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The Search for a High Yield Well
By Kathy Jesperson • On Tap Editor
It takes a lot of energy to get water out of the ground and into cities, homes, and farms. Before a groundwater source can be developed into a community supply, well performance and aquifer capabilities must be adequately assessed. That means studying an area’s geology, drilling test wells, and possibly even performing computer modeling, to name just a few things that must be done.
Wells come in different shapes and sizes, depending upon the type of material the well is drilled into and how much water is being pumped out. For example, some people want to drill a municipal well that can pump thousands of gallons per minute (gpm). Other people want a domestic well that only pumps 10 gpm. How much water a well yields is associated with the size and kind of aquifer you drill into.
Before beginning any well drilling project, it’s important to understand some well terminology, such as well yield, specific capacity, zone of influence, static water level, pumping water level, drawdown, and cone of depression.
Well yield is a measure of how quickly and how much water can be withdrawn from the well over a period of time. For instance, a small well’s yield is typically measured in gpm or gallons per hour (gph). For large wells, yield may be measured in cubic feet per second (cfs).
Specific capacity is an important term, referring to whether the well will provide an adequate water supply. Specific capacity is calculated by dividing how much water has been taken out of the well—or the well’s drawdown—by well yield. A sudden drop in specific capacity indicates that there may be trouble brewing, such as pump problems, screen plugging, or other possible serious problems.
The zone of influence is the area affected by the drawdown and extends out from the well a distance that depends upon porosity—the empty space between soil particles—and other factors.
The static water level is the level of water in the well when no water is being taken out. Pumping water level, on the other hand, is the level when water is being drawn from the well. The cone of depression occurs during pumping when water flows from all directions toward the pump. The water’s surface takes on an inverted cone shape.
Once the terms begin to make sense, it makes the job of figuring out where to drill a new high yield well a little easier. From that point, it’s a matter of collecting information before beginning the job.
“The most important key to locating and designing high yield wells is understanding the subsurface geology,” says Dale Ralston, president of Ralston Consulting Services. “You need to develop a conceptual model of the geologic conditions with a special emphasis on controls for groundwater flow.” Ralston says that the following steps can help maximize results:
1. Assemble available geologic informa- tion, such as published maps and reports and use the available information to identify potential aquifers and confining layers.
2. Develop a general understanding of where and how recharge occurs in these aquifers as well as aquifer discharge.
3. Use well logs (generally available from the state water resource agency) to get more site specific information on subsurface conditions in your area of interest.
4. Use all of this information to write a brief analysis of well development potential, including potential well drilling sites.
The Dry Hole - And Other Low Yield Well Problems
The well’s gone dry. After days of drilling a new one, little or no water can be found. What’s going on?
“A ‘dry well,’ generally, is a misnomer,” says Dale Ralston, president of Ralston Consulting Services. “You can find saturated ground at some level at any location. So if you dig deep enough, you can find water anywhere. The real concern is whether or not there’s water there, but how much water is there?”
Ralston says that when people claim their well is dry, they usually mean that the well is not producing enough water to meet their needs.
“There are areas where it is nearly impossible to construct a well that has a long-term yield of more than five gallons per minute (gpm),” he explains. “ But other aquifers lie under areas that will allow construction of 2,000 gpm wells at almost any location.”
Despite water being practically everywhere, communities need to be sure their wells can supply customers with the water they need. And when there is no water, a community needs to figure out why. That can be done in a number of ways.
“Sometimes, a well can be drilled into a formation with an adequate water supply, but cracks in the formation surrounding the well have become plugged or are too small to allow water to flow toward the well,” says David Terry, hydrogeologist with Leggette, Brashears, and Graham, Inc. “ In such cases, it may be possible to use a hydrofracture technique to enhance these openings so that the water can flow freely to the well.”
Hydrofracture techniques have been used for years in the oil drilling industry to overcome well-bore damage. In recent years, researchers have found that this method can be used to increase the production of low-yield wells. Hydrofracturing uses high-pressure pumps to overcome the pressure of overlying rocks and to inject fluids into newly opened fractures.
“In other cases, the nature of the formation is such that there is not an adequate supply of water in the aquifer, or the aquifer is too fine-grained to transmit usable quantities of water to a well,” he continues. “
In these cases, it is more difficult to rectify the dry well problem. The best insurance is to understand what you are drilling into then the odds of drilling
a successful well will increase before you start.”
Find the Reasons for the Absence of Water
If this is a newly drilled well, reasons for the lack of water may include:
• No aquifer is present (only aquicludes or aquitards).
• The well borehole is not intercepting the water that is present, either through plugging or inadequate fracture apertures (openings, slits, holes) at the well.
• You did not drill the hole deep enough.
• The well borehole was poorly drilled, or the driller used an inappropriate drilling technique.
• You drilled too deep and encountered a zone through which water can exit the well at a faster rate than it flows in.
Many productive wells also fail or decline substantially in yield. This can be caused by other factors, such as:
• biofouling of the well intake area or formation,
• mineral encrustation at the well borehole,
• drought—long or short-term overdrafting,
• someone else has drilled a well that interferes with the well, and
• the well is not really deep enough to sustain yield.
Wells decrease in yield over time for a range of reasons. In many cases, well rehabilitation efforts can restore nearly the original well yield. Aquifers can be recharged artificially in a couple of ways, including:
1. Spreading water over the land in pits, furrows, or ditches or erecting small dams in stream channels to detain and deflect surface runoff, allowing it to infiltrate to the aquifer, or
2. Constructing recharge wells and injecting water
directly into the aquifer.
Some recharge projects have been successful, but others haven’t worked out as intended. The lesson is that researchers still have much to learn about how
to recharge groundwater supplies.
For more information about groundwater recharge, write to the U.S. Geological Survey, Regional Hydrologist, Southeast Regional Office, 3850 Holcomb Bridge Road, Suite 160. Norcross, GA 30092 or go to www.usgs.gov.
Use Existing Data
State and federal geological and water resource agencies are the first places to look for information about where to drill a new well. In most states, data from previous well logs have been used to draw maps of an area’s geology and water-bearing layers. The agencies also can help identify how productive an aquifer can be and which one is capable of providing the amount of water that the community needs.
Besides helping detect a well’s productivity, these agencies have information about an aquifer’s water quality, such as whether it contains iron and manganese, sulfur, nitrates, or radionuclides. They also may have information about chemical contaminants.
Public and private well owners who surround the area of interest are another source of information. There’s likely a wealth of information from these existing wells. Data collected during the drilling operations can give the community a good idea of what a new well will produce. Beware, however, to only use data from the wells drilled into the aquifer that the community is considering for the new well. Data from other area aquifers will not provide any usable information.
And don’t forget about the people who drilled those wells. Local well-drilling contractors can provide practical information about where wells can be developed along with the quantity and quality of the water.
Hydrogeologists also are a good source and can provide information about where to locate a well. This is especially true in areas where water is scare or no large aquifers exist. “Surface geophysical techniques can be used to supplement this approach in some environments,” Ralston explains. “However, the expense associated with geophysical analysis limits its application to probably less than 10 to 20 percent of all locations.
“In most areas, an experienced hydrogeologist can give the probability of obtaining target yields from a production well at a specific location (i.e., 50 percent probability of getting 1,000 gpm but 80 percent probability of getting at least 500 gpm),” he says. “This allows the owner to make rational decisions relative to well construction.”
Geophysical exploration means exploring what’s underground, such as water-bearing faults and fracture zones in the bedrock. Several common methods can be used for geophysical exploration, such as seismic and resistivity methods.
Seismic methods measure the speed at which a sound wave travels through the earth. This measurement can then be correlated to geologic formations that may contain groundwater.
Resistivity methods measure the ground’s electrical resistance with depth into the ground. In general, the lower the resistance, the greater the probability that water is present. “The methods used are highly dependant on the hydrogeologic setting in the area of the water system,” says David Terry, Ph.D., hydrogeologist with Leggette, Brashears, and Graham, Inc. “You might be able to generalize this by the physiographic setting of the system (e.g., Coastal Plain, Piedmont, Highlands).
“In areas where unconsolidated sediments predominantly underlay the surface (e.g., broad valley floors or Coastal Plain areas), the objective is to find the thickest and coarsest water-bearing sedimentary unit available. This can be done using available literature, geophysical testing, or through test drilling. Knowledge of what information is available is especially useful, along with an understanding of what site-specific investigative approach will work best.
“In areas where fractured rock or karst (limestone) predominantly underlay the surface, generalized studies are less useful,” says Terry. “In those areas, geologists generally obtain an understanding of the location and pattern of bedrock fractures and fracture systems because most of the water flows through cracks rather than the matrix of the rock itself. Geophysical testing can be useful here because it can help to locate and identify fracture patterns.”
Searching For Groundwater with a Stick
Have you heard of dowsing? Maybe you’ve heard it called water witching, divining, or doodlebugging? Whatever the term, this practice is the ancient “gift” of finding water, metal, or other objects underground.
Dowsing, common since European medieval times, is still practiced the same as it was 1,000 years ago. While there’s some confusion about how long ago dowsing was first practiced, some say that the first evidence of dowsing was discovered on African cave walls that were more than 6,000 years old.
How does dowsing work?
Many dowsers use tools such as divining rods and pendulums. Other dowsers say the tool is only an “interface” or “communication device” that acts as a link between water and the subconscious. Therefore, the choice of dowsing tool doesn’t really matter, and some dowsers operate totally free of a dowsing rod or other tool.
Supposedly, when the dowser nears water, the tools move—indicating to the dowser that they’ve found water. Skeptics, however, think that dowsers involuntarily move the tools themselves.
Dowsers believe that objects, including water, possess a natural magnetic, electromagnetic, or other unknown “energy” that their senses can detect. To a dowser, sensing energy is a natural process that can be developed through practice.
Although some dowsers claim they have a special ability to detect electrostatic fields associated with groundwater, skeptics say that without scientific instruments (such as a magnetometer) it is impossible for a person to detect minute differences in magnetic or electric fields that may be associated with groundwater.
But the lack of scientific evidence doesn’t hold water with those who believe in this ancient divining technique.
Einstein Was a Believer
Even Albert Einstein believed in dowsing. He also believed that it could be explained. He said, “I know very well that many scientists consider dowsing as they do astrology, as a type of ancient superstition. According to my conviction this is, however, unjustified. The dowsing rod is a simple instrument, which shows the reaction of the human nervous system to certain factors that are unknown to us at this time.”
Whether you believe it works or not, dowsing is still practiced. And, like many other beliefs, it probably will be for quite a while.
For more information about dowsing, visit the American Association of Dowsers Web site at www.dowsers.org. The National Ground Water Association provides a skeptical analysis at www.ngwa.org.
Finding the Water
Groundwater is most likely found under valleys. Valleys that contain permeable soil that has washed down from the mountains usually have productive aquifers. In some areas of the country, only groundwater found in river valleys is of usable quality. Also, bedrock that lies beneath valleys is often fractured and water bearing. Coastal terraces as well as coastal and river plains may have good aquifers.
Any evidence of surface water, such as streams, lakes, springs, or swamps, is a good indication that groundwater is present—although not necessarily in usable quantities. Sometimes vegetation is a good indicator of the presence of groundwater. A thick overgrowth may mean shallow groundwater.
Drilling a Test Well
A test well is another way to explore the area. Test wells can provide abundant information about the most likely place to drill the production well. “Hopefully, by the time you are drilling a test well, you have already considered where the best locations might be,” says Terry. “In that case, you are trying to confirm the depth and the character of the aquifer materials, but you are first interested in well yield.
“Test wells also can identify the presence/absence of confining layers, and the depth, location, and orientation of bedrock fractures,” he explains. “You can sample water in the test well for water quality parameters to ensure that water in this zone will be of potable quality. You can conduct aquifer testing (pumping tests) to establish quantitative information about the yield potential of the aquifer and potential effects of pumping the well on other users or the environment.”
Computer modeling allows hydrogeologists to evaluate many complex stresses and effects on an aquifer, such as areas where several wells are drilled into the same aquifer. Computer modeling also allows for gaining information about rapid recharge and withdrawal. Then forecasts can be made about the most appropriate location for additional wells.
Terry adds that it’s also important to use a drilling method appropriate to the formation being drilled and to use the most experienced driller that you can find in the area. Drilling methods are as varied as site locations. “Drilling high yield wells is somewhat different from drilling ordinary domestic wells,” says Terry. “The techniques used to drill these wells are often different, and experience and procedure often influence the final result. It is important to use a driller and consultant familiar with high yield wells when conducting this work.”
Drilling methods include:
• cable tool—a percussion drilling method that has been used for years to drill wells of various sizes;
• rotary hydraulic—a process that uses a variable speed, spinning, cylinder- shaped bit;
• reverse-circulation—a method that differs from rotary hydraulic in that the drilling fluid, typically water, is circulated in the opposite direction;
• California—an approach sometimes called the stovepipe method that is similar to cable tool except a bucket is used as both bit and bailer;
• rotary air—a technique similar to rotary hydraulic except the drilling fluid is air, and
• down-the-hole hammer—a method that uses a pneumatic hammer unit attached to the end of the drill pipe.
Another good way to estimate well yield is drill cuttings. “Drill cuttings are the subsurface material that comes to the surface while drilling a well, says Terry. “They can tell you the type of material through which you are drilling and whether that material is saturated or unsaturated.
“By looking at the cuttings, you may be able to tell whether productive material is being encountered, how thick it is, and when to stop drilling. You may also be able to tell how deep a surface casing should be set to prevent surface contamination from entering the well. In addition, any cuttings can be fully analyzed. “Sand samples are analyzed to determine the grain size distribution for screen selection,” says Ralston. “Cuttings from fractured rock wells allow stratigraphic identification.”
How porous is it?
Within an aquifer, water is stored between grains of sand, rock, or soil. It fully saturates pores and cracks. The amount of open space between pores and cracks in an aquifer refers to its porosity. Because some areas are likely to be more porous than other areas, groundwater is generally unevenly distributed in quantity and quality.
Porosity or pore space refers to the amount of empty space between soil particles. The shape and arrangement of soil particles determine porosity. Infiltration, groundwater movement, and storage occur in these void spaces. The porosity of soil or geologic materials is the ratio of the volume of pore space in a unit of material to the total volume of material.
The arrangement or packing of the soil particles plays a role in porosity. Not all particles are spheres or round. Particles exist in many shapes and these shapes pack in a variety of ways that may increase or decrease porosity. Generally, a mixture of grain sizes and shapes results in lower porosity.
One important point to remember is that the diameter size of the grain does not affect porosity: Porosity is a ratio of void space to total volume. A room full of ping-pong balls would have the same porosity as a room full of basketballs—as long as the packing or arrangement were similar.
In sedimentary rocks, (such as sandstone), gravel, clay, and silt, porosity is a function of the size, shape, and sorting of the grains as well as to what degree they are bonded together. In limestone and other types of sedimentary rocks, porosity is a function of the size of openings, such as cracks, fractures, crevices, and caverns.
“Porosity is the volume of the voids divided by the total volume and, in saturated conditions, gives a measure of the amount of water in storage in the ground,” says Ralston. “Specific yield gives a measure of the amount of water that can be withdrawn from an unconfined aquifer.
“Hydraulic conductivity [permeability] is a more important term than porosity,” he says. “Hydraulic conductivity gives a measure of the water movement through the ground. A high hydraulic conductivity aquifer of considerable thickness will support high well yields.”
Groundwater on the Move
Permeability refers to groundwater’s occurrence and movement as well as to a porous material’s ability to allow fluid to flow through it. For example, fine-grained sediments, such as clay and silt, have high porosity. But, because of their small size, they tend to be closer together, and do not allow much water to flow through them. Therefore, they have low permeability and do not readily transmit water.
“In groundwater applications, we use the term ‘effective porosity’ or ‘drainable porosity,” says Terry. “This is the amount of interconnected porosity in the rock, which can store and transmit water to wells.
“Another important porosity concept is ‘primary porosity’ versus ‘secondary porosity.’ Primary porosity is a property of the rock itself, such as the spaces between grains of sand in sandstone.
“Secondary porosity is formed by cracks, which form in the rock, such as faults or joints in the rock. Secondary porosity is generally the more important factor in most types of bedrock, such as shale or granite, while primary porosity may dominate in sandstone,” he says.
“It is important to understand which type of porosity dominates in a particular area, because it will dictate where the highest- yielding well zones are located, and therefore direct you as to how best to locate them.”
For more information about locating high yield wells, contact Dale Ralston at ralston@ moscow.com or David Terry at email@example.com. Other good sources of information are the University of Kansas at www.kgs.ukans.edu/HighPlains/atlas/apdrdwn.htm; the University of Nebraska at www.ianr.unl.edu/pubs/irrigation/g358.htm; the National Ground Water Association at www.ngwa.org; and the American Water Works Association at www.awwa.org.
About the Author: On Tap Editor Kathy Jesperson has begun to work toward a masters in public health. She hopes to finish the degree in the next three years.
If you have an article idea for her, please e-mail her at firstname.lastname@example.org.
American Water Works Association. 2003. Water Sources. Third Edition. Denver: AWWA.
Buddemeier, R.W. 2003. “Water ‘Table Drawdown and Well Pumping.” University of Kansas. www.kgs.ukans.edu/HighPlains/atlas/apdrdwn.htm.
Ibid. 2003. “Groundwater Storage and Flow.” Univeristy of Kansas. www.kgs.ukans.edu/HighPlains/atlas/apdrdwn.htm.
Ibid. 2000. “Aquifer Types and Terminology.” University
of Kansas. www.kgs.ukans.edu/HighPlains/atlas/apdrdwn.htm.
Driscoll, Fletcher. G. 1995. Groundwater and Wells. Second Edition. St. Paul, MN: U.S. Filter and Johnson Filter Screens.
Pederson, Darryll. and Deon D. Axthelm. “Artesian (Confined) Aquifers and Effect of Pumping.” University of Nebraska. www.ianr.unl.edu/pubs/irrigation/g358.htm.
U.S. Geological Survey. 2003. “Aquifer Basics.” www.usgs.gov.