Citizen’s Guide to Denver Basin Groundwater
This Citizen’s Guide to Denver Basin Groundwater (2007) is the seventh in a series of educational booklets designed to provide Colorado citizens with balanced and accurate information on a variety of subjects related to water resources. Copyright 2007 by the Colorado Foundation for Water Education. ISBN 978-0-9754075-6-1
Acknowledgements
The authors and the Foundation are solely responsible for the contents of this Guide.
Authors: Ralf Topper, Colorado Geological Survey
Bob Raynolds, Denver Museum of Nature & Science
Design: R. Emmett Jordan
The Colorado Foundation for Water Education thanks the people and organizations who provided review, comment and assistance in the development of this Guide.
The Foundation extends a special thank you to the Colorado Geological Survey, Colorado Water Conservation Board and Suncor Energy Inc. for their generous financial support that made this publication possible.
Colorado Foundation for Water Education
1580 Logan St., Suite 410, Denver, Colorado 80203 • 303-377-4433 • www.cfwe.org
Officers
President: Diane Hoppe
1st Vice President: Justice Gregory J. Hobbs, Jr.
2nd Vice President: Matt Cook
Secretary: Wendy Hanophy
Assistant Secretary: Rita Crumpton
Treasurer: Chris Rowe
Assistant Treasurer: Ken Lykens
Board of Trustees
Steve Acquafresca
Becky Brooks
Rep. Kathleen Curry
Alan Hamel
Taylor Hawes
Lynn Herkenhoff
Sen. Jim Isgar
Rod Kuharich
Veva McCaig
Margaret Medellin
Dale Mitchell
John Porter
John Redifer
Rick Sackbauer
Robert Sakata
Reagan Waskom
Staff
Don Glaser, Executive Director
Jeannine Tompkins, Office Manager
Mission Statement
The mission of the Colorado Foundation for Water Education is to promote better understanding of water resources through education and information. The Foundation does not take an advocacy position on any water issue.
All photographs and illustrations are used with permission and remain the property of the respective photographers (©2007). All rights reserved. Denver Public Library Western History Collection–cover (top), p.1 (bottom), p.12 (top 2), p.15, p.21. Brian Gadbery–cover (bottom), p.10, p.22, p.24, p.28. Ruth Wright–p.1 (top). Emmett Jordan–p.2, p.16, p.17, p.18, p.28, p.29. Eric Wunrow–p.4. Bob Raynolds–p.6 (3), p.8, p.33 (background). iStockPhoto. com–p.32. Jim Richardson–p.33.
About the cover: Inset above homes in Castle Rock, two men pose with their horse-powered well drilling rig.
Recent archaeological investigations have unearthed ancient Puebloan reservoirs within Mesa Verde National Park. To support a population that grew into the thousands, the ancestral Puebloans of southwestern Colorado learned to plan, build, and operate public works projects to collect and store water. Some evidence suggests that their disappearance by A.D. 1300 may have been in response to decades of successive drought cycles and the subsequent loss of their water supply. Like our predecessors, we need to develop long-term water supply strategies to serve and sustain our growing population. Fortunately, we have more technical and physical resources available to meet our water needs. The challenge will be to develop and implement effective water supply strategies in a timely and responsible manner.
munity-wide water planning efforts proved vital to their success. In the semi-arid west, water is viewed by some as an under-valued commodity for which our expectations should be modest and our willingness to pay, higher.
Many residents of the greater Denver metropolitan area are relying on non-renewable groundwater from deep aquifers in the Denver Basin for their water supply. Increased reliance on this resource has resulted in significant water level declines in some areas.
Population growth and increasing prosperity result in increasing municipal water demands. Projections from the State Demographer suggest that 2.8 million new residents will call Colorado home by the year 2030. Approximately two-thirds of these new citizens will reside along the Front Range. Population growth in Colorado will influence changes in lifestyles, land use, politics, personal values, and economics.
Perhaps we can learn from the early Puebloans whose com-
Our modern society has more resources than the Puebloans and can better adapt to changes in the distribution of our natural resources. The administration and management of water for sustained future growth will become increasingly complex. This will require more reliance on strategies such as aquifer storage and recovery, conjunctive use of ground and surface water resources, conservation and reuse, and controlled groundwater pumping. Aside from these engineering and management mechanisms, political, economic, and personal behaviors may also continue to evolve and our laws will continue to adapt in order to reflect the true value of safe and secure water resources for future generations.
This Citizen’s Guide explores the nature of groundwater in the Denver Basin from a geologic, hydrologic and a sociologic perspective. Particular attention is devoted to the area of southern Arapahoe County and northern Douglas County, commonly referred to as the South Metro Area, because of its rapid growth and reliance on groundwater. Although much is known about the nature of the Denver Basin aquifers, additional understanding is necessary to effectively and wisely use this water supply. This publication presents information regarding the history, status and future of this important water resource.
Cowboys water their horses at what was then known as Mummy Lake. Excavation revealed this site was part of an ancient reservoir system and is now known as Far View reservoir.
Excavation of Far View Reservoir, Mesa Verde National Park.
WWater is the life-blood of civilization.
In semi-arid Colorado, we maintain a delicate balance between water supply and demand.
Beneath the Denver metro communities lies a tremendous groundwater resource. This resource comprises aquifers that cover an area extending from Colorado Springs on the south to Greeley on the north and from the foothills of the Rocky Mountain foothills on the west to the eastern plains near Limon. This 6,700 square mile area is commonly referred to as the Denver Basin.
In the Denver Basin, bedrock aquifers serve as a bank of stored water held in the pores of sandstones and siltstones that are more than 50 million years old. Studies show that it took tens of thousands of years or more for nature to fill this resource. Currently, many individuals and
municipalities in the Denver Basin rely on this groundwater for their primary water supply, and in some places the decline in water levels is rapid.
In the 1970s and 1980s when the state began regulating the pumping of these aquifers, it recognized that they were finite and received virtually no recharge. In fact, Denver Basin aquifer withdrawal results in mining of the aquifers that depletes the water in storage and lowers water levels (pressure head) in wells.
Most wells that tap this resource are 500 to 2,000 feet deep. As water levels decline, well production decreases and more wells may be required to meet the same demands. Water level declines of more than 20 feet per year have been observed in the primary aquifers used for public water supply in densely populated areas. Such dramatic declines are the response of confined aquifers to a focused pumping regimen. As pressure levels are reduced, these rates of decline may not be sustained into the future.
Some wells located along the western
flank of the basin near the foothills have already felt the adverse effects of the declining levels. For some, the remedy may be to drill deeper to extend the useful life of their wells. For others, this may not be a technically feasible or economically viable. In some cases, a deeper aquifer option may not exist. Still others have recognized that they are going to have to replace their groundwater supplies with renewable sources of water if they are to maintain a sustainable water supply in the long-term.
Eventually the water held in these aquifers will be so depleted that additional pumping will become physically and economically impractical. While this scenario may seem bleak, an informed populace can participate in the necessary long range planning required to assure a sustainable water supply. Factors that will influence our groundwater future in the Denver Basin include the following:
• The amount of water in storage in the aquifers;
• Rates of water withdrawal;
• Location of future population growth and development;
• Alternate sources of water to meet demands;
• Aquifer storage and recovery projects;
• New regulations and legislation;
• Water conservation measures and
• The economic value placed on the water resources
This Citizen’s Guide to Denver Basin Groundwater explores the geology and hydrology of our underground water resource, the legal framework developed for its administration and management, the current development of this resource, and its limitations and sustainability.
The greatest impacts from increased pumping of these bed rock aquifers is occurring in the South Denver Metropolitan areas of southwest Arapahoe County and northern Douglas County. For this reason, they are an emphasis of much of this document.
Groundwater Basics
Throughout the Front Range metropolitan area, both surface and groundwater are used for agriculture, municipal, industrial, and domestic use. Early settlers first diverted surface water from streams, rivers, and lakes for mining activities, then for irrigation and other farm related purposes. Surface water, with its ready access and storage capability, has historically provided about 80 percent of Colorado’s water supply.
In contrast, groundwater refers to all water beneath the land’s surface. Because it is hidden from view, many people think of it in the form of underground lakes, streams and veins. But most groundwater is located in very small water-filled pore spaces between rock grains in sedimentary rocks, between sand and gravel particles in al -
luvial deposits, or in narrow crevices such as fractures and faults in crystalline rocks. Humans have tapped these water-filled pores and fractures since early civilization. Globally, groundwater accounts for the majority of fresh water on earth.
Groundwater contained in deep sedimentary rocks such as those found in the Denver Basin, has taken thousands of years to accumulate. Because of the inter-layered nature of the sedimentary rocks that make up the aquifers, precipitation infiltrating the soil does not immediately impact the amount of water in storage in these aquifers. Natural recharge to the deeper bedrock aquifers of the basin is so slow that this groundwater is essentially non-renewable.
Groundwater found in sedimentary rock (top) is slow to recharge, while alluvial groundwater quickly interacts with surface water (above).
Water supply wells of record with the State Engineer’s Office as of Feb. 2001.
Finding Water
The Denver Basin is a structural sedimentary basin that underlies the Denver metropolitan area from the foothills to the eastern plains. This layered, multi-aquifer system is recognized nationally as a major aquifer. Geologic units underlying the Denver Basin aquifer are also rich in mineral fuels with active production of oil and gas. Historically, coal has been produced from the Laramie Formation of the aquifer system.
The structural depression referred to as the Denver Basin extends north and eastward outside of Colorado’s boundaries. This larger basin accumulated sediments as the Western Interior Seaway retreated and the Rocky Mountains rose. However, when the state was called upon to determine how the basin’s water should be used, it mapped out a smaller administrative area where it would focus its efforts. This administrative area covers 6,700 square miles extending into Weld County on the north; El Paso County on the south; Jefferson County on the west; and the eastern portions of Adams, Arapahoe, and Elbert Counties on the east. The thickest portion of the basin lies just west of the town of Parker, where the LaramieFox Hills aquifer is approximately 3,000 feet below the surface.
The Denver Basin bedrock aquifer system consists of water-yielding strata, predominantly sandstones and siltstones, of Tertiary- and Cretaceous-age sedimentary rocks deposited 65-70 million years ago. The northern part of the Denver Basin aquifer system underlies the alluvial aquifer of the South Platte River, and is hydraulically connected to that unconsolidated aquifer over part of that area.
Structural Sedimentary Basin — A topographically low area in the Earth’s crust in which sediments have accumulated by transport via streams from the adjacent hills
This electron micrograph (left) shows rock grains and the small pore spaces that provide groundwater storage in the Arapahoe aquifer. The scale bar is equivalent to .5mm (thinner than a fingernail).
Studying the Denver Basin
Denver Basin groundwater refers generally to groundwater within the Dawson, Denver, Arapahoe, and Laramie-Fox Hills aquifers. This groundwater has a special classification under Colorado water law. The Colorado legislature exercises absolute authority over how the Denver Basin bedrock aquifers are allocated, whereas surface water and tributary groundwater are subject to the Colorado constitution’s prior appropriation doctrine.
Our understanding of the hydrogeology and water availability of the Denver Basin is still evolving. Many organizations, from federal scientific agencies to individual water districts, have studied this aquifer system. During the late 1960s and early 1970s, geologists with the Colorado Division of Water Resources and the U.S. Geological Survey began to study the formations of the Denver Basin and map the distribution of the aquifers. These efforts continued into the 1980s and resulted in a number of publications.
In the early 1990s, the Colorado Water Conservation Board and the Division of Water Resources began funding data collection and the development of computer-aided modeling tools called Decision Support Systems, to help administer
water in each of Colorado’s major river basins. The South Platte Decision Support System is currently being developed. In addition to looking at the river system, this model also evaluates the Denver Basin bedrock aquifers as they interact hydrologically with the South Platte River alluvial aquifer.
As part of this Decision Support System, the U.S. Geological Survey is developing a new multi-layered numerical model of the Denver Basin. This model will enhance groundwater administrators‘ and scientists’ understanding of the impacts of well pumping, changes in storage and stream depletion. Studies by the Denver Museum of Nature & Science in the late 1990’s, supported by the National Science Foundation, indicated the geological framework of the aquifers was significantly different than previously anticipated, resulting in reductions in estimates of stored water.
Recognizing that the depletion of the Denver Basin aquifers is a regional issue, individual water suppliers that help serve the metro area have funded cooperative studies to investigate alternatives for meeting future water demands. The South Metro Water Supply Study completed in February 2004 was a joint effort between the Doug-
las County Water Resources Authority participants, Denver Water, and the Colorado River Water Conservation District.
This study concluded that water levels will continue to decline in response to projected pumping rates even with expanded conservation and reuse by water providers. The study further indicated that continued pumping of the Denver Basin aquifers will produce significant losses to well production, even with the majority of water still in storage, due to well-to-well interference. Well-to-well interference occurs when densely clustered wells compete for the same stored water.
In the southern portion of the basin, the El Paso County Water Authority has recently prepared a water report to assist its members in meeting year 2020 water demands. The water report suggests that “The concept that housing developments could be initiated with Denver Basin aquifer water and generate sufficient revenues to purchase renewable water as a long-term water supply solution does not acknowledge the complex political, environmental, and water availability issues associated with the development of renewable water resources.” Individual water districts are also conducting their own investigations in developing their Denver Basin groundwater supplies.
Most recently, the Colorado Geological Survey has been conducting detailed hydro-geologic mapping to complement its current surface geologic mapping along the western margin of the Denver Basin. The survey is refining interpretations of the Denver Basin aquifers in the Dawson Butte and Castle Rock South USGS 7.5 minute quadrangles south of Castle Rock. This area includes significant new commercial and residential developments that rely on Denver Basin groundwater, which are on the western margin of the basin where impacts from regional water level declines are currently most severe. This mapping effort will provide important information in support of critical future water management decisions.
The Geologic Story
Groundwater is in rocks. It is not in subterranean pools or in underground rivers. It occurs inside rocks, within pore spaces and between mineral grains. Porous rocks are the storage vessels for groundwater; when these rocks are water-saturated they become aquifers.
Have you ever seen the rain “wet” a rock? Once water saturated, many rocks appear darker. Even if the surface water is wiped off, the rock remains damp. A rainwet rock provides an illustration of water seeping into the pore spaces of the rock. It is easier to extract or drain water out of coarse-grained rocks like gravels or sandstones, because the pore spaces tend to be large and well connected. The rock is said to have high porosity and permeability.
By contrast, fine-grained rocks like mudstones, clays and shales yield water at much lower volumes and slower rates, because the smaller less-connected pore spaces drain less efficiently. Fine-grained rocks yield little water and may even produce impermeable or confining layers separating aquifers.
The rocks that make up the groundwater aquifers in the Denver Basin are sedimentary rocks, deposited as beaches and river channel sands as an ancient sea retreated and the Central Rocky Mountains rose. The rocks occur in logical patterns that reflect how and where they were deposited. Because the rock types influence groundwater yield, these depositional patterns directly influence the aquifer’s behavior.
These many-million year old beaches and deposited river channel sands create the geologic formations that geologists have labeled, in descending order, the Dawson, Denver, Arapahoe and Laramie Formations, and the Fox Hills Sandstone.
These paintings at left depict Ancient Denvers, scenes from the Front Range as they may have appeared over the past 100 million years. Images from the Denver Museum of Nature & Science.
How Geologists Read the Earth’s History
To place the groundwater resources of the Denver Basin into perspective, it is best to start with the container—the rocks. Come along, it’s a fun story.
The geologic history of Colorado is eloquently written in the rocks beneath our feet. Geologists learn to read this writing, and they deduce the history of the Earth from clues we can all see.
Core samples taken from well bores tell geologists that the Denver Basin aquifers are curved and layered one on top of another, like a stack of bowls. In some areas, these rocks are found in thick layers far underground. At the margins of the basin the layers outcrop, giving scientists a great opportunity to determine what the rocks of the Denver Basin look like underground, and how the aquifers formed.
In the Denver area, for example, the steeply tilted sandstone ridge west of town at Dinosaur Ridge, near Red Rocks Park, reveals dinosaur bones and foot prints, ripple marks, and fossil plants. Remains of mangrove-type swamps and tidal estuaries can be seen on the rocky ridge.
Knowing what modern beach and near-shore systems look like, geologists can deduce that the Dinosaur Ridge outcrops are the remains of 100 million yearold shoreline landscapes.
Throughout the ensuing 30 million
years, more than a mile of marine shale, rich in sea shells and fossil fish, accumulated on top of these sediments. These fossils are easy to find on the edges of the Denver Basin and are on display in museums, such as Denver’s Museum of Nature & Science.
As the ocean retreated over the next million years, a geological layer known as Fox Hills Sandstone accumulated on top of the marine shale. Immediately over this sandstone layer, another geological layer full of coal beds, sandstone, shale and mudstone beds soon accumulated, representing a swamp-like environment. This is called the Laramie Formation.
The porous units of these two formations make up the oldest and deepest of the Denver Basin aquifers—the LaramieFox Hills Aquifer.
Some 68 million years ago, the Central Rocky Mountains were being uplifted by tectonic forces. Tumbling wild from the mountains came rivers full of the ingredients for a series of rock layers that spread across and filled the Denver Basin. This variable debris was the genesis for the most water-rich rock layers of the Denver Basin. Over the years, geologists have named these rocks from older to younger—the Arapahoe, Denver and Dawson Formations.
The Arapahoe Formation is older, representing the onset of significant mountain
uplift. The Arapahoe tends to be gravel-rich, particularly in the western part of the basin.
Detailed mapping and correlation of data from water wells show that the Arapahoe Formation was, in part, deposited by rivers as a giant apron or fan of debris that came out of a vanished canyon located west of Castle Rock.
Over geologic time, these deposits were buried and eventually subsided to depths of 2,000 feet or more, where they make up the water-saturated sandstone beds of the Arapahoe aquifer. This is one of the most important aquifers in the basin and is the major source of groundwater for municipal water users in Douglas County.
The Denver and Dawson formations overlie the Arapahoe Formation and intermingle with one another. The Dawson tends to be rich in granite fragments, while the Denver is rich in volcanic components. The diversity of the geologic environments at the time of deposition results in variable quality aquifers depending on location within the Denver Basin. Generally, the western portion of the aquifers are better water producers than their equivalent units in the central or on the eastern side of the basin. The location of a well with respect to the ancient sediments is critical to its water producing potential.
Visitors to Dinosaur Ridge near Morrison view 100 million year old Iguanadontid footprints.
Aquifer Descriptions
Arapahoe
In the 1980s, when the State Engineer’s Office was asked to classify the aquifers in the Denver Basin in order to regulate their use, it divided these sedimentary layers into four aquifers: the Dawson, Denver, Arapahoe and Laramie-Fox Hills. The aquifer units imperfectly mimic the distribution of the geologic formations of the same names. The four Denver Basin bedrock aquifer layers are stacked like the layers of an onion. The uppermost aquifer covers the smallest area. Successive underlying aquifers cover larger and larger areas. While the onion analogy accurately represents the stacked geologic unit/aquifer concept, the basin has an asymmetrical bowl shape in crosssection that is approximately one-half mile thick and 70 miles wide. The asymmetry is expressed by low-angle dips of the aquifers along the northern, eastern, and southern margins and high-angle dips along the western margin. For administrative purposes, their boundaries are defined by the Denver Basin Rules promulgated by the State Engineer in 1985. From the top down they are:
Rock Names
Dawson
This shallow uppermost aquifer covers an area of approximately 1,400 square miles in Douglas County, northern El Paso County, and western Arapahoe and Elbert counties. This aquifer is most commonly tapped by shallow domestic wells. Water is found in discontinuous lens-shaped sandstone beds deposited by ancient meandering rivers. The rocks are typically gravel-like and composed of weathered granite with typical well yields up to 300 gallons per minute. There are approximately 1,900 highcapacity wells completed in the Dawson aquifer that have been permitted to withdraw 30,800 acre feet annually.
Denver
Underlying the Dawson aquifer, the Denver aquifer extends further north, east, and south and covers an area of approximately 3,500 square miles. It typically contains fewer sandstone beds than the Dawson aquifer because the ancient meandering rivers were less common, and mudstone beds often predominate. Sandstone and pebble compositions are typically volcanic. Wells commonly have yields of between 50-150 gallons per minute. Approximately 800 high-capacity wells have been completed in the Denver aquifer with total annual permitted withdrawals of more than 72,600 acre feet.
Arapahoe
The Arapahoe aquifer includes rock units on the west side of the basin deposited near the mountains by an ancient river fan system. These units possess splendid aquifer characteristics. This aquifer underlies an area of approximately 4,700 square miles. Deep municipal wells along the I-25 corridor are often drilled in this aquifer and good quality wells yield up to 800 gallons per minute. The aquifer also includes rocks deposited farther east (away from the mountains) but their fine-grained nature results in less productive wells, as found in eastern Elbert County. The State Engineer’s Office has issued permits for more than 1,000 high-capacity wells in the Arapahoe aquifer with maximum total annual withdrawals of more than 168,700 acre feet.
Laramie-Fox Hills
The Laramie-Fox Hills aquifer underlies the entire 6,700 square mile administrative area of the Denver Basin. It is made up of both the beach sandstones of the Fox Hills and the overlying river sandstone beds in the lower Laramie Formation. Coal beds commonly occur in the lower part of the Laramie Formation and water from this aquifer can be high in sulfur. Well yields of 350 gallons per minute are typical. As the deepest of the four aquifers, it is often considered to be the aquifer of last resort for drinking water supplies. The Laramie-Fox Hills aquifer has seen the development of 490 high-capacity wells with a permitted cumulative maximum withdrawal of 51,600 acre feet.
Fountains of Water: Water Use and Regulation
Artesian wells were found in many places in Colorado, including this well near Montrose (top). Pressure from artesian wells in Denver (right) were used, among other things, to operate the organ bellows in Trinity Methodist Church (above).
Artesian well or artesian spring — A well or spring that taps groundwater under pressure such that the water rises above the top of the aquifer, but may not come to the surface, without pumping. If the water naturally rises above the surface, it is known as a flowing artesian well.
Colorado’s early water development history is tied to the need for irrigation in a semi-arid climate. The diversion of river water for irrigation was first practiced in the mid1800s by two groups of settlers—Hispanic settlers in southern Colorado and pioneers from the eastern states and northern Europe who settled in the South Platte and Arkansas River basins.
Legal rights to use water diverted from the rivers and creeks of Colorado became codified into territorial law. Ultimately a system of prior appropriation was devised to govern the use of all surface water in the state, and this concept was included in Colorado’s 1876 Constitution. A simple way to explain this system is “first come, first served,” that is, whoever diverts unappropriated water first for beneficial use, has a senior right. A majority of the laws regarding the use and development of water in Colorado are based on this system of prior appropriations for protection of water rights in order of their priority dates.
When many of these laws were created, groundwater use was minimal in most areas. Wells were generally shallow and hand-dug. Windmills dotted the landscape, particularly on the high plains, and pumped limited quantities of groundwater for livestock and homesteads. Almost a century would pass before groundwater pumping would increase dramatically, and require extensive regulations to govern its use.
Colorado water law has largely been developed through the interaction between water users, water officials, the legislature and the judiciary. Judicial rulings have frequently identified and highlighted emerging groundwater issues. Rules governing the use of Denver Basin groundwater have evolved over time as our understanding and use of these aquifers has increased.
Denver Basin Groundwater Development
Colorado Scientific Society members researched and documented the artesian wells of Denver as early as 1884. According to historical accounts, R. R. McCormick was boring for coal in a ravine by St. Luke’s Hospital in north Denver, when he struck the first artesian water. Due to its purity and superiority over the water furnished from the South Platte River by the Denver Water Company, his discovery soon led to more interest in widespread groundwater development.
Much discussion ensued in the scientific and business community about the potential source of this gushing water. Many thought the water was derived from seepage from reservoirs nearby, but other scientists correctly asserted this water was artesian–emanating from deep sandstone layers under considerable pressure.
The geology of the countryside around Denver was first described and mapped by Ferdinand V. Hayden in the 1870’s with
funds from the Department of the Interior. In 1873, this study became known as the United States Geological and Geographical Survey of the Territories, or the “Hayden Survey.” His work showed artesian groundwater to be common along the western edge of the Denver Basin from the towns of Marshall (near Boulder) to Sedalia, in areas well known for their tilted sandstone outcrops. The outcrops measured 200 to 600 feet higher than the City of Denver, and were thought to be areas of aquifer recharge whose elevation could account for the pressure in the artesian wells of Denver.
By 1890, numerous artesian wells had been drilled in the area. At the lowest elevations in the basin, artesian pressure pushed water more than 100 feet above the ground. Two main water-bearing sandstone layers had been identified at depths of 375 and 600 feet in downtown Denver.
The fabulous artesian pressures from these wells were put to many innovative uses. They helped produce the decorative fountains at Union Station, provided a power assist for the elevators at the Brown Palace Hotel, and operated the organ bellows at the Trinity Methodist Church. By 1895, nearly 400 wells had been drilled in the vicinity of Denver and surrounding areas.
But they soon began to play out. By the mid-1890s considerable pressure losses had already been observed in the city wells and official investigations began as to its cause. To some, this free-flowing water was being wasted by extravagance and ignorance, and there was talk of new, special legislation to prevent its waste. However, it would not be until the 1950s that new technology, population growth and drought would combine to push the state’s first set of extensive groundwater regulations into place.
Confined or Artesian Aquifers
The term water level ( ) is used in this document in reference to a completed well, whether it is the water table of an unconfined aquifer or the artesian head of a confined aquifer.
Confined or artesian aquifers are completely saturated geologic units in which the water is under pressure (artesian head) as a result of an overlying low permeability confining layer that prevents the free movement of air and water. As the early geologists surmised, the Denver Basin aquifers were under artesian pressure because the recharge areas were several
hundred feet higher in elevation than most of the basin. Over most of the basin, the Denver, Arapahoe, and Laramie-Fox Hills aquifers are confined such that water levels in wells rise above the top of the aquifer. The water produced from wells drawing on the artesian head represents a very small amount of the total water in storage as the aquifer is not being physically drained.
Widespread Use of Wells and Drought Conditions Require Groundwater Management
Although groundwater use in Colorado dates back to the late 1800’s, it was not until the 1950s that well drilling became widespread. The loss of surface water supplies during the drought of the 1950s, together with expansion of rural electric co-ops and the development of vertical turbine pumps, led to expanded groundwater pumping east of the Continental Divide between 1943 and 1969.
Existing vested surface rights holders in the South Platte and Arkansas river basins became concerned that un-administered diversion of tributary groundwater through wells could be intercepting water needed to fulfill senior surface water rights. This led to statutes and court decisions in the 1950s and 1960s requiring the State Engineer to administer tributary groundwater under the doctrine of prior appropriation.
In the high plains area of Colorado, intensified irrigation pumping led to a
Designated Groundwater under natural conditions does not recharge or supplement to any significant degree continuously flowing surface streams. It is regulated by the Colorado Ground Water Commission.
realization that the Ogallala aquifer, in particular, was being mined well beyond its recharge capacity. In 1957, the General Assembly created the Colorado Ground Water Commission for the purpose of examining and possibly regulating critical groundwater areas. In 1965, it gave the Commission authority to designate groundwater basins where the principal reliable source of water supply is groundwater with little connection to surface water.
The Commission has established eight designated groundwater basins in eastern Colorado, four of which (Lost Creek, Kiowa Bijou, Upper Big Sandy, and Upper Black Squirrel Creek) cover the rural eastern 47 percent of the Denver Basin. By establishing the designated basins, the State General Assembly acknowledged that some groundwater had little or no connection to the sur-
face streams. Creating the category “designated groundwater” was the first statutory departure from the “all groundwater is tributary to streams” rule recognized by the Colorado Supreme Court to protect appropriators of natural stream waters.
Unlike tributary groundwater or surface water, designated groundwater is regulated by the Ground Water Commission and is not subject to court adjudication. The Ground Water Commission uses a modified appropriation system to allocate designated groundwater outside of the Denver Basin on a permit by permit basis. But, the General Assembly has directed that designated groundwater within the Denver Basin shall be allocated to the owners of the overlying land, based on the hundred year pumping regime that applies to the four aquifers of the Denver Basin.
The Two Main Types of Groundwater
It is important to understand the two main categories that are used to talk about groundwater: Tributary and Non-tributary.
Tributary groundwater is hydraulically connected to a surface stream and can influence the amount or direction of flow of water in that stream. Water in sand and gravel alluvial aquifers adjacent to major rivers is an excellent example of tributary groundwater.
Non-tributary groundwater is typically produced from aquifers geologically confined such that they have little physical connection to surface waters. With the exception of the uppermost portion of the Dawson aquifer, groundwater contained under confined conditions in the Denver Basin aquifers is considered to be non-tributary.
Colorado Designated Groundwater Basins
Deep Groundwater Gets a 100-Year Minimum Aquifer Life
Between 1965 and 1973, many new wells were being drilled into the deep bedrock aquifers of the Denver Basin. This included areas in the South Platte River basin and in the south Denver metro area.
Concern over the amount of water being pumped and the life of these aquifers led to Senate Bill 213 passed in 1973. This legislation established how water pumped from deep and potentially nonrenewable aquifers should be managed
and set criteria for the State Engineer to follow in issuing well permits in these bedrock aquifers.
For the first time, the general assembly spelled out that landowners had the right to develop deep non-tributary groundwater underlying their property. To set a limit on pumping rates, the legislature adopted a 100-year pumping regime. Regulations were implemented that allowed landowners to withdraw water at the rate of one percent of the aquifer resource under their property per year.
Does a Well Permit Guarantee a 100-year Water Supply?
A Denver Basin well permit does not guarantee a 100-year water supply. Actually, the well permit gives a person the legal right to drill for water. Even when water is encountered, an estimate of available water is made, and a one percent withdrawal rate determined. Because of the uncertainty of available water estimates, the supply may last more or less than 100 years.
A group of Montrose, Colorado, citizens (circa 1885) pose in front of equipment used to drill an artesian well in Montrose County.
Regulating the Denver Basin Aquifers
In the late 1970s, groundwater speculation as well as large new housing developments that applied for groundwater permits precipitated concerns about sustainable water supplies for the Front Range.
The state legislature and Denver Regional Council of Governments created a groundwater task force charged with making recommendations about pumping water from the area’s deep aquifers. The following were among the panel’s main conclusions:
• The bedrock aquifers underlying the greater metro area contained large amounts of water that could be a great asset to supplement water supplies for the Front Range;
• Artesian wells and wells drilled at the edge of the Denver Basin, where the aquifers are thin, may be depleted, need to be re-drilled, or run dry before others;
• Economics would likely be the deciding factor in limiting this water resource;
• Decreased yields could cause serious social and economic problems, but this should not prohibit development of the Denver Basin aquifers.
In the mid-1980s a separate “Blue Ribbon” committee was also convened by Gov. Richard Lamm to develop a set of recommendations to the legislature regarding the future management of this resource. Among their findings are the following:
• The prior appropriation doctrine does not work for deep non-tributary groundwater. Preventing injury to senior water rights is central to the prior appropriation system. However, the interdependent and artesian nature of the Denver Basin aquifer system makes that protection impossible if the resource is to be developed. After the first well starts pumping, all subsequent wells will naturally deplete that first well’s original artesian pressure. Protecting “senior” rights from any depletion just isn’t possible.
• Deep bedrock aquifers should be pumped with special care, because they are essentially non-renewable, or renew over very long time frames.
• No groundwater is completely nontributary and if it does adversely affect surface streams, water rights holders should be compensated by returning some water to the stream.
• The interests of overlying landowners should be clarified.
Statutory Groundwater Definitions
Designated Basin Groundwater—Designated groundwater is water that under natural conditions would not recharge or supplement continuously flowing surface streams. It is specific to deep groundwater underlying the eight “designated basin” areas created by the Colorado Ground Water Commission, located on Colorado’s eastern plains. This is considered nontributary and is regulated under specific designated basin rules.
Tributary Groundwater—Water hydraulically connected to a surface stream that can influence the amount or direction of flow of water in that stream. It is regulated by the prior appropriation system, like other surface water rights.
Non-Tributary Groundwater—Water the pumping of which in 00 years will not deplete the flow of a natural stream at an annual rate greater than /0th of one percent of the annual rate of withdrawal from the well.
Not-Nontributary Groundwater—Denver Basin groundwater that is connected with surface streams or the deeper aquifers where they outcrop. If pumped, these withdrawals would deplete the flow of a natural stream at an annual rate greater than /0th of one percent the annual rate of withdrawal from the well.
Augmentation plans are court approved plans under the priority system to protect senior surface rights and are generally developed by engineers, lawyers, and other consultants. They allow for out-of-priority diversions by replacing water in the stream that junior users consume.
The Denver Basin Rules
In 1985, complex legislation commonly known as Senate Bill 5 took into consideration the recommendations of the “blue ribbon” committee. Specific statutory language and definitions were developed to address the allocation of the Denver Basin and Dakota aquifers, as well as all other non-tributary groundwater statewide. Senate Bill 5 also required the State Engineer to promulgate rules and regulations governing the withdrawal of groundwater from the Denver Basin aquifers by December 31, 1985. These became know as The Denver Basin Rules. While this legislation did not specify how groundwater in the basin should be managed over the long-term, it did provide a basic legal framework for how the groundwater of the Denver Basin should be allocated.
By enacting this legislation, the General Assembly agreed that it was acceptable to mine the Denver Basin aquifers by taking out more water than was being replaced. This was allowable even though reduction in the artesian head or water levels in the aquifers would occur, and some wells might be impaired.
The legislature also clarified that nontributary groundwater is “water which in 100 years will not deplete the flow of a natural stream at an annual rate greater than 1/10th of one percent of the annual depletion from the well.” This definition applies primarily to the Denver, Arapahoe and Laramie-Fox Hills aquifers.
The legislature also recognized that some of the deep Denver Basin aquifers were not completely dissociated from
overlying streams, and were actually not non-tributary. Unfortunately, the confusing wording “not-nontributary” stuck during the rule-making process and is still used today.
Not-nontributary groundwater refers to those parts of the Denver Basin aquifers which are in some way connected to surface streams or the aquifers where they outcrop. Examples that contain not-nontributary groundwater are areas along the South Platte River or the various streams within the basin such as Monument Creek, Plum Creek and Cherry Creek.
For parts of the Denver Basin not within a designated basin, the water court has jurisdiction to enter decrees for the use of water. In the designated parts of the Denver Basin, the 1/100th per year pumping rule applies and the Ground Water Commission makes all determinations regarding the allocation and use of water.
Replacing Depletions from Streams
Though the General Assembly defined non-tributary groundwater as not connected to surface water, they realized some hydrologic connection may be evidenced over very long timeframes. Consequently, Senate Bill 5 provided that not all the water withdrawn from the non-tributary Denver Basin aquifers could be consumed; two percent had to be replaced. The State Engineer generally assumes that this provision is met by return flows from outdoor watering or other sources.
The not-nontributary designation in
Areas along the South Platte River contain not-nontributary groundwater.
Senate Bill 5 required that the Denver Basin aquifers be evaluated for their potential hydraulic connection to surface water. This was accomplished by the State Engineer’s Office through modeling. To avoid having to evaluate how to replace this water on a case-by-case basis, a blanket strategy for replacing water potentially removed from local rivers by well pumping was implemented. The legislation required judicial approval of plans for augmentation at different standards of four percent of withdrawals or actual depletions depending upon the aquifer and the distance from the stream contact. These replacement requirements do not apply to groundwater within the designated basins.
Municipal Rights to Denver Basin Groundwater
As development of the Denver Basin aquifers progressed, cities and water districts wondered what rights they have to develop groundwater beneath their communities. Initially, water was allocated to the overlying landowner at the rate of one percent of the aquifer resource per year. Landowner consent is normally required to extract Denver bedrock water from under a person’s property.
SB 5 clarified that municipalities and water districts by implied consent may utilize the Denver Basin groundwater resources within their boundaries, if they provide a reasonable alternative water supply to the land owners.
How Much Groundwater is Left?
Current estimates are that the basin contains over 200 million acre feet of recoverable water in storage. Since most of the water production to date is from the artesian portion of the aquifers, we’ve used less than one percent of the water in storage. Yet, in the most heavily developed aquifers water levels (artesian pressures) are declining at rates of one inch per day (30 feet per year). More important than how quickly they are dropping is how long it will be economically feasible to continue significant pumping of these resources.
Each year, groundwater measurements from select wells in the Denver Basin are published by the Colorado Division of Water Resources. The validity of these data is sometimes questioned because the wells measured are not dedicated monitoring wells and vary significantly in age and how they are pumped, among other differences. However, consistent annual trends in the data allow engineers and hydrologists to infer the gain or loss of water storage in these aquifers.
The data indicate that depending on the aquifer and the well’s location, water levels in the Denver Basin aquifers are either relatively stable, declining, or even rising.
Water Level Trends
For the shallower Dawson and Denver aquifers, water level data show both rises and declines depending upon location. With the housing development boom of the last decade, lawn irrigation recharge
has become a significant input of water for portions of these two aquifers and may account for some of the observed water level increases.
Water level trends in the dominant municipal water supply aquifers, the Arapahoe and Laramie-Fox Hills, are not favorable. Over the past 10 years, water levels have declined throughout the Arapahoe aquifer. Between 1990 and 2000, development in the south Denver metro area of northern Douglas County and southern Arapahoe county has resulted in declines from 100 to almost 300 feet. With local decline rates of up to 40 feet per year, the future prospects for this aquifer are of great concern to water managers.
Annual decline rates of this magnitude were simulated by computer models as part of the 2004 South Metro Water Supply Study—a joint effort between the Douglas County Water Resources Authority, Denver Water and the Colorado River Water Conservation District to look at alternative sources of water supply.
Model results predicted the Arapahoe aquifer in southern Arapahoe and northern Douglas counties along the I-25 corridor will become unconfined by the year 2020, or sooner. This means that the artesian pressure head of the aquifer will have been drawn off, and wells will begin physically draining the pore space of the aquifer.
Throughout the past 10 years water levels in portions of the Laramie-Fox Hills aquifer have also declined. The Laramie-Fox Hills
aquifer is used as a municipal water supply in the southeast Denver metropolitan area, resulting in localized water-level declines of up to 125 feet in the past decade.
Annual water level changes for the Denver, Arapahoe, and Laramie-Fox Hills aquifers are shown in the sidebar on page 24. These patterns emphasize that in areas of high demand along the Front Range, groundwater resources are being withdrawn from the Denver Basin bedrock aquifers at rates in excess of recharge, resulting in a mining condition that depletes the groundwater in storage and lowers water levels in wells.
By contrast, in certain areas in the central and eastern portions of the aquifer, water levels are either stable or may be rising.
It is important to note, however, that the hydrogeological characteristics of the Denver Basin aquifers localize the effects of pumping. Pressure changes are not being felt over the entire basin as conventional theory would predict. The sedimentary rock formations that hold water in these aquifers do not allow significant groundwater movement laterally across the basin over short periods.
This means that even if groundwater levels are not dropping in the eastern portion of the basin, in terms of human life-spans, those portions of the aquifer cannot compensate for the significant water draw downs created by the pumping stresses of the south metro area.
Water Level Changes in the Arapahoe Aquifer 1983-2006
Graphs illustrating the falling water levels/artesian head in municipal water wells in the Arapahoe aquifer as reported by the Colorado State Engineer. Each graph shows the elevation (in feet) of the water level plotted against time, and each represents a well located at the spot indicated by the arrow to the map. The red line is the best fit line showing the long term rate of fall. As water level data is not reported systematically, some wells have more measurements than others.
How Long Will the Denver Basin Aquifers Last?
State statutes presume the productive life of the Denver Basin aquifers to be at least 100 years, and permit well pumping up to one percent of that supply per year. But well permits do not guarantee water will be available for a century or more. Well permits actually only grant the right to drill for water and pump at the stipulated rates.
Much time and money is spent trying to understand how and when water supplied from the Denver Basin aquifers will change from confined to unconfined conditions. These include extensive computer models developed by the USGS, the Division of Water Resources, and various academic and consulting groups.
Such models were used when the regulations allocating Denver Basin groundwater were first developed in the mid-1980s. They estimated, for example, the amount of recoverable water from each aquifer.
Over the past decade, a somewhat refined understanding of the geological characteristics of these aquifers has resulted
in substantial revisions to estimates of the amount of groundwater available for extraction. In the late 1980s, the actual amount of water stored in the Denver Basin aquifer system—from the foothills to east Elbert County—was estimated by the U.S. Geological Survey to be 467 million acre feet, with 269 million acre feet being recoverable by pumping. While gravity will help drain out much of the water, some will necessarily remain in the rock clinging to particles. That is why recoverable reserves are always less than actual reserves.
Today, new data indicates that actual aquifer yields may be one-third less than previously predicted. In 1999, a continuous core drilled in the central part of the Denver Basin by the Denver Museum of Nature & Science provided a series of geologic samples. Analysis of these samples and other recent data have helped to refine estimates of recoverable water. Current estimates are that about 200 million acre feet may be recoverable. Analysis of permitted wells recorded
Confined v. Unconfined Aquifers
In certain locations on the edges of the Denver Basin, the aquifers outcrop to the surface. This connection to the surface, lack of overlying confining layers, and a water table that is directly influenced by precipitation, creates what hydrogeologists call unconfined conditions along the edge of the basin.
This is in contrast to the majority of the Denver Basin where
at the State Engineer’s Office indicates exempt and non-exempt Denver Basin wells are authorized to withdraw as much as 350,000 acre feet of groundwater per year from all four aquifers. Assuming this rate of production is accurate and fixed, then 200 million acre feet of water in storage could last approximately 570 years.
Although a 570-year aquifer life sounds promising, scientists and water managers are concerned that these estimates may be misleading. Evaluating the recoverable storage of these aquifers still assumes that the aquifers will be drained almost completely. However, wells are physically incapable of draining an aquifer to that extent. In addition, much of the estimated recoverable resource is spread across the eastern part of the basin, where demand is minimal and the cost of extraction and conveyance is presently prohibitive. Because of uncertainty in the basin hydraulics there are many wells in the Denver Basin that may not have a 100year useful life.
confining pressures.
Increased groundwater withdrawals from the Denver Basin’s confined aquifers (the Denver, Arapahoe, and Laramie-Fox Hills systems) are beginning to convert these aquifers from confined to unconfined conditions.
As an aquifer becomes unconfined, it is no longer under pressure and its water begins to drain from the pore spaces between the rock grains. It is theorized the rate of water level decline in the aquifer should decrease as the previously confined aquifer goes unconfined.
Shale
Dropping Artesian Pressure
Artesian pressures dropped in the Denver Basin aquifers as soon as widespread pumping began more than 100 years ago. And, it will continue to happen in response to pumping because of the unique hydraulics of the basin.
Back in the 1970s and 1980s when scientists, engineers, lawyers and others were struggling to come up with a set of rules to regulate pumping in the Denver Basin, the drawdown of water levels (artesian head) and potential injury to some wells owners was understood as inevitable.
And over time, dropping artesian pressures has made extraction of water less efficient and more costly in some areas. Artesian pressure assists well pumps by reducing the total elevation the pumps must lift water. But with declining water levels, larger pumps and motors, as well as increasing energy usage will be required to produce the same amount of water. This problem has already started to occur in some south metro areas of the basin that are being drawn down by municipal pumping.
Economic Considerations
At the end of the day, the amount of water available from the Denver Basin may be limited by economics, as much or more than by state regulations or the amount of water in storage.
While it may seem that giving a resource a 100-year or even 400-year life is short-sighted, some also see it as an important compromise that allows development of the south metro area, while buying time for those same developments to generate the revenues needed to acquire renewable water supplies—or encourage annexation of developments by municipal providers.
This interim use/revenue generating concept, however, does not necessarily factor in all the complex political, social, economic and environmental issues associated with the development of re -
newable water resources. The acquisition of new water supplies is not simply a financial transaction.
It is likely that economics will prevent the Denver Basin aquifers from being completely exhausted. Over time, large-capacity pumping may become so expensive that it simply becomes too costly to drill more wells or to keep pumping existing wells with diminishing returns.
Cascading Reduction in Well Yield
To illustrate the cascading reduction in well yield and requirement to drill more and more wells to maintain a specific production requirement, we reference an example from the US Geological Survey.
Assume that well A produces 40 acre feet per year when initially completed, and it experiences a fixed rate of water level decline that causes the aquifer to be dewatered in 100 years. If the required application needs 30 acre feet per year, well A needs to be augmented in 25 years. So we drill well B, and the combined yield of both wells far surpasses our fixed yield, but the combined well yield decreases more rapidly. In another 27 years a third well (C) is required to maintain the required production capacity. With three wells operating we again exceed our water need, but due to declining water levels and well-to-well interferences it will only be 13 years before a fourth well is needed. You can see from the graph above that this sequence will continue at an ever increasing frequency even though the water level is declining at a constant rate.
In this example, if the cost of constructing and equipping each well is $ 500,000 then the water cost for the first well is $267 per acre-foot. Because of reduced total production as more wells are added, the water cost for the sixth well is $13,500 per acre-foot. This simple example does not include increased operation expenses. Clearly the economics of relying on non-renewable groundwater supplies as a primary resource are not favorable in the long-term.
Artesian pressures dropped in the Denver Basin aquifers as soon as widespread pumping began more than 100 years ago.
Regional Water Level Decline and Well to Well Interference
Drilling more and more wells is not necessarily a viable long-term solution. duces a cone of depression in the ground water around that well. Cones of depression from multiple wells in a well field overlap and accentuate each other resulting in a greater regional water level decline.
water, water levels are declining—in some cases quite dramatically. In areas where there are fewer wells or production rates are lower, such as in the majority of the eastern half of the basin, water levels are falling more slowly or remaining stable.
Computer modeling results from the 2004 South Metro Water Supply Study indicated that the combined effect of re gional water level declines and well-towell interferences may result in produc tion loss of as much as 85 percent in the Arapahoe aquifer by 2050.
Location, Location
Determining how much water will be available to current and future groundwa ter users in the Denver Basin is dependent on the aquifer tapped, and the location of the well.
In high population areas such as Park er and Castle Rock where there are many municipal wells pumping large volumes of
The Denver Basin aquifers also differ in physical character and water holding capacity from one side of the basin to the other. For example, the sandstones within the Arapahoe aquifer are thick and have the best water storage properties near the mountains on the west side of the basin. However, they become thin to nearly insignificant on the eastern edge of the basin.
The area “in the margin” or on the outer edges of the bowl—if one thinks of the Denver Basin aquifer as a series of stacked bowls—are the most vulnerable to dropping water levels. For example, some well users living on the western margin of the Denver Basin in Douglas County, have already been forced to
change
wells in the aquifers of the Denver Basin.
Denver Aquifer
Arapahoe Aquifer
Laramie-Fox Hills Aquifer
In Parker and Castle Rock where municipal wells are pumping large volumes of water, water levels are declining.
Douglas County
The orange area has been defined by Douglas County commissioners as Margin A, where dried up wells and lowered pressure have forced limits on development.
The yellow area is in less danger but still faces problems.
Water table declines are so prevalent in some areas of Douglas County that the county has developed maps highlighting these areas, and requires developers in these areas to find renewable, sustainable water supplies. These regions are referred to as Water Supply Overlay District Margins A and B (see map). It is anticipated that at the current rates of decline, many of the well users in these areas may not have available groundwater supplies in 15 years.
deepen their wells or pumps in an attempt to find more water.
In contrast, outside of these marginal areas, there are many well owners with shallow wells which tap Denver and Dawson aquifers that are not experiencing any water level declines. Most are relatively shallow, low-yield wells drilled by individual landowners or developers to serve household needs or a smaller subdivision. Because the draw on these shallow aquifers is spread over large areas, and extraction rates are lower, water levels in these aquifers are stable or falling at a slower rate. Although homeowners may
have to deepen their wells, the sustainability of this resource is promising.
It is important to note that vertical transmission of water between the Denver Basin aquifers is extremely slow. For this reason, the deeper aquifers being tapped and depleted by municipal users are not drawing water in any meaningful way from the overlying aquifers being tapped by individual homeowners.
Arapahoe Aquifer Dropping Fastest Efficient well production combined with good water quality makes the Arapahoe aquifer the most desirable of the Denver
Basin aquifers for municipal water supply. As a consequence, its water levels are dropping the fastest, particularly in the south metro area where high-volume municipal wells are operating. These wells are often 1,500 to 2,000 feet deep and can pump at rates of 700-800 gallons per minute or more. Sustainable water supply strategies are being developed and implemented to maximize the useful life of these wells. The strategies include water conservation, conjunctive use of surface groundwater and groundwater recharge.
Homeowners Hunt for Water
Plum Valley Heights, a small subdivision of horse properties tucked in the foothills between Sedalia and Highlands Ranch, has just 29 homes and is still dependent on non-renewable groundwater. And it’s running out.
For Jack McCormick and his neighbors, that spells problems in so many ways. They aren’t connected to a surface water supply, their aquifer levels are falling, Colorado water law doesn’t protect them, and other booming metro communities might or might not be amenable to helping.
“It’s a crisis for us in the margin,” McCormick says one misty spring afternoon, as he speaks with visitors in his dining room. He’s got maps to show where the basins and urban areas are, as well as the small communities in the Chatfield area cut off from the urban infrastructure. “We’re in the most tenuous part of the county.”
Plum Valley Heights is on the rim of the stacked and tilted bowls of the Denver Basin aquifers and is the first to feel the effects of a diminishing groundwater supply.
McCormick’s first well was drilled 408 feet into the shallow Dawson aquifer. It initially produced 5 gallons a minute, but production finally dropped to a point where it was no longer useful. Next, McCormick drilled a new well into the lower Arapahoe aquifer. Since 1987, the well’s static level has dropped 7 to 10 feet a year. Four times since then the pump’s been lowered, chasing the dropping water table. He estimates the well’s life may be only 1015 more years, depending on technology.
“Less than a half mile away, one of my neighbors has his pump as low as it can go,” McCormick says. “These folks up here,” he says pointing to a small enclave north of his property. “They’ve been hauling water for years.”
“One of the things we need for certain is a remedy,” McCormick says. “I’d be hard pressed to prove Highlands Ranch is pumping our water. We need legislation that adequately defines injury.”
McCormick and his neighbors have been “working the issue.” They aren’t rookies. The residents of Plum Valley and the surrounding communities have been talking to county commissioners, lobbying for legislation, searching for renewable water sources in conjunction with new developers, and teaming up with their 30,000 or so rural neighbors in Louviers, Chatfield Basin and Meridian to bring in a surface water supply.
Plum Valley, McCormick says, is hanging some hopes on developers who are mulling plans for Sterling Ranch, a 2,000-acre tract immediately to the east. If the area is developed for housing, Plum Valley and its neighbors could tap into its infrastructure for surface water. They’re already saving money for the possibility.
“We’re not looking for others to solve our problems,” McCormick says. “We want to come to the table and participate.”
Population Growth & Development
Denver Basin population by county from Colorado Dept. of Local A airs
Douglas County has captured national headlines for being one of the most rapidly growing counties in the nation. Between 1990 and 2000, its population grew by 191 percent and local municipalities and water districts have worked hard to acquire new water sources to meet this growing demand. Development has continued even in areas lacking a renewable and sustainable water supply. Because groundwater has been plentiful, is of good quality, and is inexpensive to produce, development has used this resource as its primary source of water supply. However, municipal water providers are beginning to recognize that continued dependence on groundwater will come at an ever-increasing price.
Colorado experienced accelerated growth in the 1990s with influxes of people and a large construction boom that created 10,000 new jobs each year. According to the Colorado Department of Local Affairs, each of the Denver Basin counties experienced significant population increases since 1985 with resultant increases in water demands.
Municipal water providers must secure adequate supplies to meet peak demands within their service area. Providers generally plan on an annual water demand of 0.6 acre feet per household. In 2004, Elbert, Douglas, Weld, Adams, El Paso, and Arapahoe counties had approximately 778,000 housing units combined, which
Total Housing Units by
County
would equate to a water demand of about 467,000 acre feet per year.
Results from the recently completed Statewide Water Supply Initiative indicate that Colorado’s population is expected to increase by 65 percent between 2000 and 2030. An additional 2.8 million people are expected to call Colorado home by 2030, with over 80 percent of these new residents living along the Front Range urban corridor. Planning officials in Douglas County have projected that their population will nearly double by the year 2025. Where will municipal suppliers find a sustainable, renewable water supply required to meet these increased future demands?
These three maps show the increase in developed areas from 1957 to 1977 to 1997, respectively (United States Geologic Survey).
Fort Collins
Sustainability
The sustainability of natural resources is difficult to quantify, but it is often generalized that we must meet the social, environmental, and economic needs of the present without compromising the ability of future generations to meet their own needs.
In some ways, this concept of sustainability appears to be at odds with Colorado’s own water resource doctrine: “The right to divert the unappropriated waters of any ‘natural stream’ to beneficial uses shall never be denied.” The courts have determined that maximum utilization of stream water is implicit in the Colorado Constitution.
The problem is that maximum utilization of groundwater resources in the Denver Basin is not sustainable. These ancient aquifers are a finite resource that replenishes only on the scale of hundreds or thousands of years. Yet by allocating this resource based on a 100-year life, the state legislature acknowledged this was
not a renewable resource, and could not be expected to last forever.
Although there is significant debate regarding how much water is available in different parts of the Denver Basin, it is important to note that aquifer drawdown and a 100-year aquifer life are central parts of how the state decided to allocate this complex resource.
The result is that depending on location and the aquifer tapped, south metro area businesses and homes may have serious water supply challenges to address in the near future. These challenges can be framed with certain scientific evidence.
Current well logs and measurements show that the Denver Basin aquifers are reacting variably to the pressures of development. Where demand is modest and the resource large, the aquifers show few declines and may provide many decades of adequate supply. This
Aquifer volume drained by pumping
generally applies to private exempt domestic wells except for those located along the basin’s western margins.
In contrast, municipalities like Castle Rock that are pumping from the deep Arapahoe aquifer have experienced annual declines of 40 feet or more, and it is anticipated that these wells will start to have diminished water flow rates within the next decade.
Rural well owners on the western margins of the Denver Basin between Denver and Colorado Springs are particularly at risk. Not only are they in a clearly identified area of significant groundwater level declines, but they must find their own solutions, through the formation of rural water users associations, long-range planning or resigning themselves to trucking in water.
We have previously stated that the estimated recoverable water supply of 200 million acre feet in storage could last approximately 400 years under the current maximum permitted rate of withdrawal. However, the estimate of the amount of water storage is based on an average value of water holding capacity (specific yield) for each of the four major aquifers. We have also stressed that this aquifer property is very location dependent being a function of geology. This graphic illustrates a simple conceptual model of the Arapahoe aquifer and the relation between geology and the amount of water recoverable. On the west side of the basin where the aquifer is dominated by sandstones, the amount of water physically capable of draining from the saturated pore spaces may be as much as 25 percent of the rock volume. The eastern portion of the aquifer volume may only yield 10 percent. With only two borings in the Denver Basin from which aquifer core samples have been recovered and analyzed, our knowledge and estimate of the average storage coefficient is limited.
Aquifer volume drained by pumping
Converting to Renewable Water
Ron Redd is in his sixth year as utilities director for Castle Rock. Bisected by Interstate 25, the former quarry town is one the Front Range’s fastest growing communities.
The 38,000-person town now depends almost entirely on non-tributary, non-renewable groundwater from the Denver Basin aquifers. But faced with a build-out population of some 104,000 people, the town’s declining wells may be unable to produce enough water. That’s why in the next 20-30 years Castle Rock plans to convert to a water supply that is more than 75 percent renewable water. Town planners estimate they will need an additional 18,800 acre feet of water per year.
In preparation, in early 2006 the town increased its water resource impact fees
on new development to $19,500, from $11,500 for each new residential building permit. The additional money raised will help fund new renewable water sources. Their end goal is to provide a transition and “soft landing” for the town as it moves away from the exclusive use of non-renewable groundwater, and sustains itself through the integrated use of ground and surface water supplies. Part of the solution is water conservation.
Currently, the town’s water conservation target is to reduce per capita consumption by 18 percent. Castle Rock is already on its way toward that goal. So far, residents have decreased their average daily usage to 135 gallons per person per day, 35 gallons below the Front Range
Regional Water Master Plan
The South Metro Water Supply Authority (SMWSA) adopted a Regional Water Master Plan in June 2007. This plan will guide the participating water providers as they reduce their reliance on deep groundwater and expand the role of renewable water supplies in meeting current and future water needs. Although they currently hold non-tributary groundwater rights for about 111,000 acre feet per year, the Authority’s members intend to reduce their sustained use of non-tributary groundwater to less than 15,000 acre feet per year at buildout.
average of 170 gallons per day.
In the meantime, Redd and his staff are developing strategies to invest in renewable sources. Increasing and banking the impact fees, along with pumping differently to extend the aquifer’s life are two ways to meet current and future water needs.
“We’re considering two dozen alternatives for renewable sources,” Redd says. “Basically, we priced them all out, and they came in costing about $300 million.”
Options include buying into the newly constructed Reuter-Hess reservoir in Parker, pumping water from as far away as Sterling or the Arkansas River, or buying into Denver Water’s Green Mountain pump-back water project for $25,000 to $30,000 per acre foot, among other scenarios.
The plan quantifies additional renewable water supply goals. The goals are based on projected water demands less projected available supply from current and identified sources. The aggregate new renewable water supply goal increases from an additional 4,000 acre feet per year in 2010 to 43,400 acre feet per year at buildout. An additional 10 percent in renewable supply at buildout is needed to address uncertainties and potential future participants.
The South Metro Water Supply Author-
The plan identifies demands and provides strategies to meet water needs in 2010 (interim), 2020 (mid-term), 2030 (long-term), and at buildout. The projected demands take into account savings associated with current and anticipated water conservation programs.
ity members include Arapahoe County Water and Waste Water Authority, Castle Pines Metropolitan District, Castle Pines North Metropolitan District, Centennial Water & Sanitation District, Cottonwood Metropolitan District, East Cherry Creek Valley Water & Sanitation District, Inverness Water & Sanitation District, Meridian Metropolitan District, Parker Water & Sanitation District, Pinery Water & Waste Water District, Roxborough Park Metropolitan District, Stonegate Village Metropolitan District and the Town of Castle Rock.
The Regional Water Master Plan is available at the South Metro Water Supply Authority Web site www.southmetrowater.org.
Preserving the Aquifers
In 1973, the General Assembly established a minimum Denver Basin aquifer pumping life of 100 years. More recently, in an effort to promote a longer aquifer life, counties such as Elbert, Adams, Weld, and El Paso have enacted a 300-year rule. All new development in those counties must show they can provide a sufficient water supply to last at least 300 years.
This means if a developer or overlying landowner is relying on groundwater, the total annual volume of groundwater available to them is 0.33 percent of the calculated water in storage. The assumption is that with reduced annual pumping, there will be less drawdown and less wellto-well interference, prolonging the life of the aquifer.
It is important to note that the volume of total recoverable water is an estimate, a number that has been very hard to quantify.
Looking for Renewable Water Supplies
In the long-term, sustainable solutions for water supplies in much of the south metro area will require at least a partial shift to surface water resources. But in this day and age, finding unclaimed surface water to divert on a reliable basis can be an expensive and challenging proposition.
To the west, additional renewable supplies may be acquired through transbasin diversions, but the construction of storage, conveyance, treatment, and distribution facilities is expensive, difficult to permit, and has potentially negative environmental ramifications. It would also be unpopular in the basin of origin, an area that may have its own water issues. Construction of additional storage projects like the proposed Two Forks Dam on the South Fork of the South Platte River has historically been unpopular.
To the south, the over-appropriated Arkansas River which supports both growing municipalities and existing farm communities provides limited new water for Front Range communities.
To the east, very few significant surface water resources exist and data suggests that on the eastern edge of the Denver Basin the quality of the aquifer is so poor that only limited amounts of water could be extracted and imported.
To the north, it has been suggested that Denver Water might some day be induced to share water with the south metro area. This is possible, but it was brought home by the 2002 drought that
even Denver’s renewable surface water supplies have their limits.
Transferring water from agriculture, on a willing buyer-willing seller basis, is one of the most likely sources of new water for these areas. Currently, agriculture accounts for more than 85 percent of the water delivered for use in Colorado. While this option has historically meant drying up irrigated lands, recent legislation has authorized interruptible agricultural transfers and crop rotation options. Any conversion of agricultural water should be pursued in a thoughtful and respectful manner reflecting the importance of and need for continued viable agriculture and the open space it represents. In addition, water providers in the South Metro area are individually, or in partnership, developing local water storage solutions. Although RueterHess, the City of Parker’s new reservoir, is the largest and most known, several other storage alternatives are under way. Centennial Water is adding over 6,200 acre feet of new surface storage below Chatfield Reservoir. Also, a number of the water suppliers in the South Metro area are participating in the reallocation of storage in Chatfield Reservoir to provide for additional surface storage of domestic water. The future benefit of on-stream storage and the significant role it will play in managing future supplies of renewable water for the region is noteworthy.
Transferring water from agriculture (above), on a willing buyer-willing seller basis, is one of the most likely sources of new water for urban areas. Conservation measures include irrigating golf courses (right) and parks with treated, but non-potable water.
Alternative Management Strategies
Alternatively, water managers are looking at ways to reduce groundwater demands, and to diversify their options for managing existing groundwater supplies. A partial list of options includes:
• Water conservation;
• Conjunctive use of surface water and groundwater;
• Potable and non-potable water reuse; and
• Control of non-native phreatophytes, high water-use plants.
Conservation programs have been effective in both reducing water demand through changes in consumer behavior and improving water use efficiency. Tiered rate pricing and mandatory watering restrictions during the recent drought decreased Denver Water’s demand by almost 20 percent. The town of Castle Rock is hoping it can get its customers to reduce their water use by 18 percent over the long-term. Parker Water and Sanitation District has been very successful in reducing demand through pricing and conservation, reducing their water delivery to 0.4 acre feet per household. Conservation can also be achieved through removal and control of non-native water-loving vegetation, or phreatophytes, that consume water that could otherwise be put to beneficial use. Tamarisk is an invasive plant species of particular concern in Colorado.
Reuse water may involve recycling return flows after infiltration into the ground, or effluent from wastewater treatment plants. Many municipalities are reusing treated but non-potable water for irrigation
of golf courses, parks and open-space.
Conjunctive use manages surface water and groundwater supplies jointly to produce a larger, more reliable supply than either water supply could generate alone. The goal is to allow water providers to extend the life of their aquifers while fully using their surface water rights, managing short-term shortages, and minimizing the need for new, above ground storage reservoirs. For example, water providers like Centennial and East Cherry Creek Valley Water have contracted to purchase available surface water from Denver, and only use groundwater to augment their peak demands or in times of drought.
Actively recharging aquifers is an emerging strategy to not only manage water supply, but also restore and protect aquifers. During wet years with aboveaverage precipitation and runoff, surface water is stored for later use by injecting it into groundwater aquifers. In this scenario, a deep confined aquifer is used for storage in much the same way as a surface reservoir. A water management district might also direct water from streams, lakes, or reservoirs to permeable areas of a groundwater basin where the water can infiltrate the soil through recharge ponds. In either case, the stored water can be withdrawn at some future point when surface sources are in short supply.
Centennial Water District has been implementing an aquifer storage and recovery (ASR) project in the Denver Basin for 14 years. Direct injection through existing production wells is a proven technology for putting additional water in storage and restoring groundwater levels. They have 23 wells permitted for injection into the Denver, Arapahoe, and Laramie-Fox Hills aquifers when surplus water is available. ASR projects are also being implemented by Consolidated Mutual Water Company in the Lakewood area and by Colorado Springs Utilities. To be effective in addressing declining water levels, aquifer recharge must be implemented on a regional scale
and will require cooperation among many water districts and providers.
Sustainable water management in this arid state relies on the ability to store water. Storage projects involve the construction of new reservoirs, conversion of storage facilities, and enlargement or rehabilitation of existing reservoirs. The conversion of gravel pits to gravel lakes is another option for development of new storage capacity. However, limited surface water flows and protection of existing water rights in the Denver Basin make the construction of new on-channel reservoirs difficult.
Off-channel reservoirs require the construction of diversion and pumping facilities to deliver the water to storage. Parker Water and Sanitation District is constructing the off-channel Reuter-Hess Reservoir to hold water that may include a blend of deep Laramie-Fox Hills groundwater, recycled effluent and alluvial groundwater.
Communities around Parker are considering participation in the project, and expansion of the reservoir is being considered even prior to its completion. The decade-plus permitting process required for this construction project illustrates the time challenge of constructing major new infrastructure.
Individual homeowners will be able to supply their water needs in areas where aquifers are thick by continuing to deepen their wells. However, those located on the western fringe of the basin where aquifers are thin, may face the need to convert to alternate supplies.
Finally, some experts suggest that changing the rates or spatial patterns of groundwater pumping can help extend the life of the aquifer. The development of satellite well fields is proposed to reduce the impact of high density, large capacity wells.
Clearly, water providers in the Denver Basin can not meet all of their objectives with a single water strategy. Sustainable solutions will require them to continue to develop collaborative integrated use strategies to meet the current and future water needs of the basin.
Timeline of Colorado Groundwater Law
The following timeline sets forth, in summary form, major events in the establishment of Colorado groundwater law.
1876 Article XVI, Sections 5 and 6 of the Colorado Constitution declare that the un-appropriated water of every “natural stream” is the property of the public dedicated to the beneficial use of the people of the state by priority of appropriation.
1903 Colorado General Assembly provides that any water right derived from any “natural stream” is subject to court adjudication, 1903 Colo. Sess. Laws, Ch. 130, 297-98.
1914 Colorado Supreme Court confirms that the constitutional term “natural stream” subjects to the rule of prior appropriation all sources of stream supply, including percolating groundwater that is tributary to a surface stream, German Ditch & Reservoir Co., 56 Colo. 252, 270-71 (1914).
1919 Colorado General Assembly provides that all claims to prior appropriation water rights shall be filed within two years; if not, their priorities shall be postponed to those water rights that are adjudicated by the courts, 1919 Colo. Sess. Laws, Ch. 147, 487-96.
1951 Colorado Supreme Court holds that Colorado law includes a presumption that all groundwater is tributary to and subject to appropriation and administration as part of the waters of a surface stream, unless a person proves by clear and satisfactory evidence that the groundwater is not tributary, Safranek v. Town of Limon, 123 Colo. 330, 333 (1951).
1957 Colorado General Assembly provides that: (1) all users of groundwater must file a statement of use with the state engineer, (2) new wells shall not be drilled without a permit from the state
engineer, (3) a well permit “shall not have the effect of granting or conferring a groundwater right upon the user,” (4) the priority date of a “groundwater appropriation shall not be postponed to a time later than its true date of appropriation by failure to adjudicate the right in a surface water adjudication,” and (5) the newly-established Ground Water Commission shall identify critical groundwater areas that “have approached, reached or exceeded the normal annual rate of replenishment,” 1957 Colo. Sess. Laws, Ch. 289, 863-73.
1965 Colorado General Assembly adopts the Ground Water Management Act that: (1) authorizes the Colorado Ground Water Commission to create designated basins for groundwater that has little or no connection to a surface stream, (2) provides for the Ground Water Commission to allocate and regulate designated groundwater through a permit system on a modified prior appropriation basis for economic development through the maintenance of reasonable pumping levels, (3) authorizes the creation of local groundwater management districts for regulation of designated groundwater, (4) requires all new wells, wherever they may be located in the state, to obtain a construction permit from the state engineer, and (5) provides that a state engineer well construction permit “shall not have the effect of granting nor conferring a groundwater right upon the user,” 1965 Colo. Sess. Laws, Ch. 319, 1246-68.
1965 Colorado General Assembly, by a separate act from the Ground Water Management Act, requires State Engineer to administer tributary groundwater in accordance with the doctrine of prior appropriation that is applicable to the distribution of surface water, and adopt rules and issue orders necessary to enforce this responsibility, 1965 Colo. Sess. Laws, Ch. 318, 1244-45.
1968 Colorado Supreme Court states that “implicit” in the Colorado Constitution’s prior appropriation provisions are the propositions that: (1) “along with vested rights, there shall be maximum utilization of the water of this state” and (2) administration of water in the second century of prior appropriation law involves how maximum utilization of surface water and tributary groundwater can be integrated into the law of vested rights, Fellhauer v. People, 447 P.2d 989, 995 (Colo. 1968).
1969 Colorado General Assembly adopts the Water Right Determination and Administration Act of 1969 which, among other provisions, states that (1) tributary groundwater and surface water shall be administered according to the doctrine of prior appropriation, in order to maximize beneficial use, (2) vested surface water and tributary groundwater rights shall be protected in order of their decreed priorities, (3) wells that have not obtained adjudication of their priorities have a period of two years in which to file for their original appropriation date and, if not, their priorities shall be postponed to other priorities that have been adjudicated by the courts, and (4) augmentation plans may be decreed to allow out-of-priority diversions that are not subject to state engineer curtailment, if sufficient replacement water is provided to alleviate material injury to adjudicated water rights, 1969 Colo. Sess. Laws, Ch. 373, 1200-1224.
1973 Colorado General Assembly provides that non-tributary groundwater outside of designated groundwater basins shall be subject to state engineer well construction permits and rules that provide for overlying landowners, or those acting with the consent of overlying landowners, to use this type of groundwater which underlies their lands on the basis of a “minimum useful life of one hundred years,” 1973 Colo. Sess. Laws, Ch. 441, 1520.
1974 Colorado Supreme Court holds that the “tributary character” of water that “takes over a century to reach the stream” is “de minimus” and is “not part of a surface stream” as contemplated by the Colorado Constitution’s prior appropriation provisions, Kuiper v. Lundvall, 187 Colo. 40, 44 (1974).
1977 Colorado General Assembly repeals legislation it had enacted in 1974, 1974 Colo. Sess. Laws, Ch. 111, 440-42, that had allowed the State Engineer to approve temporary augmentation plans while the water court was adjudicating applications for augmentation plans, 1977 Colo. Sess. Laws, Ch. 483, 1702-04.
1983 Colorado Supreme Court holds that: (1) designated groundwater and nontributary groundwater are not subject to the prior appropriation provisions of the Colorado Constitution, and the General Assembly may use its plenary authority to decide how these public waters shall be allocated and administered, and (2) the 1969 Act applies only to surface water and tributary groundwater, State v. Southwestern Colorado Water Conservation District, 671 P.2d 1294 (1983). The General Assembly responds promptly with legislation that (1) recognizes and enforces prior water court decrees adjudicating nontributary groundwater outside of designated basins and (2) allows the water courts to adjudicate to overlying landowners the right to extract nontributary groundwater outside of designated basins under their lands, 1983 Colo. Sess. Laws, Ch. 516, 2079-80.
1985 Colorado General Assembly provides that nontributary and not-nontributary groundwater in the Denver Basin bedrock aquifers of the Dawson, Denver, Arapahoe, and LaramieFox Hills formations shall be allocated to overlying landowners, or those
acting with the consent of the overlying landowners, to be extracted at a rate of no more than 1/100ths percent per year, 1985 Colo. Sess. Laws, Ch. 285, 1160-69.
1988 General Assembly clarifies that the Ground Water Commission, when issuing permits for the beneficial use of designated groundwater in the four Denver Basin aquifers, shall allocate this water on the same basis as provided in the 1985 act for non-designated portions of the Denver Basin, namely “upon the basis of ownership of overlying land” and “an aquifer life of one hundred years,” 1988 Colo. Sess. Laws, Ch. 258, 1238.
2000 Colorado Supreme Court holds that all water within Colorado constitutes a public resource consisting of: (1) waters of the natural stream, which includes surface water and groundwater that is tributary to the natural steam, (2) designated groundwater, (3) nontributary groundwater outside of designated groundwater basins, and (4) nontributary and not-nontributary Denver Basin groundwater of the Dawson, Denver, Arapahoe, and Laramie-Fox Hills aquifers, Upper Black Squirrel Creek Ground Water Mgmt. Dist. v. Goss, 993 P.2d 1177, 1182 (Colo. 2000).
2001 Colorado Supreme Court holds that through the 1969 Act (1) the General Assembly created a new statutory authorization for water uses that, when decreed, are not subject to curtailment by priority administration, (2) this statutory authorization is for out-of-priority diversions for beneficial use that operate under the terms of decreed augmentation plans, (3) plans for augmentation allow diversions of water out-of-priority while ensuring the protection of senior water rights through a replacement water supply that offsets injurious out-of-priority depletions, and
(4) injurious depletions not adequately replaced shall result in curtailment of the out-of-priority diversions, Empire Lodge Homeowners’ Association v. Moyer, 39 P.3d 1139, 1150 (Colo. 2001).
2002 Colorado General Assembly (1) authorizes State Engineer to approve substitute supply plans for out-of-priority tributary groundwater diversions under limited circumstances while augmentation plan applications are pending in the water court, and (2) approves the Arkansas river basin amended rules governing the diversion and use of tributary groundwater in that basin, 2002 Colo. Sess. Laws, Ch. 151, 459-64.
2003 Colorado Supreme Court holds that proposed State Engineer 2002 South Platte Basin rules allowing outof-priority diversions under replacement plans, in the absence of an augmentation plan application pending in water court, were contrary to statute and in excess of his authority, Simpson v. Bijou Irrigation Co., 69 P.3d 50, 67 (Colo. 2003).
2004 Colorado General Assembly allows South Platte tributary groundwater wells to operate out-of-priority under State Engineer approved substitute supply plans, with provisos that (1) augmentation plan applications must be filed in Division No. 1 Water Court by December 31, 2005, and (2) wells not included in an adjudicated augmentation plan or State Engineer approved substitute supply plan shall be “continuously curtailed” from operating out of priority, 2004 Colo. Sess. Laws, Ch. 316, 1205.
Timeline excerpted from “An overview of Colorado Groundwater Law” prepared by Justice Greg Hobbs for the Geological Society of America Symposium on Groundwater Mining and Population Growth.
Glossary
Alluvial aquifer An aquifer formed by geologic sediments deposited in a stream channel or on a floodplain.
Aquifer Storage and Recovery Storage of water in a suitable aquifer through direct injection in a well when water is available and later recovery of the water from the same well when it is needed.
Artesian head The elevation, above the top of the aquifer, to which water will rise in a well completed in a confined aquifer due to hydrostatic pressure. It defines the potentiometric surface.
Bedrock aquifer An aquifer within the solid rock that underlies any unconsolidated sediment or soil.
Cone of depression The draw down of the potentiometric surface due to the pumping of a water well. The amount of draw down diminishes away from the well so that the resulting configuration of the modified groundwater surface resembles a cone.
Confined aquifer An aquifer that is overlain by a confining bed.
Conjunctive use Coordinated use of surface and groundwater supplies to meet demand so that both sources are used more efficiently.
Return flows Surface water or groundwater that returns to rivers and shallow aquifers after being put to beneficial use, such as irrigation. In river basins all around Colorado, the same water is diverted and returned to the river and shallow aquifers three to seven times or more before it leaves the state.
Specific yield Represents the volume of water that can drain from a unit volume of saturated material by gravity. The storage property of an unconfined aquifer, whose value is often on the order of 0.15.
Storage coefficient Represents the volume of water yielded from the combined elastic properties of water and the aquifer skeleton, from a unit volume of the aquifer. The storage property of a confined aquifer, whose value is often on the order of 0.0001.
Structural sedimentary basin A topographically low area in the Earth’s crust in which sediments have accumulated by transport via streams from the adjacent hills.
Unconfined aquifer An aquifer having a water table, whose surface is at atmospheric pressure.
Alley, W. M., Reilly, T. E., and Franke, O. L., 1999, Sustainability of Ground-Water Resources: US Geological Survey Circular 1186, 79 p.
CDM in Association with Meurer & Associates, 2007, Regional Water Master Plan: South Metro Water Supply Authority, 128 p. Available at www. southmetrowater.org.
Colorado Foundation for Water Education, 2004, Citizen’s Guide to Colorado’s Water Heritage: Colorado Foundation for Water Education, 34 p.
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Not that many years ago, settlers turned to water dowsers to find what they could not see, groundwater. Prosperity, if not survival, rode on a “sweet water” well. A “dry hole” could dash a homesteaders dreams. Science has replaced the dowser, but not lessened our dependence on groundwater.
This Citizen’s Guide to Denver Basin Groundwater explores the geology and hydrology of our underground water resource, the legal framework developed for its administration and management, the current development of the resource, as well as its limitations and sustainability.