U.S. Department of the Interior—U.S. Geological Survey Prepared in cooperation with Bureau of Land Management Investigation of Salt Loss from the Bonneville Salt Flats, Northwestern Utah James L. Mason and Kenneth L. Kipp, Jr. decreasing thickness region. The playa is flanked by the and extent of the salt Silver Island Mountains on the crust are caused by northwest, and the land surface brine withdrawal for gradually slopes upward for many mineral production. miles to the northeast, east, and south. Situated at the lowest altitude in the Mining of halite study area, the salt crust would be the (common table salt) final destination for surface runoff. from the Bonneville Salt Runoff from the flanks of the adjacent Flats began in the early Silver Island Mountains generally 1900s. Extraction of occurs during intense summer thun- potash from brine derstorms and flows in distinct beneath the Bonneville channels from the mountains toward Salt Flats began in 1917, the playa. These flows cross the when supplies of alluvial fans but mostly disappear into potassium salts from large surface fractures before reaching Germany were inter- the playa. Runoff during wetter-than- Drilling on the salt crust, Bonneville Salt Flats, Utah. rupted during World normal conditions can migrate several Measurements of Salt Crust War I. The area from which brine is withdrawn for potash production was Great Indicate a Decrease in Thickness expanded in 1963, when mineral leases Salt Great Salt and Extent on 25,000 acres of Federal land were Lake Lake UTAH issued. Desert The Bonneville Salt Flats study area Salt Lake City is located in the western part of the The Bonneville Salt Flats is a playa, Great Salt Lake Desert in northwestern which is a topographically low, flat Utah, about 110 miles west of Salt Lake area where evaporation is the only City (fig. 1). The salt crust covers about form of water loss from this arid 50 square miles, but the extent varies yearly as a result of salt being dissol- EXPLANATION ° ved by the formation and movement 4210 Approximate potentiometric contour— 113 45' of surface ponds during the winter Shows approximate altitude at which water Y and redeposited with the evaporation level would have stood in tightly cased wells when referenced to an average FLOATING of these ponds during the summer. density. Contour interval, in feet, is ISLAND T. 2 variable PLAYA BOUNDAR N. A decrease in thickness and extent Approximate location of of the salt crust on the Bonneville Salt ground-water divide 40°52'30" Brine-collection ditch Flats has been documented during Playa boundary 1960-88 (S. Brooks, Bureau of Land Well—Shallow monitoring 4213 Management, written commun., 1989). well completed to a Maximum salt-crust thickness was 7 depth of 25 feet or less MOUNTAINS T. feet in 1960 and 5.5 feet in 1988. No 1 N. 4210 definitive data are available to identify ISLAND and quantify the processes that cause 4211 salt loss. More than 55 million tons of 4213 UTAH SILVER salt are estimated to have been lost NEVADA 114°00' from the salt crust during the 28-year 4212 Bonneville Racetrack 4213 (abandoned) period. The Bureau of Land Manage- Access road ment needs to know the causes of salt 4214 Brine-collection ditch 40°45' loss to make appropriate management T. Wendover 1 decisions. 80 S. Union Pacific Railroad 42104208 The salt crust has been used for 4212 4210 4204 4208 4212 high-speed automobile racing since 42134214 1914. The racing community has been Salt evaporator R. 19 W. R. 18 W. R. 17 W. R. 16 W. concerned about the progressive 0 2 4 6 MILES deterioration of the racing surface. 0 2 4 6 KILOMETERS They have suggested that the Figure 1. Water-table configuration based on an average density for September 1992 in the shallow- brine aquifer, Bonneville Salt Flats, Utah. basin-fill aquifer, and in a shallow- been withdrawn through the brine- brine aquifer. Brine of economic value collection ditches. The general is present in the shallow-brine aquifer. configuration and altitude of the Ground-water densities in the three contours are very similar to those aquifers vary from less-dense brackish mapped by Lines (1979, fig. 35) for the water in the alluvial-fan aquifer to same time of year. This comparison more-dense brines in the shallow-brine indicates that there is no long-term aquifer. change in water levels within the shallow-brine aquifer despite seasonal The shallow-brine aquifer is com- and yearly fluctuations in fluid inflows posed of a halite and gypsum crust at and outflows. The horizontal move- the surface and is surrounded by ment of brine through the shallow- carbonate mud, both on the surface brine aquifer is from the center of the and immediately beneath the crust. playa, where ground-water levels are About 3 feet, or 1 meter Vertical fractures in the shallow-brine high, toward the northwestern margin aquifer can be seen along the exposed of the playa and toward the brine- sides of the brine-collection ditches collection ditches on the east and and appear as salt ridges in a poly- south, where water levels are low. gonal pattern on the salt-crust surface. These fractures, which can be up to an Water-level and density data inch wide, are probably the principal collected at five locations during 1993, conduits through which brine moves. representative of wetter-than-normal The spacing between fractures varies; conditions, indicate that a potential a 1-foot interval is common. for downward movement existed between the shallow-brine aquifer and Large surface fracture in vegetation zone along Previous studies of the hydrology northwestern margin of the playa. View is toward the underlying ancient Lake of the Bonneville Salt Flats were done northwest, Silver Island Mountains in background. Bonneville sediment in the upper part by Turk (1969) and Lines (1979). Data of the basin-fill aquifer. Despite used by Turk were limited to areas measurements that indicate downward miles from the northeast, east, and adjacent to brine-collection ditches and movement, contradictory evidence south. Slight changes in altitude of the evaporation ponds. Lines estimated indicates possible upward movement land surface (less than 1 foot), how- the amount of brine and associated salt of brine in the area near the center of ever, can create small depressions that flows into and out of the shallow- the salt crust. where saline water can accumulate. brine aquifer. The current study uses Runoff toward the salt crust is some- computer simulations of ground-water Tritium values were determined for times impeded by these temporary flow and transport of salt in solution brine collected from several wells storage depressions that are easily in the shallow-brine aquifer to demon- completed in the shallow-brine aquifer. breached when the water level rises strate salt and fluid movement within Tritium is an unstable isotope of as a result of additional precipitation this dynamic ground-water system. hydrogen that decays with a half-life or when strong winds move sheets of of 12.3 years. The tritium value of water across the playa surface. Some Field Study and Computer ground water that is isolated from of the runoff toward the salt crust is Simulations Quantify Salt Loss mixing with other water will decrease impeded by man-made structures such by one-half of its original value after as the brine-collection ditch and This study was designed to define 12.3 years. Tritium occurs naturally in associated berm located east of the salt the hydrology of the ground-water the atmosphere, but the largest source crust, and the embankments created system and its relation to the natural was generated during atmospheric for Interstate Highway 80 and the and human-induced processes causing nuclear testing from 1952 to 1969. railroad tracks to the south. salt loss. Specific topics of study Present (1992-93) atmospheric tritium included the transport of salt in Ground water in the Bonneville Salt values are almost the same as those solution by ground-water flow and the Flats study area is present in an measured prior to atmospheric nuclear migration of ponded water. Data alluvial-fan aquifer along the north- testing, about 10 tritium units. Tritium collected during this study were used western margin of the playa, in a deep values measured in brine collected to make hydrologic interpretations and during this study range from 0 to 24 to develop computer tritium units. Tritium values of about simulations to achieve a better 10 tritium units measured in brine understanding of the fluid collected from the shallow-brine and salt-transport processes aquifer are indicative of water seeping in the shallow-brine aquifer. into the subsurface within a few years Water-level and brine- prior to sampling. Tritium values of density data were used to greater than about 10 tritium units are contour the seasonal water- indicative of water seeping into the table surfaces of the shallow- subsurface several years prior to brine aquifer. The water-table sampling. Of greatest importance are configuration (fig. 1) tritium values of almost 0 tritium units represents conditions near the measured in brine collected from the end of the evaporation season, shallow-brine aquifer beneath the center of the salt crust. These values Brine-collection ditch, located east of the salt crust, Bonneville after most of the brine used Salt Flats, Utah. for mineral production has indicate mixing with older brine that has a tritium value of 0 tritium units. more than double the 680 acre-feet with variable density is by computer The only possible source for this brine reported by Lines (1979, p.