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tv o ~ ,.;:.. ! NTRODUCTION CHAPTER I The Problem Salinity is a major issue in the Lower' Colorado River Basin. A criterion for flow- weighted average annual salinity concen- tration of 879 mg/l was established in 1976 as a maximum for flows at Imperial Dam. Three years before, the seven basin states had formed a Colorado River Basin Salinity Control Forum to coordinate salinity control efforts. A provision, known as Minute 242, in an agreement with Mexico, assured that waters delivered to the Mexican diversion point would have an annual average salinity of no more than 115 ppm 0'.7"'1 t'ha t of wa ter arriving at Imperial Dam. wLdle average annual salinities have decreased from 890 ml'/l in 1970 to a little below 800 mg/l in 1981, a decline probably associated with the filling of Lake Powell, the expectation for the long run is for increasing salinity levels unless an effective control program is established. Any major future increases in salinity would only add to already major lOSR€S to agriculture and damages to munici- pal and industrial water users (U. S. Depart- ment of the Interior 1974 and Andersen and Kleinman 1978). ~ ~ f I , Multiple methods are being explored to hold down salinity concentrations. Two principal alternatives exist. One is to remove salt from the water through construc- tion of a desalting complex as has been authorized by PL 93-320 for the United States to fulfill its obligation with Mexico. A potentially less expensiv~ alternative is to reduce the concentratlon of salt reaching the mouth of the Colorado. The concentration may be reduced either by adding to the water or by reducing the salt. The high economic value of water in the Lower Basin makes using more to transport salt unattractive and focuses attention on ways to reduce the salt content. One approach to reducing salt content is to reduce the amount of salt leaving the Upper Basin either by augmenting natural salt precipitation processes or by finding an economically attract ive use for salt brine. Explored options include salt precipitation in reservoirs (Messer et a1. 1981), export of salt brines as the conveying fluid in coal slurry pipelines (Israelsen, et a1. 1980), and use of the salt for electric power production in salt-gradient solar ponds (Riley and Batty 1982). All three have cost l)r technical feasibility problems. Alternatives for reducing the or.iginal salt loading entering the river system are even more difficult to evaluate because the salt sourqes are so many and so diffuse. Salts enter the Colorado River after being leached from irrigated soils, concentrated by evapotranspiration, and returned as agri- cultural drainage. Municipal and industrial uses add salts from extracted gr.oundwater, expose salt bearing materials to weathering, and increase leaching as a result of outside water uses in residential areas. Fossil fuel extraction and processing in the Upper Basin are being particularly watched as future threats. All of tnese man-caused sources of salt loading adcl to the larger natural salt loading. Mineral springs and natural groundwater seeping from marine formations abound. Natural diffuse sources are scat- tered over vast areas of open land. Blackman et a1. (1973) estimate that 37 percent of the total salt loading to the Colorado River occurs from diffuse sources in the Upper Basin. Mountainous areas yield most of the river flow from a rela'tively small fraction of the catchment and supply relatively high quality water. As the streams traverse the immense, semiarid lowlands, little flow is added and water quality deteriorates as water is used con- sumptively and the streams interact with na~ural salt bearing geological formations. The Price River subbasin of Central Utah (Figure 1.1) is a miniature of this salt loading pattern. Relatively high quality flow (less than 10DO mg/l TDS or total dissqlved solids) originates in mountainous headwater areas. After emerging from the mountains, the river traverses an irrigated area amounting to about 2 percent of the total catchment. Further downstream, it crosses large areas of natural and range lands. It contacts a marine formation high in soluble salt content called the Mancos Shale. Finally, it reaches Woodside with an averagB dissolved solids concentration of about L500 mg/l, This most downstream river section, where the Price River flows through arid range lands having an average annual precipi- tation of only about 8 inches, provides a setting to study and quantify natural salt loading. Hopefully, the relationships derived and the understanding gained from 1