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w o tv N United States Geological Survey Colorado River Basin Salinity Control Program Accomplishments for Fiscal Year 1996 Arizona Well Inventory .ofthe Lower Colorado River Flood Plain and Adjacent Areas in Arizona, . California, Nevada, and Utah The U.S. Supreme Court decree, 1964, Arizona versus California, is specific about the. responsibility of the Secretary of the Interior to account for consumptive use of water from the. mainstream; consumptive use is defined to include, "water drawn from the mainstream by underground pumping." Water pumped from wells on the flood plain is presumed to be Colorado River water and the accounting surface can be used to identify wells outside the flood plain in and near the lower Colorado River valley that yield water that will be replaced by water from the river. The objective of this cooperative project between the U.S. Geological Survey and the Bureau.of Reclamation is to inventory wells on the lower Colorado River flood plain and adjacent areas in . Arizona, California, Nevada, and Utah. Work on the project began in April 1994 to locate wells, provide current information for each well, and provide precise position information in .order to apply the accounting-surface method and include the appropriate wells in water accounting along the river. Water levels are required, where possible, for all wells in areas adjacent to the flood. plain. In Arizona and California, over 1,200 wells have been inventoried in the Yuma area, over 500 wells in and near the Colorado River Indian Reservation, and about 125 wells on the Chemehuevi Indian Reservation. Data have been updated or entered into the.U.S. Geological Survey ground-water data base on over 1,250 wells. California Three indicators of salinity have been obtained as part of a two-year binational toxins monitoring. program on the New and Colorado Rivers. They are 1) Residue on evaporation at 180 degrees Centigrade; 2) Calculated sum of major constituents; and 3) Specific conductance. Samples were collected in June 1995 from the All-American Canal below the Imperial Dam diversion and from the Colorado River at the Northerly International Boundary (NIB) near Yuma (above the Morales Dam diversion to the Mexicali Valley). The salinity values are typical for the Colorado River, but deceptive at the downstream site in this case. Last year the Gila Riverwas still contributing flow to the NIB site, which would ordinarily result in lowered salinity. In this case, however, salinity at the two sites is not very different; perhaps because it is well after the wet season when water in the Gila River gradually increases in salinity. This year, the most recent data indicates that the water in the river is even less influenced by the Gila River. Differences between the two sides are especially evident in the isotope data. Gila River water is enriched in deuterium and oxygen-18 and is depleted in tritium relative to upstream Colorado River water. 8