More Than a Grain of Salt: The Salt Crust on Great Salt Lake’s North Arm
By Andrew Rupke and Taylor Boden
About the Authors
Andrew Rupke joined the Utah Geological Survey in 2010 as an industrial minerals geologist after working in the mining industry for 6 years. His research primarily focuses on Utah’s industrial minerals, including potash, limestone, phosphate, and others. This work often leads him to Great Salt Lake, an important mineral resource for Utah, and he also coordinates the Survey’s Great Salt Lake brine sampling program.
Taylor Boden joined the Utah Geological Survey in 2004, working as a geologist for the Energy and Minerals Program. He has worked on a wide range of projects involving Utah’s industrial and metallic mineral resources.
The saltiness of Great Salt Lake (GSL) is its defining characteristic, but many people do not realize that some areas of the lake are saltier than others. Prior to the construction of the rock-fill railroad causeway that now separates the north and south parts (arms) of GSL, the lake had, more or less, one salinity level. Since the causeway was completed in 1959, Gilbert and Gunnison Bays (the south and north arms of the lake, respectively) have had distinctly different salinity levels. Because flow of brine through the causeway is restricted and because GSL receives most of its fresh water into the south arm, the north arm of the lake has become much saltier than the south arm (see Survey Notes, v. 34, no. 1, p. 1–4, 2002, and v. 47, no. 1, p. 8–9, 2015, for more information on the causeway).
Shortly after these salinity differences began to manifest, the Utah Geological Survey (UGS) started to systematically sample and analyze the lake brine, and we continue to do so. Nearly 50 years of data show an average salinity of 12 percent in Gilbert Bay, but Gunnison Bay’s average is much higher, about 25 percent. To put those numbers in context, ocean water salinity is about 3.5 percent.
For much of the past 50 years, Gunnison Bay has been salty enough to cause the formation of a salt crust on the floor of the lake. Modeling of past salinity levels in GSL by scientists at the U.S. Geological Survey (USGS) and Utah State University suggests that a salt crust has been present in Gunnison Bay since the mid-1960s with the exception of a short time period from the late 1980s to early 1990s when lake levels were exceptionally high and, as a result, salinity levels were low.
Formation of a salt crust over hundreds of square miles is significant because it reduces the overall saltiness of the lake by confining hundreds of millions of tons of salt to the north-arm lake bed that would normally be all or partially dissolved in the lake brine. And, of course, the salinity levels of the lake have an impact on its ecology and industry. For instance, brine shrimp thrive in a certain range of salinity levels, and a reduced salinity in the south arm can have negative impacts on the mineral industry that operates in that part of the lake. While estimates indicate that the amount of salt in the crust fluctuates, at times the crust has probably stored about 20 percent of the salt in the GSL system.
Given the significance and volume of salt in the crust, surprisingly little research has focused on the crust itself (as mentioned, previous work relied on modeling salinity data—not direct observations of the salt crust). However, in the early 1970s the UGS conducted a core-drilling program across Gunnison Bay to gain information on the thickness, extent, and composition of the salt crust, but the data from that study were never published. Other studies occasionally included some limited information on the salt crust.
In 2015, with the help of funding from the Utah Division of Forestry, Fire and State Lands, we began a study that combines old data and new information on the salt crust. One objective of our work is to provide an updated overview of what is known about the salt crust and make that information more broadly available in a single report, and another aspect is to generate some new data on the salt crust from field observations.
Our field study focused on nearshore investigation of the salt crust, and we examined it at several locations around the perimeter of the north arm. The recent low lake levels have exposed wide stretches of salt crust around the lake— ideal for scientific observation. The crust is generally a solid mass of coarse salt crystals that can be up to about half an inch long. The salt crystals are primarily halite, a mineral composed of sodium chloride—more popularly known as table salt. Chemical analyses indicate that the salt crust is about 99 percent sodium chloride with minor amounts of calcium, magnesium, potassium, and sulfur. The salt crystals typically grow in place on the bed of the lake, but delicate “rafts” of salt crystals also form on the surface of the water during the hot, dry summer months, which eventually sink and accumulate on the lake bottom.
As part of the project, we developed a method for measuring the salt crust thickness, and we found crust up to 1.9 feet thick not far from the shore. Our data, in conjunction with past data and aerial photography, indicate that the salt crust covers nearly the entire lake bed of the north arm. Recent aerial photography shows a white rim of salt all the way around the north arm of the lake, and past data show thick salt in the central part of the lake. Although the scope of our project did not allow for measuring the crust thickness in the central parts of Gunnison Bay, the core drilling conducted by the UGS across Gunnison Bay in 1970 and 1972 showed salt crust up to 4.6 feet thick in more central parts of the lake, and boreholes completed during oil and gas exploration of the lake in 1974 showed salt crust locally up to 8 feet thick.
Using aerial photography, we mapped the outer extent of the salt crust and estimate that during late summer 2014 it covered a minimum area of 414 square miles. Where we measured the thickness of the salt crust, the salt was generally about one foot thick within a short distance from the water’s edge. Using our data and published bathymetry from the USGS, we developed a contour around the north arm estimating where the salt is one foot thick. From the area within the 1-foot contour (349 square miles), we estimate that at least 436 million (short) tons of salt was present on the floor of the north arm during the late summer and fall of 2015. We estimate that another 20 million tons is present between the edge of the crust and our 1-foot contour for a total of 456 million tons of precipitated salt. Our estimate represents a minimum tonnage for the salt crust, because past data strongly suggest that the crust is much thicker in the central part of the lake where we were unable to measure it. Estimates for the total amount of salt in the GSL system are around 4.5 billion tons, so at least 10 percent, but likely much more, of that is sitting at the bottom of the north arm.
An important aspect of our work is that we have established a set of baseline data that we can use to monitor the salt crust and see if it is precipitating or dissolving. Over time we can reoccupy various sites that we have previously measured and determine if the crust is getting thicker or thinner. Our baseline data collection is timely because a bridge in the railroad causeway is scheduled to open during the second half of this year (it may be open by the time this article is published). This new bridge will allow increased flow of brine through the causeway, and will almost certainly affect salinity and water levels in both Gilbert and Gunnison Bays. If enough lower-salinity water flows into the north arm, the salt crust could begin to dissolve. If this happens, we hope to detect it when we perform periodic measurements of the salt crust.
Monitoring the salt crust, coupled with continued brine sampling and salinity measurements, will help us and the broader scientific community to better understand how salt cycles through the lake. Ultimately, this improved understanding will allow us to be more prepared to help the various entities that manage the lake to make informed and prudent decisions.
If you are interested in checking out the salt crust for yourself, the easiest place to access it is at the Spiral Jetty (see Survey Notes, v. 35, no. 1, p. 10–11, 2003), which is south of the Golden Spike National Historic Site. The area is remote and requires driving several miles down an unpaved road, so be prepared. Once at the Spiral Jetty you may need to walk several hundred yards to reach the water’s edge if water levels are very low, as they currently are. You will know you are on the salt crust once you are walking on a very solid surface. During the summer and fall you should be able to observe the coarse crystalline salt in the shallow water, and if you are there during the right conditions you may see some salt rafts forming on the surface of the water.
The Other Salt Crust: Another salt crust in Utah, famous as a racing surface and a movie setting, is also currently being studied. The racing community is concerned that the Bonneville Salt Flats (BSF), where many land speed records have been set, are deteriorating, and Dr. Brenda Bowen and some graduate students from the University of Utah have begun evaluating the area to understand what factors are affecting the salt surface. Cancellation of racing events at the BSF in 2014 and 2015 have heightened these concerns.
The BSF began as a remnant of Pleistocene Lake Bonneville, but the salt crust is currently sustained as shallow, briny groundwater wicks to the surface and evaporates, leaving the salts behind. The BSF represent a complex and dynamic geologic system, and both natural and anthropogenic (human-caused) forces may play a role in changes being observed on the raceway.
The study will investigate potential effects caused by a potash mining operation located south of the BSF that uses some of the shallow brine as their feedstock, as well as climatic and other natural factors. Hopefully, the results of the University of Utah’s detailed study will provide some definitive conclusions on what factors are affecting the salt crust and what steps might help preserve the raceway.
Survey Notes, v. 48 no. 3, September 2016