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CHAPTER III - WATER QUALITY STANDARDS FOR SALINITY Cu Criteria .....:1 00 The Forum developed and agreed upon basinwide water quality N standards for salinity, including numeric criteria and a plan of implementation for salinity control in 1975 (1975 Forum report). The standards were pursuant to federal regulations published in the Federal Register on December 18, 1974. I~ order to provide for sound water quality objectives, based on a basinwide approach, numeric criteria were estab;Lished at three key stations (below Hoover, below Parker, and at Imperial Dams). The key stations were selected due to their proximity to major diversions in the Lower Basin. The State of Nevada diverts Colorado River main stem water from Lake Mead for municipal and industrial uses in the Las Vegas area. The Metropolitan Water District of Southern California and the Central Arizona Project divert water from Lake Havasu for all uses. The large agricultural areas in the Imperial and Coachella Valleys in California and the Yuma area in Arizona and California are served by diversions at Imperial Dam. The flow-weighted average annual salinity for the year 1972, as determined by Reclamation from daily flow and salinity data collected by the USGS and Reclamation, were selected as the numeric criteria. EPA published regulations in 1974 stating "The flow- weighted average annual salinity in the lower main stem of the Colorado Riv~r ~~stem is to be maintained at or below the average value found during 1972." The states adopted the EPA regulation. There is no inference that 1972 represents a typical or average year. The average daily flow of the river past each of these three measuring points is determined, and the average concentration of salts in the water in milligrams per liter (mg/l) is also determined each day at each of these three measuring points. Each average daily flow is then multiplied by the daily salinity concentratior.s.. ....p.sul ting in a flow/salinity mass I)Jlmbe:r:'. For all 365 days of the year, the daily mass numbers are added together, resulting in an annual mass number. The sum of average daily flow (in cfs) of the river past the gage for the entire year is also calculated. The total mass number is then divided by the sum of each average daily flow for the year, and the resulting figure is the flow-wei~hted average annual salinity for the calendar year at that station. It should be noted that in regulations, 1972 was the year upon The numeric criteria are as follows: conformance with the 1974 which to base the standards. 4 See Appendix B, Forum letter to EPA dated February 26, 1990, and EPA Regulation 40 CFR, Part 120. 18