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. ~ Q ~ ~ . . CAUSES OF SALINITY (continued) Salt balance conditions eKist 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 total dissolved-sol ids load in the return flow over the total load in the applied water. Salt pickup chargeable to inigat ion would be only that addit ional which occurs as a result of irrigat ion and should not include the amount resulting from natural sources. The small amount of data presently available gives indications of much variation in the amount of pickup from land due to irrigation. The esti- mated salt pickup in this report for projects without specific information is based on values of 0 and 2 tons per acre (0 and 4.5 t/ha) per year from newly irr igated 1 and. Zet-o or min imum cond it ions occur generally after initial leaching in areas whe["e soils are loose and contain vet.y little salt and a salt balance of inflow and outflow has been reached. The 2 tons per acre (4.5 t/ha) value was selected as the higher end of the range for the average annual pickup over a project area. It was also assumed in this report no additional pickup would result from supplemental water applied to presently irrigated lands. On projects where detailed. information on salt pickup was available through use of models or by special investigations, these more spec ific figures we.re used. 3. Municipal and Industrial Sources of Salinity Salt loads contributed to the CoLnado River system by municipal and industrial sour::es are generally minor, totaling about I percent of the basin salt load. Future increases in salt loads from these Sources are eKpected to be small relative to the total basin salt burden and will have only a minor effect on salinity levels. With the except ion of concent rated return flows from the Las Vegas area, 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 dischaL-ged (i.e., industrial brines) waL-rant such control. The energy shortage has caused an increase of interest for construc- tion of large energy producing industries within the Colorado River Basin. With emphas is placed on improving the water qual ity in the basin, these industries have been IJnder pressure by State and Federal agencies to prevent the return of salts to the river by consuming all water diverted for use. 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 quality than in downstream reaches produces a concentrating effect on the waters of 32