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Show Utah Geological and Mineralogical Survey, Water- Resources Bulletin 14, 1970 In the north part of the lake, the concentration of dissolved solids does not change significantly with season or lake level. The major dissolved ions, sodium and chloride ( about 85 percent of the total), apparently are in equilibrium with the salt crust ( Hahl and Handy, 1969, p. 15). Assuming that stratification does not occur, the north part will remain at a nearly constant concentration as long as the salt crust is not totally dissolved. Some of the inflow to the north part may evaporate before it mixes with the more dense brine in the north part. That part of the brine from the south part which does mix, however, temporarily reduces the concentration of the water in the north part below the saturation concentration for sodium chloride. However, the brine then redissolves a small amount of salt crust and again returns to the saturation concentration. During the season of high evaporation, the rate of evaporation exceeds the rate of inflow and salt is precipitated, adding to the salt crust. The net change in the amount of salt crust depends on the amount dissolved during inflow periods compared to the amount precipitated during evaporation periods. Chemical Composition The brine of the lake, prior to the construction of the causeway, had a nearly uniform chemical composition. When large amounts of sodium chloride precipitated from solution, however, the ratio of ions in solution changed significantly. Because sodium ( Na) and chloride ( CI) were the only ions to precipitate in any quantity, the percentage of the other ions in solution with respect to sodium and chloride increased. The chemical composition can be affected temporarily due to the precipitation of Glauber's salt ( Na2SO4- 10H2O) during the winter months; however, this salt is re dissolved each spring as the lake warms, and there is no net change in ratio of ions in solution. The south part of the lake now receives almost all the fresh- water inflow that previously fed the entire lake. However, the fresh- water inflow still does not significantly affect the ionic ratios in the south part because the total amount of dissolved ions in the fresh- water inflow is still extremely small compared to that in the brine ( Madison, 1969, p. 142). The ratio of ions in solution in the south part is now dependent primarily on the interchange of load of dissolved ions through the causeway. For example, the ratio of the concentrations of magnesium ( Mg) to chloride ( CI) in the south part at the end of any stipulated period of time will be: ( Initial load Mg + load Mg gained from north part - load Mg lost to north part) v ( Initial load CI + load CI gained from north part - load CI lost to north part) or ( Initial load Mg ± net change in load of Mg) •?• ( Initial load CI ± net change in load of CI). As was the case with dissolved solids, the net movement of individual ions through the causeway is dependent on the ratio of the discharges in each direction and the ratio of concentrations of the ions. In the north part of the lake, the concentrations of sodium and chloride remain essentially constant regardless of load exchange, because the concentration is kept at or near saturation with respect to these ions as a result of precipitation or solution of the salt crust. Thus, the ratio of any other ion to sodium or chloride ( for example, magnesium to chloride) will change primarily in response to changes in concentration of the other ion ( magnesium). If the net movement of magnesium load through the causeway is to the north, then for a given lake stage the magnesium- chloride ratio will increase because the magnesium concentration will increase even though the chloride concentration remains constant. The magnesium- chloride ratio in the north part can also change with volume changes even if net movement of magnesium load through the causeway is zero. This can occur because volume change will change the concentration of magnesium but not that of chloride, which remains constant as indicated above. Figure 8 shows the ratio of magnesium to chloride for samples collected before and after construction of the causeway. The data for 1967- 69 are mostly from samples collected within 2 miles of the causeway. However, a comparison of these data with data from a few samples collected simultaneously at other points in the south part of the lake showed no significant areal differences in magnesium- chloride ratios. The variations in the magnesium- chloride ratios shown in figure 8 are due partly to precipitation of salt and partly to interchange of brine through the causeway. Data are not available at the present time ( 1969) to define how much of the change in ionic ratios has been the result of salt precipitation and how much the result of interchange through the causeway. 17 |