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PART III CAUSES AND IMPACTS OF SALINITY e Studies prior to irrigation would be helpful to determine con- tribution from irrigation, but they have not been made in most areas. The amount of salt from this source must, therefore, be estimated or determined by detailed investigations, possibly with the use of simula- tlon models. Salt balance conditions exist when the amount of dissolved solids carried off the land is 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 Salt loads contributed to the Colorado River system by munici- pal and industrial sources are generally minor, totaling about I per- cent of the basin salt load. Future increases in salt loads from these sources are expected to be small relative to the total basin salt burden and will have only a minor effect on salinity levels. e Most municipal and industrial wastes are relatively low in total salt load in comparison with natural and agricultural sources, and complete elimination of such waste discharges would have little effect on salinity concentrations in the main river system. Since these wastes are point sources of salinity, control could be achieved if salinity levels in the waste being discharged (i.e., industrial brines) warrant such control. Development of oil and gas, oil shale, and mineral resources in the basin also 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 salts, flowing to the surface through abandoned oil wells. B. Increased Concentration from Water Depletions Addition of salts to the river system is not the only cause of in- creased salinity concentrations. 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. e 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