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. . . ....~ l\:I Ul t,,) PART III CAUSES AND IMPACTS OF SALINITY Studies prior t9 irrigation would be helpful to determine con- tribution from irrigation, but they have not been made in most areas. The amount of aait from thia aource must, therefore, be estimated or determined by detailed investigations, possibly with the use of simula- tion models. Salt balance conditiona exiat when the amount of dissolved solids carried off the land ia equal to that amount added. Pickup of salt as used in this report represents an unbalanced condition shown by the increase of TDS load in the return flow over the total load in the applied water. Salt pickup attributed to irrigation is only that addi- tional amount which occurs as a result of irrigation and does not in- clude the amount resulting from natural sources. 3. Municipal and Industrial Sources of Salinity pal and cent of sources and wi 11 Salt loada contributed to the Colorado River system by munici- industrial sources are generally minor, totaling about 1 per- the basin salt load. Future increasea in salt loads from these are expected to be amall relative to the total basin salt burden have only a minor effect on salinity levels. Most municipal and industrial wastes are relatively low in total aalt load in comparison with natural and agricultural sources, and complete elimination of such waate dischargea would have little effect on salinity concentrations in the main river system. Since these wastes are point sources of aalinity, control could be achieved if salinity levels in the waate being discharged (i.e., induatrial brines) warrant such control. Development of oil and gas, oil shale, and mineral resources in the basin alao has the potential to increase salt loading. Many saline aquifers are static (very little water movement) until they are disturbed by drilling or mining activities. An example is the Meeker Dome Salinity Control Unit, described in Part VII, which came about as the result of deep ground water, high in dissolved salta, flowing to the surface through abandoned oil wells. B. Increased Concentration from Water Depletions Addition of salts to the river syatem is not the only cause of in- creased salinity concentrationa. The depletion of water of better qual- ity in the Upper Basin produces a concentrating effect on the waters of the downstream reaches. This concentrating effect occurs to a greater degree when the diverted salts return to the river than when they are depleted along with the water. Since the Lower Basin has already developed most of its water supply with the exception of the Central Arizona Project, most of the additional future depletions will be developed in the Upper Basin. 'Depletions in the Upper Colorado Basin were 4 million acre-feet in 1983. 13 .