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Show Utah Geological and Mineralogical Survey, Water- Resources Bulletin 14, 1970 which allow brine to flow through the causeway ( see figure 2). The culverts were placed where the lake is deepest. The bottoms of the culverts are about 10 feet above the lake bottom, but the bottom of the east culvert is about 4 feet lower than that of the west culvert. Although brine moves through the permeable fill and the culverts, the causeway acts as a partial dike dividing the lake into north and south parts. A little more than one- third of the lake lies north of the causeway. The causeway interrupts the formerly free movement of brine within the lake. This, along with the fact that more than 95 percent of the fresh- water surface inflow enters the lake south of the causeway, has resulted in substantial changes in the hydrology and chemistry of the lake. Hahl and Handy ( 1969) made a fairly detailed study of the chemistry of the lake and pointed out some of the basic changes that had taken place in the lake- the brine in the south part had become more dilute, the brine in the north part had become more concentrated, and a salt crust of undetermined thickness and extent had been precipitated on the bed of the lake in the north part. A sampling network was established to define any chemical changes taking place in the immediate vicinity of the causeway ( figure 1), and samples for chemical analysis were collected at high and low lake stages and during evaporation and inflow periods. Results of these analyses are given in tables 6 and 7 ( appendix C). Measurements of flow in the culverts were made 15 times from October 1968 to September 1969. During late 1969, a dye tracer study was made in an attempt to define brine movement through the permeable fill ( appendix A). The collection of data for the investigation was greatly facilitated by personnel of the Southern Pacific Co., who allowed access to the causeway and provided other assistance on many occasions. Special thanks are due Mr. Lee Schaub of the Ogden Office of the Southern Pacific Co., without whose excellent cooperation the investigation could not have been accomplished. HYDROLOGY OF THE LAKE PRIOR TO CONSTRUCTION OF THE CAUSEWAY Prior to 1957, the hydrologic characteristics of Great Salt Lake were typical of a closed lake. The lake stage ( altitude of water surface) rose or fell over a period of several years in response to the balance between the amount of water evaporated from the surface and the amount of water contributed to the lake by surface runoff, ground- water inflow, and precipitation on the surface. During periods of falling lake stages, surface area and volume decreased and concentration of dissolved solids increased; conversely, during rising lake stages, surface area and volume increased and concentration decreased. In addition to long- term changes- because of a series of wet or dry years- the stage, surface area, volume, and concentration were continually changing because the period of maximum inflow differs from the period of maximum evaporation. At lake stages below about 4,195 feet above mean sea level, the concentration of sodium chloride reached saturation and salt was precipitated. When lake stages again rose, the precipitated salt was redissolved. Before the causeway was constructed, the dis- solved- solids concentration in all parts of the lake probably was generally similar, although differences in inflow, evaporation, currents, wind, and density have always caused some variation in the concentration from place to place. During the summer, brine in the large shallow areas of the lake became more concentrated than brine in deeper parts of the lake because of evaporation; but periodic mixing during storms probably kept the composition of the lake water fairly uniform. Although the concentration varied with lake stage and volume, the total load of dissolved solids changed little from year to year because the total salt load in the lake is more than 2,000 times the amount of salt added annually by surface inflow. The relative ionic composition of the dissolved solids also remained relatively constant ( Hahl and Langford, 1964, p. 25); sodium and chloride were the principal constituents, with sulfate, magnesium, and potassium present in lesser amounts. Calcium, bicarbonate, and other substances were present as minor constituents. HYDROLOGY OF THE LAKE AFTER CONSTRUCTION OF THE CAUSEWAY Initial Sequence of Events During 1959, the average lake stage was about 4,195.5 feet above sea level. At this stage, the original undivided lake would have contained about 350 g/ 1 ( grams per liter) of dissolved solids and would have been at or near saturation with respect to sodium chloride. When the causeway was closed, the south part of the lake was diluted because almost all the fresh- water inflow was entering south of the causeway. The north part of the lake remained near saturation because its major inflow was brine flowing through the causeway 9 |