Hydrogeology of the Denver Basin Aquifer


Jake Stevenson

A Student Presentation from
Emporia State University
Earth Science Department

This webpage project was created in partial completion for the Geohydrogeology class at Emporia State University Spring 2006. The assignment was to demonstrate knowledge of hydraulogy techniques as well as to give some insight on a particular aquifer system.

The Denver Basin Aquifer

Picture from National Altas 2006

Table of Contents

Hydrologic Setting

The Denver Basin Aquifer underlies an area of about 7,000 square miles in Colorado which extents from Greeley south to near Colorado Springs and from the Front Range east to near Limon. Within this basin are four separate aquifer systems, these aquifers lie within five geologic formations. The four systems being in order from youngest to oldest are; the Dawson aquifer, the Denver aquifer, the Arapahoe aquifer, and the Laramie-Fox Hills aquifer. The basins name is derived from the city of Denver which predominates the land. (Robson and Banta, 1995)

The Denver Basin has a semiarid climate in which potential annual evaporation is about five times larger than annual precipitation. The majority of precipitation that falls in the area runs off in streams, is evaporated from the soil surface, or is consumed by vegetation. A small portion, however percolates through the soil and eventually finds its way into the aquifer. (Robson and Banta, 1995)

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Hydrologic and Geologic Properties

Considered by the United States Geological Survey (USGS) as sandstone aquifers, the Denver Basin system is generally under confined conditions. "Regional, intermediate and local flow is present in the sandstone aquifers in the western United States, except for those in Oklahoma, where flow is mostly local" (Miller, J.A., 1995).

The Dawson is the uppermost and least extensive water yielding aquifer in the Denver Basin. Covering an area of about 1,200 square miles, "the sediments that form the Dawson aquifer primarily consist of coarse-grained, poorly to well consolidated sandstones interbedded with conglomerate, siltstone, and shale" (Robson and Banta, 1995). The saturated thickness of the aquifer is 300 to 400 feet in the center partition of the aquifer.

The Denver formation is 600-1,100 feet thick and contains the Denver Aquifer. Covering about 3000 square miles, underlying Denver CO, the aquifer lies in consolidated, interbedded shale, claystone, siltstone, sandstone, and coal sequences. Water yielding layers of sandstone and siltstone occur in poorly defined irregular beds which due to the prevalence of claystone and shale, creates an aquifer with a leaky confining layer between the overlying Dawson and the underlying Arapahoe Aquifers. (Robson and Banta, 1995)

The Arapahoe formation contains the Arapahoe aquifer which covers nearly 4,300 square miles. This aquifer sits within 400-700 feet of interbedded conglomerates, sandstones, siltstones, and shale. The aquifer is defined by the base of shale beds underlying the Denver formation to the top, and the upper portion of shale, coal seams, and thin beds of sandstone and siltstones in the underlying Laramie formation to the bottom. A 300 to 400 foot thick layer in the upper part of the Laramie formation creates a nearly impermeable layer which impedes water movement between the Arapahoe and the Laramie-Fox Hills aquifer which lies below. (Robson and Banta, 1995)

The Fox-Hills aquifer is located in the lower part of the Laramie formation and is formed from the Fox-Hills and other sandstones. Covering nearly all of the approximately 7,000 square miles, the aquifer is underlain by the nearly impermeable Pierre Shale. This shale formations forms, essentially the sink bowl of the entire Denver Basin Aquifer system. The Fox-Hills aquifer consists of fine to very fine grained sandstones and siltstones ranging from 0-300 feet in thickness. (Robson and Banta, 1995)

click on the image to enlarge Picture taken from http://capp.water.usgs.gov/gwa/ch_c/jpeg/C081.jpeg

Transmissivity of the Denver Basin aquifers generally ranges from about 10 to 1,000 feet squared per day. Although the transmissivity of the Denver Basin aquifers is relatively small, these extensive aquifers are major sources of water in a semiarid region, with nearly 270 million acre feet of recoverable ground water available from the four aquifers. Differences within each aquifer are present as well. For example "the transmissivity of the Laramie-Fox Hills is less than 5 feet squared per day near the northwestern margin of the aquifer and is less than 100 feet squared per day over much of the remaining area. However, transmissivity exceeds 600 feet squared per day in an area of south of Denver" (Robson and Banta, 1995).

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Water Resources

Being a semiarid climate in which potential annual evaporation is nearly five times larger than annual precipitation, only a small portion of the total water budget is able to replenish the aquifers. Most precipitation is lost due to streams, evaporation, and vegetation. The majority of recharge occurs between stream channels in the topographically higher southern part of the basin. Precipitation is greatest in this area, which, combined with the permeable soils allows for deep percolation into the aquifers. Recharge can occur both on a local and regional scale in this area. Locally water moves from highland recharge areas through shallow sandstone beds to then discharge into nearby stream valleys. Regionally, water can move from the recharge areas into deeper parts of the aquifer, from which the water can move vast distances before discharging many miles away. Water is also able to travel between the aquifers allowing for recharge or discharge. "In the central part of the basin, water levels in shallower wells generally are higher than those in deeper wells, creating the potential for water to move downward from the shallower aquifers to the deeper aquifers"(Robson and Banta, 1995).

click on the image to enlarge Picture taken from http://capp.water.usgs.gov/gwa/ch_c/jpeg/C085.jpeg

The main sources of ground water discharge from the Denver Basin aquifer system are from two sources. The most prevalent means of discharge from the aquifer system comes from "inter-aquifer movement of water from bedrock to overlying alluvial aquifers" (Robson and Banta, 1995). The second source of ground water discharge is by humans, either for municipal or for agriculture.

For the region an average of about 5 million acre-feet of water falls as precipitation a year. Of this 5 million acre feet, nearly 99.2% or 4.96 million acre feet, is lost due to evaporation, transpiration by plants, or surface runoff. The remaining .8% or 40,000 acre feet of water recharges the four Denver Basin Aquifers. During a 20 year period from 1958-78, about 45,000 acre-feet of water was discharged yearly from the aquifers. 57.7% or 26,000 acre-feet was by natural discharge to surrounding alluvial aquifers, and springs. A portion was also lost as evaporation. 42.2 % or about 19,000 acre-feet was discharged to pumping wells. (Robson and Banta, 1995)

According to Robson and Banta, in the Ground Water Atlas of the United States, "Denver has a long history of water-level decline in wells. Ground-water withdrawal has caused these water-level declines and the resulting decrease in the volume of ground water in storage. Between 1884 and 1980, about 750,000 acre-feet of water was withdrawn from the basin aquifers in the Denver metropolitan area." As large as this number might seem it only accounts for about 0.3% of the nearly 270 million acre feet of recoverable ground water from the 4 aquifers. However during this time period it amounted to roughly 30 percent of the 3.8 million acre feet of recharge from precipitation the basin experienced, causing withdrawal to exceed recharge. This imbalance caused declines exceeding 500 feet in some wells.

Near downtown Denver between 1884 and 1960, the only long-term hydrograph of water-level change recorded a decline of about 400 feet in the Arapahoe aquifer. Also shown from the hydrograph is a recovery since 1960 attributed to decreased withdrawal from the aquifer. (Robson and Banta, 1995)

click on the image to enlarge Picture taken from http://capp.water.usgs.gov/gwa/ch_c/jpeg/C088.jpeg

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Ground Water Contamination

For most of the Denver Basin Aquifer system the water quality meets drinking water regulations set forth by the United States Environmental Protection Agency (EPA). Water in the basin contains small dissolved solids with concentrations ranging from less than 100 milligrams per liter in the Dawson aquifer to about 2,000 milligrams per liter in the Laramie-Fox Hills Aquifer. (Robson and Banta, 1995)

Water in the Denver Aquifer is much like the Dawson with dissolved solids concentrations of 100 to 1,000 milligrams per liter. "As water moves laterally toward the margins of the aquifers, the quality of the water is degraded by surface recharge that contains dissolved sulfate ad other chemical constituents leached from soluble minerals in the overlying soil and rock. The increase in sulfate concentrations is particularly evident near the margins of the Denver, Arapahoe, and Laramie-Fox Hills aquifers" (Robson and Banta, 1995).

The Arapahoe aquifer has somewhat softer water in comparison to the Denver aquifer. This is due largely to ion exchange and dissolution. Dissolved solids in the Arapahoe aquifer range in concentration values from 200 to 1,400 milligrams per liter. (Robson and Banta, 1995)

The Laramie-Fox Hills Aquifer has soft water towards the center of the aquifer and hard water to the edges. The water is of sodium bicarbonate or sodium sulfate type. The dissolved-solids concentrations of water in this aquifer range from about 200 to 2,000 milligrams per liter; larger concentrations are near the aquifer margins. Along with the water there are some gasses which exist within the aquifer. These gasses are hydrogen sulfide and methane, it is because of these gasses that cause some of the water to have a putrid odor and have little value for most uses, such as drinking water. (Robson and Banta, 1995)

Generally, dissolved-ion concentrations in the Denver Basin Aquifers range from 20 to 200 micrograms per liter of water. Although, apparently in response to chemical environments near the well, ranges from 7,000 to 85,000 micrograms per liter of water have been reported. When these high concentrations of dissolved micrograms are exposed to oxygen the dissolved iron reverts back to an insoluble form, visible as black to reddish-brown precipitates. These precipitates cause the water to appear cloudy and can discolor and stain porcelain sinks and laundry. (Robson and Banta, 1995)

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The future of the Denver Basin Aquifer is limited. With a growing population and the current discharge from the system, there is only a matter of time before the well runs dry.

In 1985 a total of roughly 12,000 wells were pumping water from the system, currently there are about 33,000 or nearly 175% more wells. From 1990 to 1997 a project titled "The Denver Basin Aquifer Recharge Demonstration Project" was completed by the U.S. Department of the Interior, Bureau of Reclamation. The six year project showed researchers the potential for aquifer storage and recovery in the Denver Basin. After all the research was conducted on the Denver Basin Aquifer, it was found that it would be economically feasible to buy or divert water, inject it into the Arapahoe aquifer, store it in that aquifer and then later recover the water for residential or agricultural use. All this could be done for as little as $2.45 per 1000 gallons or $0.65 per cubic meter. (Halepaska, Et Al.)

This is the type of thinking and attitudes that need to be adopted in order to retain the Denver Basin Aquifer system as a renewable water source, in a region that lacks much excess water. This type of action would allow for a sustainable aquifer system allowing many generations use.

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Denver Museum of Natural Science, "Denver Basin Aquifer Project", Available on World Wide Web, homepage: http://www.dmns.org/main/minisites/denverbasin/index.html. Retrieved 4/18/05.

Fetter, C.W., Applied Hydrogeology, 4th ed., Prentice Hall, Inc. Upper Saddle River, New Jersey, 2001.

Halepaska, John C. et al., Case History of Aquifer Storage and Recovery in the Denver Basin Aquifers, Available on World Wide Web, URL http://www.hydrologyconsultants.com/AEGpaper.pdf Retrieved 4/11/06.

Miller, J.A., 1995, Ground Water Atlas of the United States, "Introduction and National Summary". Available on World Wide Web, URL: http://capp.water.usgs.gov/aquiferBasics/sandstone.html Retrieved 4/16/06. Updated 01/31/05

Robson, S.G and Banta, E.R., 1995, Ground Water Atlas of the United States, "Arizona, Colorado, New Mexico, Utah, HA730-C". Available on World Wide Web, URL: http://capp.water.usgs.gov/gwa/ch_c/C-text6.html Retrieved 4/18/06.

U.S. Department of Interior, National Atlas 2006, “Principle Aquifers Wall Maps”. Available on World Wide Web, URL: http://www.nationalatlas.gov/wallmaps.html#aquifers Retrieved 04/19/2006. Updated 4/14/06

U.S. Geological Survey, Ground Water Atlas of the United States, “Imagery”. Available on World Wide Web, URL http://capp.water.usgs.gov/gwa/ch_c/C-Denver_Basin.html Retrieved 4/16/06.

Related Links

Emporia State University Earth Science at ESU
Introduction to Hydrogeology U.S. Geological Survey
AQTESOLV Applied Hydrogeology
For more information email stevenson_jacob@stumail.emporia.edu.