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CHAPTER 3 - NUMERIC CRITERIA FOR SALINITY o N ~ I-' Overview As discussed earlier in this report, the EP A promulgated a regulation which set forth a basinwide salinity control policy for the Colorado River Basin. This policy required that the flow- weighted average annual salinity in the lower mainstem of the Colorado River be maintained at or below the 1972 levels. The Basin states, acting through the Forum, addressed this requirement in its first Review entitled Water Quality Standards for Salinity Including Numeric Criteria and Plan of Implementation for Salinity Control- Colorado River System dated June 1975. In the 1975 Review, the Forum proposed three stations as appropriate points in the lower mainstem of the Colorado River at which to measure the flow-weighted average annual salinity. These stations are located at the following points: (1) below Hoover Dam; (2) below Parker Dam; and (3) at Imperial Dam. The basis for selecting these stations is their proximity to key diversion facilities on the lower Colorado River. Nevada diverts Colorado River mainstem water from Lake Mead for use in the Las Vegas area. The Metropolitan Water District of Southern California and the Central Arizona Project divert water from Lake Havasu, impounded behind Parker Darn, for millions of water users in southern California and central Arizona, respectively. The large agricultural areas in the Imperial and Coachella Valleys in California and the Yuma area in Arizona are served by diversions at Imperial Dam. The flow-weighted average annual salinity for these stations was determined by Reclamation from daily flow and salinity data collected in 1972 by USGS and Reclamation and became the numeric criteria. The fact that 1972 was chosen as the basis for establishing the numeric criteria creates no inference that 1972 represented a typical or average year from either a hydrologic or water quality perspective. The numeric criteria for each of those stations is as follows: Below Hoover Dam Below Parker Dam At Imperial Dam 723 mg/L 747 mg/L 879 mg/L The criteria were established to protect infrastructure and crop production rather than human health or fish and wildlife values. The salinity concentrations that are anticipated in the future, even without salinity control efforts, have not been shown to have adverse effects on human health or wildlife. Thus, the Colorado River Salinity Control Program is different from most other water quality standards compliance programs. 3-1