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<br />than at present because the level of the river was approximately 7 <br />feet higher at the Reservation than at present. The flooding acted to <br />flush accumulated salts from the flood plain soil, In addition, occa- <br />sional high water in the 1940's and 1950's fluctuated ground-water <br />levels, which also aided in flushing accumulated salts. For estimating <br />purposes it was assumed that the salts in soils under natural vegeta- <br />tion in 1974 accumulated during the last 35 years, <br />The vegetation in the undeveloped area south and east of Poston <br />had an estimated consumptive use of 2 acre-feet per acre each year <br />which had a salt content estimated at 1,2 tons per acre-foot [4], <br />Because of the concentrating effect of water use, it was estimated <br />that approximately 30 percent of the salts formed insoluble precipi- <br />tates and the rest was susceptible to flush out when the area was <br />initially irrigated, This produced an accumulation of 59 tons of <br />soluble salt per acre under vegetation as of 1974. The quantity of <br />salt per acre was adjusted for lands that were cleared prior to 1974 <br />or that had a greater rate of evapotranspiration than 2 feet per year. <br />The time required for soluble salt from new land to reach the <br />river was estimated according to the following conceptual relation- <br />ships. Soluble salt is flushed from the root zone, flows through the <br />ground-water aquifer, and subsequently discharges into surface <br />drains, Most of the salt leaves the root zone during the first 3 to 5 <br />years but some takes years longer, After reaching the water table, <br />some of the salt emerges relatively quickly but some travels a longer <br />path deep in the ground-water aquifer. The combined effect of these <br />mechanisms is to spread the salt discharge over a period of years. A <br />cursory evaluation of each mechanism was made to arrive at a series <br />of percentages which, when multiplied by the tonnage of salt in a <br />tract of new land, would produce a salt discharge for each future <br />year starting with the initial year of irrigation. The resulting series <br />of percentages provided for about half of the salt to reach the drains <br />in the first 8 years, and it was assumed the rest of the salt would <br />reach the drains during the following 42 years in order to arrive at a <br />round number of 50 years for the process. 002252 <br />Estimates were then made of annual salt discharge from new <br />lands in the recent past by applying these percentages to the tonnage <br />18 <br />