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Show Madison- Effects of Causeway on Chemistry of Great Salt Lake These computations have been made for a range of south part concentrations of from 50 to 450 tons per acre- foot ( 37- 330 g/ 1) for a range of the net annual inflows to the north part of 400,000 to 2 million acre- feet ( table 2). The figure of 400,000 acre- feet represents the approximate quantity of inflow required to maintain the north part at about 4,193 feet above mean sea level under average climatic conditions. The figure of 2 million acre- feet represents the approximate quantity of inflow required to maintain the north part at a stage of about 4,200 feet. If, for example, the average concentration of dissolved solids in the south part for a given period of time were 200 tons per acre- foot, or about 147 g/ 1 ( table 2, column 1), there would be zero net load movement through the causeway when the average ratio of discharge ( Vj / V0) for the same period was 2.4: 1 ( column 2). For the same concentration, if the average ratio of discharge were greater than 2.4: 1, the net load movement would be to the north. If the average ratio of discharge were less than 2.4: 1, the net load movement would be to the south. The figures enclosed in parentheses in table 2 are for concentration and discharge that approximate those estimated for the 1969 water year. On the basis of these figures, it would appear that load loss from the south part will continue unless the average ratio of discharge decreases; or, if the ratio of discharge does not change, until the average dissolved- solids concentration in the south part decreases. The information presented in table 2 indicates that extremely low ratios of discharge northward to discharge southward are probably unlikely because of the exceptionally large quantities of discharge involved. Although the available data do not permit accurate prediction of the amount of load loss or gain which will occur over a given period of time or for a specific lake stage, the data do make it possible to show the theoretical effect that a given change in load will have on the south part of the lake ( See table 3). For example, if 50 million tons of dissolved solids were lost from the south part as a result of interchange through the causeway, the concentration of dissolved solids in the south part would decrease by an amount ranging from 3.8 to 6.5 g/ 1, depending upon the lake stage. If the interchange of water through the causeway resulted in an increase of 50 million tons in the south part, the concentration would increase by the corresponding amount. By contrast, it should be noted that at the present ( 1969) average concentration of dissolved solids in the south part ( about 220 g/ 1), the normal seasonal change in lake stage of about 2 feet in itself would result in a concentration change of 20- 30 g/ 1 in the south part. INDUSTRIAL WITHDRAWAL OF BRINE The withdrawal of brine from the Great Salt Lake for industrial use may become a significant factor affecting the chemistry of the lake. Withdrawals have increased rapidly during the last few years. In 1969 these withdrawals amounted to less than 50,000 acre- feet, a relatively small amount compared to the approximately 7 million acre- feet of brine in the lake. New developments in the mineral- extraction industry, however, could easily double withdrawals in the near future. Withdrawals of this magnitude could be significant in the computation of gains or losses of dissolved minerals from each part of the lake. For example, an annual withdrawal of 100,000 acre- feet of brine from the lake would result in a loss of about 47 million tons of dissolved solids from the north part of the lake or about 28 million tons from the south part at their present concentrations. This compares with a total estimated load loss of 300 million tons from the south part for the 7- year period, 1963- 69 ( figure 12). NEEDS FOR ADDITIONAL STUDY Reliable predictions of future changes in the hydrology of Great Salt Lake cannot be made without the collection of additional data and the analyses of these data. Analysis of presently available data has resulted in a definition of some of the factors that control the hydrology of the lake. Rough estimates have been made of the interchange of brine through the causeway and of the possible effects of this interchange on the chemistry of the lake. If reliable predictions of future changes are required for economic or other considerations, it will be necessary to continue and enlarge the scope of collection of field data and then to analyze all available data. A program of future data collection should include: 1. Continuous measurement of lake stage north and south of the causeway. 2. Continuous measurement of quantity of discharge, both north and south, through the two existing culverts in the causeway. 3. Measurement of discharge, both north and south, through the causeway fill. 28 |
