Publication n°121 of the International Association of Hydrological Sciences Proceedings of the Anaheim Symposium, December 1976

LAID SUBSIDENCE IN CENTRAL ARIZONA

Carl C. Winlkka and Fata D. Wold Administrator, Arizona Resources Information System, Phoenix, Arizona Geologist, U.S. Bureau of Reclamation, Phoenix, Arizona

Abstract Land subsidence and earth fissures occurring in parts of central

Arizona are related to water-level declines caused by large-scale ground­water withdrawal. Differential land subsidence and earth fissures have damaged Picacho Reservoir, agricultural lands, water-distribution systems, water wells, buildings, interstate highways, county roads and streets, and have necessitated rerouting of a proposed major aqueduct. Earth fissures, probably associated with the land subsidence, were first reported in 1927, and land subsidence was first measured in 19^8.

The maximum documented subsidence from 19̂ +8 to 1967 was 2.29 m near the town of Eloy. Since 1967, more than 0.9 m of subsidence has been measured along Interstate Highway 10 near Picacho.

Between 19^8 and 1967, in the lower Santa Cruz Basin, about 570 square kilometers have subsided more than 0.15 m, and about 155 square kilometers have subsided more than 0.9 m in the Casa Grande-Eloy area. In the Stanfield-Maricopa area, approximately 200 square kilometers have subsided more than 0.3 m.

In the Salt River Valley, the land surface has subsided as much as l.l6 m near Queen Creek, 0.6U m west of Luke Air Force Base, and 1.13 m east of Mesa, between 19^8 and 1967. Between 1971 and 1975, approximately 0.3 m of subsidence was measured southeast of Mesa.

By 196k, large-scale pumping, principally for irrigation, had lowered water levels more than 110 m in parts of the area, and numerous earth fis­sures were observed. Since 196U, water levels have declined more than 2k m in local areas. Compaction-recorder data indicate long-term water-level declines correspond with land subsidence. Seasonal water-level fluctua­tions correspond with seasonal sediment compaction and expansion; however, the amount of compaction greatly exceeds the expansion. Between April 1965 and April 197^, measured compaction in the upper 253 m of sediment in the Santa Cruz Basin near Eloy accounts for 0.1*1 m (63%) of the O.65 m total surface subsidence at the compaction-recorder site.

Earth fissures, as much as 13.8 km long, occur in the alluvial sedi­ments on the periphery of the subsiding areas, transect natural drainage-ways, and act as drains. The fissures intercept surface runoff in unde­veloped areas, and capture irrigation water traversing cultivated lands. Downward and lateral water movement in the fissures causes rapid near-surface widening—partly by slumping but mainly by erosion of the sides.

In 1976, renewed fissurlng near the Picacho and Santan Mountains and several areas of previously unmapped fissures were mapped on orthophoto quads using a helicopter. New and renewed fissurlng, measured water-level declines and sediment compaction, together with the limited leveling data available indicate land subsidence continues to occur over large areas of central Arizona.

Introduction Major dams constructed on rivers in central Arizona provide the water

necessary for extensive agricultural and industrial development. This development coupled with a mild, dry, sunny climate have made Arizona an

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increasingly more desirable place to live and development has continued at a rapid pace. From 1970 to 1975» Arizona's increase in population was over 25 percent, the highest growth rate of any State in the Nation.

To meet water requirements of this growing area, ground water is being pumped in addition to that supplied through the reservoir system. The volume of ground water withdrawn now greatly exceeds the rate of recharge. Land subsidence and earth fissures occurring in parts of central Arizona are related to water level declines caused by large-scale ground-water withdrawal (Figure l).

Ground-water withdrawal in Arizona now exceeds the recharge by 2,700 cubic hectometers per year. The Central Arizona Project is expected to lessen this overdraft by approximately 1,500 cubic hectometers of water per year beginning in the 1980's. In the meantime, substantial subsidence and the intensification of earth fissuring in subsiding central Arizona basins is continuing. The Central Arizona Project, new highway construction, and flood control projects will all be affected by this continuing subsidence process. Residential, industrial, and commercial expansion which follow will also be affected not only by the physical hazards involved but also by the depreciation effect earth fissures have on property when occurring in or near development (Figure 2) or irrigated farmland (Figure 3).

At present, the extent to which declining ground water levels, sub­sidence, and related earth fissures constitute environmental hazards is minimal. Subsidence effects are gradual and take place generally in agri­cultural or undeveloped desert areas.

While fissures normally occur in natural desert areas they have also occurred across interstate highways, in irrigated fields, across pipelines, and frequently across unpaved local roads. In two known Instances, earth fissures are in residential areas. Subsidence has caused the collapse of well casings and in some places apparent extrusion of well casings from the ground surface. Subsidence has thus required well replacement or modifi­cation.

The subsidence process in Arizona transects both the scope and geo­graphical extent of agency responsibilities. As a result, the total effects are not known and any assessment of the effects on future projects requires a considerable data gathering effort. Records of subsidence are scarce and existing level lines must be carefully evaluated prior to use for subsidence measurements. A continuity of elevation change measurements is made increasingly difficult due to the destruction of bench marks and the rising cost of field surveys.

Earth fissures not shown on existing maps were discovered during studies for the Bureau of Reclamation's Central Arizona Project. Other interested Federal and State agencies, with a common need for this infor­mation, joined the Bureau of Reclamation in mapping all visible earth fis­sures in the 2,000 square kilometer area in three major basins south and east of Phoenix, Arizona (Figure l).

This paper reports on a joint effort by several agencies to update and bring together information to better understand the interrelationship between ground-water declines, subsidence, and earth fissures.

Techniques Mapping of the earth fissures was done in two stages. First, l:2l*,000

and larger scale color infrared aerial photographs were taken of all known fissure areas. A total of ikk aerial photographs were obtained. The air­craft and camera were provided by Arizona Department of Transportation and the film by the U.S. Geological Survey. All identified fissures, as well as probable fissures, were transferred from this photography to l:2i*,000

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-60~ Contour of water -level decline (meters) 1923-1972

• Earth Fissures, 1976

FIGURE 1

LOCATION OF WATER-LEVEL DECLINES AND EARTH

FISSURES IN CENTRAL ARIZONA.

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Figure 2 - New earth fissure heading toward housing development south of the Sacaton Mountains.

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scale orthophotoquads (Wlnlkka-Morse 197*0 furnished by the Arizona Resources Information System.

Second, these fissures were checked and the entire area of approxi­mately 2,000 square kilometers was searched by two observers and a recorder in a four place helicopter. Fissures were spotted from the air and checked on the ground. Locations were determined by using the orthophotoquads as base sheets. This procedure took three days for a total of about 15 hours flying time.

Ground-water decline and measured subsidence information was gathered from State and Federal agency records. Recent information was found to be available from the U.S. Geological Survey and the Arizona Department of Transportation.

Water-Level Declines Substantial water-level declines are continuing in a number of ground­

water dependent areas in central Arizona (Laney-in preparation). While these areas are predominantly agricultural, urban areas are not excluded (Figure l). Maximum values of water-level decline are lhO meters near Stanfield, 110 meters east of Mesa, 98 meters southeast of Eloy, south of Queen Creek, and at Scottsdale. The decline at Scottsdale is recent, having occurred since 1952 (USBR-in preparation). In all of the above areas declines are increasing in magnitude and in total area affected.

Subsidence Land subsidence in Arizona related to ground-water withdrawal (Poland-

Schumann 1969) is continuing. Subsidence has a relationship to ground­water declines In the Casa Grande-Pieacho section of Pinal County (Figure k). When declines and subsidence are plotted against time, a strong linear relationship is found at Picacho (Figures 5 and 6).

Information available on four bench marks in the Toltec-Eloy area of Pinal County shows subsidence of these marks from i960 to 1975 to range from 0. i+U m to 0.69 m. In the Picacho area several marks have subsided more than 0.70 m from 1967 to 1975 with the result that the area of maximum measured subsidence has shifted approximately 6 kilometers southeasterly along the Casa Grande-Pieacho Peak profile since I967 (Figure h). The maximum known subsidence measured in Arizona between 1967 and 1975 has occurred at Picacho where a 1967 National Geodetic Survey Bench Mark, X363, subsided 0.93 m. During that same time period, other marks in the area subsided by amounts exceeding 0.70 m, as illustrated In Figures k and 6.

The longest earth fissure in Arizona, 13.8 kilometers in length, which also has a vertical displacement (Figure h) , crosses Interstate Highway 10 between bench marks B and C. The southeastern side of this fissure is now subsiding at the same rate as the northwestern side, 0.09 m per year. An intensified fissure pattern is developing north and south of Interstate Highway 10 within 3 kilometers of the existing long fissure.

The subsiding areas of central Arizona are substantial as shown by Schumann 197^- In Pinal County within the lower Santa Cruz Basin from 19^8 to 1967, an area of about 570 square kilometers subsided more than 0.15 m and about 155 square kilometers of this area has subsided more than 0.9 m-In the Stanfield-Maricopa area approximately 200 square kilometers has subsided more than 0.3 m.

Earth Fissures Earth fissures have existed in parts of Arizona for many years

(Schumann-Poland 1969) and (Schumann 197^). The new mapping of these fis-

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WATER-DECLINE, METERS

30

SUBSIDENCE, METERS

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1952 BASE

""̂ ~ y

FIGURE 5. WATER-LEVEL DECLINE IN PICACHO VICINITY 1952-1972

1952 BASE

^ . ^

TOTAL SUBSIDENCE AT THREE BENCH MARKS IN

PICACHO VICINITY. 1952-1975. (FOR LOCATION

SEE FIGURE 4 )

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sures confirms the fact that they are continuing in or near areas of sub­stantial subsidence (Figure l). Three forms of intensification were noted during the mapping. First, fissures have occurred in new areas, second, fissures are developing en echelon with or parallel to older fissures, and third, older fissures have reopened. These forms of intensified fissuring are expected to continue unless stabilized ground-water levels are achieved.

Conclusions The coincidence of areas of continued ground-water declines, continued

subsidence, and increased earth fissuring support the view that there is a causal relationship begun by ground-water declines. These phenomena are having an adverse impact on habitation and agriculture.

The full significance of ground-water withdrawal, subsidence, and earth fissuring is relatively unknown at this time. Much additional information, primarily costly field surveying, is needed to adequately define this potentially severe problem.

The ground-water withdrawal, subsidence, and earth fissuring occurring in Arizona are broader in scope than existing agencies are able to address.

Acknowledgment s Data presented were collected by persons from several agencies. The

following assisted in mapping: Robert L. Laney and Thomas L. Holzer of the U.S. Geological Survey; Edward A. Nemecek and Phillip C. Briggs of the Arizona Water Commission. Robert L. Laney and his staff prepared the water-level declines maps. Richard H. Raymond of the Bureau of Reclamation was of much assistance in all phases of gathering and interpreting data. Herbert H. Shcumann of the U.S. Geological Survey furnished valuable background infor­mation.

References

Laney, R.L. (in preparation): Water-level Decline, Land Subsidence and Earth Fissures in South-Central Arizona. Twentieth Annual Watershed Symposium Proceedings-Arizona Water Commission.

Schumann, H.H. (197*0: Land Subsidence and Earth Fissures in Alluvial Deposits in the Phoenix Area, Arizona, U.S. Geological Survey Miscellaneous Investigations Map I-81+5-H.

Schumann, H.H. and Poland, J.F. (1969): Land Subsidence, Earth Fissures and Groundwater Withdrawal in South Central Arizona, U.S.A., International Symposium on Subsidence, Tokyo, pp-295-302.

U.S. Bureau of Reclamation (in preparation): Geology and Ground-Water Resources Report, Central Arizona Project, Arizona Projects Office, Phoenix, Arizona.

Winikka, Carl C. and Morse, Samuel A., Jr. (197*0: Orthophotoquads for Arizona Land-Use Mapping and Planning, Journal of the Surveying and Mapping Division, ASCE, Vol. 100, Proc. Paper 10525, May 197*+, pp. 1-6.

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