Published online August 3, 2006
Salt Mineralogy of Las Vegas Wash, Nevada: Morphology and Subsurface Evaporation
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
Brenda J. Buck,* Katherine Wolff, Douglas J. Merkler, and Nancy J. McMillan
ABSTRACT
ization also increased the impermeable surface area of
the LVW drainage, and after 1980 significant erosion
and incision of the LVW began (Jackson and Patten,
1988). Rapid urban and suburban growth in Las Vegas
Valley for about 30 yr has involved consistent disruption
of natural drainage paths and associated salt flux processes in the native soils. For overland and open channel
flow, the net effect has been to collect and channelize
flow through urban portions, totally impede and block
sheet flow, and reduce the discharge capacity of lesser
tributaries. Incision into the underlying unconsolidated
Neogene Muddy Creek Formation is greater than 6 m
in places and has drained much of the lower LVW
marshes and reduced the extent of the hydrophytic vegetation, such as cattail (Typha domingensis), common reed
(Phragmites australis), and Baltic rush (Juncus balticus),
and has lowered water tables (Jackson and Patten, 1988).
The lowered water tables have caused spatial shifts in
vegetation. Hydrophytic vegetation, such as Sedge (Carex
spp.), Baltic rush (Juncus balticus), and cattail (Typha
domingensis) occurs where water tables are within approximately 60 cm of the surface and are replaced by
phreatophytes salt cedar (Tamarix ramosissima) and arrowweed (Pluchea sericea) when lowered (Burbey, 1993).
Because the LVW drains into Lake Mead, the source for
approximately two thirds of the city of Las Vegas’ drinking
water (Burbey, 1993), there are many relevant environmental concerns. Some of these include managing water
quality standards (primarily to reduce the contribution of
salts and contaminants to the Colorado River), flood control, maintenance as an artificial wetland for recreational
and educational activities and wildlife habitat, selenium
levels in the LVW, and water rights for return flow to Lake
Mead. Critical to addressing these issues is an understanding of the soil properties in the LVW.
The soils in the LVW are heavily concentrated with
salts as a result of high evapotranspiration rates combined with throughflow and runoff from calcic and gypsic
parent materials, including Neogene Muddy Creek and
Horse Springs Formations, Triassic Moenkopi and Chinle
Formations, and a complex assemblage of Paleozoic marine carbonates (Page et al., 2003). Las Vegas has an average annual precipitation of approximately 10 cm yr21,
with an estimated evaporation rate of over 200 cm yr21.
The sampled soils in the LVW were mapped as part of the
Land series: fine-silty, mixed, superactive, thermic Typic
Aquisalids, in the 1977 Soil Survey of Las Vegas Valley.
The Land series also occurs along two other southern
Nevada perennial riparian systems: the Virgin and Muddy
rivers. The occurrence of a surface crust of salt minerals
because of capillary fringe evaporation is a typical feature
Las Vegas Wash drains the Las Vegas Basin in Nevada by capturing
a series of tributaries and ending in Lake Mead and is being developed
into an urban wetland. The soils are part of the Land series and contain high concentrations of pedogenic salts because of local sulfate-rich
parent materials and high evapotranspiration rates. These salts cause
damage to property, affect plant communities in the wetlands, and
contribute to salinity in the Colorado River System. To gain a better
understanding of these salts, two soil profiles were described. Whole
soil samples were analyzed for ammonium acetate–extractable and
water-soluble elemental analyses, cation exchange capacity (CEC),
pH, electrical conductivity, and particle size. Salt minerals were
analyzed using scanning electron microscope (SEM)/energy dispersive
spectrometer (EDS) and x-ray diffraction (XRD). Results of XRD
analyses indicate hexahydrite, bloedite, mirabilite, gypsum, thenardite, halite, vivianite, and sepiolite. SEM/EDS analyses found bloedite,
eugsterite, halite, hexahydrite, gypsum, thenardite, and possibly
kainite. Gypsum occurs at the surface and in all subsurface horizons.
The more soluble salts occur at the surface and in two subsurface
horizons. This study documents the third occurrence of eugsterite in
the USA and is the first study to document salts other than gypsum
forming snowball morphology. We interpret the subsurface zones of
soluble salts to represent relict water tables where capillary action,
combined with subsurface evaporation, has concentrated Na-Mg sulfates and halite. These relict water tables may represent previous highlevel water tables that were lowered because of increased urbanization
resulting in flooding, erosion, and incision of the Las Vegas Wash.
A
within Las Vegas Valley,
Nevada, drains water into Las Vegas Wash (LVW)
and finally into Lake Mead and the Colorado River.
The LVW encompasses a saline meadow ecosystem that
is being developed as an urban wetland. Before 1940,
LVW was an ephemeral stream characterized by desertscrub vegetation typified by honey mesquite, Prosopis
glandulosa Torr. and fourwing saltbush, Atriplex canescens (Malmberg, 1965). Population growth in the Las
Vegas Valley during the 1950s increased wastewater
and industrial discharges until flow in the LVW became permanent (Jackson and Patten, 1988). Cattail and
reed marshes dominated the LVW by 1970 (Jackson and
Patten, 1988). Continued urbanization increased the
flow velocity in the LVW through urban runoff and increased irrigation of lawns and golf courses, sewage
treatment discharges, and ground-water seepage. UrbanSYSTEM OF TRIBUTARIES
B.J. Buck, Dep. of Geoscience, Univ. of Nevada Las Vegas, 4505
Maryland Pkwy., Las Vegas, NV 89154; K. Wolff, Nicholas School of the
Environment and Earth Sciences, Duke Univ., Durham, NC 27708;
D.J. Merkler, Natural Resources Conservation Service, 5820 S. Pecos Rd.
Bldg. A. Suite 400, Las Vegas NV 89120; N.J. McMillan, Dep. of Geological Sciences, New Mexico State Univ., Las Cruces, NM 88003. Received 19 Aug. 2005. *Corresponding author ([email protected]).
Published in Soil Sci. Soc. Am. J. 70:1639–1651 (2006).
Soil Mineralogy and Urban Soils
doi:10.2136/sssaj2005.0276
ª Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
Abbreviations: CEC, cation exchange capacity; EC, electrical conductivity; EDS, energy dispersive spectrometer; LVW, Las Vegas Wash;
LVW1, Las Vegas Wash 1; LVW2, Las Vegas Wash 2; SEM, scanning
electron microscope; XRD, x-ray diffraction.
1639
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
1640
SOIL SCI. SOC. AM. J., VOL. 70, SEPTEMBER–OCTOBER 2006
of the Land series. These salt minerals contribute to the
destruction of concrete structures due to salt weathering,
salt heaving, and hydrocollapse (Chapman, 1980; Hawkins
and Pinches, 1997; Hellmer, 2001).
The accumulation of sulfate minerals in soils is common in arid and semiarid regions throughout the world.
Common pedogenic sulfate minerals include gypsum,
glauberite, eugsterite, mirabilite, epsomite, hexahydrite,
thenardite, and bloedite (Eswaran and Carrera, 1980;
Eswaran et al., 1981; Dan et al., 1982; Doner and Lynn,
1989; Eswaran and Zi-Tong, 1991; Kohut and Dudas, 1993;
Boettinger, 1997; Buck and Van Hoesen, 2005). The dominant controlling mechanisms for the accumulation and
formation of these salts in soils are twofold: (i) the source
of the sulfate ion, and (ii) the semi-soluble to extremely
soluble nature of these sulfate minerals. The sulfate ion
can be derived from in situ weathering of parent material
(Carter and Inskeep, 1988) or sulfide minerals (Mermut
and Arshad, 1987), fluvial or eolian input (Taimeh, 1992;
Buck and Van Hoesen, 2002), and an atmospheric
source from seawater (Podwojewski and Arnold, 1994;
Toulkeridis et al., 1998; Rech et al., 2003). The specific
controls affecting the amount, type, and position of sulfate
minerals within the soil profile depend heavily on physical
and chemical conditions at the macro- and micro-scale.
Some of these conditions include soil pH, temperature,
pCO2, amount and type of cations and anions available,
amount and chemistry of soil water, and atmospheric and
soil humidity. Understanding the interactions among these
different sulfate minerals, soil microorganisms, and the
numerous physical and chemical parameters is difficult
in soils because in most cases these are open systems, with
continuous fluxes of energy and matter. Research into the
behavior of sulfate minerals is important because these
minerals can have an enormous effect on a soil’s physical
and chemical characteristics, which usually result in adverse affects on agricultural and engineering uses. Identifying sulfate minerals in the field is an important first step
for any field of research or use of a particular soil. Buck
and Van Hoesen (2002) found that, unlike calcite, pedogenic gypsum occurs on a macro-scale as spherical accumulations 0.5- to 3.0-mm in diameter, termed snowballs.
They found that this special characteristic of pedogenic
gypsum may be related to soil microorganisms (63% of
snowballs contained actinomycetes; 100% contained
organic material) and that the microorganisms may be
attracted to the hygroscopic nature of the gypsum mineral.
Scanning electron microscopy (SEM), especially when
equipped with an energy dispersive spectrometer (EDS),
is an important tool for identifying pedogenic salts in
a soil profile. Other methods, such as x-ray diffraction
(XRD), when used alone, are not as useful because many
salt minerals have similar peaks to common soil constituents such as feldspars and quartz. In addition, the identification of accessory minerals (those that constitute less
than 5% of the volume of the soil) in diffraction patterns
is difficult because the high-intensity peaks of accessory
minerals are often overprinted by the numerous lowintensity peaks of the dominant minerals. Therefore, to
gain a better understanding of soil processes (chemistry
and water movement), SEM/EDS analysis can be an
attractive method to use in conjunction with other common soil analyses, such as water-soluble elements in
addition to XRD analyses of surface salt crusts. The
objective of this study was to use SEM/EDS analyses to
determine the salt mineralogy in soils and to test the
hypothesis that Stage I snowballs are an indicator of
pedogenic gypsum, as previously described in Buck and
Van Hoesen (2002).
MATERIALS AND METHODS
Two soil profiles, approximately 16 km upstream from Lake
Mead, in the LVW were excavated, described (according to
standard USDA techniques outlined in Schoeneberger et al.,
1998), and sampled (LVW1 in March 2001; LVW2 in July 2001)
(Fig. 1). Both pedons LVW1 and LVW2 occur in a map unit
dominated by the Land series and were assumed to be wet
within 1 m of the surface during field work on the Las Vegas
Valley Soil Survey in 1977. This was an artificial or induced
condition resulting from increased runoff and effluent return
flow to the Las Vegas Wash. Erosional entrenchment of the
axial stream channel from high volume storm flows in the
1980s drained much of the floodplain and lowered the water
tables within these pedons below 1 m. The use and management of these soils within the floodplain has remained relatively constant. As a result of this investigation, these pedons
should be considered taxajuncts to the Land series. This raises
questions about the assumed water table depths noted in the
original mapping. LVW1 does not have a diagnostic salic horizon, failing to meet the thickness requirement of 15 cm. LVW1
is classified as a fine-silty, mixed, active, thermic Typic Haplogypsid. LVW2 is classified as a clayey over loamy, mixed, semiactive, thermic Gypsic Haplosalid. These sites were chosen
to coincide with research and evaluation of selenium mobility
in the Las Vegas Wash. Soil texture was estimated in the field
according to feel (Thien, 1979).
Additional soil textural analyses were not performed because there are no methodologies available to accurately measure soil texture without removing salt crystals. To ensure
accuracy in identifying salt mineralogy, samples were taken
directly from the field into the laboratory, and analyses were
Fig. 1. Location of study site, Las Vegas Wash, eastern Las Vegas,
Nevada.
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
BUCK ET AL.: SALT MINERALOGY OF LAS VEGAS WASH, NEVADA
performed as quickly as possible. The time period for laboratory analyses varied between 1 h and 5 wk after field sampling.
A few analyses (|20 SEM images) were performed 4 mo after
sampling. No changes in mineralogy were observed in the later
analyses, suggesting that accuracy was maintained. A stereomicroscope was used to determine salt characteristics, including shape and orientation of crystals within snowballs. Dental
tools and a stereomicroscope were used to obtain intact
Stage I snowballs or portions of Stage II nodules (Buck and
Van Hoesen, 2002) as small peds. These were attached to aluminum studs with carbon glue and sputter coated with gold. To
overcome the problem of charging during SEM analyses, it
is essential to provide the optimum contact for the electrons
across the surface of the sample through careful mounting
and gold coating. Too much gold coating can result in highintensity gold peaks, thus masking EDS analyses of the salt
chemistry. Therefore, in this study, initial gold sputter coating
was 75 s, and additional gold was added to individual samples
at 10 s intervals, and/or carbon glue was added at the base or
sides of the sample until optimum SEM/EDS analyses were
achieved. Sixty-two samples of small peds were extracted and
mounted for SEM/EDS analyses, which were performed on a
JEOL 5600 scanning electron microscope and an Oxford ISIS
energy dispersive X-ray system at the Electron Microanalysis
and Imaging Laboratory at University of Nevada Las Vegas. A
total of 369 SEM images and 438 EDS analyses were used to
determine the composition and mineralogy of pedogenic salts
in these profiles. Within each SEM image, multiple EDS analyses were obtained to assist in the identification of all minerals
present within each image. For example, if one SEM image
provided evidence for three different salt minerals because of
crystal habit and supporting EDS analyses, then three observations would be recorded from that single SEM image. If a
mineral displayed dissolution features within a SEM image,
then one observation of a dissolution feature for that mineral
was also recorded. The total number of observations per horizon was combined, and the percentages of salt minerals or
dissolution observations were determined by their frequency
of occurrence within the total SEM images analyzed. These
results should be understood to be semiquantitative because of
possible user bias in selecting areas within the sample that may
be more photogenic.
Surface salt crusts were analyzed in the XRD laboratory at
New Mexico State University using a GeigerFlex diffractom-
1641
eter (Rigaku, The Woodland, TX). Analytical conditions were
Cu tube run at 40 kV and 30 mA, 2u range of 2 to 908, scan rate
18 min21, time constant of 1 s. Three aliquots of each sample
were analyzed separately and mounted with double-sided
tape. This method ensured that (i) no solvent (including water)
could cause a reaction in the salt samples and (ii) that, although relatively little powder was analyzed in each aliquot, all
minerals would be included in at least one analysis. The three
sets of peak positions and intensities were combined for each
sample for mineral identification using the Joint Committee
for Power Diffraction Studies powder diffraction file.
Ammonium acetate extractable elements, CEC, and watersoluble elemental analyses were conducted at Utah State University Analytical Laboratory on whole-soil samples from each
horizon according to the Western States Laboratory Plant, Soil
and Water Analysis Manual (Gavlak et al., 1994). The University of Nevada Las Vegas Pedology Laboratory determined
soil pH and EC from a saturated paste according to USDA
methods 8C1b and 8A1a (Soil Survey Staff, 1996).
RESULTS
Field descriptions of LVW1 and LVW2 indicate similar soil profiles (Fig. 2 and 3). At the time of sampling,
the water table in LVW1 was observed at 130 and 147 cm
in LVW2. Redox features present include rare manganese oxide occurring as irregular masses (1–10 mm) near
roots. Pedogenic salts in these profiles occur as (i) individual crystals in the soil matrix, (ii) small filaments
(0.5–1 mm), (iii) small snowballs (0.5–1 mm) (Buck and
Van Hoesen, 2002), (iv) Stage II nodules (2–3 cm), and
(v) incipient Stage III partially indurated horizons (similar to the stages of carbonate formation) (Gile et al.,
1966; Reheis, 1987; Buck and Van Hoesen, 2002). The
snowball morphology is the most common and occurs
throughout both profiles (Fig. 2, 3, and 4 [left]). The Stage
II nodules have diffuse borders and increase with depth,
and at the base of both profiles these nodules grade
into a partially developed, incipient Stage III gypsumcemented horizon. In all forms, these salts vary between
very friable to friable. There was no visible evidence for
salts more soluble than gypsum in either profile except
Fig. 2. Soil description of LVW1 indicating increased salt mineral concentration with depth from few snowballs near the surface to incipient Stage III
pedogenic gypsum above the water table. A 2-cm-thick salt crust occurs at the surface.
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
1642
SOIL SCI. SOC. AM. J., VOL. 70, SEPTEMBER–OCTOBER 2006
Fig. 3. Soil description of LVW2 indicating increased salt mineral concentration with depth. At the time of sampling all snowballs seemed to be
gypsum, and only after SEM/EDS analyses were performed were the two Bz horizons identified.
for the surface salt crusts present at both profiles. Because of this, the soil horizons were originally labeled
with a subscript ‘y’. The ‘z’ subscripts were added after
SEM/EDS analyses indicated that these highly visible
(snowballs, Stage II nodules) salt concentrations were
composed of soluble salts and not gypsum. The ‘n’ subscripts were added after chemical analyses indicated accumulation of exchangeable sodium.
SEM/EDS analyses were used to determine salt mineralogy via crystal habit and chemical analyses. Additionally, XRD data from the surficial crusts were compared
with the SEM/EDS data to assist in mineral interpretation. This was useful in distinguishing between salt
minerals that have similar chemical signatures (e.g., epsomite and hexahydrite). Through these methods, SEM/
EDS analyses indicate that LVW1 contains bloedite,
halite, hexahydrite, eugsterite, and mirablilite at the surface and gypsum in all of the subsurface horizons (Tables 1
and 2; Fig. 5). SEM/EDS analyses of LVW2 found
bloedite, eugsterite, halite, hexahydrite, and gypsum
(Tables 1 and 3; Fig. 5). A sodium-sulfate phase was
identified, but the anhedral crystal habit prevented the
distinction between thenardite and mirabilite. Additionally, an uncommon Mg-K-Cl-SO4 phase was indicated using EDS and is tentatively identified as kainite.
Because the electron beam in the SEM/EDS analyses
can penetrate up to the 2.5-mm depth (modeled depth
using Casino 2.42 based on penetration through gypsum; Drouin et al., 2001), this signature could be the result
of a combination of more than one mineral occurring
within the area of the beam. However, the visual appearance of the mineral in the SEM suggested that the
EDS signature was measuring a single thick crystal,
and therefore kainite is tentatively identified. SEM/EDS
analyses identified gypsum in all of the subsurface horizons of LVW2. Halite, hexahydrate, and bloedite reap-
Fig. 4. Photograph of (left) snowball morphology in LVW1 (61–98 cm); (right) microsnowballs in LVW2 surface crust.
BUCK ET AL.: SALT MINERALOGY OF LAS VEGAS WASH, NEVADA
1643
Table 1. Mineralogy and Chemical Formulas for Minerals found
in this study.
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
Mineral
Chemical formula
Gypsum
Hexahydrite
Mirabilite
Thenardite
Eugsterite
Bloedite
Halite
Kainite†
Sepiolite
Vivianite
CaSO4!2H2O
MgSO4!6H2O
Na2SO4!10H2O
Na2SO4
Na4Ca(SO4)3!2H2O Na2Mg(SO4)2!4H2O
NaCl
KMgClSO4 3H2O
Hydrous Mg-silicate
Fe3(PO4)2!8H2O
† Tentative identification based only on EDS data.
pear at 62 to 110 cm and again occur at 127 to 150 cm
(Table 3).
The presence of hexahydrite, sepiolite, bloedite, mirabilite, and gypsum in LVW1 surface crust was confirmed
by XRD analyses. X-ray diffraction analysis for LVW2
surface crust indicates the presence of hexahydrite, gypsum, bloedite, vivianite, thenardite, and halite. Analyses
of EC and water-soluble ions indicate the highest amounts
of water-soluble elements at or near the surface in both
profiles, but LVW2 additionally shows two subsurface
zones with elevated EC, sulfur, sodium, chloride, magnesium, and potassium (Tables 4 and 5). Soil pH varies
between 7.6 and 8.6 (Tables 4 and 5). In LVW1, pH values
decrease with depth. In contrast, in LVW2, pH values
are more variable, with the lowest value in the 62- to
110-cm horizon.
The crystal habits of the salt minerals vary with mineralogy and depth (Tables 6 and 7). Gypsum occurs as
euhedral tabular pseudo-hexagonal, tabular hexagonal,
lenticular, lath, bladed, and anhedral massive (Fig. 6A–C).
Bloedite occurs as tabular pseudo-hexagonal, bladed,
foliated, skeletal, lath, columnar, tabular rhomboidal,
tabular hexagonal, and anhedral massive (Fig. 6D–F;
7C, D, F; 8F). In addition, in the surface crusts only,
bloedite occurs as groups of interlocking, spheroidal, 10to 20-mm euhedral tabular crystals that form microsnowballs that vary in size from 100 to 500 mm (Fig. 4b,
6D, 7A–C). Eugsterite always is acicular (Fig. 7D–F).
Hexahydrite is most commonly anhedral massive but is
also tabular pseudo-hexagonal (Fig. 8A, C, E). Halite
occurs primarily as anhedral massive, but hopper and
cubic forms are also present (Fig. 8B, D). Evidence of
dissolution (pits, corrosion of crystal edges) was measured using the SEM data (Tables 2 and 3).
Table 2. LVW1 mineralogy distribution and dissolution evidence–
based on SEM data.
Horizon
Azn 0–2 cm
Ay 222 cm
By1 22–61 cm
By2 61–98 cm
2By3 98–140 cm
Mineral
Percent†
bloedite
halite
hexahydrite
eugsterite
mirabilite
gypsum
gypsum
gypsum
gypsum
77
13
4
3
3
100
100
100
100
Percent dissolution
per horizon†
49
69
93
94
100
† Percentage determined by frequency of occurrence within the total SEM
images analyzed.
Fig. 5. EDS results for (A) Mg-sulfate. X-ray diffraction analyses identified the Mg-sulfate phase as hexahydrite; (B) Na-Ca-sulfate. Crystal habits in SEM analyses indicate eugsterite; (C) Na-Mg-sulfate.
X-ray diffraction analyses identified this phase as bloedite.
DISCUSSION
Morphology of Sulfate Minerals
The macromorphology of pedogenic gypsum has
been described in Buck and Van Hoesen (2002). They
determined that the snowball morphology is unique to
pedogenic gypsum. Their results suggested that soil microorganisms play a role in the development of the snowball morphology, possibly because of the hygroscopic
nature of gypsum: water is attracted to the surfaces of
the gypsum crystals, and in arid environments this may
create a niche for microorganisms. However, in this study,
we found that numerous sulfate minerals (in addition to
gypsum) can display this morphology, and, in contrast to
the Buck and Van Hoesen (2002) study, no evidence for
microorganisms was found in any of the SEM analyses.
Halite can be present within the snowball morphology,
but it is never found exclusively forming snowballs. Thus,
1644
SOIL SCI. SOC. AM. J., VOL. 70, SEPTEMBER–OCTOBER 2006
Table 3. LVW2 Mineralogy Distribution and Dissolution Evidence
based on SEM data.
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
Horizon
Azn 0–1 cm
Ayzn 1–9 cm
Byzn1 9–20 cm
Byzn2 20–62 cm
2Byzn3 62–110 cm
2Byzn4 110–127 cm
2Byzn5 127–150 cm
Mineral
Percent†
bloedite
eugsterite
hexahydrite
halite
gypsum
thenardite
kainite
gypsum
gypsum
gypsum
halite
hexahydrite
bloedite
gypsum
gypsum
hexahydrite
bloedite
halite
gypsum
74
17
3
3
2
trace
trace
100
100
100
52
26
17
5
100
57
26
9
8
dance are controlled by environmental fluctuations primarily associated with changes in temperature and soil
water status. However, other factors, such as the presence of organic matter, the amount and types of ions
in solution, and pH, can affect crystal habits (Edinger,
1973; Jafarzadeh and Burnham, 1992; Amit and Yaalon,
1996; Buck and Van Hoesen, 2002). Depending on the
solubility of the individual minerals, pedogenic sulfate
crystals in the Las Vegas Wash profiles vary between
euhedral (for the less-soluble minerals or recently precipitated minerals), subhedral, and anhedral (for the
highly soluble minerals such as halite). There are numerous different crystal habits found in this study, but
the greatest variety is found at or near the surface
(Tables 6 and 7). This is probably a reflection of the increase in number of minerals found at the surface and the
fact that this portion of the profile experiences the greatest
number and degree of environmental changes. Dissolution at the surface horizons of both profiles is partly a
function of the high solubilities of the minerals present
and of increased fluctuations in relative humidity at the
surface (Tables 2 and 3). Dissolution features in the salt
crystals in LVW1 increases with depth. In all but the surficial salt crust horizon, dissolution occurs only in gypsum crystals, reflecting proximity toward the water table
(Table 2). This profile was sampled and described in
March, when soil water contents are likely to be near their
highest. Additionally, the lack of salts more soluble than
gypsum in the subsurface horizons may reflect their complete dissolution and removal. In LVW2, evidence of dissolution varies according to mineralogy in the subsurface
zones of soluble salts (Table 3). In contrast to LVW1,
where gypsum displayed evidence of dissolution, 0% dissolution was found in the Ayzn, Byzn1, and 2Byzn4 horizons in LVW2, which are composed entirely of gypsum.
The lack of dissolution features in these horizons may also
be an indication that the salts more soluble than gypsum
have been removed.
Percent dissolution
per horizon†
21
0
0
10
48
0
39
† Percent determined by frequency of occurrence within the total SEM
images analyzed.
we conclude that the snowball morphology is unique to all
pedogenic sulfate minerals, not just gypsum in particular,
and although soil microorganisms may also occur with
the snowballs, they are not active players in producing
this morphology. Why sulfate minerals display this unique
morphology in soils is unknown. All of the sulfate minerals identified in this study also display the other types of
macromorphologies similar to gypsum (Stage I filaments,
Stage II nodules, and incipient Stage III indurated horizon) (Harden et al., 1991; Buck and Van Hoesen, 2002).
Micromorphology and Crystal Habits
A morphology similar to a snowball, but at a smaller
scale, is found within the surface crusts of both profiles.
Bloedite occurs as groups of interlocking, spheroidal,
10- to 20-mm, euhedral tabular crystals that form microsnowballs (Fig. 4 [right], 6D, 7A–C). These micro-snowballs vary in size from 100 to 500 mm. Because these
micro-snowballs are found only in the surface horizons,
we suggest that these spheroidal accumulations of crystals
are formed by kinetic effects of supersaturation and rapid
crystallization associated with rapidly changing surficial
conditions. The rapidly precipitating sulfate crystals precipitate out of solution around an initial crystal nucleus,
creating these spheroidal clusters. Therefore, at two distinct scales, sulfate minerals are found to precipitate in
spherical accumulations: snowballs and micro-snowballs.
The crystal habits of the salt minerals in this study
vary considerably. The crystal habits and relative abun-
Mineralogy
Minerals found through SEM/EDS analyses include
gypsum, bloedite, hexahydrite, mirabilite, eugsterite, halite,
thenardite, and possibly kainite (Tables 1–3). Minerals
found with XRD analysis include gypsum, bloedite, hexahydrite, mirabilite, thenardite, halite, sepiolite, and vivianite. Although eugsterite is not identified in the XRD
analysis and vivianite is not found in the SEM/EDS analyses, these results show good agreement between the
two methods. Some differences in the results between
the two methods could have been caused by several fac-
Table 4. LVW1 Chemical Data†.
H2O soluble ions
Depth
Azn
Ay
By1
By2
2By3
cm
0–2
2–22
22–61
61–98
98–140
pH
8.3
8.4
7.9
7.7
7.6
CEC
meq 100 g
12.7
13.2
10.7
8.8
7.3
EC
21
mS cm
97.5
19.8
11.9
10.6
7.9
† CEC, cation exchange capacity; EC, electrical conductivity.
Ca
Mg
Na
21
12.4
7.6
13.7
12.9
14.1
655.0
9.8
31.4
21.6
19.1
644.2
22.1
63.1
41.8
36.8
K
mmol L
55.0
1.9
4.3
3.4
2.9
B
S
Cl
1.4
0.3
1.2
9.8
0.1
877.3
28.6
56.7
43.8
39.9
448.0
32.2
29.1
16.5
15.2
21
1645
BUCK ET AL.: SALT MINERALOGY OF LAS VEGAS WASH, NEVADA
Table 5. LVW2 chemical data.†
H2O soluble ions
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
Depth
Azn
Ayzn
Byzn1
Byzn2
2Byzn3
2Byzn4
2Byzn5
cm
0–1
1–9
9–20
20–62
62–110
110–127
127–150
pH
CEC
‡
8.3
8.6
8.4
7.9
8.0
8.1
meq 100 g21
14.1
11.2
11.4
14.5
3.7
14.3
10.2
EC
Ca
Mg
Na
K
21
mS cm
‡
101.8
57.9
49.6
66.5
26.7
50.1
B
S
Cl
4.8
4.4
1.1
0.4
0.5
0.3
0.3
1506.6
934.0
217.9
96.3
546.4
78.0
115.7
1168.7
1701.6
475.1
207.5
258.5
153.5
223.9
21
8.8
13.9
17.3
16.4
23.3
14.2
15.4
1134.3
799.4
176.3
62.2
281.3
47.9
180.3
1135.7
1014.4
392.7
198.3
676.0
154.4
213.0
mmol L
137.0
183.9
62.0
27.6
73.8
19.4
27.9
† CEC, cation exchange capacity; EC, electrical conductivity.
‡ Not measured.
tors. First, the XRD analyzes a whole soil sample from
the surface salt crust, whereas the SEM method analyzes
small, visible accumulations of salt crystals (snowballs).
Some of the salts found exclusively in the XRD analyses
could be salts acting as cementing agents between soil
particles, which would likely be excluded in SEM/EDS
analyses. Second, some of these highly soluble minerals
may have dissolved and/or precipitated during changes
in temperature and humidity during shipping/handling
before analyses. Finally, in SEM/EDS analyses, the peak
for phosphorus is overprinted by the peak for gold (coating used in the SEM/EDS analysis). Therefore, it is unlikely that vivianite could have been positively identified
using this method. A carbon coating can be substituted
for the gold, but this can complicate positive identification of carbonate minerals.
Eugsterite (Na4Ca(SO4)3!2H2O) was identified in
this study using SEM/EDS analyses, which show an
euhedral, acicular crystal habit and a Na-Ca-SO4 composition (Fig. 5B, 7D–F). Although eugsterite may dehydrate to glauberite [Na4Ca(SO4)3], Shahid (1988),
Qingtang (1989), and Shahid and Jenkins (1994) found
glauberite to precipitate as planar, rhombohedral crys-
tals, whereas Vergouwen (1981), Shahid (1988), and Mees
and Stoops (1991) have found eugsterite to form as acicular crystals. In this study, all SEM/EDS analyses showing
Na-Ca-SO4 compositions are found with euhedral, acicular crystals, which we conclude are eugsterite. This is only
the third time that pedogenic eugsterite has been identified in the USA. Skarie et al. (1987) found eugsterite
in North Dakota, and Buck et al. (2004) found it in the
Mojave Desert of California. Skarie et al. (1987) suggested that significant levels of Cl were necessary for
its formation. In both profiles in this study, eugsterite is
Table 7. LVW2 crystal habits.
Horizon
Mineral
Crystal habit
Percent†
Azn 0–1 cm
bloedite
eugsterite
hexahydrite
halite
gypsum
thenardite
kainite
Ayzn 1–9 cm
gypsum
Byzn1 9–20 cm
gypsum
Byzn2 20–62 cm
gypsum
2Byzn3 62–110 cm
halite
hexahydrite
bloedite
gypsum
2Byzn4 110–127 cm
gypsum
2Byzn5 127–150 cm
hexahydrite
bloedite
halite
gypsum
anhedral massive
tabular rhomboidal
bladed
anhedral massive
columnar
hollow triangle
foliated
lath
tabular hexagonal
acicular
tabular pseudo-hexagonal
anhedral massive
anhedral massive
tabular pseudo-hexagonal
anhedral massive
tabular pseudo-hexagonal
tabular pseudo-hexagonal
tablular hexagonal
lenticular
tablular hexagonal
tabular pseudo-hexagonal
lenticular
lenticular
lath
tabular pseudo-hexagonal
anhedral massive
hopper
cubic
anhedral massive
tabular pseudo-hexagonal
tabular pseudo-hexagonal
bladed
tabular pseudo-hexagonal
lenticular
lath
tabular pseudo-hexagonal
anhedral massive
tabular pseudo-hexagonal
tabular pseudo-hexagonal
bladed
lenticular
anhedral massive
tabular pseudo-hexagonal
100
19
15
13
7
6
3
3
2
100
33
67
100
100
100
32
57
29
14
50
25
25
50
38
12
73
23
4
91
9
86
14
100
60
28
12
92
8
50
33
17
100
100
Table 6. LVW1 Crystal Habits Determined by SEM data.
Horizon
Azn 0–2 cm
Mineral
Crystal habit
Percent†
bloedite
halite
hexahydrite
eugsterite
mirabilite
bladed
tabular
foliated
hollow triangular
tabular pseudo-hexagonal
anhedral massive
anhedral massive
hopper
cubic
anhedral massive
tabular pseudo-hexagonal
acicular
subhedral tabular
tabular hexagonal
lenticular
tabular pseudo-hexagonal
tabular hexagonal
lenticular
tabular pseudo-hexagonal
tabular hexagonal
lenticular
lath
tabular pseudo-hexagonal
tabular hexagonal
lenticular
andhedral massive
48
16
12
11
11
2
79
14
7
75
25
100
100
83
17
57
36
7
55
18
18
9
65
11
12
12
Ay 2–22 cm
gypsum
By1 22–61 cm
gypsum
By2 61–98 cm
gypsum
2By3 98–140 cm
gypsum
† Percentage determined by frequency of occurrence within the total SEM
images analyzed.
† Percentage determined by frequency of occurrence within the total SEM
images analyzed.
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
1646
SOIL SCI. SOC. AM. J., VOL. 70, SEPTEMBER–OCTOBER 2006
Fig. 6. SEM images of (A) lenticular gypsum from LVW1 By2 horizon (61–98 cm); (B) tabular pseudo-hexagonal gypsum from LVW1, By3 horizon
(98–140 cm); (C) lath gypsum from LVW1, By2 horizon (61–98 cm); (D) euhedral, tabular pseudo-hexagonal bloedite from LVW2 surface salt
crust (0–1 cm); (E) euhedral bladed bloedite from LVW1 Azn horizon (0–2 cm); (F) twinned bladed bloedite from LVW1 Azn horizon (0–2 cm).
found within the surface crusts, which also contain the
highest levels of Cl (Tables 4 and 5) and may partially
explain its presence. Vergouwen (1981) suggested that
the successful precipitation of eugsterite could be aided if
Na/Ca ratios were greater than 4. In the surface horizon
of LVW1 (0–2 cm), the Na/Ca ratio is 30, and in LVW2
surface crust, the ratio is 74. Therefore, the presence of
eugsterite in the surface samples of both of these profiles may have been aided by high Na/Ca ratios and increased Cl levels.
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
BUCK ET AL.: SALT MINERALOGY OF LAS VEGAS WASH, NEVADA
1647
Fig. 7. SEM images of (A) bloedite microsnowballs sealing the LVW2 surface salt crust (0–1 cm); (B) microsnowball of euhedral and subhedral
tabular bloedite from LVW2 surface salt crust (0–1 cm); (C) euhedral tabular pseudo-hexagonal bloedite forming a series of microsnowballs and
sealing LVW2 surface salt crust (0–1 cm); (D) backscatter image of acicular eugsterite (on right) with planar bloedite base from LVW2 surface salt
crust (0–1 cm); (E) acicular eugsterite from LVW1, Azn horizon (0–2 cm); (F) acicular eugsterite with tabular bloedite from LVW2 surface salt
crust (0–1 cm).
Mirabilite and thenardite are found in the XRD and
SEM/EDS analyses of the surface crusts. Mirabilite is
the hydrated form of sodium sulfate and is more stable
than thenardite in cooler temperatures and higher relative humidities (Driessen and Schoorl, 1973; Wiedemann
and Smykatz-Kloss, 1981; Timpson et al., 1986). Thus, it
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
1648
SOIL SCI. SOC. AM. J., VOL. 70, SEPTEMBER–OCTOBER 2006
Fig. 8. SEM images of (A) subhedral and anhedral hexahydrite from LVW1, Azn horizon (0–2 cm); (B) backscatter image of skeletal (hopper)
and anhedral halite from LVW2, 2Byzn3 horizon (62–110 cm); (C) subhedral hexahydrite from LVW2 2Byzn3 horizon (62–110 cm); (D)
euhedral, skeletal (hopper), cubic halite from LVW1, Azn surface crust (0–2 cm); (E) dehydrated tabular pseudo-hexadonal hexahydrite from
LVW2 2Byzn3 horizon (62–110 cm); (F) euhedral skeletal bloedite from LVW1, Azn surface crust (0–2 cm).
was not surprising that mirabilite is found in the surface
crust in March (LVW1) and thenardite in July (LVW2).
There is some controversy surrounding the environmental controls of bloedite versus konyaite precipita-
tion in soils. Studies by Driessen and Schoorl (1973) and
Eghbal et al. (1989) found bloedite to be a primary mineral sealing the surface crust. Kohut and Dudas (1993)
found bloedite in addition to eugsterite in Canadian salt
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
BUCK ET AL.: SALT MINERALOGY OF LAS VEGAS WASH, NEVADA
efflorescences. They reported that konyaite dehydrated
rapidly to bloedite, agreeing with Van Doesburg et al.
(1982), who concluded that konyaite is unstable and dehydrates to bloedite at room temperature within a few
days to 2 yr. Timpson et al. (1986) found konyaite to transform to bloedite after 1 yr of storage in the laboratory.
Konyaite may be unstable in dry surface crusts (Van
Doesburg et al., 1982; Shayan and Lancucki, 1984). The
rate of transformation is likely due to crystal size, temperature, and relative humidity (Van Doesburg et al.,
1982; Timpson et al., 1986). Braitsch (1971) suggests that
the presence of Cl can lower the temperature at which
bloedite is a first precipitate. In contrast to those studies,
Keller et al. (1986) suggested that bloedite as a primary
precipitate in nature is rare because natural temperature
and water vapor ratios favor konyaite (Na2Mg(SO4)2!
5H2O) formation. They also note that bloedite occurring
without konyaite is rare because of the large range of
temperature and humidities in which konyaite is stable (as
compared with bloedite) (Keller et al., 1986).
In this study, bloedite is found in XRD and SEM/EDS
analyses of the surface horizons of both profiles and in
SEM/EDS analyses of two subsurface horizons in LVW2
(Tables 2 and 3). Konyaite is not present. Consistent with
the studies by Driessen and Schoorl (1973) and Eghbal
et al. (1989), bloedite is found to be a primary mineral
sealing the surface crust. The presence of bloedite and
the absence of konyaite in both of our profiles may be
explained by the primary precipitation of bloedite and/or
dehydration of konyaite before SEM/EDS and XRD
analyses. We prefer the first explanation because SEM/
EDS analyses began only hours after sampling, and if
konyaite dehydrated to bloedite, the SEM/EDS analyses indicate that in doing so it nearly always formed
perfect, euhedral crystals (Fig. 6D–F). However, these
euhedral crystals also could be explained if, during the
process of dehydration of konyaite, the water released
caused the complete dissolution of the konyaite crystals
and was followed by precipitation of bloedite. Many of
the bloedite crystals form micro-snowballs that can be
seen with a hand lens (Fig. 4b, 7A–C). The shape and size
of these micro-snowballs did not change on transport
to the lab, again suggesting that bloedite is a primary
precipitate in Las Vegas Wash. This is also supported by
the presence of bloedite (and the absence of konyaite) in
the two subsurface horizons of LVW2. Presumably, these
horizons would be more likely to have higher water
vapor ratios than the surface crusts and thus should favor
the formation of konyaite over bloedite. The presence of
other ions in solution (including Cl) might have favored
the direct precipitation of bloedite within this study. More
research is needed to understand the factors controlling
the precipitation of these minerals.
Halite is found in the SEM/EDS analyses of both profiles and in the XRD analyses of LVW2 surface crust.
Halite occurs as anhedral masses and as cubic, and
skeletal (hopper) forms intimately mixed with the sulfate minerals. Although previous studies (Driessen and
Schoorl, 1973; Eswaran and Carrera, 1980) have reported that halite rarely exhibits perfect cubes in soils
because of its tendency to dissolve in the water it attracts,
1649
in this study the perfect cube and skeletal forms of halite are fairly common (LVW2 at 62–110 cm, 27%
occurrence, and the surface crust of LVW1, 21% occurrence). In both horizons, the majority of the cubic forms
are skeletal (hopper) suggesting precipitation from supersaturated conditions.
Unlike many areas of the Great Plains (Keller et al.,
1986; Timpson et al., 1986; Skarie et al., 1987; Arndt and
Richardson, 1989, 1992), calcite is not present in our
study. This contradicts the Hardie and Eugster (1970)
model for mineral precipitation. This model, however,
is for closed-basin brines and is not applicable in this
study because the soils in the Las Vegas Wash are in an
open system in which the soil solution is not always in
contact with all of the solids. In many cases, salt minerals
within individual pore spaces and the surface crusts form
through complete evaporation and are not in contact
with the brine; therefore, the numerous phase diagrams
developed for specific brine chemistries and temperatures are not valid for this complex soil system (Mees and
Stoops, 1991). Several explanations may address why calcite is not present in these profiles, yet dominates the
soils of the region. In the presence of gypsum, calcite
becomes less soluble because of the common ion effect (Reheis, 1987; McFadden et al., 1991; Buck and Van
Hoesen, 2002). Therefore, calcite in these soils may never
become soluble enough to accumulate in the subsurface
of these soils as it does within surrounding soils that do not
contain gypsum. However, Arndt and Richardson (1989)
suggest that the common ion effect is minimized at high
ionic strengths. Additionally, Marion and Schlesinger’s
(1994) model suggests that carbonate solubility can be increased in the presence of gypsum because the sulfate minerals lower the pH. In this study, however, the pH values
remain relatively high. Last, the availability of bicarbonate in the Las Vegas Wash system is unknown and would
also affect calcite precipitation. The absence of calcite in
this study suggests that one or more of these suggested
mechanisms are preventing its formation.
Environmental Interpretations
Changes in groundwater have long been shown to
affect the chemistry and salinity of soils (Abtahi et al.,
1979; Timpson et al., 1986; Skarie et al., 1986, 1987;
Arndt and Richardson, 1989; Eghbal et al., 1989; Last,
1989; Steinwand and Richardson, 1989; Kohut and Dudas,
1993; Salama et al., 1993). In this study, the distribution
with depth of specific minerals, their crystal habits, and dissolution features can be used to infer subsurface changes
in environmental conditions—in particular, the presence
of relict water tables. The high solubilities of these minerals result in their formation at the farthest extent of
water movement—at the surface and in the subsurface
along the capillary fringe of a water table. The solubility of
each mineral will be affected by the ratios of cations and
anions present within the soil water and can vary from
pore to pore. However, all of the soluble minerals found
in this study are at a minimum two orders of magnitude more soluble than gypsum in pure water. Therefore, the two subsurface horizons in LVW2 (at 62–110 and
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
1650
SOIL SCI. SOC. AM. J., VOL. 70, SEPTEMBER–OCTOBER 2006
127–150 cm) that contain soluble salts (halite, hexahydrite, bloedite) could only have been formed through
evaporation along the capillary fringe of previous water
tables (Tables 3 and 5). Thus, each of these subsurface
zones of soluble salts reflects levels of stabilization of
dropping water tables. Because of the absence of datable
material, we are unable to determine if these zones of
subsurface soluble salts reflect seasonal changes in the
water table or if they reflect the regional drop in water
table caused by incision due to flash flooding in the last
several decades. There are two possible reasons why
LVW1 did not show these subsurface zones of soluble
salts: (i) LVW1 was sampled in March (LVW2 sampled in
July), when seasonality of precipitation and lower
evapotranspiration rates combine to create slightly higher
water tables (130 cm in LVW1 versus 147 cm in LVW2).
Because these soluble salts would be quickly dissolved
when water tables rise, the lack of subsurface zones of
soluble salts in LVW1 may be explained by the season in
which it was sampled. However, this cannot completely
explain the higher relict water table found in LVW2 at 62
to 110 cm. (ii) LVW1 is located farther away and at a
slightly higher elevation from the LVW and its Duck
Creek tributary (Fig. 1); water table drops related to the
Duck Creek drainage may not have affected this site.
Therefore, the relict water tables found in LVW2 may be
related to transitory perched water tables related to drainage along Duck Creek, which did not affect the LVW1 site,
or to incision along Duck Creek in response to regional
incision caused by increased urbanization of Las Vegas.
We believe that the regional incision beginning in the
1980s was probably the primary factor resulting in the salt
mineral distribution found in this study because these soils
were mapped as part of the Land series (Typic Aquisalids)
in 1977 but are currently identified as a Typic Haplogypsid
(LVW1) and a Gypsic Haplosalid (LVW2). Lowering the
water table ceases the surficial accumulation of soluble
salts through seasonal capillary flow of ground water to
the surface. Additionally, a lower water table facilitates
temporal conditions of episaturation and removal of soluble salts from the upper portion of the profile. Therefore,
changes in land use (in this case, increased urbanization)
combined with the transitory nature of these soluble salts
can result in significant changes to soil classification and
map units.
CONCLUSIONS
SEM/EDS analyses of two soil profiles in the LVW
found the following: (i) distinctive zones of subsurface
accumulations of soluble salts that we attribute to evaporation from relict water tables. The lowered water tables
are the result of erosional entrenchment of the axial
stream channel caused by increased urbanization. This
disconnection of the water table from the surface has
helped form areas of soluble salt accumulation lower in
the profile which may be important as a proxy for zones
of selenium concentration. (ii) The third occurrence in
the USA of pedogenic eugesterite. It has previously been
found in North Dakota (Skarie et al., 1987) and the
Mojave Desert of California (Buck et al., 2004). (iii) The
snowball morphology (Buck and Van Hoesen, 2002) can
be formed from a myriad of pedogenic sulfate minerals,
not only gypsum, but is not found to form from other
evaporite minerals such as halite. (iv) Soil microorganisms are not necessary to form the snowball morphology
(in contrast to Buck and Van Hoesen, 2002). The highly
transitory nature of these soluble salts suggests that
diurnal through decadal fluctuations in climate and/or
urbanization has the potential to greatly affect the mineralogy, chemistry, and classification of these soils. Understanding these changes is important because these
Na-Mg-Ca-K-SO4-Cl salts affect soil pH, EC, structure,
and other parameters important to the mobility of selenium and other soil pollutants that are present in the Las
Vegas Wash. These salts can affect the distribution and
type of vegetation leading to soil erosion and atmospheric dust contributions to the Las Vegas Valley. This
study is an important first step in understanding the contributions of the Las Vegas Wash to water quality in the
Colorado River System and planning the development
of the Las Vegas Wash Wetlands habitat, including the
development of ponds for migrating birds.
ACKNOWLEDGMENTS
This research was funded by a UNLV SITE grant. We thank
Rob Fairhurst, Evelyn Coleman, Clay Crow, Leigh Justet, and
Sarah Lundberg from the UNLV Electron Microanalysis and
Imaging Laboratory (EMIL) and Mike Howell for assistance
with figures. Thanks to Joe Chiaretti, two anonymous reviewers,
and Joey Shaw for comments that improved this manuscript.
REFERENCES
Abtahi, A., C. Sys, G. Stoops, and H. Eswaran. 1979. Soil forming
processes under the influence of saline and alkaline groundwater in
the Sarvestan basin (Iran). Pedologie 19:325–357.
Amit, R., and A.D. Yaalon. 1996. The micromorphology of gypsum
and halite in reg soils: The Negev desert, Israel. Earth Surf. Processes Landforms 21:1127–1143.
Arndt, J.L., and J.L. Richardson. 1989. Geochemistry of hydric soil
salinity in a recharge-throughflow-discharge prairie-pothole wetland system. Soil Sci. Soc. Am. J. 53:848–855.
Arndt, J.L., and J.L. Richardson. 1992. Carbonate and gypsum
chemistry in saturated, neutral pH soil environments. p. 179–188.
In R.D. Robarts and M.L. Bothwell (ed.) Aquatic ecosystems in
semi-arid regions: Implications for resource management. N.H.R.I.
Symposium, Series 7, Environment Canada, Saskatoon.
Boettinger, J. 1997. Aquisalids (salorthids) and other wet saline and
alkaline soils: Problems identifying aquic conditions and hydric
soils. p. 79–97. In M.J. Vepraskas and S. Sprecher (ed.) Aquic conditions and hydric soils: The problem soils. SSSA Spec. Pub. 50.
SSSA, Madison, WI.
Braitsch, O. 1971. Salt deposits, their origin and composition. SpringerVerlag, New York.
Buck, B.J., A. Brock, W.H. Johnson, and A. Ulery. 2004. Corrosion of
depleted uranium in an arid environment: Soil-geomorphology,
SEM/EDS, XRD, and electron microprobe analyses. Soil and Sediment Contamination. Int. J. 13:545–561.
Buck, B.J., and J. Van Hoesen. 2002. Snowball morphology and SEM
analysis of pedogenic gypsum, southern New Mexico, USA. J. Arid
Environ. 51:469–487.
Buck, B.J., and J. Van Hoesen. 2005. Assessing the applicability of
isotopic analysis of pedogenic gypsum as a Paleoclimate Indicator,
southern New Mexico. J. Arid Environ. 60:99–114.
Burbey, T.J. 1993. Shallow ground water in the Whitney area, southeastern Las Vegas Valley, Clark County, Nevada. Part II. assessment of a proposed strategy to reduce the contribution of salts to
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
BUCK ET AL.: SALT MINERALOGY OF LAS VEGAS WASH, NEVADA
Las Vegas Wash. U.S. Geological Survey, Water-Resources Investigations Report 92–4051, 57 pp.
Carter, B.J., and W.P. Inskeep. 1988. Accumulation of pedogenic gypsum in western Oklahoma soils. Soil Sci. Soc. Am. J. 52:1107–1113.
Chapman, R.W. 1980. Salt weathering by sodium chloride in the Saudi
Arabian desert. Am. J. Sci. 280:116–129.
Dan, J., D.H. Yaalon, R. Moshe, and S. Nissum. 1982. Evolution of reg
soils in southern Israel and Sinai. Geoderma 28:173–202.
Doner, H.E., and W.C. Lynn. 1989. Carbonate, halide, sulfate, and
sulfide minerals. p. 331–378. In J.B. Dixon and S.B. Weed (ed.) Minerals in the soil environment. 2nd ed. SSSA, Madison, WI.
Driessen, P.M., and R. Schoorl. 1973. Mineralogy and morphology of
salt efflorescences on saline soils in the Great Konya basin, Turkey.
J. Soil Sci. 24:436–442.
Drouin, D., A.R. Couture, R. Gauvin, P. Hovington, P. Horny, and
H. Derners. 2001. Monte Carlo simulation of electron trajectories
in solids. Casino Ver. 2.42. Universite de Sherbrooke, Sherbrooke,
Quebec, Canada.
Edinger, S. 1973. The growth of gypsum. J. Cryst. Growth 18:217–224.
Eghbal, M.K., R.J. Southard, and L.D. Whittig. 1989. Dynamics of evaporite distribution in soils on a fan-playa transect in the Carrizo Plain,
California. Soil Sci. Soc. Am. J. 53:898–903.
Eswaran, H., and M. Carrera. 1980. Mineralogical zonation in salt
crust. p. 20–30. In Int. Symposium Salt Affected Soils, Karnal, India.
Eswaran, H., M. Ilaiwi, and A. Osman. 1981. Mineralogy and micromorphology of aridisols. p. 153–174. In Proc. 3rd. Int. Soil Class.
Workshop of the Soil Manag. Support Services, Damascus, Syria.
ACSAD, Damascus.
Eswaran, H., and G. Zi-Tong. 1991. Properties, genesis, classification,
and distribution of soils with gypsum. p. 89–119. In W.D. Nettleton
(ed.) Occurrence, characteristics, and genesis of carbonate, gypsum
and silica accumulations in soils. SSSA Spec. Publ. 26. SSSA,
Madison, Wisconsin.
Gavlak, R.G., D.A. Horneck, and R.O. Miller. 1994. Plant, soil and
water reference methods for the western region. Western Regional
Extension publ. 125.University of Alaska, Fairbanks, AK.
Gile, L.H., F.F. Peterson, and R.B. Grossman. 1966. Morphological
and genetic sequences of carbonate accumulation in desert soils.
Soil Sci. 101:347–360.
Harden, J.W., E.M. Taylor, M.C. Reheis, and L.D. McFadden, L.D.
1991. Calcic, gypsic, and siliceous soil chronosequences in arid and
semiarid environments. p. 1–16. In W.D. Nettleton (ed.) Occurrence, characteristics, and genesis of carbonate, gypsum and silica
accumulations in soils. SSSA Spec. Publ. 26. SSSA, Madison, WI.
Hardie, L.A., and H.P. Eugster. 1970. The evolution of closed basin
brines. Mineral. Soc. Am. Spec. Pub. 3. p. 273–290.
Hawkins, A.B., and G.M. Pinches. 1997. Understanding sulphate generated
heave resulting from pyrite degradation. p. 51–76. In A.B. Hawkins
(ed.) Ground geochemistry: Implications for construction. Proc. Int.
Conf. Implic. Ground Chem. Microbial Construction, Bristol, UK.
Hellmer, W.K. 2001. Geotechnical considerations for soils laden with
sodium sulfate. p. 739–746. In Luke et al. (ed.) Proc. 36th Annu.
Symposium on Eng. Geology and Geotechnical Eng. University of
Nevada, Las Vegas, NV.
Jackson, J.M., and D.T. Patten. 1988. Plant-soil-water relationships in
Las Vegas Wash, Bureau of Reclamation Report: REC-ERC-88–4,
30pp. Bureau of Rec., Denver, CO.
Jafarzadeh, A.A., and C.P. Burnham. 1992. Gypsum crystals in soils.
J. Soil Sci. 43:409–420.
Keller, L.P., G.J. McCarthy, and J.L. Richardson. 1986. Mineralogy and
stability of soil evaporites in North Dakota. Soil Sci. Soc. Am. J.
50:1069–1071.
Kohut, C.K., and M.J. Dudas. 1993. Evaporite mineralogy and traceelement content of salt-affected soils in Alberta. Can. J. Soil Sci.
73:399–409.
Last, W.M. 1989. Continental brines and evaporites of the northern
Great Plains of Canada. Sed. Geol. 64:207–221.
Malmberg, G.T. 1965. Available water supply of the Las Vegas groundwater basin Nevada. USGS Water Sup. Paper 1780. U.S. Dept. of
Interior, Geological Survey, Washington, DC.
Marion, G.M., and W.H. Schlesinger. 1994. Quantitative modeling of
soil forming processes in deserts: The CALDEP and CALGYP
1651
models. p. 129–145. In R.B. Bryant and R. W. Arnold (ed.) Quantitative modeling of soil forming processes. SSSA Spec. Pub. 39.
SSSA, Madison, WI.
McFadden, L.D., R.G. Amundson, and O.A. Chadwick. 1991. Numerical modeling, chemical and isotopic studies of carbonate accumulations in soils of arid regions. p. 17–35. In W.D. Nettleton (ed.)
Occurrence, characteristics and genesis of carbonate, gypsum and
silica accumulations. SSSA Spec. Pub. 26. SSSA, Madison, WI.
Mees, F., and G. Stoops. 1991. Mineralogical study of salt efflorescences
on soils of the Jequetepeque Valley, northern Peru. Geoderma 49:
255–272.
Mermut, A.R., and M.A. Arshad. 1987. Significance of sulfide oxidation in soil salinization of southeastern Saskatchewan, Canada. Soil
Sci. Soc. Am. J. 51:247–251.
Page, W.R., S.C. Lundstrom, A.G. Harris, V.E. Langenheim, J.B.
Workman, S.A. Mahan, J.B. Paces, G.L. Dixion, P.D. Rowley, B.C.
Burchfiel, W.J. Bell, and E.I. Smith. 2003. Geologic and geophysical
maps of the Las Vegas 309 3 609 Quadrangle, Clark and Nye Counties, Nevada, and Inyo County, California. U.S. Dept. of Interior,
Geological Survey, Washington, DC.
Podwojewski, P., and M. Arnold. 1994. The origin of gypsum in
Vertisols in New Caledonia determined by isotopic composition of
sulfur. Geoderma 63:179–195.
Qingtang, Y. 1989. The origin and sedimentary environment analysis
of Glauberite. Acta Sediment. Sinica 7:137–143.
Rech, J.A., J. Quade, and W. Hart. 2003. Isotopic evidence for the
source of Ca and S in soil gypsum, anhydrite and calcite in the
Atacama Desert, Chile. Geochim. Cosmochim. Acta 67:575–586.
Reheis, M.C. 1987. Soil survey laboratory methods manual. Soil Surv.
Invest. Report 42, version 3.0. U.S. Dept. of Interior, Geological
Survey, Washington, DC.
Salama, R.B., P. Farrington, G.A. Bartle, and G.D. Watson. 1993. The
chemical evolution of groundwater in a first-order catchment and
the process of salt accumulation in the soil profile. J. Hydrol. 143:
233–258.
Schoeneberger, P.J., D.A. Wysocki, E.C. Benham, and W.D. Broderson. 1998. Field book for describing and sampling soils. NRCS,
USDA, National Soil Survey Center, Lincoln, NE.
Shahid, S.A. 1988. Studies on the micromorphology of salt-affected
soils in Pakistan. Ph.D. diss. Univ. College of North Wales, Bangor,
United Kingdom.
Shahid, S.A., and D.A. Jenkins. 1994. Mineralogy and micromorphology
of salt-crusts from the Ounjan, Pakistan. Develop. Soil Sci. 22:799–810.
Shayan, A., and C.J. Lancucki. 1984. Konyaite in salt efflorescence
from a Tertiary marine deposit near Geelong, Victoria, Australia.
Soil Sci. Soc. Am. J. 48:939–942.
Skarie, R.L., J.L. Richardson, A. Maianu, and G.K. Clambey. 1986.
Soil and groundwater salinity along drainage ditches in eastern
North Dakota. J. Environ. Qual. 15:335–340.
Skarie, R.L., J.L. Richardson, G.J. McCarthy, and A. Maianu. 1987.
Evaporite mineralogy and groundwater chemistry associated with saline soils in eastern North Dakota. Soil Sci. Soc. Am. J. 51:1372–1377.
Steinwand, A.L., and J.L. Richardson. 1989. Gypsum occurrence in
soils on the margin of semipermanent prairie pothole wetlands. Soil
Sci. Soc. Am. J. 53:836–842.
Taimeh, A.Y. 1992. Formation of gypsic horizons in some arid regions
of Jordan. Soil Sci. 153:486–498.
Thien, S.J. 1979. A flow diagram for teaching texture-by-feel analysis.
J. Agron. Educ. 8:54–55.
Timpson, M.E., J.L. Richardson, L.P. Keller, and G.J. McCarthy. 1986.
Evaporite mineralogy associated with saline seeps in southwestern
North Dakota. Soil Sci. Soc. Am. J. 50:490–493.
Toulkeridis, T., P. Podwojewski, and N. Clauer. 1998. Tracing the
source of gypsum in New Caledonian soils by REE contents and
S-Sr isotopic compositions. Chem. Geol. 145:61–71.
Van Doesburg, J.D.J., L. Vergouwen, and L. Van der Plas. 1982.
Konyaite, Na2Mg(SO4)2 ! 5H2O, a new mineral from the Great
Konya Basin, Turkey. Am. Mineral. 67:1035–1038.
Vergouwen, L. 1981. Eugsterite, a new salt mineral. Am. Mineral. 66:
632–636.
Wiedemann, H.G., and W. Smykatz-Kloss. 1981. Thermal studies on
thenardite. Therm. Acta 50:17–29.
1
Table 1. Mineralogy and Chemical Formulas for Minerals found in this study.
Mineral
2
Chemical Formula
Gypsum
CaSO4•2H2O
Hexahydrite
MgSO4•6H2O
Mirabilite
Na2SO4•10H2O
Thenardite
Na2SO4
Eugsterite
Na4Ca(SO4) 3•2H2O
Bloedite
Na2Mg(SO4)2•4H2O
Halite
NaCl
Kainite†
KMgClSO4 •3H2O
Sepiolite
Hydrous Mg-silicate
Vivianite
Fe3(PO4) 2•8H2O
† tentative identification based only on EDS data.
3
30
1
Table 2. LVW1 Mineralogy Distribution and Dissolution Evidence based on SEM data.
Horizon
Azn
Ay
0-2 cm
2- 22 cm
Percent†
Percent Dissolution Per Horizon†
Bloedite
77%
49%
Halite
13%
Hexahydrite
4%
Eugsterite
3%
Mirabilite
3%
Gypsum
100%
69%
By1
22-61 cm
Gypsum
100%
93%
By2
61-98 cm
Gypsum
100%
94%
Gypsum
100%
100%
2By3 98-140 cm
2
Mineral
†percent determined by frequency of occurrence within the total SEM images analyzed.
3
4
31
1
Table 3. LVW2 Mineralogy Distribution and Dissolution Evidence based on SEM data.
Horizon
Azn
Ayzn
0-1 cm
1-9 cm
Percent†
Percent Dissolution Per Horizon†
Bloedite
74%
21%
Eugsterite
17%
Hexahydrite
3%
Halite
3%
Gypsum
2%
Thenardite
trace
Kainite
trace
Gypsum
100%
0%
Byzn1
9-20 cm
Gypsum
100%
0%
Byzn2
20-62 cm
Gypsum
100%
10%
Halite
52%
48%
Hexahydrite
26%
Bloedite
17%
Gypsum
5%
2Byzn4 110-127 cm
Gypsum
100%
0%
2Byzn5 127-150 cm
Hexahydrite
57%
39%
Bloedite
26%
Halite
9%
Gypsum
8%
2Byzn3 62-110 cm
2
Mineral
†percent determined by frequency of occurrence within the total SEM images analyzed.
3
32
1
Table 4. LVW1 Chemical Data†
Depth
pH
(cm)
CEC
EC
H2O Soluble Ions
(meq/
(mS/cm)
(mmol/L)
100g)
2
Ca
Mg
Na
K
B
S
Cl
Azn
0-2
8.3
12.7
97.5
12.4
655.0
644.2
55.0
1.4
877.3
448.0
Ay
2-22
8.4
13.2
19.8
7.6
9.8
22.1
1.9
0.3
28.6
32.2
By1
22-61
7.9
10.7
11.9
13.7
31.4
63.1
4.3
1.2
56.7
29.1
By2
61-98
7.7
8.8
10.6
12.9
21.6
41.8
3.4
9.8
43.8
16.5
2By3 98-140 7.6
7.3
7.9
14.1
19.1
36.8
2.9
0.1
39.9
15.2
†CEC, cation exchange capacity; EC, electrical conductivity.
33
1
Table 5. LVW2 Chemical Data†
Depth
pH
(cm)
CEC
EC
H2O Soluble Ions
(meq/
(mS/cm)
(mmol/L)
100g)
Ca
Mg
Na
K
B
S
Cl
Azn
0-1
‡
14.1
‡
8.8
1134.3
1135.7
137.0
4.8
1506.6
1168.7
Ayzn
1-9
8.3
11.2
101.8
13.9
799.4
1014.4
183.9
4.4
934.0
1701.6
Byzn1
9-20
8.6
11.4
57.9
17.3
176.3
392.7
62.0
1.1
217.9
475.1
Byzn2
20-62
8.4
14.5
49.6
16.4
62.2
198.3
27.6
0.4
96.3
207.5
2Byzn3
62-110
7.9
3.7
66.5
23.3
281.3
676.0
73.8
0.5
546.4
258.5
2Byzn4
110-127
8.0
14.3
26.7
14.2
47.9
154.4
19.4
0.3
78.0
153.5
2Byzn5
127-150
8.1
10.2
50.1
15.4
180.3
213.0
27.9
0.3
115.7
223.9
2
†CEC, cation exchange capacity; EC, electrical conductivity; ‡ not measured.
3
34
1
Table 6. LVW1 Crystal Habits Determined by SEM data.
Horizon
Azn
0-2 cm
Mineral
Bloedite
48%
Tabular
16%
Foliated
12%
Hollow Triangular
11%
Tabular Pseudo-Hexagonal
11%
Anhedral Massive
2%
Anhedral Massive
79%
Hopper
14%
Cubic
7%
Anhedral Massive
75%
Tabular Pseudo-Hexagonal
25%
Eugsterite
Acicular
100%
Mirabilite
Subhedral Tabular
100%
Gypsum
Tabular Hexagonal
83%
Lenticular
17 %
Tabular Pseudo-Hexagonal
57%
Tabular Hexagonal
36%
Lenticular
7%
Tabular Pseudo-Hexagonal
55%
Tabular Hexagonal
18%
Lenticular
18%
Hexahydrite
By1
By2
2- 22 cm
22-61 cm
61-98 cm
Percent (%)†
Bladed
Halite
Ay
Crystal Habit
Gypsum
Gypsum
35
2By3
1
98-140 cm
Gypsum
Lath
9%
Tabular Pseudo-Hexagonal
65%
Tabular Hexagonal
11%
Lenticular
12%
Andhedral Massive
12%
†percent determined by frequency of occurrence within the total SEM images analyzed.
2
36
1
Table 7. LVW2 Crystal Habits
Horizon
Azn
Ayzn
Byzn1
0-1 cm
1-9 cm
9-20 cm
Mineral
Bloedite
Crystal Habit
Percent (%)†
Tabular Pseudo-Hexagonal
32%
Tabular Rhomboidal
19%
Bladed
15%
Anhedral Massive
13%
Columnar
7%
Hollow Triangle
6%
Foliated
3%
Lath
3%
Tabular Hexagonal
2%
Eugsterite
Acicular
100%
Hexahydrite
Tabular Pseudo-Hexagonal
33%
Anhedral Massive
67%
Halite
Anhedral Massive
100%
Gypsum
Tabular Pseudo-Hexagonal
100%
Thenardite
Anhedral Massive
100%
Kainite
Anhedral Massive
100%
Gypsum
Tabular Pseudo-Hexagonal
57%
Tablular Hexagonal
29%
Lenticular
14%
Tablular Hexagonal
50%
Tabular Pseudo-Hexagonal
25%
Gypsum
37
Byzn2
20-62 cm
2Byzn3 62-110 cm
Lenticular
25%
Lenticular
50%
Lath
38%
Tabular Pseudo-Hexagonal
12%
Anhedral Massive
73%
Hopper
23%
Cubic
4%
Anhedral Massive
91%
Tabular Pseudo-Hexagonal
9%
Tabular Pseudo-Hexagonal
86%
Bladed
14%
Gypsum
Tabular Pseudo-hexagonal
100%
Gypsum
Lenticular
60%
Lath
28%
Tabular Pseudo-Hexagonal
12%
Anhedral Massive
92%
Tabular Pseudo-Hexagonal
8%
Tabular Pseudo-Hexagonal
50%
Bladed
33%
Lenticular
17%
Halite
Andhedral Massive
100%
Gypsum
Tabular Pseudo-Hexagonal
100%
Gypsum
Halite
Hexahydrite
Bloedite
2Byzn4 110-127 cm
2Byzn5 127-150 cm
Hexahydrite
Bloedite
1
*percent determined by frequency of occurrence within the total SEM images analyzed
38