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....-4 I~ c L 6. Land Subsidence and Earth Fissures :,' :~ Ground-water withdrawal has resulted in water level declines throughout the area. These water-level declines have caused land subsidence and earth fissures. Subsidence occurs through compaction of the sediments in and below dewatered zones. Earth fissures or cracks generally occur around the periphery of basins with ground-water declines and subsidence. The greatest water-level declines have been in the basins surrounding Eloy, Pi cacho, Ma ri copa, and Stanfi e 1 din Pi na 1 County; and Queen Creek, Mesa, Scottsdale, Luke Air Force Base, and Sun City in Maricopa County. The use of CAP water will cause the water level to decline less rapidly or stabilize. This will decrease the amount of subsidence that will take place and decrease the occurrence of fissures. In areas where the water level is stabilized, subsidence will slow and gradually stop. The water-level change predictions for the various water- allocation alternatives in Appendix F indicate that the water level will continue to decline to some degree in most areas. Correspondingly, subsidence will conti nue to some degree also. The di fferences between the plans with respect to subsidence are differences in distribution of the subsidence that will take place. All of the plans will considerably reduce the amount of subsidence that will take place without CAP. I'; ...):. .. .<; '0- % (' '.: " - '.~ 7. Soils and Irrigated Agriculture "; ., The valley soils of the project area are generally highly suited to irrigated agriculture. A land classification completed in 1972 by the Bureau found the majority of the farmland to be arable and irrigable under Bureau specifications. The Department of Agriculture has designated nearly all of the irrigated farmland in the project area as prime. Under any alternative allocation, approximately 100,000 acres of prime farmland on the Salt River Project and about 34,000 off the Salt River Project would be converted to urban use to accommodate population growth. In response to each alternative allocation, additional prime farmland ranging from 1,800 to 7,000 acres would be retired to obtain ground-water supplies for M&I use. This acreage to be urbanized over the 50-year project period represents about a 15 percent decrease from the estimated 1 million acres having a recent history of irrigation. However, this general level of urbanization would occur under any of the allocation alternatives. The maximum difference in estimated urbanization of prime farmland among the allocation alternatives is 532 acres. :.: Colorado River water is suitable for growing salt-sensitive crops on soils have good leaching capability or for growing moderately to 1 erant crops on soil s wi th poor 1 each i ng capabi 1 ity (soil s havi ng both capabilities are found in the project area). Since the CAP water supply from the Colorado River is not estimated to approach the lower salinity limit for sustained irrigation, no significant adverse effect on crop production is anticipated. Likewise, it is not anticipated that irrigated agriculture on the Salt River and Gila River Reservations would suffer adverse effects as a result of subst i tuti ng treated effl uent for CAP water, although the soc i a 1 acceptance of tribal members would likely not be obtained. 9