soil mineralogy effects of salt mineralogy on dust ... · salt mineral assemblages and crystal...

1958 SSSAJ:Volume75:Number5•September–October2011

Soil Sci. Soc. Am. J. 75:1958–1972 Posted online 8 Aug. 2011 doi:10.2136/sssaj2011.0049 Received 8 Feb. 2011. *Corresponding author ([email protected]) © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

EffectsofSaltMineralogyon DustEmissions,SaltonSea,California

SoilMineralogy

The details of how salt crusts affect dust emissions are not well understood, but we hypothesized that the vulnerability of surfaces to eolian erosion may be

partly controlled by the salt mineralogy. Previous research has described some halite salt crusts as “puffy” or “hairy” (Eswaran, 1984; Teller and Last, 1990; Vizcayno et al., 1995; Goodall et al., 2000; Smith et al., 2004). Although these researchers did not consider the effect of salt characteristics on dust emissions, their descriptions suggest that such surfaces may be more susceptible to eolian erosion. In contrast, salt crusts have also been known to form a hard surface that strongly resists defla-tion (e.g., Gillette et al., 1980, 1982; Nickling, 1984; Watson, 1985; Argaman et al., 2006; Mees and Singer, 2006). Experimental results have also shown variability in emissions when salt minerals are present (e.g., Nickling and Ecclestone, 1981).

Gaining a better understanding of these processes is becoming increasingly important because some of the most emissive surfaces in the world are saline dry lakes (e.g., the Aral Sea and Owens Lake) (Cahill et al., 1996; Singer et al., 2003). In California, increased demand for water from the Salton Sea is exposing vast new shorelines. Evaporation at the soil–atmosphere interface leads to salt precipitation. Capillary rise of the very shallow water table (0.01 to >1.00 m, depending on dis-tance from the lake) along these newly exposed shorelines can lead to significant salt crust formation because of the continuous supply of water and dissolved salts.

Brenda J. Buck*Dep. of GeoscienceUniv. of Nevada-Las Vegas4505 Maryland Pkwy.Las Vegas, NV 89154

James KingVicken Etyemezian

Desert Research Institute755 E. Flamingo Rd.Las Vegas, NV 89119

Some of the most emissive surfaces on Earth are dominated by salt minerals. We hypothesized that the vulnerabili-ty of surfaces to eolian erosion may be controlled by salt mineralogy and crystal habit. We used x-ray diffractometry (XRD) and scanning electron microscopy–energy dispersive x-ray spectrometry (SEM-EDS) analyses to measure salt mineral assemblages and crystal habits along exposed shorelines of the Salton Sea, California. Potential dust emissions were also measured using the Portable In-Situ Wind Erosion Lab (PI-SWERL). Results indicate that surfaces with the highest emissions, up to ~1 mg m−2 s−1, are composed of hydrous/anhydrous salt minerals and minerals with acicular or prismatic crystal habits. Hydrous/anhydrous minerals (mirabilite/thenardite, eugster-ite/glauberite, gypsum/bassanite, and numerous Mg sulfates) are more unstable under changing environmental conditions, are likely to dissolve and reprecipitate repeatedly, form less cohesive tiny individual crystals or small aggregates, and are therefore more likely to result in highly emissive surfaces. Salt minerals with acicular or pris-matic habits are more likely to be disruptive, enhance salt heave, lessen the degree of interlocking precipitates, and form loose, “puffy” crusts that are highly emissive. Low-sloping surfaces near the shoreline had greater fluctuations in water content and relative humidity, triggering frequent salt mineral dissolution–precipitation and increased emissions. A high water table also allowed a continuously replenishing supply of salt crystals, increasing the poten-tial for extensive dust emissions. Surfaces containing salt minerals are incredibly dynamic, but understanding the processes that control surface characteristics is an important step in mitigating dust emissions.

Abbreviations: PI-SWERL, Portable In-Situ Wind Erosion Lab; PM10, particulate matter with a £10-mm aerodynamic diameter; SEM-EDS, scanning electron microscopy–energy dispersive x-ray spectrometry; XRD, x-ray diffractometry.

SSSAJ:Volume75:Number5•September–October2011 6

Eolian erosion of these surfaces leads to significant dust emis-sions of both salt and silicate minerals. Downwind deposition of eolian salts from these surfaces can degrade groundwater and soil quality (e.g., Wood and Sanford, 1995; Eckardt et al., 2001; Mees and Singer, 2006). Additionally, emissions from these sur-faces can have an enormous negative impact on air quality and human health. Exposure to particulate matter is associated with an increased risk of cardiovascular and respiratory morbidity and mortality, asthma, lung cancer, inflammation, and increased mortality (e.g., Dockery and Pope, 1994; Donaldson et al., 2000; Ichinose et al., 2005; Craig et al., 2008; Soto-Martinez and Sly, 2010; Tong et al., 2010; Peters, 2011). In this study, we show for the first time the linkages among salt mineralogy, crystal habit, and the potential for dust emissions on newly exposed shore-lines of the Salton Sea, which is expected to become a large dust source as water levels continue to drop.

The presence of salt minerals can greatly affect the amount of fine material that is available for eolian erosion. New salt minerals can continuously be brought to the surface through capillary water movement and evaporation. This supply of fine particulates can be replenished indefinitely as long as a solute-containing water source is present, such as a high water table. In addition, the processes of salt mineral precipitation–dissolution and thermal expansion combine to cause salt heaving (Buck et al., 2006a). Salt crystallizes in a displacive manner (i.e., Wellman and Wilson, 1965; Amit and Yaalon, 1996; Aref et al., 2002; Buck et al., 2006a), which both increases the amount of fine par-ticles in the soil (e.g., new salt minerals) and pushes to the surface other subsurface minerals (e.g., silicates) that become available for continued eolian erosion. The resulting dust emissions are composed of both salt and silicate minerals.

Salt minerals are common components of arid soils, and their high solubility can result in significant variation in soil characteristics (e.g., bulk density, pH, electrical conductivity, and available ions) in very short time periods and distances (Buck et al., 2006b). Because of their high solubility, salt minerals can be both added to or removed from soils very quickly if the condi-tions affecting the amount or quality of the soil water change. The types of salt minerals in surface soils can vary greatly depend-ing on the regional geology, dust contributions (e.g., sea spray, volcanic, or industrial contributions), climate (i.e., temperature, relative humidity, and precipitation), water table(s), slope and aspect, vegetation, soil pH, and soil texture (among others). Because soil crusts are formed in open systems such as soil pores, phase diagrams and models using closed systems (e.g., evapora-tion of seawater) cannot be used effectively to predict mineralogy (Miller et al., 1989; Mees and Stoops, 1991; Buck et al., 2006b).

Identical salt minerals exhibit many different crystal habits (i.e., the size and shape of the crystals). Crystal habits of salt minerals can vary considerably and are controlled by the environmental character-istics occurring at the time of mineral formation (e.g., Driessen and Schoorl, 1973; Eswaran et al., 1980; Tursina et al., 1980; Jafarzadeh and Burnham, 1992; Vizcayno et al., 1995; Amit and Yaalon, 1996; Buck and Van Hoesen, 2002; Buck et al., 2006b). For salt miner-

als, these include changes in temperature, soil water, the presence of organic matter, the amount and types of ions in solution (i.e., de-gree of supersaturation), and pH (see Edinger, 1973; Folk, 1974; Jafarzadeh and Burnham, 1992; Amit and Yaalon, 1996; Buck and Van Hoesen, 2002; Buck et al., 2006b).

The minerals most likely to respond quickly to changing en-vironmental conditions are hydrous/anhydrous minerals. These include thenardite/mirabilite, glauberite/eugsterite, gypsum/bassanite/anhydrite, konyaite/bloedite, and the myriad of hy-drated Mg sulfate minerals. Anhydrite is generally not very com-mon in the southwestern United States because its stability re-quires high temperatures and low relative humidities. Konyaite is also generally thought to be relatively unstable (Van Doesburg et al., 1982; Shayan and Lancucki, 1984; Kohut and Dudas, 1993) and has not been found in this region. Thus, the most common sets of hydrous/anhydrous minerals in the southwestern United States are thenardite/mirabilite, glauberite/eugsterite, and gyp-sum/bassanite (± anhydrite).

The transition temperatures for these minerals can vary greatly depending on the relative humidity, crystal defects, and the presence and concentrations of other dissolved salts (e.g., Weidemann and Smykatz-Kloss, 1981; Steiger and Asmussen, 2008; Warren, 2010). Surface environmental conditions change frequently and at many different temporal scales (i.e., diurnally, seasonally, or passing storm events); salt crystals can dissolve and reprecipitate in response to these changes. The resulting salt min-eral assemblage is often highly transitory; mineral types and con-centrations can vary as environmental conditions change. This also results in mineral assemblages that vary spatially with depth. Salt crusts on surface soils are commonly composed of complex mineral assemblages and crystal habits that change within only a few millimeters of depth (Eswaran and Carrera, 1980; Tursina, 1980; Tursina et al., 1980; Mees and Stoops, 1991; Vizcayno et al., 1995; Buck et al., 2006b; Mees and Singer, 2006).

Generally such mineralogical changes cannot be measured through XRD alone because it is extraordinarily difficult to physically separate these soft friable salt crusts. Moreover, XRD analysis is generally unable to identify minerals that make up <5% (v/v) of a sample (Soukup et al., 2008a). Scanning electron microscopy–energy dispersive x-ray spectrometry analyses, how-ever, have been used to distinguish the mineralogy, crystal hab-its, and surface morphology in layered salt crusts (e.g., Eswaran and Carrera, 1980; Mees and Stoops, 1991; Buck et al., 2006b). Although SEM-EDS analyses cannot directly differentiate be-tween hydrous and anhydrous minerals, the mineral habit can sometimes be used to make this distinction. Therefore, the com-bination of XRD and SEM-EDS is currently the most effective method both for determining salt mineralogy and for learning about the physical characteristics of these surfaces (Buck et al., 2006b; Soukup et al., 2008a).

GEoloGical SEttinGThe Salton Sea lies in an internally drained basin in south-

ern California (Fig. 1). It was accidentally formed in the early


part of the 20th century when flooding on the Colorado River broke through irrigation canals and filled the basin. The level of salinity has since continued to increase through evaporation, ag-ricultural irrigation runoff, and salt contributions from geologic deposits and groundwater. Although initially a freshwater lake, high evaporation rates, combined with limited inflows (primari-ly agricultural runoff ), continue to increase the salinity (current-ly ?25% greater than ocean water) while decreasing the lake’s size. High levels of nutrient runoff from adjacent agricultural lands have caused algal blooms and eutrophication (Robertson et al., 2008). These conditions have negatively impacted wildlife use of the Salton Sea, with numerous widespread die-offs of fish and birds. Water levels are expected to continue to drop as water inflow to the Salton Sea is decreased according to the Colorado River Quantification Settlement Agreement. Up to 34,000 ha of lake-bed sediments may become exposed, and windblown dust emissions may increase significantly (Case and Barnum, 2008). The levels of particulate matter with a £10-mm aerodynamic di-ameter (PM10) in this region are already some of the highest in the nation (Chavez et al., 2008).

MatErialS and MEthodSTwelve sites surrounding the Salton Sea were sampled for salt and

soil characterization between 9 and 11 Feb. 2007 (Fig. 1). To determine

if the salt crust mineralogy or other characteristics affect dust emissions, nine of those 12 sites (PL-1, PL-2, PL-4, PL-5, PL-7, PL-8, PL-9, PL-12, and PA-2) were measured at the time of sampling using the Portable In-Situ Wind Erosion Lab (PI-SWERL). The PI-SWERL is a battery-powered instrument that creates a shear stress on the surface within a cylindrical enclosure using a rotating annular blade within close proxim-ity to the surface (Etyemezian et al., 2007; Sweeney et al., 2008). It mea-sures the PM10 concentration (mg m−3) at an outlet with a DustTrak nephelometer (Model 8520, TSI Inc., Shoreview, MN) that records at 1 Hz while a blower vents clean air through the PI-SWERL at a constant rate (m3 s−1). The potential PM10 dust emissions (mg m−2 s−1) are es-timated by maintaining a constant speed for a set amount of time (s). Each site was tested systematically five to eight times to get a representa-tive potential emissions value for the range of soil surfaces encountered and for a range of possible wind speeds at the selected site.

Climate data from the Thermal, CA, station for the sampling period recorded mean maximum and minimum temperatures of 22 and 5.5°C, respectively, and no precipitation during the entire month. At each of the 12 sites, a shallow soil pit (~0.3 m) was excavated and described us-ing standard NRCS techniques outlined in Schoeneberger et al. (2002). A field spring penetrometer was used to measure the amount of force required to break the surface crust (15 replicate measurements per site). Samples of intact surface crust, bulk surface crust, and a bulk sample of the top intact layer of soil were used for the following laboratory analyses.

Fig. 1. location of sampling sites around the Salton Sea, california.


All mineralogical tests were performed on samples that were not pretreated in any way. Mineralogic analyses were conducted in Las Vegas under environmental conditions that were similar to those of the field study area. Sample bags were left open to atmospheric conditions so that condensation and dissolution did not occur. Similar samples stored un-der these conditions have been reanalyzed after up to 3 yr of storage and have been found to be unaltered (Buck, unpublished data, 2004). Mineralogic analyses were performed as quickly as possible after field collection. No water was used in sample preparation. The XRD and SEM-EDS analyses were conducted on only the uppermost portion of the soil or crust (~1 cm). Preparations for these analyses were done ac-cording to the methods of Soukup et al. (2008b). The XRD analyses of the powdered samples were performed by step scanning using a X’Pert PRO diffraction system (PANalytical BV, Almelo, the Netherlands) at the University of Nevada-Las Vegas (UNLV). Diffraction patterns were imported into the High Score software (PANalytical BV) to determine the mineralogy and are presented as semiquantitative numbers associ-ated with the amount by volume.

Fifty-six samples were prepared for SEM-EDS analyses (Soukup et al., 2008b). Analyses were performed on a JEOL 5600 scanning electron microscope ( JEOL USA, Peabody, MA) and an ISIS energy dispersive x-ray system (Oxford Instruments, Oxford, UK) at the UNLV Electron Microanalysis and Imaging Laboratory. A total of 229 SEM images and 324 EDS analyses were used to determine the salt mineralogy and com-position. Gravimetric water content was determined using National Soil Survey Laboratory Method 3C3 (Soil Survey Staff, 2004). The presence of hydrous salt minerals will overestimate the gravimetric water content and therefore these data should be considered unreliable.

Soil samples were sieved using a 2-mm sieve before each of the fol-lowing pretreatments or analyses were performed. These pretreatments were performed to determine the pH, electrical conductivity (EC), or-ganic matter content, concentration of soluble salts, CaCO3, and tex-ture of the insoluble fraction. Both pH and EC measurements were ac-quired with a 1:1 saturated paste using National Soil Survey Laboratory methods 4F2 (saturated paste), 4C1a2a1 (pH), 4F2c (EC extraction), and 4F2b1 (EC measurement) (Soil Survey Staff, 2004). Organic mate-rial was removed by combustion in a 400°C muffle furnace. The organic matter concentration was calculated from pre- and post-combustion sample weights. The presence of hydrated soluble salts will cause over-estimation of the organic matter concentration; therefore these data should be considered unreliable. The concentration of soluble salts and carbonate were determined by gravimetric loss after dissolution with reverse osmosis water (soluble salts) or NaOAc buffer solution (carbon-ate) according to the methods of Soukup et al. (2008b) and Kunze and Dixon (1986). Shell fragments (barnacles and ostracods) are common and therefore the concentration of CaCO3 does not reflect pedogenic processes. Soil texture was determined using the hydrometer method (modified from Gee and Bauder, 1986). Current accepted methods of measuring soil texture require the removal of all cementing agents, including salts (Soukup et al., 2008a). Therefore, these methods report only the size fractions of the insoluble minerals. Currently, there are no well-established techniques to measure the grain sizes of soils composed primarily of salt minerals (Soukup et al., 2008a).

rESUltSThe 12 sites studied are located around the perimeter of the

Salton Sea (Fig. 1) and range from areas recently exposed to ar-eas that have not been inundated for centuries. Of the 12 sites, 11 contained salt crusts at the surface (Table 1). The salt crusts varied between 0.2 and 1.5 cm in thickness (Table 1). Some sites contained loose, very fine salt crystals beneath the cemented salt crust, and several contained vesicular pores (Table 1). Results from the XRD analysis indicated that all salt crusts were composed of a complicated mixture of several salt minerals, but all contained halite (NaCl) and bloedite [Na2Mg(SO4)2×4H2O] (Table 2). Other salt minerals present in some locations but not others in-cluded glauberite [Na2Ca(SO4)2], thenardite (Na2SO4), hexa-hydrite (MgSO4×6H2O), mirabilite (Na2SO4×10H2O), gypsum (CaSO4×2H2O), eugsterite [Na4Ca(SO4)3×2H2O], and bassan-ite (2CaSO4×H2O) (Table 2). The only site not dominated by salt minerals was PA-2 (Bombay Beach), where quartz (SiO2) was the major mineral at the surface (Table 2).

The PM10 emissions from PI-SWERL measurements indi-cated that sites containing salt crusts can be grouped into two categories: those with very high emissions (PL-1, PL-4, PL-7, and PL-8), and those with moderate to low emissions (PL-2, PL-5, PL-9, and PL-12) (Table 3). The lowest emitting surface, PA-2, did not contain a salt crust (Table 3).

Multiple SEM-EDS analyses showed consistent results for each site: similar mineralogy, crystal sizes and habits, composi-tion and sizes of salt mineral aggregates, smoothness of surface, and degree of cementation. The salt crusts contained a complex mixture of salt minerals at all scales (i.e., Fig. 2b, 2c, 3a, 3c, 4, and 5). Commonly, the salt crusts were layered with distinct sets of salt minerals present at the surface and different sets of minerals at the base (Table 4). Examples of this include Sites PL-4, PL-7A, PL-9, and PL-12. At Site PL-4, the surface of the crust was composed of hexahydrite and gypsum, whereas the base was pri-marily glauberite, bloedite, and halite. Site PL-7A had a surface crust composed of thenardite and glauberite, with minor halite and bloedite, and a basal crust primarily composed of halite. At both PL-9 and PL-12, the surface was composed of thenardite, halite, and bloedite, but the base of the crust was primarily eugs-terite/glauberite, thenardite, or both (Table 4).

The crystal habits of individual salt minerals varied among sites and within surface crusts (Table 4). Halite occurred as anhe-dral masses that varied in size from <1 up to >100 mm, cementing a variety of other minerals into aggregates or occurring as thin coatings draping other minerals (Fig. 4b, 4e, 4f, and 5a). Halite was also present as subhedral to euhedral cubic crystals that var-ied in size from <1 m up to >100 mm (Fig. 2a–2d, 4b, and 4f ). These forms of halite occurred both at the surface of the salt crust as well as at the base. Bloedite generally occurred in a tabular form (<1–20 mm), often as euhedral pseudohexagonal crystals or rarely as rhomboidal crystals (Table 4; Fig. 2b). Bloedite was also found to occur as anhedral to subhedral masses and coatings (Fig. 4e, 4f, and 5a). Thenardite occurred only as subhedral to euhedral dipyramidal crystals (<1 to >100 mm) or as anhedral


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0.2


masses that could not be distinguished from mirabilite (Table 4; Fig. 2a, 2b, and 3). Mirabilite occurred as fine tabular (<1–3-mm) crystals (Fig. 5a). Hexahydrite always occurred as subhedral to anhedral masses or as tiny (<1–2-mm) pseudohexagonal tabu-lar crystals. Most commonly, the hexahydrite occurred as mas-sive cements or as a coating, often with tension cracks formed from dehydration (Fig. 5c). The dehydration was likely to have occurred during SEM analyses, but natural environmental condi-tions (e.g., increased temperatures or decreased humidity) at the site could also produce these features. The two Na–Ca sulfate minerals gluberite and eugsterite are difficult to differentiate us-ing only SEM-EDS analyses. Previous studies have found, how-ever, that glauberite tends to form stout, planar rhombohedral or prismatic crystals, whereas eugsterite is known to form acicular crystals (Vergouwen, 1981b; Skarie et al., 1986; Shahid, 1988; Qingtang, 1989; Mees and Stoops, 1991; Shahid and Jenkins, 1994; Schiefelbein et al., 2002; Buck et al., 2004, 2006b; Mees and Singer, 2006; Morton et al., 2006; Mees and Tursina, 2010). The Na–Ca sulfate crystals in this study occurred as euhedral, prismatic crystals (<1 mm wide by <20 mm long) arranged in ra-

diating, fan-shaped clusters. This evidence, combined with the XRD results, suggests that the majority of these crystals were probably glauberite (Fig. 2c, 2e, 3c, 4c, 4d, 5d, and 5f ) (Table 2). Gypsum occurred as subhedral lenticular to tabular (20–50-mm) crystals. Bassanite was identified at two sites (PL-7A and PL-6) using XRD analyses. It was not identified using SEM analyses at those sites, however, possibly because it occurred as massive an-hedral forms or it was not chemically distinguishable from other salt minerals if it occurred as coatings, masses, or tiny crystals in mixed aggregates.

diScUSSionhigh-Emitting Surfaces

The highest emitting surfaces were PL-8, PL-1, PL-7, and PL-4 (Table 3; King et al., 2011). The surfaces with high emissions had the following in common: the presence of hydrous/anhydrous salt minerals, loosely cemented (noninterlocking) salt crystals or aggregates, acicular or prismatic crystal habits, and a low slope. There were no geographic trends with emissions (see Fig. 1).

table 2. X-ray diffractometry results for surface crust from 12 sites.

Mineral Pa-2 Pl-1 Pl-2 Pl-3 Pl-4 Pl-5 Pl-6 Pl-7 Pl-7a Pl-8 Pl-9 Pl-12

———————————————————— % (v/v) ———————————————————————Halite (NaCl) 41 18 35 31 21 25 54 42 35 26 39

Bloedite [Na2Mg(SO4)2·4H2O] 17 27 20 19 14 21 8 14 17 14 48 25

Glauberite [Na2Ca(SO4)2] 19 21 25 49 19

CaCO3 8 9 19 7 10 13 4

Thenardite (Na2SO4) 5 8 28 16

Quartz (SiO2) 40 8 13 3 7 6 9

Gypsum (CaSO4·2H2O) 3 19 7 4 1 1 2 5 5

Hexahydrite (MgSO4·6H2O) 20 14 28 19

Mirabilite (Na2SO4·10H2O) 16 19 18

Eugsterite [Na4Ca(SO4)3·2H2O] 6 5

Bassanite (2CaSO4·H2O) 5 5

Volkonskoite [Ca0.3(Cr,Mg,Fe)2(Si,Al)4 O10(OH)2·4H2O] 12Ca Al silicate [Ca3Al2 (SiO4)3] 23

table 3. Surficial soil texture and analysis results.

Site initial wt. Waterorganic matter

Soluble salts caco3

texture analysis

ph Ec†PM10

emissions‡Sand Silt clayUSda

classification

g —————————————————— % —————————————————— dS m−1 mg m−2 s−1

PA-2 105.2 1.0 1.6 1.1 3.1 30 30 40 clay loam 7.4 1.3 6.5

PL-1 102.6 8.0 13.5 29.7 6.4 5 53 42 silty clay 7.3 137.6 279.6

PL-2 104.2 7.3 16.0 37.8 6.8 21 49 30 clay loam 7.3 135.2 20.7

PL-3 101.2 8.4 14.1 36.4 5.8 5 50 45 silty clay 7.2 134.1 –

PL-4 104.3 8.1 9.2 37.9 6.4 34 27 39 clay loam 7.3 116.0 83.4

PL-5 102.8 2.5 4.5 58.6 3.8 3 27 70 clay 7.6 130.7 26.4

PL-6 98.0 8.2 10.5 35.4 16.1 19 39 42 clay 7.4 118.8 –

PL-7 104.3 6.9 9.9 52.2 11.8 16 42 42 silty clay 7.4 127.5 277.7

PL-7a 103.9 7.1 7.0 46.4 6.6 13 56 31 silty clay loam 7.3 141.5 –

PL-8 110.6 4.7 12.6 60.3 4.9 22 39 39 clay loam 7.2 111.7 927.2

PL-9 103.8 16.8 15.4 51.3 4.6 22 40 38 clay loam 6.8 125.4 7.8PL-12 106.7 11.9 11.2 52.0 4.0 30 15 55 clay 7.1 136.4 8.2

† Electrical conductivity.‡ Particulate matter with a £10-mm aerodynamic diameter; average emissions are given for a corresponding shear velocity of 0.56 m s−1.


The salt crust at Site PL-8 was composed of masses of loose prismatic glauberite crystals and loosely cemented aggregates that formed an irregular “puffy” crust (Fig. 4c and 4d). These crystal morphologies result in decreased cohesiveness and high

emissions. This area has a very low slope and a large area will be exposed by only a small decrease in the Salton Sea water level, potentially exposing a significant dust emission source.

Fig. 2. Scanning electron microscope (SEM) images of the surface of Site Pl-12: (a) wide-angle view of well-cemented, large, smooth subhedral halite crystals interspersed with smaller aggregates of bloedite and thenardite crystals, (b) large (10–50 mm) subhedral cubic halite with smaller (1–10-mm) euhedral to subhedral pseudohexagonal tabular bloedite and (1–5-mm) subhedral dipyramidal thenardite crystals, (c) smooth, interlocking subhedral to euhedral cubic halite crystals with very fine (<1-mm) prismatic bloedite, and (d) cubic halite; (e) SEM image of base of salt crust at Site Pl-12 composed of loose mats of prismatic glauberite crystals; and (f) SEM image of the surface of Site Pa-2 composed of grains of quartz and other silicate minerals covered with a thin coating of silicate clays, creating a smooth, tightly cemented crust.


At Site PL-1, prismatic glauberite and acicular eugsterite could be commonly seen interspersed in a loose matrix of partial-ly cemented aggregates of mirabilite, bloedite, and an unknown K salt, cemented by anhedral halite and hexahydrite (Fig. 5d and

5f ). The base of this crust showed prismatic glauberite occur-ring in loose aggregates (Table 4). It is probable that this surface could become even more emissive with increased temperatures or decreased relative humidity because the eugsterite would de-

Fig. 3. Scanning electron microscope images of the surface of Site Pl-9: (a) well-cemented, large (100-mm) thenardite crystals overlain by aggregates of pseudohexagonal and lenticular bloedite (5–20 mm) and subhedral thenardite/mirabilite crystals (1–5 mm) and (b) well-cemented aggregates (10–50 mm) created by intergrown euhedral and subhedral dipyramidal thenardite crystals (<5 mm); and the surface of Site Pl-7: (c) close-up view of loosely interlocking euhedral dipyramidal (<1–5-mm) thenardite crystals with tiny (<1–10-mm) radiating acicular to prismatic eugsterite/glauberite crystals, (d) a wider view of the same crystals, forming a rough, highly porous surface composed of interlocking aggregates of loosely cemented euhedral dipyramidal thenardite crystals (<1–5 mm), forming arches and bridges, creating the characteristic “puffy” crust, and (e,f) continuing wider angle views of the puffy crust.


hydrate to glauberite and the mirabilite to thenardite—both of which can have acicular habits and would thus tend to form loose individual crystals or small aggregates of crystals. Additionally, fracturing caused by the dehydration of hexahydrite can turn

formerly stable aggregates into smaller, loose components when exposed to high temperatures and low humidities (e.g., diurnal or seasonal). This site also has a very low slope toward the Salton

Fig. 4. Scanning electron microscope images of (a) the surface of Site Pl-7a composed of mats of organic filaments binding dipyramidal thenardite (<5 mm), prismatic glauberite (<15 mm) crystals, and 20- to 200-mm-diameter salt mineral aggregates cemented with anhedral to subhedral halite and bloedite; (b) the base of salt crust at Site Pl-7a composed primarily of well-cemented anhedral to euhedral cubic halite up to 50 mm in diameter; (c,d) the surface of Site Pl-8 composed of loosely cemented prismatic glauberite (<1–33 mm wide by <15 mm long); and (e,f) the surface of Site Pl-2 composed of tightly interlocking crystals of cubic halite, with tiny (<1–5-mm) anhedral to subhedral bloedite.


Sea, which provides a continuous source of salts through capil-lary movement and evaporation of the shallow water table.

The surface at Site PL-7 was extremely irregular and porous, with primarily thenardite (and to a lesser extent, halite) forming

arches and bridges, creating a characteristic “fluffy” surface and causing it to have one of the highest emissions (Fig. 3c–3f; Table 3). The base of the crust was also composed of similar loose, po-rous aggregates, and if exposed would also result in large emis-

table 4. Salt crust scanning electron microscopy–energy dispersive x-ray spectrometry results.

Site Position Mineralogy crystal habit and size

PL-1 top glauberite euhedral to subhedral prismatic (<1–3 mm wide, <20 mm long)mirabilite euhedral tabular (<1–3 mm)halite anhedral cement

hexahydrite anhderal cementbase glauberite euhedral prismatic (<1–3 mm wide, <20 mm long)

bloedite euhedral tabular (<1–3 mm)halite anhedral to subhedral cubic (<10 mm)

PL-2 top halite tightly interlocking euhedral cubic (up to 100 mm)bloedite subhedral to anhedral (<1–5 mm)glauberite/eugsterite euhedral acicular to prismatic (<1–10 mm)

PL-3 top halite smooth anhedral to subhedral cubic (>100 mm)glauberite subhedral prismatic (<1 mm wide, 10 mm long)bloedite subhedral tabular (<5 mm)gypsum subhedral lenticular (<10 mm)

PL-4 top hexahydrite suhedral to anhedral massive, dehydrated with tension cracks

gypsum subhedral lenticular to tabular (20–50 mm)base glauberite loose euhedral prismatic (1–2 mm wide, <20 mm long)

bloedite euhedral to subhedral platy (1–7 mm)halite subhedral to euhedral cubic (<10 mm)

PL-5 top mirabilite euhedral tabular (<1–2 mm) forming loose aggregatesbloedite subhedral (7–12 mm)halite anhedral coatings

base halite euhedral to subhedral cubic

thenardite/mirabilite subhedral dipyramidal to anhedral occurring as a cement

PL-6 top halite smooth anhedral

bloedite euhedral tabular pseudohexagonal and rhomboidal crystals (10–20 mm).thenardite euhedral dipyramidal(3–7 mm)

base thenardite euhedral dipyramidal (30–100 mm)

halite anhedral coatings in places cementing (100–500-mm diam.) aggregates of tiny (<1–5 mm) mixed salts and euhedral cubic (>100 mm in diameter)

glauberite euhedral prismatic (<1–2 mm wide, £15 mm long) in radiating, fan-shaped clustersPL-7 top thenarite euhedral dipyramidal thenardite (1–8 mm)

halite anhedral coatings

glauberite/eugsterite euhedral acicular to prismatic (<1–10 mm) in radiating clustersgypsum suhedral lenticular/tabular (>20 mm)

PL-7a top thenardite euhedral dipyramidal (<5 mm)glauberite euhedral prismatic (<15 mm)halite and bloedite aggregates anhedral/subhedral masses (<1–3 mm) cemented to form 20–200-mm-diam. aggregates

base halite anhedral to euhedral cubic (<50 mm)PL-8 top glauberite euhedral prismatic (<1–33 mm wide, < 15 mm long), loosely interconnected in 50–100-mm clusters

halite subhedral cubic to anhedral (<10–30 mm)PL-9 top

thenardite euhedral and subhedral dipyramidal (<5-mm single crystals loosely cemented to form 10–50-mm-diam. aggregates, or interlocking >100-mm crystals)

halite euhedral cubic (10–100 mm) and anhedral cementbloedite subhedral skeletal (10–20 mm)

base glauberite/eugsterite loose euhedral prismatic (<1 mm wide by <20 mm long)PL-12 top halite euhedral to subhedral cubic (10–50-mm diam.)

bloedite euhedral to subhedral pseudohexagonal tabular (<1–10 mm)thenardite subhedral dipyramidal (<1–5 mm)

base glauberite euhedral prismatic (<1 mm wide, <20 mm long)thenardite euhedral dipyramidal (2–10 mm)


sions. This site also has a very low slope, therefore a slight water level lowering will expose large areas to eolian erosion.

Site PL-4 had the lowest emissions of the four sites grouped into the highest emissions category (Table 3). The salt crust was composed of nearly equal amounts of halite and hexahydrite

Fig. 5. Scanning electron microscope (SEM) images of (a) the surface of Site Pl-5 showing a loose network of small (>1–2 mm) tabular mirabilite crystals, subhedral bloedite (7–12 mm), and anhedral halite acting as a coating and cement; (b) the surface of Site Pl-5 showing larger subhedral halite and salt cemented silicate grains; (c) the surface of Site Pl-4 composed of dehydrated hexahydrite with overlying loose gypsum and silicate mineral aggregates; (d) the surface of Site Pl-1 showing loosely connected aggregates of euhedral prismatic glauberite (<1–2 mm wide by <15 mm long), thin coatings of halite, and anhedral crystals of mixed salts; (e) the surface of Site Pl-1 showing thin euhedral tabular to anhedral crystals of mixed mineralogy (hexahydrite, bloedite, halite, mirabilite/thenardite, and unknown K salt) interspersed in a loose fabric of prismatic and acicular glauberite/eugsterite; and (f) the surface of Site Pl-1 composed of prismatic radiating clusters and individual crystals of glauberite.


(Table 2). Gypsum crystals were cemented and coated with hexa-hydrite (Fig. 5c). In most places the hexahydrite was dehydrated, creating tension cracks. Many of the gypsum and silicate grains were lying loosely on top of the smoother hexahydrite and halite crust. These loose grains, in addition to the larger plates created by the desiccation of the hexahydrite, probably contributed to the increased emissions. This site also has a very low slope toward the waterline, providing a high water table and a continuous source of salt minerals.

Moderate- to low-Emitting SurfacesThe four sites that had moderate to low emissions were PL-

5, PL-2, PL-12, and PL-9 (Table 3). These surfaces had the fol-lowing in common: tightly cemented (interlocking) salt crystals or aggregates with massive, smooth, interlocking crystal habits. High clay contents in the insoluble fraction might also have con-tributed to decreased dust emissions (Table 3).

Site PL-5 had moderate to high PM10 emissions (Table 3) and was comprised of a thin layer of fairly loose, fine mirabilite crystals (Fig. 5a and 5b). Clay- and silt-sized silicate grains were strongly cemented by salt minerals (bloedite, halite, and mira-bilite). Therefore, this site probably had relatively low emissions because the surface was strongly cemented by mostly anhedral and smooth salt crystals. Sodium sulfate dominated the surface of the crust at this site, however, so increased emissions are pos-sible from repeated dehydration and rehydration of the mirabil-ite/thenardite, which could result in a “fluffy” heaved surface if thenardite were to precipitate as acicular crystals. The high clay content in this crust may act as a buffer and retain moisture, thus limiting the degree of Na sulfate hydration and dehydration.

Despite having a low slope, which was a common factor for the four highest emission sites, PL-2 had moderate to low emissions (Table 3). This surface had some areas of loose crystals (primarily eugsterite/glauberite) that overlaid a tightly cemented and interlocking salt crust (Fig. 4e and 4f ). The bundles of looser crystals and aggregates should be very emissive; however, once those have been removed, the underlying tightly cemented sur-face is fairly stable. Therefore, although this site has a low slope similar to the highly emissive sites, the degree of interlocking salt minerals appears to have been the controlling factor limiting emissions at the time of sampling. The interlocking crystal hab-its may be a function of environmental conditions not measured in this study, such as the relative humidity and temperature at the time of sampling. The thin layer of cyanobacteria (Table 1) found a few millimeters below the surface at this site has been described in other salt crusts ( Joeckel and Ang Clement, 1999; Dong et al., 2007) and may also have affected the crystal habits and lessened emissions at this site. The presence of the hydrous/anhydrous minerals (eugsterite/glauberite and thenardite/mi-rabilite) suggests, however, that this surface has the potential to become more emissive with changing environmental conditions (e.g., diurnally or seasonally). The very loose sediment at the base of the salt crust suggests that if the basal crust is exposed through disturbance, very high emissions could occur.

Site PL-12 had one of the lower dust emissions (Table 3). The top of this crust was composed of a significant amount of smooth, large halite crystals (Fig. 2a–2d), which combined with the high clay content (Table 1) were probably the control-ling factors behind the lower dust emissions; however, thenard-ite comprised a significant portion of the surface of this crust. Environmental changes could significantly increase the dust emissions at this site—especially if the thenardite dissolved and later reprecipitated in an acicular habit. Additionally, dis-turbance or erosion of the surface crust will expose the loose, prismatic glauberite crystals at the base (Table 4), leading to in-creased dust emissions.

Site PL-9 had the lowest dust emissions of any surface with a salt crust (Table 3). The surface was dominated by tightly ce-mented aggregates of tiny thenardite crystals or larger dipyrami-dal crystals that were well-cemented by halite and bloedite (Fig. 3a and 3b). Because the base of the salt crust was dominated by uncemented prismatic glauberite crystals, if the upper part of the crust were to be broken or eroded, thus exposing this lower crust, emissions could be substantially higher.

The lowest emitting surface was PA-2 (Table 3). This site is located approximately 1 km away from the shoreline and had no visible salts at the surface. Its thin, smooth clay crust (Fig. 2f ) suggested a surface with very little moisture cycling and low salt content. This resulted in a typical dry playa surface that would be relatively immobile by most winds if undisturbed. Seasonal allu-vial deposition of fine material on the surface of this playa could result in increased emissions (Reynolds et al., 2007).

controls on dust EmissionThe results of this study show that specific crystal habits and

the degree of interlocking crystals play the greatest roles in af-fecting dust emission. Although they did not link these charac-teristics to dust emissions, as early as 1973, Driessen and Schoorl noticed that the effectiveness of surface crust sealing depended on the crystal morphology and not the chemical composition. Acicular and prismatic crystal habits have been found to have a greater disruptive effect to the media in which they precipitate because of increased crystal pressures during mineral precipita-tion (Grattan-Bellew and Eden, 1975; Goudie, 1977; Sperling and Cooke, 1985). These crystal habits also are less likely to form interlocking, smooth surfaces, but instead grow as elongated nee-dles, often forming radiating clusters (e.g., Shahid and Jenkins, 1994). Therefore, such crystal habits may lessen the degree of interlocking precipitates, enhance salt heaving in surrounding materials, and lead to greater emissions.

Through crystal habit, salt mineralogy can indirectly af-fect dust emissions. In this study, thenardite, eugsterite, and glauberite were found to have acicular or prismatic habits and be associated with increased potential emissions. Other studies have found that halite, anhydrite, bassanite, trona, and gypsum also can precipitate in these habits (Milton, 1942; Hanna and Stoops, 1976; Grattan-Bellew and Eden, 1975; Eswaran et al., 1980; Vergouwen, 1981a; Eswaran, 1984; Shahid and Jenkins,


1994; Amit and Yaalon, 1996; Joeckel and Ang Clement, 1999; Datta et al., 2002; Mees and Stoops, 2003). Therefore, mineral-ogy alone cannot predict dust emissions because different miner-als can have the same crystal habit and individual minerals can also display many different habits—all dependent on the envi-ronmental conditions present at the time of crystallization.

Unfortunately, the specific environmental conditions that determine the salt mineralogy, the formation of certain crystal habits, and their arrangement within the soil remain largely un-known, limiting the potential for a model that would predict which exposed surfaces are likely to be emissive and for how long. It is known that changes in temperature, soil water, the presence of organic matter, the amount and type of ions in solution, soil pH, and other environmental conditions affect salt mineralogy and crystal habits (see Edinger, 1973; Jafarzadeh and Burnham, 1992; Vizcayno et al., 1995; Amit and Yaalon, 1996; Buck and Van Hoesen, 2002; Buck et al., 2006b). Because soils are open systems and environmental characteristics can fluctuate enor-mously and rapidly at many different temporal and spatial scales, it is extremely difficult to determine which specific factor(s) con-trol these characteristics. Additionally, this study only looked at small samples from single-point sites at one point in time around the Salton Sea. More research is needed to understand how these surfaces vary spatially and temporally and how the characteristics of the salt crusts affect dust emissions.

Factors other than crystal habit that affected emissions in-clude the proximity to the lake and the depth to the water table. Surfaces near the lakeshore or with a shallow water table are more susceptible to dramatic and frequent changes in relative hu-midity and water content. Such changes are likely to result in the frequent dissolution and precipitation of salt minerals, especially hydrous/anhydrous salt minerals. Rapid mineral precipitation is more likely to result in smaller crystals, which, if not tightly ce-mented together, are more emissive. Periods of decreased water contents will lessen the cohesive properties of these crusts and can also lead to increased emissions. In contrast, topographically higher (older) shorelines with deeper water tables can eventu-ally lose the surface salt crust as that salt is redistributed deeper into the soil profile (i.e., Site PA-2). These older shorelines have a greater potential for interaction with fresher groundwater derived from local and regional rain (i.e., Yechieli and Ronen, 1997). These more steeply sloping shorelines also have deeper water tables. Therefore, capillary rise may not reach the surface and salt minerals will precipitate in the subsurface, or at the sur-face only seasonally (i.e., Buck et al., 2006b). Eventually fresh-water inputs at the surface will dissolve salt minerals and flush them deeper into the soil. These changes can occur very quickly (decades in the work of Yechieli and Ronen, 1997) and result in a long-term change in the salt regime or can be transient on a seasonal basis (i.e., Cahill et al., 1996; Gillette et al., 2001).

Salt-dominated playa systems such as the Salton Sea and Owens Lake pose special problems with regard to dust emissions. Unlike nonsaline or dry playas, where increased soil moisture (e.g., high water tables) can increase vegetation and decrease dust emis-

sions (Pye, 1987; Reheis, 2006; Reynolds et al., 2007; Elmore et al., 2008), saline and sodic soils are hostile environments for most types of vegetation. High water tables in such areas promote sur-face salt accumulation, which together with salt heaving, provide a continuous source of fine materials for dust emissions. Therefore, in these environments dust emissions can be lessened by man-agement techniques that lower water tables so that salt minerals precipitate deeper in the soil profile and not at the surface. This interpretation is supported by other studies indicating that in-creased dust emissions occur on saline wet playas during periods of increased precipitation during El Niño events (Reheis and Kihl, 1995; Reheis, 2006; Reynolds et al., 2007). In contrast, high water tables in nonsalt playas can decrease dust emissions by increasing sediment cohesiveness or increasing the vegetation density (Pye, 1987). Understanding how saline and nonsaline soils respond dif-ferently to changes in land use and climate is urgently needed to protect public health and to lessen environmental degradation.

conclUSionSIn salt-dominated systems, salt mineral dynamics—disso-

lution, reprecipitation, and the resulting changes in abundance, mineralogy, and crystal habits—control dust emissions. Surfaces with the highest emissions will be those that are (i) composed of minerals with acicular or prismatic crystal habits, which promote noninterlocking, loosely cemented crusts, (ii) contain hydrous/anhydrous salt minerals, and (iii) have a high water table or a high seasonally retarded or perched water table.

Salt minerals with acicular or prismatic habits are more likely to be disruptive (i.e., acting as a wedge pushing particles apart). This enhances salt heaving, lessens the degree of interlocking pre-cipitates, and forms loose, “puffy” crusts that are highly emissive. Salt mineralogy indirectly affects dust emissions in that thenard-ite, eugsterite, and glauberite all exhibit acicular or prismatic hab-its and are associated with surfaces that have high emissions. Salt crusts composed of hydrous/anhydrous minerals—mirabilite/thenardite, eugsterite/glauberite, gypsum/bassanite, and numerous Mg sulfates—are more likely to dissolve and reprecipitate repeat-edly (i.e., diurnally or seasonally), form tiny, individual crystals or small aggregates, and be less cohesive and more likely to result in highly emissive surfaces. Low-sloping surfaces with shallow water tables near the shoreline have greater fluctuations in water content and relative humidity, which can lead to frequent salt mineral dis-solution and precipitation (e.g., diurnally or even more frequently). Rapid mineral precipitation under such conditions is more likely to form small crystals, which, if not interlocking, will increase emis-sions. Additionally, the high water table allows a continuously re-plenishing supply of salt crystals that can become available for eo-lian erosion. Salt heaving also brings additional silicate minerals to the surface. Therefore, in salt-dominated playa systems where surfi-cial sand is not a controlling factor, dust emissions can be reduced if water tables are lowered so that salt mineral deposition occurs in the subsurface. This prevents the continued emplacement of fine salt mineral particulates at the surface as well as stops salt heaving from bringing upward additional, potentially wind-erodible silicate


minerals. Lowered water tables combined with freshwater inputs (either through precipitation or irrigation) will flush the existing surface salts into the subsurface, leaving only silicate minerals at the surface. Although such surfaces can still be highly emissive, they are not emissive to the same degree as salt-dominated surfaces (e.g., Owens Lake and the Aral Sea) (Cahill et al., 1996; Reheis et al., 2002). This conceptual model for the effects of salt on dust emis-sions has been developed from measurements and insights gained at the Salton Sea. It is critical that the model and underlying hy-potheses be tested at other suitable locations.

acKnoWlEdGMEntSThis research was funded by the California Department of Water Resources, under the Salton Sea Authority. We thank Jan Morton and Michael Howell for helping to collect laboratory data, George Nikolich for help collecting PI-SWERL data, and Rich Reynolds for helpful suggestions that improved this manuscript.

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