Subsidence and its Effect on the Tucson Basin
Melinda R. Koslow
Abstract
Tucson, Arizona is affected by ground subsidence each year because of removal of underground water. The formations in many valleys in Arizona are alluvial and lacustrine, ranging in depth from about 50 to 350 meters. Declines in water have reached more than 150 feet. This causes a compaction that leads to a "falling" of the land about 0.5 centimeters per year. The annual rainfall of about 15.5 inches per year is not enough to replenish the water source. Cracks in the earth called earth fissures result from subsidence ranging to almost a thousand feet in length and about thirty feet in width. These cause many problems in structures and piping. Piping can break and leak and cause health problems. Maximum and minimum subsidence projections by the United States Geological Survey show that potential water-level declines for 1940-2024 ranged from 411 to 438 feet in the Tucson Basin and could lead to subsidence of 10 feet or more in some locales.
Introduction
Ground subsidence is a large change on the earth's surface, which can be caused by humans. Ground subsidence is the downward movement or sinking of the earth's surface caused by removal of underlying support. Tens of thousands of square miles on the earth have subsided(5). Every year the city of Tucson, Arizona grows in population. As the city of the desert grows, water usage becomes more and more a problem.
Right now, in 1998, Tucson is using water at a faster rate than can be replenished. The main source of water in the Tucson area is an extensive alluvial-aquifer system. Ground subsidence occurs from the extensive underground pumping.
Subsidence happens especially in basins filled with shallow marine deposits, unconsolidated alluvial, or lacustrine deposits. Water removal is the cause of subsidence, because almost all spaces between rocks in the sediments contain water. When the water is removed, the unconsolidated deposits no longer receive buoyant support and this leads to a compaction of the land. For example a person sipping soda with ice through a straw, causes the liquid to disappear, so the ice on top falls along with the remaining soda.
The compaction occurs in two steps. First is the compaction of the clay in the sediments that takes place because of the creation of void spaces caused by water removal. The second is a readjustment of the unconsolidated sand, silt, and clay particles in order to fill the void spaces.
Problems that may occur due to subsidence include changes in elevation, destruction of underground piping, strain on buildings and other structures, and a change in ability to continue pumping groundwater(2). Subsidence makes it harder to pump underground water as the bedding compresses. The changes in elevation are not as obvious as the other problems because it happens over a large time span, however it affects piping and puts a strain on structures. Piping containing water, oil, and sewage may break which is costly and causes health and safety hazards.
Cracks in the earth called earth fissures also result from subsidence(3). Subsidence stretches the sediments, which causes horizontal stresses. Earth fissures initially crack less than an inch wide and then may stretch to around thirty feet wide. They also may become thousands of feet in length. Earth fissures are widened by erosion, which has a serious economic impact as structures are damaged, property values decrease, and the cost of new construction increases.
Materials and Method
To retrieve the information on subsidence, I started at the University of Arizona library located in Tucson, Arizona. I searched for books on subsidence and subsidence in the Tucson, Arizona area using Sabio, located on the Internet. Many books on subsidence, the effect on buildings, in the Tucson Basin, and on the earth can be found in the science and engineering library. I then talked to my Professor, Professor Riley, about other possible resources. He gave me three reports done by different departments in Arizona.
Results
Noticeable changes of the land in Tucson started in the early 1900's. The water level of Tucson has declined over 100 feet(1). Scientists from the National Oceanic and Atmospheric Administration have found that Tucson is dropping about 0.5 centimeters per year. Substantial land dropping can be found along the Santa Cruz River from the intersection of interstates 10 and 19 through the city of Sahuarita. In the area of Sahuarita, which is south of Tucson, declines in water of more than 150 feet have been observed(6).
Areal recharge includes mountain-front recharge, irrigation return flow, streamflow infiltration, and mine return and sewage-effluent return flow. The average annual rainfall for the basin is 15.5 inches. Mountain-front recharge involves the highest mountains. For example, mountains like the Santa Catalinas, or Rincons receive the highest amount of precipitation. Irrigation return flow is a source of recharge from farms and other places that irrigate their lands. Recharge is decreasing, however, because of increased efforts to improve irrigation efficiency and decreased irrigation acreage.
Stream-flow recharge is increasing along with the building of canals and channels which "catch" the water during flash floods or other heavy rains. Sewage effluent recharge has risen from about 5,600 acre-feet/year in 1950 to more than 49,000 acre-feet/year in 1985, and continues to be a significant source of recharge (1).
A general increase in pumpage for livestock, domestic, municipal, industrial, and agricultural uses began around 1940. The steady increase reached about 170,000 acre-feet in 1954(1). Since then the increase/decrease has fluctuated, but reached about 269,000 acre-feet in 1975, and has continued to rise since(1). Agricultural use was the biggest user of groundwater until the 1970's-1980's. Today the general public in households, schools, and businesses use the most water. The United States Geological Survey's projection is that the land will fall about a maximum of 10ft in Tucson due to groundwater withdrawals.
Discussion
Subsidence affects many areas around the world like the countries of Australia, Japan, and Italy. Southwestern states like California, Texas and Nevada are also "falling". Citizens and businesses of Tucson are using groundwater mainly, because they cannot agree on another solution. The state of Arizona has an area of about 3,000 square kilometers, including the metropolitan cities of Phoenix and Tucson, that has been affected by subsidence(5). Subsidence causes many problems, therefore it needs to be studied and the problems may be solved.
Conclusion
The problem of land subsidence affects many places around the world including Tucson, Arizona. Subsidence is the compaction of underground sediments caused by high usage of underground water. Some may say it is mainly caused by ignorance and neglect. However, it affects the city of Tucson by 0.5 centimeters of compaction per year. It also causes earth fissures which crack and hurt pipelines, structures, the economy, and the environment. The U.S. Geological Survey's forecast of potential land subsidence can be seen on the chart on page 8 of this report. Potential water declines for 1940-2024 ranged from 411 to 438 feet for the maximum and minimum subsidence projections, 10 ft and 0.5 ft, respectively.
Tucson can conserve the water by following simple techniques like turning off the water while brushing your teeth and using water-saving toilets. Other ways of conserving ground water are controversial ways like using the Central Arizona Project (CAP) or using chemically treated wastewater. They are controversial because people believe that they are not clean enough or are too expensive.
References
1. Benedict, J.F. and Hanson, R.T. Stimulation of Ground Water Flow and Potential Land Subsidence, Upper Santa Cruz Basin, Arizona. Tucson, U.S. Geological Survey, 1994.
2. Institution of Civil Engineers. Ground Subsidence. London: Institution of Civil Engineers, 1977.
3. Jackson, Garrett W., Pearthree, Philip A., Slaff, Steven. Development of Earth
Fissures in Picacho Basin, Pinal County, Arizona from 1959 to 1989.
Tucson: Arizona Geological Survey, 1989.
4. Poland, Joseph F. Guidebook to Studies of Land Subsidence due to Ground- Water Withdrawal. Paris: United Nations, 1984.
5.Slaff, Steven. Land Subsidence and Earth Fissures in Arizona. Arizona: Arizona Geological Survey, 1993.
6. United States Department of Commerce. Subsidence Monitoring for the State of Arizona. United States: National Oceanic and Atmospheric Administration, 1983.
This is a student paper written in the Fall of 1998 in a First-Year Colloquia Course on Water Resources in the Tucson Basin at the University of Arizona. For more information contact: jjriley@ag.arizona.edu