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Show Utah Geological and Mineralogical Survey, Water- Resources Bulletin 14, 1970 North brine Bottom of culvert Lower south brine Upper south brine Lower south brine North brine B Dashed lines represent position of interface Figure 6. Schematic drawing of Southern Pacific Co. causeway showing ( A) direction of flow through the culverts and ( B) probable direction of flow through the permeable fill. Figures are not representative of quantities of discharge. Trends Since 1963 Although the lake is divided into two parts, each with an outlet through the causeway, the lake as a whole is still a closed lake with no outlet. The fresh- water inflow carries a relatively insignificant amount of dissolved salt, thus the total load of salt in the lake does not change significantly over a period of a few years. However, the total dissolved- salt load and the chemical composition of each part may change because of interchange of load through the causeway and because of precipitation or solution of the salt crust. Dissolved Solids A change in concentration of dissolved solids in either part of the lake may be considered as the change in dissolved- solids load divided by the change in volume. Volume changes resulting from inflow and evaporation can create seasonal changes in the dissolved- solids concentration in the south part of the lake. Long- term changes in lake stage and volume ( related to climatic cycles) will cause corresponding changes in dissolved- solids concentration. Figure 7 shows a comparison between the actual dissolved- solids concentration in the south part for the period 1964- 69 and the theoretical dissolved- solids concentration if volume change had been the only factor. The concentration in the south part, however, also can increase or decrease over a period of several years, depending on the net exchange of load of dissolved solids through the causeway. The net exchange of load through the causeway is dependent upon the relationship between the ratio of discharge in each direction and the ratio of the concentration of dissolved solids in the water. This relationship may be expressed by the equation: L = VTCT- V0C0 L = Net exchange of dissolved- solids load Vj = Volume of brine discharging into the north part from the south part through the causeway Cj = Average concentration of Vj V0 = Volume of brine discharging out of the north part into the south part through the causeway Co = Average concentration of V0 If, for instance, the ratio of discharge was equal to the inverse of the ratio of concentrations, Vj_ = Co V0 = Q then net exchange of load ( L) would be zero ( VjQ - V0C0 = 0); and concentration of the south part would be strictly a function of volume change. If Vi / V0 is greater than C0/ Ci, then net load movement will be to the north; and the concentration of the south part for a given lake stage ( volume) will decrease with time. If Vi / V0 is less than CQ/ Ci, net load exchange will be to the south and the south part will gain concentration for a given lake stage. 15 |