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Show Utah Geological and Mineralogical Survey, Water- Resources Bulletin These totals give a general idea of the probable amounts of brine and dissolved salts that moved in each direction through the causeway during the 1969 water year. The totals are based on a small amount of data collected during a short period of time. The measurements of discharge in the culverts represent only lake conditions during relatively calm days. During storms or when the lake is rough, the flow in the culverts is much more turbulent than during the calm periods. The relationships used to compute discharges may not be fully applicable under turbulent conditions. Althrough the actual discharge reported may be in considerable error, they are believed to be of the correct order of magnitude. Because of the large volumes of water involved, however, even the relatively small inaccuracies inherent in chemical analysis can have a large effect on estimated loads of dissolved solids. POSSIBLE FUTURE EFFECTS OF THE CAUSEWAY ON THE CHEMISTRY OF GREAT SALT LAKE The present investigation has shown that the hydrology of Great Salt Lake is now much more complex than it was prior to construction of the causeway. The movement of brine, and hence the movement of dissolved minerals, in each direction through the causeway is the controlling factor in determining the total mineral content ( dissolved load) of each part of the lake. Estimates for the 1969 water year show that the ratio of northward to southward discharge was such that some load loss occurred from the south part during that year. Moreover, the scanty chemical- quality data available from 1963 to 1969 indicate a tendency toward load loss from the south part for the entire period ( figure 12). The effect of this load loss on the concentration of dissolved solids in the south part has been masked somewhat because the lake stage rose continually during the same period. The total change in volume due to rising lake stage over the 7- year period has caused a much larger decrease in concentration of dissolved solids than has the apparent load loss. Figure 13 shows the observed relationship of volume to concentration of dissolved solids in the upper layer of brine in the south part of the lake for the 7- year period. The dotted line in figure 13 indicates the estimated concentration of dissolved solids had there been no load loss through the causeway and had volume change been the only factor. This dilution curve may not exactly represent the true dissolved- solids concentration in the upper layer of brine had there been no load change. The actual concentration for a given volume would probably be a small percentage greater because of salts redissolved from dryland areas as the lake rises. 14,1970 The data in figure 12 for dissolved load in the entire lake do not indicate any upward or downward trend. The small seasonal changes in load appear to indicate that some salt may have been precipitated or dissolved seasonally. There is no indication, however, that large amounts of salt have been precipitated or that large amounts of salt crust have been redissolved during the period 1963- 69. Whether or not the load loss will continue in the south part in the future depends on what happens to the ratio of discharge north and south through the causeway. This ratio will be dependent on the pressure gradient across the causeway, which in turn is a function of stage difference across the causeway, density, and lake stage. If the average lake stage rises as the result of increased inflow, the difference in stage between the north and south parts should increase ( figure 4), thus tending to create a greater pressure gradient to the north and increasing the ratio of flow northward ( Vj) to flow southward ( V0). At the same time, however, the increase in lake stage ( volume) will result in a decrease in the density of the brine in the south part, thus creating a greater density difference between the north and south parts. This will cause a decrease in the pressure gradient from south to north, and tend to decrease the ratio of Vj to V0. By contrast, if the average lake stage falls, decreases in the stage difference would tend to decrease the ratio of Vj to V0, while decreases in the density difference would tend to increase it. The loss or gain of load will also affect the density of the south part, which in turn will affect the ratio of discharge regardless of lake stage. The lake will be continually rising and falling; therefore, the ratio of discharge and consequently the amount of load movement will be continually changing. The data presently available do not allow an exact definition of the complex interrelationships between water movement and density, stage difference, and lake stage. However, the data allow us to set approximate ranges of values for two of the factors that control the ratio of discharge; thus, we can place some limitations on what may or may not happen in the future. If the water in the north part of the lake continues to remain at a constant concentration, then we can compute the ratio of flows required for zero net load movement through the causeway for the range of concentrations which may occur in the south part. Also, we can compute, for the expected range of annual inflow to the north part, the approximate quantity of water which must move through the causeway for a given ratio of discharge. 25 |