria. ■ Date of well drilling and the must observe the requirements of ■ Studies conducted at the property owner at the time the well code. University of Wisconsin-Stevens ■ Distance of structures such as Choosing the ÒbestÓ well depth Point and the Centers for the septic tank from the well at is not an easy matter. And, if land Disease Control show that col- the time of drilling uses change, a well that once pro- iform bacteria contamination is duced good quality water could ■ Diameter and depth of the hole more common in drilled wells become contaminated. However, ■ than in driven wells. All the Type of casing and other mate- looking at the direction of ground- reasons for this difference are not rials used water flow, the surrounding land yet clear. However, the caps on ■ Type and depth of soil and rock uses, any existing groundwater con- many older drilled wells have formations tamination, and the natural quality of groundwater in the area will all openings which allow insects to ■ Depth of water table enter, which may introduce bac- help you to choose a well depth that ■ Water yield teria into the wells. Since will provide the best quality water February 1991, Òvermin-proofÓ Since 1936, licensed well drillers for you and your family. well caps have been required for have been required to file well con- new drilled wells. struction reports for drilled wells with state agencies. ■ A well drilling license is not The Wisconsin Geological and required to install a driven well. Natural History Survey (address on A person inexperienced in the back page) will attempt to locate the installation of such wells could report for you for a small fee. make errors that would con- Provide a legal description of your tribute to future water quality property (town, range, section, problems. quarter-section and quarter-quarter Although there are differences, section), and the year the well was itÕs not true that a drilled well installed or the home built, along always produces better quality with the ownerÕs name. water. A driven well can serve as a safe and adequate water suply. This information is not intended as a comprehensive evaluation of all the differences between drilled and driven point wells. For a more detailed analysis, refer to the publications listed at the end of this publication, or contact your DNR private water supply spe- cialist.

7 For more information Publications from the The Wisconsin Geological Publications from the Wisconsin Department of and Natural History Survey is University of Wisconsin- Natural Resources are available located at: Extension are available from your from DNR district offices or at this 3817 Mineral Point Road county Extension office or from: address: Madison, WI 53705 Extension Publications WDNR (608) 262-1705 Room 170 Box 7921 640 West Mifflin Street Madison, WI 53707 Madison, WI 53703 Well Construction and Pump (608) 262-3346 Installation NR 812 (Wisconsin Improving Your Drinking Water Administrative CodeÑState Well Quality (G3378). 1986. Code) Maintaining Your Home Well Water You and Your Well System (G3399). 1987. Driven-Point (Sand-Point) Wells Keeping Your Home Water Supply Safe Well Abandonment (G3558-1). 1993. Private Well Construction in Granite Evaluating the Condition of Your Formations Private Water Supply (G3558-2). Bacteriological Contamination of 1993. Drinking Water Interpreting Drinking Water Test Nitrate in Drinking Water Results (G3558-4). 1993. Pesticides in Drinking Water Choosing a Water Treatment Device (G3558-5). 1993. Radium in Drinking Water Lead in Drinking Water Iron Bacteria Problems in Wells Sulfur Bacteria Problems in Wells Volatile Organic Chemicals in Drinking Water Answers to Your Questions about Groundwater Radon in Private Well Water Iron in Drinking Water

Authors: Christine Mechenich is an Extension groundwater education specialist with the Central Wisconsin Groundwater Center, College of Natural Resources, University of Wisconsin-Stevens Point. Byron Shaw is a professor of soil and water science 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 WisconsinÐExtension provides equal opportunities in employment and programming, including Title IX and ADA requirements. If you need this information in an alternative format, contact the Office of Equal Opportunity and Diversity Programs or call Extension Publications at (608)262-2655. This publication is available from your Wisconsin county Extension office or from Cooperative Extension Publications, Room 170, 630 W. Mifflin Street, Madison, WI 53703. Phone: (608)262-3346.

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