A Dusty Tale: Determining Sources for Elements Found in Salt Lake Dusts

1. A Dusty Tale: Determining Sources for Elements Found in Salt Lake Dusts Urban Nelson Eby & Benjamin Nye Environmental, Earth and Atmospheric Sciences University of Massachusetts Lowell, MA Playa Wasatch Front

2. Dust samples were collected from • Playas using a Big Springs #8 sampler (green symbols) • Urban sites using passive samplers placed on the tops of buildings (orange symbols) • Snow by digging a pit and sampling dust layers (magenta symbols) • Samples were collected by Greg Carling and students at Brigham Young University Big Springs Number Eight Sampler Urban passive dust collector

3. Time Integrated Wind Velocities and Directions for the Great Basin of Utah SLC = Salt Lake City Airport, D = Delta, GSLD = Great Salt Lake Desert, TDL = Tule Dry Lake, SDL = Sevier Dry Lake, and MF = Milford Flat. Figures are from: Hahnenberger, M., Nicoll, K. (2012) Meteorological characteristics of dust storm events in the eastern Great Basin of Utah, U.S.A. Atmospheric Environment 60, 601-612.

4. Thirty-two dust samples (17 playa, 10 urban, 5 snow) were analysed for 48 elements using Instrumental Neutron Activation Analysis (INAA) INAA involves the interaction of thermal neutrons with target nuclei resulting in the production of gamma-ray emitting radioactive progeny. The intensity of the characteristic gamma rays is used to quantify the elemental concentrations. Neutron source Gamma Ray Spectroscopy lab

5. Results of Factor Analysis Using a Subset of 20 Elements Almost 90% of the variance can be explained by 4 components. These components may be related to the following sources: Elements associated with crustal sources Elements associated with anthropogenic sources Na and Cl from evaporites Arsenic, U, and Sr which have mixed sources Extraction Method: Principal Component Analysis. Rotation Method: Varimax with Kaiser Normalization. Rotation converged in 7 iterations.

6. Origin of Minerals • Quartz and Feldspar – rock weathering (Lithogenic component) • Halite, gypsum, and the carbonate minerals – evaporation (Evaporite component) • The mineralogy defines two distinct origins for the dust particles. The anthropogenic inputs are superimposed on these basic groups. Mineralogical Composition of Play, Urban, and Snow Dusts Figure S2 - “Mineral composition of ten representative samples based on x-ray diffraction (XRD) patterns. Fine playa, urban, and snow samples contained the same suite of minerals at different relative abundances”. From Goodman et al. (2019) Atmospheric deposition along the Wasatch Front (Utah, USA) reflects regional playa dust and local urban aerosols. Submitted to Chemical Geology.

7. Iron concentrations are related to weathered rock while Ca and Na reflect relative contributions of halite versus the Ca-containing minerals - calcite, dolomite, and gypsum. Note that the snow dusts are distinct from the playa and urban samples. The Th-La-Sc plot involves immobile elements that are associated with the chemistry of the parental rock material. The tight cluster for the dust samples indicates that the lithogenic component is the same throughout the region. The SLC and Saltaire exceptions are due to an anthropogenic input. Iron and Sc are closely related and Fe will be used as a reference element on subsequent plots.

8. Chondrite-normalized REE patterns have similar shapes and negative Eu anomalies. Compared to most of the dust sample patterns, Saltair shows an enrichment in the LREE’s and the Salt Lake City samples show a significant La enrichment.

9. Playa, urban, and snow dust samples define a constant La/Yb ratio with two notable exceptions. The two Salt Lake City dust samples and the Saltair dust sample plot at significantly higher La/Yb ratios. Note the proximity of these sites to the Chevron Oil refinery. La is used as a catalyst in the cracking of petroleum and the refinery is the source of the elevated La values. Thus, the La/Yb ratio can be used to fingerprint a refinery input. Bingham Canyon as seen from space

10. Evaporite Components • The Na:Cl atomic ratio is 1:1 for the playa (with the exception of Sevier Dry Lake) and urban samples. Halite is the major Na-containing phase in these samples. • Chlorine content for the snow dusts are relatively low and the Na content diverges significantly from the Na:Cl 1:1 line. In these samples Na is mostly contained in the feldspars. • Assuming that the Cl is balanced 1:1 by Na • Playas - 80 to 100% of the Na is contained in halite. • Urban dust - 10 to 80% of the Na is contained in halite. • Snow dust – 5 to 40% of the Na is contained in halite. • The evaporite component of the dusts decreases towards the east away from the playas.

11. Evaporite Components • Strontium is significantly enriched in the playa dust relative to Rb. The Sr resides in the carbonate phases. • The Ba/Rb ratio is essentially constant for the playa and snow dust samples. This ratio is largely controlled by the trace element chemistry of the feldspars. • Barium is slightly enriched in the urban dusts suggesting a very minor input of Ba in the urban environment.

12. Anthropogenic inputs • Urban dusts are significantly enriched in Br which was used in leaded gasoline. • Playas also have high Br content, but with correspondingly high Cl content. In the case of the playas the Br was concentrated by evaporation. • Given the range of Br and Cl concentrations in the playa and urban dusts, it is difficult to establish reference values. • Using average values for the playa and urban dusts and minimum values for the snow dust (solving three simultaneous linear equations), playa component = 1 to 2% and urban component = 7 to 15% of the snow dusts (in terms of Br and Cl).

13. Anthropgenic inputs • Br, As, Cr, Sb, Se, and W • Assume three sources – playas, urban areas, and mineral extraction. • Average values calculated for playa and urban areas. Saltair is used as a proxy for mineral extraction. • Least squares solution for the proportion of end members in each snow dust sample. SD3 site is in the Uinta mountains. SD4 site is due east of the Bingham Canyon copper mine.

14. Summary • Dusts deposited on Wasatch snow come from three sources – playas, urban areas, and activities associated with Bingham Canyon copper mine. • The silicate component of the dusts is homogeneous across the study area. • Ca, Mg, Na, Cl are concentrated by evaporation in playa dusts. Less than 3% of this material is ultimately deposited with the snow dusts. • La is enriched in dusts in the immediate vicinity of the Salt Lake City oil refinery. La is used as a catalyst in petroleum refining. • Br is enriched in the urban dusts (from the combustion of gasoline) and the urban areas are the major source of Br in the snow dusts. • As, Sb, W, Se, and Cr are concentrated in urban dusts and dust from the copper mining activities (except Cr). With one exception the concentration of these elements in the snow dusts can be explained by variable mixtures of playa and urban sources.