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<br />- <br /> <br /> <br />w <br />w <br />~ <br />c..~ <br /> <br />The CRSS model presented on page 31 predicts future salinity trends given <br />e;(pected depletions and salt loading schedules. Reclamation is also now <br />using the computer extensively for long-term specific ion analysis and <br />reservoir studies to improve these predictions. <br /> <br />Salinity concentrations at Imperial Dam have decreased in an apparent <br />steady deel ine from 1970 to 1979. dropped notably in 1980, and increased <br />sharply in 1981. as shown in figure 4. Generally, the data are showing <br />the buffering effect that over 50 million acre-feet of reservoir storage <br />can have on the salinity concentration at Imperial Dam. Although the data <br />vary within the range of expected values. the recent decline of TDS con- <br />centrations at some of the stations on the Colorado River has led to the <br />hypothesis that some mechanism(s) may be decreasing the sal inity of the <br />Colorado River. However, several factors complicate the analysiS of the <br />decline and leveling off of salinity concentrations. <br /> <br />The most significant of these are the natural variations in concentrations <br />stemming from variations in runoff in wet and dry years and the effects of <br />in i t i a 1 fill i ng and oper at i on of the ma ins tern reservoi rs such as Lake <br />Powell. There is evidence that Lake Powell has stored higher TDS water and <br />routed the lower TDS spring runoff downstream from 1965 throu9h 1980. This <br />selective salinity routing in the 1970's may not be representative of <br />future conditions. In addition, the sedimentation processes in the reser- <br />voirs may influence both TDS and the ratio of dissolved ions. Suspended <br />sed iment, subj ect to mechan i c a 1 degr adat i on in a river env ironment. may <br />continue to release salts and exchange ions (SOdium exchanged for calcium); <br />however, once sett 1 ed out in the reservo i r, these salt sand ion exch ange <br />capabilities may be isolated. Precipitation of calcium carbonate from <br />biological/chemical influences in the reservoirs has been hypothesized to <br />limit salinity. A number of mechanisms exist by which calcium and carbon- <br />ate may be reduced. <br /> <br />The overall significance of these mechanisms needs to be defined in terms <br />of total salt reduction and economic benefits. It would be fortunate if <br />future salinity conditions were partially alleviated by some natural <br />mechanisms as hypothesized. However, it is highly unlikely that these <br />natural mechanisms can be depended upon to maintain sal inity below the <br />standards under future depletion conditions. <br /> <br />The downward fluctuation of salinity at Imperial Dam during the 1970's is <br />within the expected range and therefore cannot be judged as a significant <br />long-term trend. Therefore, the long-range CRSS predict ions for sal inity <br />increases need to take precedence. <br /> <br />Economic Viability <br /> <br />Only in the past 5 years has sufficient site-specific information been <br />generated to obtain reliable cost data on salinity control measures. The <br />highest costs for structural controls are associated with desalination <br />plant sand 1 i ned ev apor at ion ponds. The desalt i n9 and pond costs are <br />directly responsive to the steady increases in costs of energy and petro- <br />chemicals as well as to new design requirements to meet imposed Federal and <br />State regulations. <br /> <br />13 <br />