Aeolian Research xxx (2011) xxx–xxx
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Aeolian Research
journal homepage: www.elsevier.com/locate/aeolia
Arsenic concentrations in dust emissions from wind erosion and off-road vehicles
in the Nellis Dunes Recreational Area, Nevada, USA
Deborah Soukup a,⇑, Brenda Buck a, Dirk Goossens a,b, April Ulery c, Brett T. McLaurin d, Dirk Baron e,
Yuanxin Teng a
a
Department of Geoscience, University of Nevada Las Vegas, 4505 Maryland Parkway, Las Vegas, NV 89154-4010, USA
Physical and Regional Geography, Department of Earth and Environmental Sciences, Katholieke Universiteit Leuven, Celestijnenlaan 200E, 3001 Heverlee, Belgium
Department of Plant and Environmental Sciences, New Mexico State University, MSC3Q, P.O. Box 30003, Las Cruces, NM 88003-8003, USA
d
Department of Geography and Geosciences, Bloomsburg University of Pennsylvania, Bloomsburg, PA, USA
e
Department of Physics and Geology, California State University, 9001 Stockdale Highway, Bakersfield, CA 93311, USA
b
c
a r t i c l e
i n f o
Article history:
Received 5 June 2011
Revised 15 November 2011
Accepted 21 November 2011
Available online xxxx
Keywords:
Arsenic
Soil
Dust emission
Wind erosion
Off-road vehicles (ORVs)
a b s t r a c t
Field and laboratory experiments were performed in the Nellis Dunes Recreational Area near Las Vegas,
NV, USA to evaluate arsenic concentrations associated with dust emissions from wind erosion and offroad vehicles. Soil samples were collected from 17 types of desert surfaces and five unpaved parking
lot locations for analyses. The surface units are based on surficial characteristics that affect dust emissions. Arsenic concentrations were also measured in dust emitted from each surface unit using a Portable
In Situ Wind Erosion Laboratory (PI-SWERL). Emissions were measured from ORV trails and undisturbed
terrain. Concentrations of As in the soil and parking lot samples ranged from 3.49 to 83.02 lg g 1 and
from 16.13 to 312 lg g 1 in the PI-SWERL samples. The lower concentrations in the soil samples are
expected because of the larger particle sizes (<2 mm) as compared to the PI-SWERL samples (<10 and
10–60 lm). Soluble As in the PI-SWERL samples was as high as 14.7 lg g 1. In the Nellis Dunes area
the emission rates for As for wind-induced emissions (wind erosion) are highest for the surfaces with
significant amounts of sand. Surfaces rich in silt and clay, on the other hand, produce nearly no arsenic
during wind erosion but can emit substantial arsenic concentrations when driven on by off-road vehicles.
The elevated arsenic emissions from the Nellis Dunes area are of great concern because the site is located
in the immediate vicinity of the city of Las Vegas, and utilized by over 300,000 visitors annually.
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
Exposure to arsenic (As) has been strongly linked to health
problems such as heart disease, hypertension, peripheral vascular
disease, diabetes, immune suppression, acute respiratory infections, intellectual impairment in children, and skin, lung, prostate,
bladder, kidney, and other cancers (Chen et al., 1992; Abernathy
et al., 1999; Järup, 2003; Tseng et al., 2003; Smith et al., 2006;
von Ehrenstein et al., 2007; Kozul et al., 2009a). Additionally,
arsenic has been found to be uniquely harmful to lung tissue by
inhibiting wound repair and altering genes associated with
immune functions in lung tissue (Olsen et al., 2008; Kozul et al.,
2009a,b).
Exposure to arsenic may occur through ingestion of contaminated groundwater, food containing inorganic or organic arsenic,
or by inhalation of airborne dust (Mandal and Suzuki, 2002). The
majority of atmospheric arsenic is highly respirable inorganic
⇑ Corresponding author.
E-mail address: [email protected] (D. Soukup).
arsenic particulate matter smaller than 2.5 lm (ARB, 1990). Combustion of fossil fuels, geothermal steam development, and arsenical pesticide/herbicide use are the largest sources of inorganic
arsenic emissions to the atmosphere. Other sources of inorganic arsenic emissions are mining and quarry operations, cement manufacturing, glass manufacturing, agricultural burning, waste
incineration, and secondary lead smelting (ARB, 1990). Considering
these sources, it is not surprising that most studies on arsenic concentrations in air have been performed on or near polluted sites, or
in cities.
Another source of airborne arsenic is windblown dust. Several
studies have documented the dispersal of arsenic through airborne
dust at the local, regional, or even continental scale. Morman
(2010) described African dusts bringing arsenic to the Caribbean
and southeastern USA, and expressed potential concern regarding
arsenic-laden dust in the southwestern USA. Reynolds et al.
(2008), Reheis et al. (2009), and Breit et al. (2009) described the potential for naturally-derived dusts from playas in the southwestern
USA to contain high arsenic concentrations. Arsenic contamination
from dust is now being recognized in some of the most remote
1875-9637/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.aeolia.2011.11.001
Please cite this article in press as: Soukup, D., et al. Arsenic concentrations in dust emissions from wind erosion and off-road vehicles in the Nellis Dunes
Recreational Area, Nevada, USA. Aeolian Research (2011), doi:10.1016/j.aeolia.2011.11.001
2
D. Soukup et al. / Aeolian Research xxx (2011) xxx–xxx
Fig. 1. Study area location map.
locations in the world: Yeo and Langley-Turnbaugh (2010) found
As concentrations that are above US EPA drinking water guidelines
in snow samples on Mount Everest in the Himalayas.
Barren desert surfaces and dry lake beds have long been identified as important dust sources (Gill and Gillette, 1991; Mulitza
et al., 2010). Natural wind erosion is the dominant mechanism for
dust production in these areas. However, many desert surfaces
are increasingly disturbed by human activity. One type of disturbance that requires special attention is off-road vehicular (ORV)
activity. ORV driving is one of the most prevalent and fastest growing leisure activities on public lands worldwide (Cordell, 2004;
Cordell et al., 2008). In southern Nevada (USA) the number of offroad drivers has quadrupled in the last few years (Spivey, 2008).
Dust emissions created by ORV activities require special attention
because ORV driving is a non-selective process. This means that
components that normally stay fixed in the soil may be released
and inhaled. This is a special concern if ORV-driven substrata contain potentially hazardous chemicals, minerals, or organisms.
The results of several recent investigations have documented
elevated concentrations of arsenic in dust and surface sediments
in southern Nevada and California (Gill et al., 2002; Reynolds
et al., 2008; Rojo et al., 2008; Reheis et al., 2009; Breit et al.,
Please cite this article in press as: Soukup, D., et al. Arsenic concentrations in dust emissions from wind erosion and off-road vehicles in the Nellis Dunes
Recreational Area, Nevada, USA. Aeolian Research (2011), doi:10.1016/j.aeolia.2011.11.001
D. Soukup et al. / Aeolian Research xxx (2011) xxx–xxx
3
Fig. 2. Occurrence of the 17 surface units at the Nellis Dunes Recreation Area.
2009). This study investigates the occurrence of arsenic in the Nellis
Dunes Recreational Area (NDRA), one of the most popular destinations for ORV activity in this region. NDRA is visited by over 300,000
people annually and located only 6 km from the northeastern portion of the conurbation of Las Vegas, north Las Vegas, and Hender-
son, Nevada. Research performed within the NDRA has shown
that large amounts of dust are emitted year round from this site,
and that some of this dust blows into the city (Goossens and
Buck, 2011a). Given the significant increase in ORV activity in
southern Nevada in recent years and the proximity of the NDRA
Please cite this article in press as: Soukup, D., et al. Arsenic concentrations in dust emissions from wind erosion and off-road vehicles in the Nellis Dunes
Recreational Area, Nevada, USA. Aeolian Research (2011), doi:10.1016/j.aeolia.2011.11.001
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D. Soukup et al. / Aeolian Research xxx (2011) xxx–xxx
Table 1
Overview and characteristics of the 17 surface units in the Nellis Dunes area.
Map
unit
Description
Sand and sand-affected areas
1.1
Active dunes without vegetation. Decimeter to several meters thick.
1.2
1.3
1.4
1.5
Active dunes with vegetation. Coppice dunes <50 cm may be
present.
Anthropogenic disturbed sand surfaces. Typically <2–3 cm thick
loose sands overlying petrocalcic horizons or bedrock.
Patchy, shallow (1–3 cm thick), loose sand overlying silty/rocky
subsoil
Very fine sand and coarse silt outcrops. Commonly badlands
Silt/clay areas
2.1
Silt/clay outcrops with biological crust
2.2
Silt/clay outcrops with gravel
2.3
Aggregated silt deposits, commonly badlands, aggregates <5 mm
diameter
2.4
Anthropogenic disturbed silt surfaces
Rock-covered areas
3.1
Well-developed desert pavements with underlying silty Av horizon
Rock fragments
Surface crust
Vegetation
Sparse; may have exposed petrocalcic
horizons
Sparse; <5% rock cover
Absent
Absent
Absent
Isolated shrubs
Common, mixed with 2–3 cm thick
loose sand overlying bedrock
Common, not interlocking, rocks in
subsoil are exposed at surface
Absent
Absent
Absent
Absent
Isolated shrubs
Physical
Mostly absent
Sparse, <3–4% rock cover
Common, <15%, not interlocking
Absent
Biologic
Physical
Physical, patchy
distribution
Absent
Isolated shrubs
Usually absent
Absent
Variable, not interlocking
Absent
3.2
Rock-covered surface with silt/clay
Abundant: tightly interlocking rock
fragments, nearly 100% surface cover
Many: 60–80%, poorly interlocking
3.3
Rock-covered surface with sandy loam
Many: 60–80%, poorly interlocking
3.4
Rock-covered with encrusted sand and biological crusts
Common: 20–30%, poorly interlocking
Physical between rock
fragments
Physical and biological
between rock fragments
Physical and biological
between rock fragments
Biological, continuous
3.5
Bedrock and/or exposed petrocalcic horizons
Continuous rock outcrop
Absent
Rare, isolated
shrubs
Common, shrubs
(10–15%)
Common, shrubs
(10–15%)
Common, shrubs
(10%)
Rare shrubs
Abundant: 90–100%, non-interlocking
gravel clasts
Abundant: 70–80% with sand mixture
Common: 30–60%, poorly interlocking,
with silt mixture
Absent
Absent
Absent
Physical
Absent
Common, shrubs
(10–30%)
Active drainages
4.1
Gravelly drainages, without fine sediment
4.2
4.3
Gravel and sand drainages
Gravel and silt/clay drainages
to Las Vegas and the surrounding urban area, a study was undertaken to characterize the chemical constituents in dust emissions
from the NDRA.
2. The study area
The NDRA is located approximately 6 km northeast of the
northeastern boundary of Las Vegas, Nevada, USA (Fig. 1). This
37 km2 area is managed by the Bureau of Land Management
(BLM) and is the only area in southern Nevada that is legally accessible to the public for off-road vehicular use. According to the BLM,
over 300,000 people visit the area annually to drive their ORVs in
the dunes, washes, desert pavements, and rock-covered hills that
are characteristic of this part of the Mojave Desert (Goossens and
Buck, 2009a).
The Las Vegas Valley is located within the Great Basin region of
the Basin and Range physiographic province. It is an intermountain
valley, surrounded by generally N–S trending mountain ranges between 450 and 2100 m above the valley floor in the N and E, and up
to 3000 m above the valley floor in the west. The NDRA is located
on the eastern side of the valley, between the Las Vegas and Dry
Lake Ranges (to the N) and the Sunrise and Frenchman Mountains
(to the S). It is primarily composed of incised fan remnants and
exposed Neogene and Quaternary sediments, except for the mountains in the northeast, which are Paleozoic and composed predominantly of limestone (Goossens and Buck, 2009b). The Neogene
deposits within the field area are believed to be the Muddy Creek
Formation (10 to 5 Ma). They consist of a 2–50 m thick limestone
overlying, and partially interbedded with, a marl succession up to
10 m thick. The marl locally contains limestone rock fragments and
thin layers of gypsite (Castor and Faulds, 2001). The limestone and
marl is underlain by a fine-grained sandy interval. Neogene to
Quaternary fan remnants and inset fans occur throughout the field
area. These alluvial gravels are capped by extensive petrocalcic
horizons and overlie the Muddy Creek Formation. Extensive incision, particularly in the northern portion of the field area, has exposed the fine-grained Muddy Creek Formation. The middle of
the southern portion of the field area is occupied by an extensive
zone of dune sands, which cover the Neogene deposits. Although
much of the sand is generally less than a meter thick, many highly
active reversing dunes (oriented NW–SE) are present. These dunes
may be up to 250 m long and are among the most popular off-road
driving zones within the area (Goossens and Buck, 2009b).
Soil development is negligible in the active sand dunes and
areas where bedrock is exposed, including the badlands of the exposed Muddy Creek Formation. Surficial characteristics in these regions are controlled by the underlying bedrock geology or dune
sand characteristics. In the remaining areas, primarily the fan remnants, the soils are characterized by thin (0–10 cm), platy, alkaline,
Av (vesicular) horizons containing low amounts of organic matter
overlying calcic and/or petrocalcic horizons. Vesicular A (Av) horizons are generally associated with desert pavements. In many
areas, particularly in the western portion of the field area, the surface horizons are eroded and petrocalcic horizons are exposed at
the surface. Most of the surface gravels are composed of broken
fragments of the petrocalcic horizons in these areas. Soils in the
study area are classified as Typic Haplocalcids, Calcic Petrocalcids,
and Typic Torriorthents (Goossens and Buck, 2009b).
The climate within the study area is arid, and summers are long,
hot, and dry with average daily maximum temperatures over 40 °C.
In contrast, winters are mild, with an average daily maximum temperature in January around 13.5 °C. The average annual temperature is 19.5 °C (Lazaro et al., 2004). Mean annual precipitation is
Please cite this article in press as: Soukup, D., et al. Arsenic concentrations in dust emissions from wind erosion and off-road vehicles in the Nellis Dunes
Recreational Area, Nevada, USA. Aeolian Research (2011), doi:10.1016/j.aeolia.2011.11.001
5
D. Soukup et al. / Aeolian Research xxx (2011) xxx–xxx
105 mm, but may vary substantially. Monthly average precipitation ranges from 2 mm in June to 14 mm in February. Scattered
thunderstorms typically occur during July and August.
The average annual wind speed in the NDRA is about 3.3 m s 1
and gusts can be up to 25 m s 1 (Goossens and Buck, 2011a).
Winds blow primarily from the northeast from November through
March and from the south from April through September. During
strong winds, blowing sand and dust are common in the NDRA
although there is considerable variation over the area. The dunes
and loose silty deposits in the west are much more active than
the stabilized silt, gravel, and bedrock substrata in the east
(Goossens and Buck, 2009b).
Previous studies of dust emissions in the NDRA (Goossens and
Buck, 2009a,b; McLaurin et al., 2011) identified and mapped four
major surface classes within the study area comprising a total of
17 types of surfaces (Fig. 2). A description of each unit is provided
in Table 1. The major surface classes include:
1. Sands and sand-affected areas: active or stabilized sands, with
or without rock fragments and/or vegetation.
2. Silt/clay areas: loose and slightly stabilized silt/clay deposits,
with or without sparse rock fragments exposed at the surface.
3. Rock-covered areas: stabilized silty or sandy silty deposits with
numerous rock fragments on top, desert pavements over a silty
sublayer, bedrock, and petrocalcic horizons.
4. Active drainages: active drainages in sand and silt areas, and
gravelly drainages.
3.2. Laboratory procedures
3.2.1. Total elemental analyses
The soil samples collected from each of the surface units were air
dried and sieved to remove coarse fragments (>2 mm). The <2 mm
fraction was then digested in accordance with EPA Method 3052
(USEPA, 1996). The digested samples were initially scanned for 66
different elements using inductively coupled plasma mass spectroscopy (ICP-MS). The purpose of the initial semi-quantitative scan
was to identify elements that might be present at concentrations
exceeding naturally occurring background concentrations reported
in soils in the area. Based on the results of the semi-quantitative
scan, 17 elements were identified as potentially exceeding reported
background concentrations and were re-analyzed quantitatively for
these elements using ICP-MS. This manuscript focuses exclusively
on the As concentrations. The remaining elements will be addressed in a separate manuscript.
To ensure quality control for the ICP-MS analyses, samples of
Buffalo River Sediment Reference Material 8704 were digested in
accordance with EPA Method 3052 and analyzed along with the
NDRA samples. Satisfactory recoveries (±10% of known values)
were found for the trace elements analyzed.
Analyses for the 17 elements were also made on dust samples
collected using the PI-SWERL. The <10 lm (PM10) and 10–60 lm
size fractions were separated by sedimentation (Jackson, 1985)
Table 2
Total arsenic concentrations in surface unit soil samples (<2 mm).a
3. Methods and procedure
Surface unit description
3.1. Field procedure
During previous studies at NDRA, field measurements were performed at 68 locations (four for each surface unit) within the area
to measure natural dust emission by wind erosion (Goossens and
Buck, 2011b). Soil samples were also collected from the upper 2–
3 cm of all surface units (minimum of four samples per surface
unit) and from five unpaved areas used for parking.
Additional dust samples were collected for the current study
from 16 of the 17 surface types and from the five parking areas using
a Portable In Situ Wind Erosion Laboratory (PI-SWERL). This instrument creates an increased wind shear near the ground producing
wind erosion under controlled conditions, and allows collection of
the emitted particles. It was developed by the Division of Atmospheric Sciences, Desert Research Institute, Las Vegas, Nevada; detailed descriptions are provided in Etyemezian et al. (2007) and
Sweeney et al. (2008). For this study, the mini-SWERL version,
which samples 0.26 m2 of the soil surface, was utilized. Six to 20
samplings were performed for each surface type until a minimum
of 10 g of airborne dust was collected. The rotational speed of the
internal ring was set at 6000 round min 1 which corresponds to a
shear velocity of 0.91 m s 1. PI-SWERL dust samples were collected
both on ORV trails and on undisturbed terrain. No samples were collected from areas of outcropping bedrock or outcropping petrocalcic
horizons, which contain negligible emittable dust.
Field experiments were also performed in the NDRA during a
previous study to measure anthropogenic dust emissions caused
by off-road vehicular activity (Goossens and Buck, 2009b). Dust
emissions from off-road vehicular use were measured using three
different types of ORVs including a four-wheeler (quad), dune
buggy, and dirt bike (motorcycle). These vehicles account for over
99% of all ORV activity in the NDRA. The purpose of these experiments was to compare dust emission from ORVs with natural wind
erosion. In the current study, the dust emission data was combined
with soil chemical data to determine the emission rates for As for
each of the ORV types.
As
(lg g
1
)
b
USEPA Regional Screening Levels
Residential soil
Groundwater protection
0.39
0.0013
Sand and sand-affected areas
1.1: Dunes with no vegetation
1.2: Dunes with vegetation
1.3: Disturbed sand surfaces
1.4: Patchy layers of sand over silty/rocky subsoil
1.5: Outcrops of very fine sand and coarse silt
4.37
3.49
6.74
4.92
46.06
Silt/clay areas
2.1: Silt/clay with crust
2.2: Silt/clay with gravel
2.3: Aggregated silt deposits
2.4: Disturbed silt surfaces
19.71
83.02
11.01
11.79
Rock-covered areas
3.1: Desert pavements
3.2: Rock-covered surfaces with silt/clay zones
3.3: Rock-covered surfaces with sandy loam
3.4: Rock-covered surfaces with encrusted sand
3.5: Bedrock and/or petrocalcic horizons
13.56
7.89
6.85
7.28
9.03
Drainage areas
4.1: Gravelly drainages
4.2: Gravel and sand drainages
4.3: Gravel and silt/clay drainages
32.36
23.39
31.45
Parking lot areas
North Parking Lot #1
South Parking Lot #1
South Parking Lot #2
Southeast Parking Lot #1
5.98
4.88
6.86
17.62
Standard samples
Buffalo River Standardc
Buffalo River Reference Material 8704 reported As
concentration
14.64
17
a
Soil samples were acid digested prior to analysis using EPA Method 3052.
The screening levels (SLs) are developed using risk assessment guidance from
the EPA Superfund program and are used for site ‘‘screening’’ and as initial cleanup
goals, if applicable. The groundwater protection concentrations shown are soil
concentrations considered to be protective of groundwater resources.
c
Buffalo River Reference Material 8704 sample that was digested in accordance
with EPA Method 3052 and analyzed with the NDRA samples.
b
Please cite this article in press as: Soukup, D., et al. Arsenic concentrations in dust emissions from wind erosion and off-road vehicles in the Nellis Dunes
Recreational Area, Nevada, USA. Aeolian Research (2011), doi:10.1016/j.aeolia.2011.11.001
6
D. Soukup et al. / Aeolian Research xxx (2011) xxx–xxx
Table 3
Non-water-soluble arsenic concentrations (EPA method 3052) in PI-SWERL samples.
Table 3 (continued)
Surface unit description
Surface unit description
Particle size
(lm)
USEPA Regional Screening Levelsa
Residential soil
Groundwater protection
As
(lg g
1
)
Drainage areas
4.1R: Gravelly drainages
0.39
0.0013
Particle size
(lm)
As
(lg g
1
<10
10–60
64.33
70.59
4.1NR
<10
10–60
43.31
30.40
<10
10–60
78.14
70.24
4.2R: Gravel and sand drainages
<10
10–60
46.56
28.37
<10
10–60
54.16
45.35
4.2NR
1.2R: Dunes with vegetation
<10
10–60
52.45
42.14
<10
10–60
44.15
41.27
4.3R: Gravel and silt/clay drainages
1.2NR
<10
10–60
48.78
36.55
<10
10–60
<10
10–60
65.13
66.00
94.09
72.20
1.3R: Disturbed sand surfaces
<10
10–60
54.14
46.34
1.3NR
<10
10–60
37.96
30.37
<10
10–60
28.09
20.03
North Parking Lot #2
1.4R: Patchy layers of sand over silty/rocky
subsoil
<10
10–60
26.32
20.46
<10
10–60
27.76
23.99
South Parking Lot #1
1.4NR
<10
10–60
27.21
19.76
<10
10–60
34.34
19.25
South Parking Lot #2
1.5R: Outcrops of very fine sand and coarse silt
<10
10–60
279.03
248.31
<10
10–60
23.56
17.10
Southeast Parking Lot #1
1.5NR
<10
10–60
290.01
312.42
<10
10–60
45.24
39.89
Sand and sand-affected areas
1.1Rb: Dunes with no vegetation
1.1: NRc
Silt/clay areas
2.1R: Silt/clay with crust
<10
10–60
87.95
79.68
2.1NR
<10
10–60
83.03
79.30
2.2R: Silt/clay with gravel
<10
10–60
145.39
130.61
2.2NR
<10
10–60
161.32
138.50
2.3R: Aggregated silt deposits
<10
10–60
18.56
24.87
2.3NR
<10
10–60
27.44
33.46
2.4R: Disturbed silt surfaces
<10
10–60
25.10
24.40
2.4NR
<10
10–60
23.54
26.02
<10
10–60
28.11
26.46
<10
10–60
24.86
22.34
<10
10–60
27.88
21.85
3.2NR
<10
10–60
18.85
16.13
3.3R: Rock-covered surfaces with sandy loam
<10
10–60
32.93
25.84
3.3NR
<10
10–60
30.98
70.64
3.4R: Rock-covered surfaces with encrusted
sand
<10
10–60
44.03
49.54
3.4NR
<10
10–60
41.74
41.43
d
Rock-covered areas
3.1R: Desert pavements
3.1NR
Rock-covered areas
3.2R: Rock-covered surfaces with silt/clay
zones
4.3NR
Parking lot areas
North Parking Lot #1
Standard samples
BRS1e
BRS2
BRS3
BRS4
BRS5
21.08
19.16
20.33
18.46
18.64
Buffalo River Reference Material 8704 reported As concentration
17
)
a
The screening levels (SLs) are developed using risk assessment guidance from
the EPA Superfund program and are used for site ‘‘screening’’ and as initial cleanup
goals, if applicable. The groundwater protection concentrations shown are soil
concentrations considered to be protective of groundwater.
b
R samples collected within ORV trails
c
NR samples collected in undisturbed areas.
d
Surface unit 3.5 (bedrock) was not sampled.
e
Buffalo River Reference Material 8704 samples that were digested in accordance with EPA Method 3052 and analyzed with the NDRA samples.
and wet sieving and acid-digested in accordance with EPA Method 3052 prior to analysis. The 60 lm limit was used as a cut-off
for total suspendable particles (TSP) because it represents the
maximum size of grains that will still be transported in shortterm suspension during average wind speed and turbulence
(Pye and Tsoar, 1990). It also nearly coincides with the maximum
diameter of silt (52 or 63 lm, depending on which criterion is
used; Goossens and Buck, 2009b). Additionally, 1:10 soil:water
extracts were prepared to determine the water soluble constituents in the <60 lm fraction of the PI-SWERL dust samples. These
samples were allowed to sit overnight and were then filtered to
obtain the supernatant. The supernatant solution was also analyzed by ICP-MS using similar instrument settings and quality
control measures.
3.2.2. X-ray diffraction (XRD) analyses
X-ray diffraction (XRD) analyses were made on all soil samples
to determine the mineralogical composition. Two size fractions
were investigated, <2 lm, and 2–20 lm, because these grain sizes
may have implications for arsenic occurrence and mobility. These
Please cite this article in press as: Soukup, D., et al. Arsenic concentrations in dust emissions from wind erosion and off-road vehicles in the Nellis Dunes
Recreational Area, Nevada, USA. Aeolian Research (2011), doi:10.1016/j.aeolia.2011.11.001
D. Soukup et al. / Aeolian Research xxx (2011) xxx–xxx
Table 4
pH, electrical conductivity, and soluble arsenic concentrations in PI-SWERL 1:10
soil:water extracts (<60 lm fraction).
Surface unit description
Sand and sand-affected areas
1.1Ra: Dunes with no vegetation
1.1NRb
1.2R: Dunes with vegetation
1.2NR
1.3R: Disturbed sand surfaces
1.3NR
1.4R: Patchy layers of sand over silty/
rocky subsoil
1.4NR
1.5R: Outcrops of very fine sand and
coarse silt
1.5NR
Silt/clay areas
2.1R: Silt/clay with crust
2.1NR
2.2R: Silt/clay with gravel
2.2NR
2.3R: Aggregated silt deposits
2.3NR
2.4R: Disturbed silt surfaces
2.4-NR
Rock-covered areasc
3.1R: Desert pavements
3.1NR
3.2R: Rock-covered surfaces with
silt/clay zones
3.2NR
3.3R: Rock-covered surfaces with
sandy loam
3.3NR
3.4R: Rock-covered surfaces with
encrusted sand
3.4NR
a
b
c
pH
Electrical
conductivity
(dS m 1)
As
(lg g
8.23
7.81
7.83
8.18
8.47
8.15
7.44
2.43
0.90
0.76
0.18
0.62
0.28
1.85
1.78
0.55
4.36
2.23
6.82
0.61
6.05
7.43
7.86
1.42
2.87
2.80
8.04
7.55
2.31
4.13
7.50
8.05
7.86
7.84
7.74
7.83
7.63
7.76
1.70
0.35
2.15
2.11
1.78
2.19
2.08
1.46
8.28
5.88
9.24
10.59
5.02
1.49
9.58
2.17
7.91
8.29
8.06
0.72
0.38
0.42
0.81
1.04
0.87
8.60
8.21
0.30
0.30
0.65
1.91
8.11
8.13
0.32
0.07
1.40
2.20
6.58
0.06
0.42
Drainage Areas
4.1R: Gravelly drainages
4.1NR
4.2R: Gravel and sand drainages
4.2NR
4.3R: Gravel and silt/clay drainages
4.3NR
7.87
8.39
7.71
7.87
8.07
8.56
2.26
0.48
1.74
1.26
0.40
0.12
3.11
3.47
2.17
14.71
3.93
9.15
Parking lot areas
North Parking Lot #1
North Parking Lot #2
South Parking Lot #1
South Parking Lot #2
Southeast Parking Lot #1
8.03
7.93
9.11
8.31
8.28
0.85
1.03
1.96
0.50
0.44
1.38
1.04
7.78
1.32
5.57
1
)
R samples collected within ORV trails.
NR samples collected in undisturbed areas.
Surface unit 3.5 (bedrock) was not sampled.
fractions were separated by centrifugation and sedimentation following rinsing with distilled water. The distilled water rinses were
necessary to remove soluble salts from the soils in order to disperse the samples prior to fractionation.
Pastes of K- and Mg-saturated clay (<2 lm) and silt (2–20 lm)
were smeared on glass slides (Theisen and Harward, 1962). The
K-saturated sample slides were examined by XRD at 25 °C and
after heating at 350 and 550 °C for 2 h. The Mg-saturated samples
were also analyzed at 25 °C and after being placed in a desiccator
containing a pool of ethylene glycol and heated at 65 °C for 2 h.
The desiccator vent was closed upon removal from the oven
and the slides stored in the desiccator at least 12 h prior to
XRD analysis. All samples were examined by XRD (Cu Ka radiation) using a PANalytical X’PERT Pro diffractometer, equipped
with an X’Celerator detector. Additional descriptions of these
7
methods can be found in Reid-Soukup and Ulery (2002) and
Soukup et al. (2008).
4. Results
The concentrations of As in the surface unit and parking lot soil
samples ranged from 3.49 to 83.02 lg g 1 or parts per million
(ppm; Table 2). The highest concentrations of As in the soil samples
occurred within the silt/clay areas (surface units 2.1–2.4), the
drainages (surface units 4.1–4.3), and surface unit 1.5 (very fine
sand and coarse silt outcrop). The As concentration in the Southeast Parking Lot #1 sample (17.62 lg g 1) was also elevated compared to the other soil samples collected from parking areas.
The As concentrations in the PM10 and 10–60 lm fractions in the
PI-SWERL dust samples ranged from 18.56 to 290.01 and from 16.13
to 312.4 lg g 1, respectively (Table 3). The highest As concentrations in both size fractions of dust were in the samples collected
from undisturbed terrain of the very fine sand and coarse silt outcrop
(surface unit 1.5). Elevated As concentrations (41.13–161.32 lg g 1;
Table 3) also occurred in the ORV trails and undisturbed terrain samples from silty/clay areas and active drainages (surface units 2.1, 2.2,
4.1, 4.2, and 4.3). The lowest concentrations occurred in the undisturbed terrain samples of rock-covered surfaces with silt/clay (surface unit 3.2).
Water-extractable or soluble As concentrations in the 0–60 lm
fractions of the PI-SWERL dust samples ranged from 0.42 to
14.71 lg g 1 (Table 4). Highest soluble As concentrations were
found in the undisturbed gravel and sand drainages (unit 4.2).
Other areas with increased soluble As include the undisturbed
gravel and silty drainages (unit 4.3), many of the silt/clay units
(2.1, 2.2, 2.4), some of the sandy units (1.2, 1.4, 1.5), and some of
the unpaved parking areas (Table 4). The pH values of the soluble
PI-SWERL extracts were near-neutral to alkaline, ranging from
6.58 to 9.11 (Table 4). Electrical conductivity (EC) values of the extracts ranged from 0.06 to 2.43 dS m 1 and document the salinity
of most of the soils in the NDRA, particularly considering the dilution factor of 10. Historically, a soil was classified as saline if the EC
of the saturation extract exceeded 4 dS m 1 (United States Salinity
Laboratory Staff, 1954). More recently, saline soils have been defined to indicate the types of crops capable of tolerating various
levels of soil salinity. Saline soils are defined as EC > 1.5 dS m 1
for sensitive crops; EC > 3.0 dS m 1 for moderately sensitive crops;
EC > 6 dS m 1 for moderately tolerant crops; and EC > 10 dS m 1
for tolerant crops (Essington, 2004).
The mineralogical composition of the clay (<2 lm) and silt
(2–20 lm) fractions of the soil samples at NDRA is dominated by
smectite with lesser amounts of palygorskite, mica/illite, kaolinite,
quartz, and calcite. Gypsum was also identified in several samples,
although it should be noted that most of the gypsum present
would have been removed during the distilled water rinses prior
to fractionation.
5. Discussion
5.1. Occurrence of arsenic at NDRA
Geographically, the highest arsenic concentrations are found in
the northern portion of the NDRA in specific exposures of the Muddy
Creek Formation: units 1.5 (fine sand and coarse silt), and 2.2 (silt/
clay with gravel). Arsenic concentrations are also high in the drainages (4.1–4.3). The very high arsenic contents in the units 1.5 and 2.2
(up to >80 ppm in the topsoil, and >300 ppm in the PI-SWERL
samples) suggests that the occurrence of arsenic in the NDRA is largely controlled by one or more geologic processes. Arsenic enrichment is frequently caused by hydrothermal alteration (Boyle and
Please cite this article in press as: Soukup, D., et al. Arsenic concentrations in dust emissions from wind erosion and off-road vehicles in the Nellis Dunes
Recreational Area, Nevada, USA. Aeolian Research (2011), doi:10.1016/j.aeolia.2011.11.001
8
D. Soukup et al. / Aeolian Research xxx (2011) xxx–xxx
Fig. 3. Arsenic concentrations in the Nellis Dunes Recreation Area. (a) Soil (0–2 mm); (b) emitted by PI-SWERL, 0–60 lm; (c) emitted by PI-SWERL, 0–10 lm (PM10);
(d) emitted by PI-SWERL, 10–60 lm.
Jonasson, 1973), or can also occur via weathering and erosion or
depositional processes (Henke, 2009). Research is on-going to better
understand the geological processes that have concentrated arsenic
in the northern NDRA.
Soluble arsenic concentrations are highest in the drainages
(units 4.2, 4.3), and are also elevated in some of the silty units
(2.1, 2.2, 2.4) and some of the sandy units (unit 1.2, 1.4, 1.5). The
soluble arsenic at NDRA is likely explained by the overall low
potential for leaching because of fine-grained texture (units
2.1–2.4), or well-developed vesicular horizons (Av) associated with
desert pavements (units 3.1–3.4) (Young et al., 2004). Additionally,
drainages in this area are commonly cemented with calcium carbonate at shallow depths, inhibiting permeability (units 4.1–4.3).
These features, combined with the arid regional climate allows
for highly soluble arsenic minerals to be retained near the surface
and subsequently remobilized later via dust deposition or during
brief periods of runoff followed by evaporation.
Arsenic content at NDRA is greatest in the finer-grained
fractions. Some of the PI-SWERL dust samples (0–10 lg; PM10
and 10–60 lm fractions) were as much as one order of magnitude
higher in arsenic than the soil samples (0–2 mm fraction) (Tables 2
and 3). For most of the PI-SWERL samples the As concentrations
were higher in the 0–10 lm fraction than in the 10–60 lm fraction. Many other studies have shown that As is often preferentially
concentrated in finer size fractions. For example, Chen et al. (1999)
reported that clay content and cation exchange capacity (CEC)
were highly correlated with As concentrations in Florida surface
soils. Van Pelt and Zobeck (2007) quantified the chemical constituents of fugitive dust in the Southern High Plains of Texas and reported that the finer particles in the source soils contained higher
concentrations of As, although at significantly lower concentrations than in NDRA, ranging from 1.13 to 3.894 lg g 1.
The increased As concentrations in the finer-grain fractions suggests adsorption of As on clay complexes, hydrous metal oxides (Al,
Mn, and Fe), carbonates, and/or concentration of As within clay or
other fine grained mineral species. XRD analyses revealed that the
mineralogical composition of the clay (<2 lm) and silt (2–20 lm)
fractions of the soil samples at NDRA is dominated by smectite
with lesser amounts of palygorskite, mica/illite, kaolinite, quartz,
calcite, and gypsum. Previous studies have reported sorption of
As onto the surface and isomorphic substitution within the
structures of both calcite (Roman-Ross et al., 2003; Di Benedetto
et al., 2006) and gypsum (Roman-Ross et al., 2003; FernándezMartínez et al., 2008). Another mechanism for concentrating As
in the fine-grained fractions of soils is As-rich clay minerals. Pascua
et al. (2005) described the occurrence of an As-rich smectite
(1500–4000 ppm) in a geothermal field in Japan. These investigators found minimal adsorption of As on smectite surfaces; instead,
the As was predominantly dissolved within the smectite or occurred within mineral occlusions. As stated previously, additional
studies are currently underway to determine the geological processes that lead to the concentration of As in the NDRA sediments.
Please cite this article in press as: Soukup, D., et al. Arsenic concentrations in dust emissions from wind erosion and off-road vehicles in the Nellis Dunes
Recreational Area, Nevada, USA. Aeolian Research (2011), doi:10.1016/j.aeolia.2011.11.001
D. Soukup et al. / Aeolian Research xxx (2011) xxx–xxx
Fig. 4. Total As concentrations (mg kg
9
1
) in soil samples from the United States (source: USDA-NRCS, 2010).
The differences in arsenic concentration with grain size also result in different areal patterns of arsenic distribution at NDRA. The
As concentrations in soil samples at NDRA are shown in Fig. 3a.
This map was constructed by combining the soil As concentrations
with the surface unit map of NDRA presented in McLaurin et al.
(2011). The As map is based on 51 data points, and the data is considered representative because the size of the arsenic concentration classes is very large compared to the variation in As
concentrations within each surface unit. Evaluation of the NDRA
data shows that spatial variation of the As concentration within a
unit falls within the width of the classes shown on the map (i.e.,
they do not change the patterns shown on the maps). Comparison
of the soil map (0–2 mm, Fig. 3a) with the maps for airborne dust
(0–60 lm, Fig. 3b; 0–10 lm, Fig. 3c; 10–60 lm, Fig. 3d) shows
some differences in patterns. Arsenic concentrations in the airborne fractions are highest in the sandy areas located in the northwest, southwest, and just north of the central sand dunes, and also,
though somewhat less, in the sand dunes themselves (Fig. 3b). A
comparison between the finest (0–10 lm; PM10) and the somewhat coarser dust fraction (10–60 lm) also shows differences,
with the sand dunes being the most emissive areas for As in the
PM10 fraction and the non-dune sand surfaces the most emissive
areas for As in the 10–60 lm fraction (Fig. 3c and d). The differences between the distribution of As in the 0–10 lm and that in
the 10–60 lm fraction is significant, because PM10 is transported
in long-term suspension whereas much of the coarser dust is transported in short-term suspension (Pye and Tsoar, 1990). Therefore,
the PM10 fraction travels farther and may potentially affect populations at greater distances from the source area compared to the
10–60 lm fraction.
In NDRA there is no clear relationship between As concentrations and location of disturbed versus undisturbed surfaces. The
As concentrations in associated disturbed and undisturbed samples
are the same order of magnitude for both the PM10 and 10–60 lm
fractions. For the PM10 samples, As concentrations in 9 of the 16
ORV trail samples were 1.56–16.18 lg g 1 higher than those measured in the associated undisturbed terrain samples, and approximately equal in one sample (Table 3). In the other six PM10
samples, the As concentrations in the undisturbed terrain samples
were 3.25–28.96 lg g 1 higher than those detected in the associated ORV trail samples. Arsenic concentrations in the 10–60 lm
fraction were 2.03–8.11 lg g 1 higher in seven of the 16 ORV trail
samples and approximately equal in three samples as compared
to the associated undisturbed terrain samples (Table 3). The As concentrations in the other six ORV trail samples were from 1.62 to
64.11 lg g 1 lower than those reported in the corresponding undisturbed terrain samples.
5.2. Regional and national distribution of As in soils
The concentrations of As in some of the soil samples at NDRA
are substantially higher (3.49–83.02 ppm) than in soils elsewhere
in the United States, where the average ranges from 3.6 to
8.8 ppm; and throughout the world where averages range from
2.2 to 25 ppm (McBride, 1994). In a 1975 study of 21 soil samples
collected in the western United States, As concentrations ranged
from non-detectable to 97 ppm with an average concentration of
6.1 ppm (Connor and Shacklette, 1975). In another study, As analyses were performed on 50 soils collected throughout California.
Arsenic concentrations in these soils ranged from 0.6 to 11 ppm,
Please cite this article in press as: Soukup, D., et al. Arsenic concentrations in dust emissions from wind erosion and off-road vehicles in the Nellis Dunes
Recreational Area, Nevada, USA. Aeolian Research (2011), doi:10.1016/j.aeolia.2011.11.001
10
D. Soukup et al. / Aeolian Research xxx (2011) xxx–xxx
Fig. 5. Potential emission rate for arsenic during wind erosion. Data are for the PM10 (0–10 lm) fraction.
with an average concentration of 3.5 ppm (Bradford et al., 1996).
Reheis et al., 2009 reported median As concentrations of 10 ppm
in surface soil samples in southern Nevada and California. However, the As concentration in five surface soil samples in that study
ranged from approximately 30 to 50 ppm.
Arsenic concentration data for the entire United States is also
available from a soil inventory prepared by the United States
Department of Agriculture Natural Resources Conservation Service
(USDA-NRCS, 2010). This database includes information on As concentrations in more than 2800 soil samples collected at over 450
different locations in the United States. Using this data, we constructed a figure showing the reported soil As concentrations
(Fig. 4). The legend of this map is the same as that shown in Fig. 3
to allow for comparison between the two. Arsenic concentrations
in the USDA-NRCS database are nearly always less than 20 ppm,
and rarely above 30 ppm. Comparing this data shows that As concentrations for most surface units at NDRA are comparable with
those measured elsewhere in the United States. The exceptions
are the drainage units (4.1–4.3) and units 1.5 and 2.2, which have
anomalously high As concentrations (Table 2).
The As concentrations in soils for NDRA units 1.5 and 2.2 are
among the highest documented in the United States to date. Breit
et al. (2009) reported As concentrations in the water soluble soil
fraction at Franklin Lake Playa (approximately 100 km southwest
of NDRA) over 400 ppm, but these values were measured at a
depth of more than 50 cm below the playa surface. Arsenic concentrations were much lower closer to the playa surface, <100 ppm at
a depth of 20 cm and <50 ppm in the uppermost 10 cm. Reynolds
et al. (2008) and Goldstein et al. (2007) reported water-soluble
salts on the ground surface in Ash Meadows and Carson Slough,
immediately north of Franklin Playa, had As concentrations as high
as 600 ppm. The Reynolds et al. (2008) and Goldstein et al. (2007)
studies are the only studies performed on non-mining sites in the
western United States that we are aware of with reported As concentrations in soil higher than those of NDRA unit 2.2.
5.3. Arsenic in airborne dust
Few studies have analyzed As in airborne dust from natural surfaces, and none have reported values as high as found in this study.
Reheis et al. (2002) studied the contributions of different local
sources to dust in the southwestern United States by comparing elemental analyses of samples collected from dust traps to analyses of
samples from potential source sediments, such as alluvial and playa
deposits. The average concentration of As in the <50 lm fraction of
dust samples in their study ranged from 5 to 25 ppm. The results of
the Reheis et al., 2002 study also showed that all dust samples were
enriched in As relative to source samples, and that dusts in the
Owens Valley (California) have higher concentrations of As than
dust samples from other areas. The highest concentrations of As occurred in Owens Valley alluvium and lake-marginal deposits away
from the dry bed of Owens Lake. The average concentration of As
in the <50 lm fraction from the Owens Valley lake bed samples
was reported to be 40 ppm, and 45 ppm in dust from elsewhere in
Owens Valley (Reheis et al., 2002). More recently, Reheis et al.
(2009) and Rojo et al. (2008) conducted a compositional study of
modern dust and surface sediments in southern Nevada and California. These investigators reported median As concentrations of
20 ppm in airborne dust (collected at a height of 2 m above the surface) and 10 ppm in surface soil samples. One outlier airborne dust
sample had an As concentration of 50 ppm. These values contrast
greatly with those at NDRA, where, for PM10 alone, arsenic concentration is 290 ppm for unit 1.5 and over 160 ppm for unit 2.2
(Table 3).
Please cite this article in press as: Soukup, D., et al. Arsenic concentrations in dust emissions from wind erosion and off-road vehicles in the Nellis Dunes
Recreational Area, Nevada, USA. Aeolian Research (2011), doi:10.1016/j.aeolia.2011.11.001
D. Soukup et al. / Aeolian Research xxx (2011) xxx–xxx
11
5.4. Arsenic hazards to health
Because arsenic has long been known to be extremely harmful
to health (Chen et al., 1992; Abernathy et al., 1999; Järup, 2003;
Tseng et al., 2003; Smith et al., 2006; von Ehrenstein et al., 2007;
Kozul et al., 2009b), the reported concentrations of As in the soil
and PI-SWERL samples were initially compared with the USEPA Region 3, 6, and 9 screening levels (SLs) for chemical contaminants in
residential soils and soil concentrations considered to be protective
of groundwater resources (USEPA, 2010). The total As concentrations in all of the samples analyzed exceed the EPA’s SL of
0.39 lg g 1 for As in residential soil by one to three orders of magnitude (Table 2).
The reported concentrations of As in all of the samples also exceed the EPA SLs considered to be protective of groundwater.
Although the reported As concentrations exceed the EPA SLs, the
potential risk to groundwater resources from leaching of As is considered to be minimal in the Nellis Dunes area. This is because
groundwater in the Nellis Dunes area is deep (>30 m below ground
surface), the arid climate and most soil characteristics minimize
leaching, and As is strongly sorbed to most soils (Matera and Le
Hécho, 2001). However, the high water-soluble concentrations of
As are of concern because of the potential for downstream contamination from runoff. Lake Mead, a major drinking water source for
Las Vegas, is located hydrogeologically downgradient of the NDRA.
The most important potential health hazard in this area is human exposure to As through inhalation of dust. In order to better
understand potential risks of As in dust emissions we calculated
PM10 emission rates for As resulting from natural wind erosion in
NDRA for each surface unit. We multiplied the emission rates for total PM10 dust (Goossens and Buck, 2011b) with the As content of
the PM10 PI-SWERL samples. PI-SWERL samples are used in the calculation because they represent the sediment fractions prone to
emission during wind erosion. The As emission rates ranged from
a low of <1 10 17 g cm 2 s 1 in surface unit 3.5 (bedrock and/or
outcropping petrocalcic horizons; classified as stable) to a maximum of 3.67 10 14 g cm 2 s 1 in surface unit 1.5 (outcrops of
very fine sand and coarse silt; classified as highly erosive). Although
As concentration was not measured in dust from bedrock surfaces
(unit 3.5; classified as stable), we could estimate it based on As concentrations in dust derived from well-developed desert pavements
(unit 3.1; classified as stable), which are a representation of local
and regional background dust (McFadden et al., 1998; Reheis
et al., 2009) and are a good proxy for dust deposited on the bedrock
surfaces. In addition to unit 1.5, emission rates for As were classified
as being very erosive or erosive in the other units containing sand
(1.1, 1.2, 1.3, 1.4, and 3.4) (Fig. 5). Note that these values represent
potential emission rates; not actual emission rates. Nonetheless,
they allow us to rank the 17 NDRA surface units according to their
capacity to emit arsenic during wind erosion (Fig. 5).
Similarly, we calculated PM10 emission rates for As resulting
from ORV activities (Fig. 6a–c). These rates were calculated by multiplying the ORV emission rates (g cm 1) for total PM10 dust (Goossens and Buck, 2009b) with the As concentration in the PM10 PISWERL samples. Here too, the numbers represent potential emission
rates; not actual emission rates. The emission rates for each ORV
activity were highest in surface units 2.2 (silt/clay with gravel; classified as highly erosive) and 3.1 (desert pavements; classified as very
erosive). The lowest emission rates occur in surface units 1.1 (unvegetated dunes; classified as stable) and 3.5 (bedrock; classified as stable). The As emission rates ranged from <1 10 9 g cm 1 to
1.89 10 6 for dirt bikes, <1 10 9 to 3.71 10 7 g cm 1 for dune
buggies, and <1 10 9 to 2.12 10 6 g cm 1 for four-wheelers.
These rates are calculated for a driving speed of 30 km h 1, which
is a conservative, but representative average for the NDRA. The
average emission rate for a dirt bike, dune buggy, and four-wheeler
Fig. 6. Potential emission rate for arsenic during ORV activity: (a) Dirt bike; (b)
dune buggy; (c) four-wheeler. Data are for the PM10 (0–10 lm) fraction and for an
average driving speed of 30 km h 1.
at 30 km h 1 is 1.24 10 7 g cm 1. At higher driving speeds,
emission rates are considerably higher. For example, for an average
Please cite this article in press as: Soukup, D., et al. Arsenic concentrations in dust emissions from wind erosion and off-road vehicles in the Nellis Dunes
Recreational Area, Nevada, USA. Aeolian Research (2011), doi:10.1016/j.aeolia.2011.11.001
12
D. Soukup et al. / Aeolian Research xxx (2011) xxx–xxx
vehicle (average of a dirt bike, dune buggy, and four-wheeler), the
emission rate is 1.89 times greater at 40 km h 1, and at 50 km h 1
more than triples (3.27 times greater).
The results of this study present an interesting dilemma for
development of a management plan for the NDRA. Dust emissions
at NDRA occur from both ORV activity and wind erosion. Previously, McLaurin et al. (2011) used dust emission data alone to suggest a management plan that directed ORV activity to stay within
the sandy surface units (1.1–1.5) and avoid the silt/clay areas with
gravel, desert pavements and the gravel silt/clay drainages (surface
units 2.2, 3.1, and 4.3). This suggestion was based on dust emission
data that showed that ORV-generated dust emission is lowest in
the sandy surfaces and highest in the fine silt and clay areas, the
desert pavements and the silty drainages. However, the As data
presented here indicate that in this case, dust emission data alone
are not enough to make a sound management plan. When including arsenic with both wind and ORV-generated dust emissions, we
find that the sandy areas emit the highest amounts of As annually
because they comprise the zones that are most susceptible to wind
erosion (Goossens and Buck, 2011b). When driving in these zones
during wind erosion, riders are exposed to increased amounts of As
that could potentially be inhaled. Therefore, until more is known
about actual human exposure and health risk at NDRA, people
should consider avoiding these sandy areas and areas downwind
during windy conditions. These conclusions are also supported
by the maps in Fig. 3, which show that for the fractions most subject to inhalation (especially PM10), the sand areas have the highest concentrations of As in emitted dust. People should consider
avoiding dust emitted from ORV driving on units 2.2 and 3.1 because this activity also generates high As emissions.
The potential health effects of the dust generated during ORV
use at the NDRA are not known because emissions vary greatly
depending on what type of vehicle is used, how intensely an area
is driven, and whether riders drive closely behind one another.
Information regarding the exact number of drivers, the length of
each drive and the specific routes followed is also unknown (Goossens and Buck, 2009b). It is also important to note that the grain
size distribution of the PI-SWERL released dust does not necessarily correspond to that of ambient dust. The PI-SWERL dust is locally
eroded dust whereas ambient dust also contains particles that
were eroded elsewhere and are in transport. Archived ambient
dust samples that were previously collected at NDRA using Big
Spring Number Eight (BSNE) passive samplers will be analyzed in
the future to evaluate whether As concentrations are similar to
those in the PI-SWERL samples. In order to determine the actual
exposures, monitoring of personal dust exposure must be performed on ORV users under different driving conditions, and on
other visitors at the site.
6. Conclusions
Elevated arsenic concentrations were measured in soil (up to
>80 ppm) and airborne dust samples (up to >300 ppm) from the
Nellis Dunes Recreation Area. To date, we are not aware of any
other studies that have reported As concentrations in dust from
natural surfaces as high as those found in the NDRA. The highest
concentrations are associated with two deposits of the Muddy
Creek Formation (surface units 1.5 and 2.2) in the northern portion
of NDRA, and in the drainages. Significant water-soluble As is present in several surface units, and runoff is likely concentrating As in
the drainages. The highest concentrations of arsenic occur in the
airborne dust, especially in the PM10 fraction, and may be as much
as one order of magnitude higher than the soil samples. The areal
distribution of As varies according to grain size, but the highest As
concentrations occur mostly in the western portions of NDRA. In
the PM10 fraction As is greatest in the sandy units (1.2, 1.3, 1.5),
two of silty units (2.1, 2.2) and all of the drainages (4.1–4.3).
The most important potential health hazard in this area is human exposure to As through inhalation of dust. Emission rates
for As resulting from both natural wind erosion and ORV activity
were calculated for all 17 surface unit in the NDRA. For PM10, during windy conditions, the sandy surfaces (units 1.1–1.5, and 3.4)
have the highest emission rates for As. In contrast, ORV-generated
emission rates for As are highest on unit 2.2 (silt/clay with gravel)
and desert pavements (unit 3.1). Arsenic emission rates vary with
type and speed of ORV: four-wheelers have the highest emission
rates, followed by dune buggies and dirt bikes. Emission rates for
As nearly double when comparing driving speeds of 30 km h 1,
to 40 km h 1 and at 50 km h 1 more than triple.
Currently, there are no EPA regulations in the United States for
As in recreational settings. To accurately evaluate the potential
health effects, monitoring of personal dust exposure must be performed on ORV users and other site visitors. The actual concentration of As in the air must be quantified, since that is what existing
standards for As exposure in the workplace are based upon. A sitespecific human health risk assessment must also be performed to
model the biological responses to all of the constituents in NDRA
dust. However, until the potential risk to human health is better
defined, site visitors should consider avoiding the sandy areas
and downwind areas during windy conditions, and avoid exposure
to ORV-generated dust on units 2.2 and 3.1. Finally, this research
indicates that a better understanding of the geochemical complexity of natural desert surfaces and their response to natural and
anthropogenic disturbances is needed.
Acknowledgements
The U.S. Bureau of Land Management (BLM) funded this project
through a Southern Nevada Public Land Management Act
(SNPLMA) Conservation Initiative, and granted permission for all
fieldwork in the Nellis Dunes Recreation Area. Lisa Christianson
(BLM) provided administrative assistance throughout the project.
The authors thank Rebecca Burt and Mike Wilson with the Natural
Resources Conservation Service for providing the NRCS arsenic
data. The authors also thank Tom Gill for sharing his expertise in
chemical analyses, and G. Breit for discussions on arsenic behavior.
The views and conclusions contained in this document are
those of the authors and should not be interpreted as representing
the opinions or policies of the U.S. Government. Mention of trade
names or commercial products does not constitute their endorsement by the U.S. Government.
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