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w w <r; :....' Table 3. - Standards for the Colorado River Station Annual flow-weighted average Below Hoover Dam Below Parker Dam At Imperi al Dam 723 mg/L 747 mg/L 879 mg/L To maintain these criteria while the Basin States continue to develop their compact-apportioned watters, an estimated 2.2 to 2.8 million tons of salt per year must be intercepted before entering the river system by year 2010. The Colorado River Basin Salinity Control Act of 1974 (Public Law 93-320) pray i des for the cons truet i on. oper at; on, and mal nten ance of cert a i n work s in the Colorado River Basin to control the salinity of water delivered to users in the United States and Mexico. Under the Act, the Secretary of the Interior was directed to expedite planning studies on 12 salinity control units of a Basinwide program to control the sal inity of Colorado River water and to construct four select units. Title I of the Act authorizes the construction of facilities to enable the United States to comply with its obligations under the agreement with Mexico of August 30, 1973 (Minute No. 242 of the International Boundary and Water Commission, United States and MexiCO). In brief, Title I provides the means of accomplishing Minute 242. which requires that water delivered to Mexico have an average annual salinity of no more than 115 plm ! 30 plm greater than the Colorado River water arriving at Imperial Dam upstream of the United States-Mexican border (p/m is a measure of the parts per million concentration of total dissolved solids, an indicator of salinity concentration essentially equivalent to concentrations expressed in mg/L). Title II of the Act provides for progrMls to control the salinity of the Colorado River upstream from Imperial Dam. Title II also authorized the construction of four salinity control units: Paradox Valley and Grand Valley Units in Colorado, Crystal Geyser Unit in Utah, and Las Vegas Wash Unit in Nevada. Title II directed the Secretary of the Interior to expedite the completion of planning reports listed in the Secretary's report "Colorado River Water Quality Improvement Prcgra1l, February 1972," which is essentially the Colorado River Water Quality Improvement Program. The Water Quality Improvement Progr am is a gener a 1 i nves t i gati on progr am deve loped in ear 1 y 1971 by the Bureau of Reclamation. Subsequent legislation, Public Law 96.375 of October 3, 1980, specifically authorized feasibility level studies on 10 of the salinity control units. The Forum (Colorado River Basin Sal inily Control Forum) approved the 1978 and 1981 revisions to Colorado River water quality standards for salinity _ including numeric criteria and a plan of implementation for salinity 9