| OCR Text |
Show EFFECTS OF A CAUSEWAY ON THE CHEMISTRY OF THE BRINE IN GREAT SALT LAKE, UTAH by R. J. Madison Hydrologist, U. S. Geological Survey ABSTRACT During 1957- 59, the Southern Pacific Co. constructed a permeable rockfill causeway to carry its railroad track across Great Salt Lake. The causeway divides the lake into two parts and interrupts the formerly free movement of brine about the lake. The causeway has caused significant changes in the chemistry of the lake, including a dilution of the brine in the south part of the lake and a concentration of the brine in the north part. Prior to construction of the causeway, the dis- solved- solids content and the chemical composition of the lake brine were controlled primarily by volume changes resulting from inflow and evaporation. The causeway created two separate but interconnected lakes with different water- surface elevations and densities. As a result, brine flows in both directions through the causeway, with less dense brine from the south part moving northward through the upper part of the causeway and more dense brine from the north part moving southward through the lower part of the causeway. The chemistry of the lake is now controlled by the interchange of dissolved- solids load through the causeway, as well as by changes in the salt crust and by volume changes. The net movement of dissolved- solids load through the causeway will be dependent upon the relationship between the ratio of water discharges in each direction and the concentration of dissolved solids. Estimates made for the 1969 water year indicated that the ratio of total flow northward to total flow southward was about 11: 1 through the culverts and about 3: 1 through the causeway fill. Discharge through the culverts accounted for about 45 percent of the total brine moving northward but only about 15 percent of the total brine moving southward. The estimated net movement of dissolved- solids load through the causeway during the 1969 water year was about 210 million tons northward. The small amount of data available for the period from 1963 to 1969 indicates continued loss in dissolved- solids load from the south part of the lake to the north part. Whether or not this load loss will continue in the future depends on what happens to the ratio of flows through the causeway. Although some limits can be placed on the possible range of these ratios, their interrelationships with other parameters of lake hydrology are extremely complex. More precise definition of the effects of the causeway on future lake hydrology will require the collection of additional field data and the analysis of all available data, preferably with the aid of a model. INTRODUCTION The division of Great Salt Lake, Utah, into two parts by a causeway has caused significant changes in the chemistry of the lake, and it is desirable to know whether these changes will persist or will be modified by time and changing events. A preliminary study of the chemistry of the lake in the immediate vicinity of the causeway was conducted from 1967 to 1969 by the U. S. Geological Survey as part of a continuing cooperative program with the Utah Geological and Mineralogical Survey. The purpose of the study was to try to define the movement of brine through the causeway and the effects of this movement on the chemistry of the lake. This report will discuss briefly the hydrology of the lake prior to the construction of the causeway, the factors which now control the chemistry, and the possible long- term effects which the causeway may have on the mineral content of the lake. During 1957- 59, the Southern Pacific Co. constructed the causeway for its railroad track across Great Salt Lake. The causeway traverses the lake in an east- west direction from Promontory Point to Lakeside ( figure 1), where the lake is about 18 miles wide. Approximately 13 miles of this distance is covered by the causeway fill emplaced in 1957- 59; and an older fill, constructed in the early 1900' s, abuts each end of the newer fill. The older fill, which has a total length of about 5 miles, formed the approaches to a trestle which crossed the lake before the new fill was emplaced. The causeway was constructed by dredging a channel 25- 40 feet deep and 150- 500 feet wide to remove bottom muds. The channel was then backfilled with sand and gravel. The causeway was raised above the lake surface with quarry- run rock and finished with riprap varying in size from 1- ton capstone 15 feet below the surface to 3- ton capstone at the top ( Cohenour and others, 1963, p. 16). The causeway is permeable and is also breached by two open culverts, each 15 feet wide, 7 |
