Basin and Range Aquifers

for Arizona

By: Spencer Musgrove

Introduction to Hydrogeology, Spring 2005



Image by Helmut Kuhn

Introduction
Basin and Range Aquifers

Basin and Range Aquifer under Arizona
Hydrologic setting
Hydrologic and Geologic Properties
Water Resource
Ground Water Contamination
Closing Remarks

    Basin and Range Aquifers

    The Basin and Range Aquifers make up about 200,000 square miles of the southwestern United States (Figure 1). It is located under seven different states, which are Arizona, California, Idaho, New Mexico, Nevada, Oregon, and Utah. Within the Basin and Range Aquifers there are three dramatically different aquifer types. The first type is made of unconsolidated basin-filled deposits and consolidated sedimentary. These deposits consist of sands and gravels that have been eroded away from the high locations to fill in the lower basins. These rocks are believed to be from the Quaternary and Tertiary ages. The second type of aquifer is made of volcanic rock, which is primarily basalt, tuff, and rhyolite of the Tertiary age. The final type of aquifer is made up of carbonate rock. This consists of dolomites and limestones, which are aged between the Mesozoic and Palezoic age (USGS, 1995). The Basin and Range Aquifers are mostly made of individual alluvial basins and are not continuous across the entire region. This is because of the complete faulting of the region, which has caused the aquifers to get pinched off. The size of the Basin and Range Aquifers are enormous; therefore I will only go into detail about the Basin and Range Aquifers that lie under the state of Arizona.

    Figure 1; Basin and Range aquifers under the southwestern United States.

    Picture from USGS, 1995 ; Modified by: Spencer Musgrove

    Click for larger picture

    Basin and Range Aquifers under Arizona

    Hydrologic setting

    The Basin and Range Aquifers occupies the southern half of Arizona (Figure 2). This is an arid region with low relative humidity, a large range in daily temperature, abundant sunshine and little precipitation. This regionís average rainfall is 4 to 8 inches in the lower basins and 16 to 30 inches in the upper mountains (USGS, 1995). Of this rainfall about 90% of it is evaporated because of the arid environment. The other rainfall flows down mountain into one of the six major river systems in the area: the San Pedro, Salt, Santa Cruz, Verde, Colorado River, and Gila. These streams are made mostly of unconsolidated, well sorted, and highly permeable sediments that flow on alluviums (USGS, 1995). This in turn allows water to infiltrate back into the aquifers. Water also infiltrates into the aquifer by fractures, however, only about 5%, or zero to a few inches of total rainfall, is recharged back into the aquifer system a year. The amount of recharge can change yearly depending on the climate condition. There is also artificial means of adding water to the aquifer. Since 1993 there has been a canal that is 336 miles long that runs from the Colorado River to 14 miles south of Tucson. This canal took twenty years to build and was built to recharge and slow the uses of water getting pulled from the Basin and Range aquifers in Arizona (CAP, 1997).

    Figure 2; Basin and Range aquifers under Arizona.

    Picture from USGS, 1995 ; Modified by: Spencer Musgrove

    Click for larger picture

    Hydrologic and Geologic Properties

    The Basin and Range aquifers under Arizona are made mostly of unconsolidated basin-filled deposits and consolidated sedimentary. These deposits are sands, silts and gravels, which are very permeable. These deposits have been eroded away from the higher elevations and have filled in old fault block valleys (Figure 3). These faults have created many different little mountains. These little mountains are made up of impermeable rocks that do not allow water to flow through them. This in turn causes the aquifers to be separated in small alluvial basins that are confined to the valleys they exist in.

    Figure 3; Characteristics of Basin-fill aquifers

    Picture from Arizona Board of Regents, 2000

    Click for larger picture

    The aquifers in Arizona can range in depth from 20 feet near the mountains to 150 feet in the center of the basins. The surface features above the aquifers are from a high elevation, 10,000 feet along the crest of mountain ranges, to a relatively low elevation, 150 feet in the southwest corner of the state. The high elevations are formed by fault blocks that have been up thrown to form a range, while the low elevations are down thrown blocks that have formed a basin (Figure 4).

    Figure 4; Diagram of Basin and Range topography and creation

    Picture from USGS, 1995 ; Modified by: Spencer Musgrove

    Click for larger picture

    The chemical characteristics of the Basin and Range aquifers differ by which location is tested. In areas of recharge there is a significant amount of calcium or magnesium cations and bicarbonate or sulfate anions, which is known as calcium magnesium, bicarbonate sulfate water type (USGS, 1995). However, in the center of many basins the water chemistry is made of sodium chloride.

    Water Resource

    The main use for water along the Basin and Range aquifers is for agricultural purposes. However, the new concern about the feature of the aquifers has to deal with the extensive growth in Arizona (Figure 5). One city that is facing this rise in population is Tucson. The city of Tucson has almost tripled in population over the last 40 years, going from 256,660 to 843,746 people(CensusScope, 2000)(Figure 6). With this growth, the Basin and Range aquifers cannot naturally recharge. As a result of this, the water level of the aquifer under the city of Tucson is dropping rapidly (Figure 7). This continual downward drop will cause wells to lose productivity, increase pumping costs, land subsidence, change in water quality, loss of vegetation, and in time, loss of drinking water.

    Figure 5; Population growth for Arizona

    Picture from CensusScope, 2000

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    Figure 6; Growth of Tucson from 1905 to 1999

    Picture from WRRC, 1999 taken from Martin Stute, 2003

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    Figure 7; Amount of water depletion in the Basin and Range aquifer under Tucson

    Picture from Martin Stute, 2003

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    Ground Water Contamination

    There have been ground water contaminations in the Basin and Range aquifers; however it has not happened on a massive scale. Most contamination happens on a local scale, but that does not mean that they are not deadly. Most of the contaminations that have taken place have happened in an industrial area. For example, the contamination of the Motorola, Inc. plant in Phoenix, happened in the middle of the city and affected the Basin and Range aquifers under it. The source was a 1,1,1- trichloroethane underground storage tank that was leaking. The decontamination for this site was to clean up a portion of the Motorola facility through soil gas extraction, groundwater pumping and treatment methods. Motorola also completed the construction of a full-scale groundwater extraction and treatment facility that pumped and treated the contaminated water (Superfund). There are local, state and government agencies that oversee operations of water from the Basin and Range aquifer in Arizona. One of the local agencies is Maricopa.Gov . This agency regulates the Drinking Water Program for public water systems that operate within Maricopa County in Arizona. The goal of the Drinking Water Program is to assure that the water provided by regulated Public Water Systems meets the requirements set forth in the Safe Drinking Water Rule. The ADEQ, Arizona Department of Environmental Quality, is a state agency that ensures that the water served in Arizona is clean and healthy through a wide variety of programs designed to prevent, detect and correct any possible contamination of drinking water or drinking water sources. USGS is a government organization that oversees the water usage for the United States by conducting different kinds of studies.

    Closing Remarks

    The Basin and Range aquifers are an enormous aquifer system, but in order to keep it clean and resourceful the public that live above it must start using it wisely. If it does not start at a local level then in time the entire aquifer that includes seven states will be without any ground water source.

    References:

    Arizona Board of Regents, Recharging the Aquifer. World Wide Web, URL: http://www.ispe.arizona.edu/pubs/cap/aquifer.html. Retrieved 4/28/05.

    CAP, Central Arizona Project. World Wide Web, homepage: http://www.cap-az.com/index.cfm. Retrieved 4/27/05.

    Census Scope. World Wide Web, URL: http://www.censusscope.org/us/m8520/chart_popl.html. Retrieved 4/28/05.

    Martin Stute, Eniromental Science for Decision Makers. World Wide Web, URL: http://www.ldeo.columbia.edu/edu/dees/U4735/lectures/08.html. Retrieved 4/27/05.

    Superfund, powered by U.S Environmental Protection Agency. World Wide Web, homepage URL: http://www.epa.gov/superfund/. Retrieved 4/28/05.

    USGS, United State Geological Survey. World Wide Web, URL: http://capp.water.usgs.gov/gwa/ch_c/C-text3.html. Retrieved 4/26/05.