McMullen Valley is located in western Arizona about 80 miles west-northwest of Phoenix. It is about 48 miles long and 15 miles wide and is drained by Centennial Wash, an intermittent tributary of the Gila.River. The annual average precipitation is 9 inches.The rock units in McMullen Valley are (in ascending order) : (1) Precambrian igneous and. metamorphic rocks, (2) Paleozoic sedimentary rocks, (3) Mesozoic(?) intrusive rocks, (4). Tertiary (?) sedimentary rocks, (5) Tertiary (?) extrusive rocks, ( 6) Tertiary (?) intrusive rocks, (7) 'Quaternary volcanic rocks, and-( 8) -Quaternary valley-fill-deposits. " The. valley-fill.-deposits are divided into,four subunits, as follows: (1) .Conglomerate, (2) .alluvial-fan deposits, (3) lake-jbed deposits, and (4) alluvium. The most conspicuous structural fea-. tures in the area are tilted horsts and grabens..The 'principal-aquifer consists of valley-fill deposits. in the trough between the mountain ranges. The buried parts of the mountain-ranges, which are the boundaries of this aquifer, are effective barriers to subsurface movement of ground water into or out of the «valley-fill deposits. The general .direction of the movement of ground water in-the valley-fill deposits is.southwestward to Salome and then southeastward through Harrisburg Valley. A, buried bedrock ridge beneath the surface outlet at the southeast end of Harrisburg Valley is a partly effective ground-water dam. (See p. 23.)The ground-water reservoir in the valley-fill deposits is recharged 'principally by seepage from streams, especially when they flood, and .to a lesser extent by seepage-from, irrigation. Prior,to the development .of irrigation, ground water was discharged from the valley primarily as underflow and by evapotranspiration; however, pumping has been relatively heavy in Harrisburg Valley so that little OE no ground water now leaves the valley as underflow, and evapotranspiration probably has Jjeen-lessened. During 1953-about.6,000 acre-feet of water was pumped for irrigation, and in 1957 the .annual pumpage had increased to 21,000 acre-feet.The-rate of discharge from irrigation wells ranges from 150 gpm (gallons per minute) to as much. as.3,500,gpm, and the specific capacity of the-wells ranges from 2 to 114 gpin-per-ft of drawdown. In the Aguila area, the wells having the higher specific capacities -produce principally from alluvium; whereas, in the Wenden and Salome:Harrisburg Valley areas, the wells having the higher specific capacities produce principally from the alluvial-fan-deposits.-The water table has declined in-areas-of concentrated pumping, particularly in the southeastern part of Harrisburg Valley and north of Aguila.The volume of the valley-fill deposits must be very large, because the deposits underlie an area of about 500 square miles and their maximum thickness exceeds 1,800 feet. The specific yield of the material is estimated to be about 15 percent, and, therefore, large quantities of water are available for withdrawal.The concentration of dissolved solids in samples of wat...
