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Selenium concentrations in water and plant tissues of a newly formed arid wetland in Las Vegas, Nevada


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There is concern that elevated levels of selenium found in the source water of a newly formed wetland park in Las Vegas, Nevada, may have detrimental effects on local wildlife. In this study, we collected and analyzed water samples monthly for a three year period from the inflow and outflow of the system. We also gathered dominant aquatic plants and selected terrestrial plants and analyzed the water and plant tissues (root, shoot, leaf and flower) for selenium by high resolution Inductively Coupled Plasma Mass Spectrometer. Except for storm events and the introduction of an alternative low selenium content source water during summer low-flow conditions, selenium in the water was relatively stable. The concentration in the outflow tended to be slightly lower than the inflow. Concentrations of selenium in the dominant plant taxa in this wetlands were typical of ecosystems in the western United States and varied by taxa, tissue type, localized conditions (e.g., contact with selenium-laden water), and to a lesser extent, seasons. Selenium in the aquatic plant spiny naiad (Najas marina) was relatively high and may pose an ecological risk to wildlife during the late spring and summer. Additional work is underway investigating aquatic food chain accumulations of selenium as well as mass balance of selenium in the system.
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Selenium concentrations in water and plant tissues
of a newly formed arid wetland in Las Vegas, Nevada
James Pollard &James Cizdziel
Krystyna Stave &Michelle Reid
Received: 9 December 2006 /Accepted: 13 February 2007 / Published online: 30 March 2007
#Springer Science + Business Media B.V. 2007
Abstract There is concern that elevated levels of
selenium found in the source water of a newly formed
wetland park in Las Vegas, Nevada, may have
detrimental effects on local wildlife. In this study,
we collected and analyzed water samples monthly for
a three year period from the inflow and outflow of the
system. We also gathered dominant aquatic plants and
selected terrestrial plants and analyzed the water and
plant tissues (root, shoot, leaf and flower) for
selenium by high resolution Inductively Coupled
Plasma Mass Spectrometer. Except for storm events
and the introduction of an alternative low selenium
content source water during summer low-flow con-
ditions, selenium in the water was relatively stable.
The concentration in the outflow tended to be slightly
lower than the inflow. Concentrations of selenium in
the dominant plant taxa in this wetlands were typical
of ecosystems in the western United States and varied
by taxa, tissue type, localized conditions (e.g., contact
with selenium-laden water), and to a lesser extent,
seasons. Selenium in the aquatic plant spiny naiad
(Najas marina) was relatively high and may pose an
ecological risk to wildlife during the late spring and
summer. Additional work is underway investigating
aquatic food chain accumulations of selenium as well
as mass balance of selenium in the system.
Keywords Selenium .Toxicity .Wildlife
Aquatic plants .Environmental assessment
Las Vegas .Ecological risk .Arid wetlands
It is well known that selenium (Se) has the potential
to accumulate to toxic levels in plants in aquatic
ecosystems and subsequently impact wildlife (Lemly
2002). Following the discovery of detrimental effects
of Se on wildlife in the Kesterson Reservoir in the
San Joaquin Valley, California, (USFWS 1990)a
survey of wetlands in the western United States
indicated most of them were contaminated with
elevated levels of Se (Presser et al. 1994;Wu2004).
This study concerns a newly constructed 130-acre
wetland in the Las Vegas, Nevada metropolitan area.
The constructed wetland, called the Clark County
Wetlands Park Nature Preserve (NP), is adjacent to
the main channel of the Las Vegas Wash in Southern
Environ Monit Assess (2007) 135:447457
DOI 10.1007/s10661-007-9664-8
J. Pollard (*):J. Cizdziel
Harry Reid Center for Environmental Studies,
University of Nevada, Las Vegas,
4505 Maryland Parkway,
Las Vegas, NV 89154-4009, USA
K. Stave :M. Reid
Environmental Studies Department,
University of Nevada Las Vegas,
Las Vegas, NV 89154-4009, USA
Nevada (Fig. 1). Construction of the Nature Preserve
wetland system was completed in April 2001.
Las Vegas Wash is a natural drainage system for
the 4,100 km
Las Vegas Valley watershed and is the
primary site for discharge of tertiary treated waste-
water. The Wash is the only major drainage for the
Las Vegas valley, currently populated by 1.8 million
residents. It empties into Las Vegas Bay in Lake
Mead, the largest man-made reservoir in the United
States. Concentrations of Se in the mainstream of the
Wash are relatively low (23μg/l), whereas con-
centrations in some tributaries, which consist primar-
ily of runoff and surfacing groundwater, are
significantly higher (1020 μg/l) (Cizdziel and Zhou
2005). Waterborne selenium at these elevated con-
centrations is of concern for the health of wildlife,
particularly birds and fish. The current EPA guidance
for aquatic life Se exposure is 5 μg/l for chronic
exposure and 20 μg/l for acute exposure (USEPA
1987). This guidance is in the process of being re-
evaluated and will probably be lowered as a result of
new data on Se effects (USEPA (U. S. Environmental
Protection Agency) 2004). The main water supply for
the NP is one of these tributaries to the Wash with
high selenium concentrations. Consequently there
was, and continues to be, considerable concern that
the NP wetlands could be a serious threat to wildlife
due to Se toxicity.
A variety of emergent vegetation species were
planted following construction of the NP, most of
which have thrived in the system, forming a well
developed wetland ecosystem. It is known that wet-
lands serve an important role in the biogeochemical
cycling of Se, with some having been shown to
volatize Se (e.g.: Hansen et al. 1998; Ansede et al.
1999; De Souza et al. 2000). Thus, it was of interest
to determine if the plants of the NP were removing Se
from the water via absorption and volatilization or if
Se was accumulating in the system, causing an
associated increased ecological risk to wildlife. To
address these issues, Se concentrations were measured
on a monthly basis in water collected at strategic
locations in the system. In addition, dominant aquatic
and terrestrial plant taxa were collected during periods
of relative senescence (fall/winter) and rapid growth
(spring/summer) and analyzed for selenium concen-
trations. This paper reports the waterborne concen-
trations of Se in the inflow and outflow from the
system over a three year period (20012003). In
addition, the distribution of Se in the plants by
species, tissue-type, and season are reported from
collections during 2002 and 2003.
Fig. 1 The Las Vegas Wash
and Nature Preserve in rela-
tion to major tributaries
and Lake Mead
448 Environ Monit Assess (2007) 135:447457
Materials and methods
Study area
The Nature Preserve is part of the 2,500 acre Clark
County Wetlands Park and is located within the
vicinity of the Las Vegas Wash (Fig. 1). The ponds
and streams of the NP were designed to be supplied
with water primarily from the nearby Monson Drain,
a tributary to the Wash. It was known prior to
construction that the Monson Drain had Se concen-
trations of >20 μg/l during some seasons and flow
conditions. During the process of performing a
National Environmental Protection Act Environmen-
tal Assessment (RECLAMATION 1999), and collec-
tion of baseline information for water quality and
biological conditions at the NP (Pollard et al. 2002), a
monitoring program was established to collect water
quality data as the wetlands developed.
Water quality sampling sites and plant collection
areas with designated station numbers were estab-
lished within the NP (Fig. 2). NP-1 is the inflow from
the Monson Drain and NP-8 is the outflow from the
system. NP-2, NP-3, NP-4, and NP-5 are located at
the outflows of individual ponds within the system
from upstream to downstream respectively. In this
report, we focus primarily on water collected from
NP-1 and NP-8. Plant samples were collected from
around the upper pond (near NP-2), middle ponds
(near NP-3, NP-4 and NP-5), and lower pond
(near NP-8).
General water quality characteristics of the system
A complete reporting of the extensive three year
water quality monitoring program can be found
elsewhere (Pollard et al. 2004). In summary, with
few exceptions (storm events and introduction of Las
Vegas Wash water) the system can be characterized as
highly saline (specific conductance 4,800 μS/cm),
relatively clear (turbidity <30 NTU), slightly alkaline
(pH 7.7), with moderate acid-neutralizing capacity
(alkalinity 200 mg/l). Temperature, dissolved oxy-
gen (DO) and flow varied by season. Temperature
ranged from 7°C (Feb.) to 27°C (July), DO
generally averaged around 10 mg/l during the winter
and dropped to lows of about 3 mg/l in the summer.
Flow rates also decreased from highs of around
2.5 cubic feet per second (cfs) in the winter to
0.2 cfs or less during the summer. Before the addition
of a pipeline in April, 2004, allowing introduction of
treated effluent from the nearby Clark County Water
Reclamation District, low flow conditions and bacte-
rial loads from stormflows during the southwestern
monsoon season resulted in stagnation of the system,
oxygen deficiencies, and fish kills. Overall, inflows
were slightly lower in specific conductance than
outflows indicating water loss through seepage,
evaporation and plant mediated evapo-transpiration.
Plant species, tissues, and collection periods
Dominant aquatic taxa were selected, including
bulrush (Scirpus californicus), cattail (Typha spp.),
alkali bulrush (Scirpus maritimus), spiny naiad
(Najas marina), and stonewart (Chara vulgaris). Four
co-dominant terrestrial taxa were also studied because
they were in close proximity to the wetland ponds and
they provide large quantities of seed and significant
forage for resident wetland wildlife. These include
common reed (Phragmites communis), quail bush
(Atriplex lentiformes), salt cedar (Tamarix chinensis),
Fig. 2 Map of the Nature Preserve showing general layout
with ponds, trails and monitoring sites. Created from Clark
County Aerial Photo taken in Spring, 2001
Environ Monit Assess (2007) 135:447457 449
and salt grass (Distichlis stricta). Roots, vegetative
segments, and flowers were chosen to reflect areas of
physiological uptake, potential accumulation and
volatilization, and wildlife consumption. Samples
were collected during two main sampling periods:
fall/winter 2002 and spring/summer 2003. These
periods were chosen to represent periods of relative
senescence (fall/winter, but mostly fall) and rapid
growth (spring/summer) in the wetland plant popula-
tions. In summary, water samples were collected at
the inflow and outflow of the park monthly for three
years (20012003) and plant tissues samples were
collected from various portions of aquatic and
terrestrial plant taxa surrounding the aquatic systems
during two seasonal periods in 2002 and 2003.
Field procedures
Water samples were collected into acid-washed
Nalgene bottles and preserved with high purity
to 1% acid. Samples were kept in a cooler
with ice until storage at 4°C at the laboratory. Plant
tissues were selected from healthy appearing sections
of mature specimens. Vegetative sections 15 cm in
length were cut from areas near, but not including the
growing ends of the plants. Roots were removed from
the same plant groups as the vegetative material and
washed in the field with NP water to remove all
visible sediment. A final rinse in the field with de-
ionized water was performed to remove any residual
NP water from the sample. Flowering portions of the
plants were removed from the same or nearby groups
of plants as the vegetative and root portions. The
samples were bagged, labeled in the field and trans-
ported to the laboratory for storage in a freezer until
processing and analysis.
The same basic field procedures were used to
collect the fall/winter and spring/summer samples
with the following exception. Based on the results of
the fall/winter analyses it was noted that individual
plants tended to have high field variability. It was
decided to composite individual plants at each of the
collection stations to allow us to obtain a complete
spatially representative data set within the project
budget. Therefore the mean concentrations presented
for the spring/summer samples are based on fewer
samples, but these samples are composites of approx-
imately the same number of plants that were collected
in the fall/winter collections and are therefore repre-
sentative of the same areas.
Sample preparation
Because of the relatively high salt content of the
water, typically >3,000 mg/l total dissolved solids,
water samples were diluted (gravimetrically) several-
fold with 1% HNO
prior to analyses, then run
directly as described below. Plant samples were
processed by placing a sample in a 250 ml Nalgene
beaker, pouring approximately 50 ml of liquid
nitrogen over the sample and grinding the sample
with a ceramic mortar and pestle. If the sample was
not sufficiently pulverized, the sample was then re-
frozen in liquid nitrogen and blended in a commercial
laboratory blender. The goal was for the sample to be
of visually uniform composition. After homogeniza-
tion samples were placed in labeled plastic bags and
returned to the freezer until digestion.
Individual plant samples and composites were
analyzed for moisture content by drying the sample
at 85°C to a constant weight (typically 24 h). This
was performed to allow conversion of selenium con-
centrations to a dry weight basis
Plant samples were digested based on a procedure
by Zhang and Combs (1996). Briefly, 0.5 g of air-
dried sample was weighed in 50 ml polypropylene
centrifuge tubes. Five ml of concentrated ultra-pure
(SeaStar, Seattle WA) was added and the vials
were transferred to a ModBlockDigestion System
(CPI International, Santa Rosa, CA). The vials were
loosely capped and the block brought to 80°C for 2 h
followed by 100°C for another 2 h. Samples were
then allowed to cool and 2 ml of concentrated ultra-
pure H
(JT Baker, Phillipsburg, NJ) was slowly
added. The vials were again loosely capped and the
block heater re-heated to 100°C for 2 h. The caps
were removed and the samples were allowed to
evaporate to a volume of approximately 0.5 ml. After
cooling, de-ionized water was added to fill the vials to
50 ml. The final digestate samples were weighed to
the nearest 0.1 mg to determine the total weight of the
final sample.
Sample analysis and quality assurance
Diluted water samples and plant digests were ana-
lyzed using a method based on U.S. EPA Method
450 Environ Monit Assess (2007) 135:447457
200.8 (USEPA 1991). The instrument employed was
the Axiom (Thermo Finnigan, San Jose, CA, USA), a
magnetic sector-field Inductively Coupled Plasma
Mass Spectrometer (ICP-MS) capable of high resolu-
tion and separation of the argon-dimer (
interference peak from the
Se peak. Analysis of
blanks indicated that the interference from
which cant be resolved by high resolution, was
negligible. Yttrium was added to each sample to 1 ppb
as an internal standard. Linear calibration curves had
correlation coefficients greater than 0.99. For quality
control, each set of samples was accompanied by a
blank and a standard reference material (SRM) from
the National Institute of Technology, to verify that
the procedure was yielding valid results, ±15% of
certified values. For water, SRM 1643d and/or 1640
were used. For plants, wheat flour (SRM 1567a) was
digested and analyzed (blind) along with other
samples within an analysis batch. The results fell
within the target range of expected values 10 out of
11 times. The mean result was 1.05± 0.08 (certified
1.1±0.2 μg/g), indicating good recoveries. Method
blanks showed contamination was negligible relative
to concentrations found in the samples. Recoveries
from spikes of samples were within 20% of the
expected value and showed there were no anomalous
matrix effects. In addition, field duplicates were
collected for both water and plant samples to deter-
mine the variability at a given site. The field variability
was higher for plants, with relative percent differences
(rpd) ranging from 3090%, compared to the water
(<10%). The precision for laboratory duplicates of
plant homogenates was <10% (n=16; seven taxa),
suggesting that the variability observed for Se be-
tween plants of the same species was real and perhaps
site specific.
Results and discussion
Selenium in the water
With few exceptions (described later) data from water
samples collected over a three year period following
wetland construction indicated that Se concentrations
in the inflow and outflow were similar and reasonably
consistent (Fig. 3). Not including the anomalous data,
the mean concentration at the inflow was 19.6± 4.0 μg/l
(range 8.627.0) and at the outflow was 17.6± 4.4 μg/l
(range 9.924.7). If Se was effectively being removed
from the source water, mitigating the relatively high
concentrations, one would expect to see outflows with
consistently lower concentrations than the inflows. We
found lower Se concentrations in the outflow compared
with the inflow for 17 out of 24 analyses. There were
6 months (Dec. 01, Jan. 02, Oct. 03, Nov. 03, Feb. 03, and
May 03) when Se concentrations were more than 5 μg/
l lower in the outflow than the inflow and only 2 months
(May 01 and Mar. 02) when the opposite was true. While
it is likely that Se is being retained and accumulated in the
system (to some extent), a more thorough investigation of
mass balance would be necessary to quantify the amounts
and resolve the issue.
The exceptions mentioned above are of interest
and serve to aid in understanding the dynamics of the
system. In March of 2001 we observed very high Se
concentrations throughout the system. This followed a
flood event which left large quantities of pooled water
over the entire NP. We hypothesized that slow surface
seepage from these pools into the upper pond carried
a large loading of re-dissolved surface salts containing
high Se concentration into the system which took
approximately a month to finish seeping into the
upper pond and moving through the system. We
intended to test this hypothesis during later storm
events, but no comparable events occurred during the
remainder of the study.
The other obvious anomaly resulted from the
introduction of Las Vegas Wash water into the system
to mitigate degrading water quality due to low inflow
Fig. 3 Selenium concentrations (mg/l) in the inflow (NP-1)
and outflow (NP-8) of the Nature Preserve for Feb. 2001
through Dec. 2003. Data is missing in 2003 for June (NP-8)
and Sept. (both sites)
Environ Monit Assess (2007) 135:447457 451
to the system and stagnation during the hot summer
months (July, August and September 2002 and 2003).
LVW water, which contained relatively low concen-
trations of Se (23μg/l), was pumped in near NP-2
or NP-4 which essentially diluted the Se in the middle
and lower ponds. The Monson Drain inflow (NP-1)
and the upper pond (NP2) did not receive input from
LVW and did not show anomalous patterns. However,
levels at the outflow decreased below that typically
found in the system during base flow conditions due
to the diluting effect of the added LVW water into the
system (Fig. 3).
Overall, these data clearly show that the water
borne Se concentrations in the NP, using the Monson
Drain as the primary water source, were chronically
above the current estimates of 12μg/l for toxic
effects on wildlife (Lemly 2002) as well as the EPA
Criterion of 5 μg/l for protection of wildlife (USEPA
1987). These data were used by Clark County Parks
and Community Services to initiate the construction
of a pipeline connecting the NP inflow (NP-1) with
treated effluent from the Clark County Water Recla-
mation District. This pipeline is capable of delivering
8 cfs to the inflow of the park and will allowed park
operations to maintain adequate flow (2.5 cfs) to avoid
stagnation in the system as well as dilution of selenium
input to the system. Monitoring of Se in the system is
continuing past the conclusion of this study to develop
a new baseline for water borne Se concentrations with
the modified inflow parameters. Future studies are
planned to observe the effects of reduced selenium
concentrations on the aquatic food chain of the system.
Selenium in the plants
Submergent taxa had high moisture content as did
most of the root samples for emergent taxa. Flowers
typically had the lowest percent moistures, while the
vegetative portions of the plants were typically in
an intermediate range between flowers and roots
Table 1 Mean dry weight selenium data (μg/g) for fall/winter sampling
Part n Mean Se SD CV % M
Emergent Taxa
Bulrush F 7 0.67 0.21 42.0 24.3
Bulrush R 5 0.74 0.04 28.6 83.3
Bulrush V 6 1.17 0.23 48.8 59.9
Cattails F 8 1.57 0.39 34.2 27.7
Cattails R 7 1.49 0.37 113.5 78.2
Cattails V 11 2.81 0.53 59.1 68.3
Alkali Bulrush F 6 1.05 0.70 91.3 26.6
Alkali Bulrush R 7 1.50 0.41 103.2 73.6
Alkali Bulrush V 4 1.51 0.35 49.1 53.0
Submergent Taxa
Chara All 1 0.85 –– 85.0
UD Pond Weed All 1 0.79 –– 92.1
Spiny Naiad All 5 1.51 0.04 40.5 93.3
Terrestrial Taxa
Common Reed F 5 1.59 1.20 106.7 29.4
Common Reed R 4 1.54 0.55 88.5 59.6
Common Reed V 10 2.03 0.55 52.4 48.4
Quail Bush F 7 0.37 0.18 122.7 59.9
Quail Bush V 11 0.54 0.11 69.6 71.2
Tamarisk F 3 0.30 0.17 75.0 24.7
Tamarisk V 3 0.87 0.01 2.2 67.0
The portion of plant analyzed (Fflower, RRoot, VVegetative Material) is presented along with the sample size (n), Standard
Deviation (SD), Coefficient of Variation (CV), and Percent Moisture (%M) for all analyses.
452 Environ Monit Assess (2007) 135:447457
(Tables 1and 2). Data are presented on a dry-weight
basis to normalize Se concentrations to the different
water contents between individual tissues and among
In this study, cattails, alkali bulrush, common reed
and spiny naiad had the highest Se content and quail
bush and bulrush had the lowest Se content within the
NP (Table 1). This was fairly consistent seasonally,
although there was considerable spatial and plant
portion variation as has been noted above. Concen-
trations of Se in the plants varied by tissue, species
and season (Tables 1and 2). For those species and
plant tissues that we were able to collect samples in
both seasons we were able to make a direct
comparison of tissue Se content. The fall samples
generally had concentrations highest in the vegetative
portions, followed by the roots and flower segments
(Table 1, Fig. 4). The distribution was somewhat
different for the spring/summer samples, which
typically had the highest concentrations in the roots,
followed by vegetative segments and flowers (Table 2,
Fig. 5), although this pattern was not entirely
consistent (see Bulrush in Fig. 5). Concentrations of
Se in the vegetative segments were consistently lower
in the spring/summer (five out of six species)
compared with the fall. The opposite was true for
roots which had higher concentrations in the spring/
summer (three out of three species). There was no
apparent trend for the flowers, which tended to have
similar concentrations by season.
Selenium volatilization
Volatilization of Se from wetlands occurs primarily
through plants, and microorganisms associated with
plants, and can be affected by not only the various
species present but by a number of physical and
chemical parameters, e.g. soil nutrients, pH, temper-
ature, sulfate concentrations, aeration (redox
conditions), and nitrate/nitrite levels (Wu 2004). Mea-
surements of most of these parameters were beyond
the scope of this work. However, we can compare our
Table 2 Mean dry weight selenium data (μg/g) for spring/summer sampling
Part n Mean Se SD CV % M
Emergent Taxa
Bulrush F 3 0.78 0.26 33.2 32.4
Bulrush R 3 0.93 0.34 36.7 85.8
Bulrush V 3 1.16 0.52 44.7 73.5
Cattails F 5 1.40 1.05 75.4 71.2
Cattails R 4 2.08 1.71 82.3 91.5
Cattails V 5 1.95 1.68 86.3 74.6
Cattails Sprout 3 1.08 0.73 67.6 81.2
Akali Bulrush F 5 0.36 0.09 25.5 39.2
Akali Bulrush R 4 2.11 2.04 96.5 85.0
Akali Bulrush V 5 1.04 0.42 40.5 71.3
Akali Bulrush Sprout 2 0.81 0.40 49.0 69.4
Submergent Taxa
Spiny Naiad All 3 4.70 2.11 44.8 92.6
Terrestrial Taxa
Common Reed F 4 1.27 0.41 31.8 10.6
Common Reed V 4 1.33 0.69 52.0 65.5
Quail Bush V 4 0.47 0.34 72.4 69.1
Quail Bush Sprout 3 0.34 0.12 35.9 76.7
Tamarisk F 4 1.38 0.34 24.6 56.6
Tamarisk V 4 2.11 1.96 92.7 61.2
Salt Grass All 2 0.71 0.69 97.2 46.0
The portion of plant analyzed (Fflower, RRoot, VVegetative Material, Sprout very young vegetative material) is presented along
with the sample size (n), Standard Deviation (SD), Coefficient of Variation (CV), and Percent Moisture (%M) for all analyses.
Environ Monit Assess (2007) 135:447457 453
plant tissue concentrations to those found in other
studies which have demonstrated phytoremediation of
Se. Hansen et al. (1998) demonstrated that similar
wetland plant species had consistently higher concen-
trations of Se in the root than in the leaves or shoot
portions of the plants. They attributed as much as
30% of the reduction in waterborne Se concentrations
exiting a constructed wetland to biological volatiliza-
tion processes. We assumed that a similar distribution
of Se in plant parts in our study would be evidence for
volatilization of this element. For example, if the
concentrations of Se were lowest in the respiring shoot
or leaf portion of a plant, it would be an indication
that Se was being volatilized via biological conver-
sions and removed from the system. On the other hand,
if Se were highest in the leaves or flowers it would be
an indication of bioaccumulation which would tend to
increase Se retention in the system over time. Applied
Fig. 4 Mean selenium
concentrations (μg/g dry
weight) in plant tissue
samples collected from the
Nature Preserve in fall/
winter, 2001
Fig. 5 Mean selenium
concentrations (μg/g dry
weight) in plant tissue
samples collected from the
Nature Preserve in spring/
summer, 2002
454 Environ Monit Assess (2007) 135:447457
to our data it appears that some plants are accumulating
Se during the fall/winter (e.g., bulrush, cattails, and
common reed) and removing it (respiring Se com-
pounds) from the system during the spring/summer
(e.g., alkali bulrush and cattails). However, the true
picture is likely more complicated. It is known that
different parts of plants tend to have different molec-
ular forms of Se in different concentrations, each with
its own volatilization properties (De Souza et al. 2000).
Because of variability within and among species we
are unable to conclude whether the wetlands as a
whole have a net loss of Se through volatilization.
Ecological risk and comparison to literature data
Plant taxa with relatively high flower or seed Se
concentrations could pose an elevated risk for birds
and other wildlife via ingestion of these parts.
Consequently, it was of interest to find which plants
currently inhabiting the wetlands have the highest
overall concentrations of Se in their tissues and
therefore represent the greatest ecological risk due to
Se. Moreover, comparison to other studies with the
same or similar plant species would allow us to
interpret our data in a broader perspective.
The majority of literature on environmental levels
of Se refers to water, sediments, fish, invertebrates,
and birds (see Hamilton 2004; Lemly 2002; and Wu
2004 for extensive reviews). The interest in Se in
plant tissues has tended to focus on the potential for
various plant taxa to concentrate Se, particularly when
the plants are a food source for agricultural forage
animals (e.g.: Wilbur 1983; Izbiki and Harms 1986;
Harms 1995). More recently, comprehensive survey
data have been compiled which document the levels
of Se in various agricultural plants as well as native
plant species (Seiler et al. 2003).
Studies of wetlands in the literature included a
constructed wetland in the San Francisco Bay
(Hansen et al. 1998) where Se ranged from 5 to
20 μg/g for shoot and root material. This was much
higher than our observations, where the highest
values were less than 4 μg/g and many observations
were less than 1 μg/g. In an area of the lower
Colorado River with similar characteristics to our
wetlands, the maximum concentration of Se in cattail
tissues was less than 0.2 μg/g (Garcia-Hernandez
2000). This was considerably less than the observed
concentrations of Se in cattails that we found in the
NP during fall or spring sampling (Tables 1and 2). In
both the Hansen and Garcia-Hernandez studies, Se in
the source waters were similar to the NP (20 ppb),
although in one case the plants were deemed to be
concentrating the Se (Hansen et al. 1998) and in the
other they were not (Garcia-Hernandez 2000).
Data for three of the plant taxa studied in the NP
(bulrush, cattails, and Quail bush) can be found in
Harms (1995), which is a comprehensive statistical
summary of all Se vegetation data collected by the US
Geological Survey for a 22 year period prior to 1995.
Bulrush and cattail data in the NP overlap those in the
Harms report. However, these taxa are represented in the
report by single samples from Merced County, California
and therefore have limited comparative value. Quail
bush leaf values presented in Harms (1995) had a much
larger range of observed values (0.087.5 μg/g) than in
our study (0.151.4 μg/g). However, the geometric
mean presented in Harms (0.43 μg/g) is similar to our
average value (0.46± 0.09), indicating that the concen-
trations found here were probably in the typical range
for this salt tolerant species.
Seiler et al. (2003) presents data on irrigation
induced Se contamination for 26 areas in the Western
United States. Median plant Se concentrations found
in that report can be compared to our data if grand
means are calculated for all plants and plant parts in
the NP. The resulting estimate of total plant Se in NP
was approximately 1.3 μg/g for spring samples and
1.2 μg/g for fall samples. These values correspond
with the following areas presented in Seiler et al.
(2003): Lower Colorado River Valley, California
Arizona; Riverton Reclamation Project, Wyoming; San
Juan River area, New Mexico; and the Sun River area,
Montana. These areas tend to have surface water
concentrations that are much lower than the NP. This
would tend to indicate that the plants in the Nature
Preserve wetlands are not generally accumulating
selenium, even though the water concentrations are well
above levels of concern.
Seiler et al. (2003) also shows typical background
levels for plant tissues to be near 1.5 μg/g and dietary
effect levels in these tissues to be near 3 μg/g Se. A
majority of the data presented in our study were
below these levels and samples that exceeded this
guidance typically were isolated samples which were
not concentrated in any one area. However, concen-
trations of Se in the spiny naiad from the spring/
Environ Monit Assess (2007) 135:447457 455
summer (mean 4.71 μg/g, dw) were consistently near
or above the dietary effect level, which could pose a
problem for birds and fish that feed on this sub-
mergent plant species. This was not as evident in fall
samples and therefore may be a seasonal effect which
might be coupled with reductions in dissolved oxygen
and pH levels created by anoxic conditions as the
system stagnates in the hot summer months. Among
the plants studied, only the spiny naiad appears to be
a potential problem and should be monitored and
discouraged if the high levels persist.
Measurements of Se in the Clark County Wetlands
Park Nature Preserve pointed out the critical impor-
tance of flow management to the ecology of the
system. Prior to April of 2004, Se concentrations in
the NP source water were generally high (20 μg/l)
and may have posed a risk to fish and other wildlife.
During the study period, disturbances from storm
events and low flow conditions interrupted this
generally stable system. Selenium concentrations in
the majority of plant tissues analyzed from the NP
area appear to be near or below typical levels found in
the Western United States. However, tissue concen-
trations in one aquatic plant taxa (spiny naiad) may
pose an ecological risk to wildlife, particularly during
the summer months. Additional studies of food chain
effects would be necessary to determine the extent of
this ecological risk and to formulate possible mitiga-
tion measures. We recommend continuing monitoring
efforts and studies on the health and status of this
unique wetland as the system evolves and matures.
Acknowledgements This study was funded by the University
of Nevada, Las Vegas Applied Research Initiative and a grant
to UNLV from the State of Nevada Division of Environmental
Protection Agency. In addition, water sampling and analysis
was funded by grants to UNLV from the U.S. Bureau of
Reclamation and Clark County Parks and Community Services.
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Environ Monit Assess (2007) 135:447457 457
... The content of Se in water at all sampling sites did not exceed the acceptable value of 2 lg Se L À1 at the time of sampling (Lemly, 1993). In the study of Pollard et al. (2007) the content of Se in water from wetland, which indirectly receives discharge of wastewaters, ranged from 9.9 to 24.7 lg Se L À1 . The values in polluted waters in the Arkansas River basin ranged from 1 to 240 lg Se L À1 (Canton and Van Derveer, 1997). ...
... The reason for that may be in its growth form, since this plant can take up elements through roots as well as through leaves, while the root uptake can be less effective in submerged plants. In the study of Pollard et al. (2007) the content of Se in the Scirpus californicus was 1.17, in Typha spp. 2.81 and in Scirpus maritimus 1.51 lg Se g À1 . ...
... We also observed some seasonal variations in the content of Se that are different between species as well as streams. In contrast, Pollard et al. (2007) did not find any seasonal variations. We can observe the trend of higher Se content in macrophytes in October and April (Fig. 4). ...
... The content of Se in water at all sampling sites did not exceed the acceptable value of 2 lg Se L À1 at the time of sampling (Lemly, 1993). In the study of Pollard et al. (2007) the content of Se in water from wetland, which indirectly receives discharge of wastewaters, ranged from 9.9 to 24.7 lg Se L À1 . The values in polluted waters in the Arkansas River basin ranged from 1 to 240 lg Se L À1 (Canton and Van Derveer, 1997). ...
... The reason for that may be in its growth form, since this plant can take up elements through roots as well as through leaves, while the root uptake can be less effective in submerged plants. In the study of Pollard et al. (2007) the content of Se in the Scirpus californicus was 1.17, in Typha spp. 2.81 and in Scirpus maritimus 1.51 lg Se g À1 . ...
... We also observed some seasonal variations in the content of Se that are different between species as well as streams. In contrast, Pollard et al. (2007) did not find any seasonal variations. We can observe the trend of higher Se content in macrophytes in October and April (Fig. 4). ...
... Although Se uptake in A. repens was high, the bioconcentration factor was below 1 (data not shown). In a study by Pollard et al. (2007), the tissues of aquatic plants growing in a swamp with the concentration of 20 g Se L −1 at the inflow, contained lower amounts of Se, with values reaching 1.16 g Se g −1 in Scirpus californicus and 4.70 g Se g −1 in Najas marina. In Ruppia maritima exposed to concentrations of 1 mg Se(IV) L −1 Se, the final content after 21 days was 100 g Se g −1 (Bailey et al., 1995). ...
Studies are scarce regarding the physiological effects of selenium treatment on antioxidant capacity of plants. In the presented in vitro experiment, Apium repens was used to investigate its ability to absorb Se(IV) at different concentrations (0.01, 0.1, 1, 10 and 50 mg L⁻¹) and its physiological response to treatment. These plants possessed a high affinity to take up Se(IV), since Se concentration measured in plants reached 730 μg g⁻¹ DM in 50 mg L⁻¹ treated plants. Lower Se(IV) concentrations (0.01–1 mg L⁻¹) promoted growth, while higher Se(IV) concentrations (10 and 50 mg L⁻¹) affected it negatively. The photochemical efficiency decreased significantly in plants treated with higher Se(IV) concentrations compared to control plants. In the roots, total cysteine and glutathione content increased gradually from 0.01 to 10 mg L⁻¹ Se treatment, when compared to the control. Enhanced total glutathione levels were also determined in the above ground parts when the plants were treated with lower Se(IV) concentrations (0.01–1 mg L⁻¹). In both roots and above-ground parts, 50 mg L⁻¹ Se(IV) treatment caused a significant degradation of total cysteine and glutathione, which was accompanied by a significantly larger oxidized glutathione pool. By applying five different concentrations of Se(IV), it was possible to identify a threshold Se content for A. repens, above which the nature of the effects induced changes from antioxidant to pro-oxidant.
... The amount of Se in the tissues of water plants growing in a swamp containing 20 μg Se L −1 , contained 1.16 μg Se g −1 in Scirpus californicus and 4.70 μg Se g −1 in Najas marina (Pollard et al. 2007). In Ruppia maritima exposed to concentrations around 6 mg Se L −1 , the content of Se was 200 μg Se g −1 (Bailey et al. 1995). ...
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In the presented in vitro experiment, the effect of selenite treatment (0.01, 0.1, 1, 10 and 50 mg L−1 Se(IV)) on Berula erecta was investigated with respect to growth, photochemical efficiency, photosynthetic pigments, anthocyanins and low molecular weight thiols. Lower Se(IV) concentrations (0.01, 0.1 and 1 mg L−1) promoted growth, while higher Se(IV) concentrations (10 and 50 mg L−1) negatively affected it. The photochemical efficiency of photosystem II decreased significantly in plants treated with higher Se(IV) concentrations, compared to that in the control plants. The content of pigments decreased in all the Se(IV) treatments. Both cysteine and glutathione showed alterations in their content and redox state depending on the Se concentration. By evaluating the glutathione/cysteine system and their redox, it was possible to identify a threshold Se content (1-mg Se(IV) L−1 treatment), above which the nature of the effects induced changes from antioxidant to pro-oxidant.
... Se concentrations in the Žerovniščica stream from 0.07 to 0.23 ng mL −1 , and in the Pšata stream from 0.03 to 0.15 ng mL −1 confirmed the range from 0.01 to 0.5 µg L −1 for typical concentrations of Se in freshwaters [51]. Nevertheless, these values of Se were lower than those in a study from a wetland which indirectly receives discharge of wastewaters, having Se concentrations from 9.9 to 24.7 µg L −1 [52]. Cd concentrations in water ranged from < 0.0 to 0.75 ng mL −1 with the highest concentration in the Ižica stream during the year 2010 and at the reference site in the year 2011. ...
Full-text available
Ten locations in the Notranjska region, Slovenia, with different land use in the catchment (town, village and agricultural areas), including reference points with different geological composition considered as unpolluted sites, were sampled for water and aquatic moss to evaluate environmental assessment in fresh water systems of the Sava River watershed. Samples of fresh water and Fontinalis antipyretica were taken in all four seasons during the years 2010 and 2012. The water chemistry of the investigated locations was dominated by [Formula: see text], while concentrations of [Formula: see text] seasonally ranged from 2.1 to 6.4 mg L(-1) and at one of the reference sites did not exceed 1.3 mg L(-1). δ(13)CDIC values seasonally ranged from -13.3 to -8.1 ‰ and indicated waters dominated by degradation of organic matter and dissolution of carbonates. δ(13)Cplant values of F. antipyretica seasonally ranged from -45 to -32.9 ‰ and of δ(15)Nplant from -0.2 to 6.5 ‰. The higher δ(15)N value of 6.5 ‰ found in F. antipyretica was related to agricultural activity in the watershed. The content of minor and trace elements in F. antipyretica ranged from 4-38 µg g(-1) for Ni, 17-105 µg g(-1) for Zn, 2-28 µg g(-1) for Pb, 0.26-1.95 µg g(-1) for Cd, 4-27 µg g(-1) for Cu, 4-49 µg g(-1) for Cr, 1-6 µg g(-1) for As and 0.33-3.24 µg g(-1) for Se. The most polluted watershed was the Pšata stream (agricultural area, cattle farm with the highest concentration of nitrate in water) also with highest values for Ni, Cr, Pb, Zn and As.
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The absorption of selenium (Se) from water by Lemna minor L. and the influence of different concentrations and forms of Se on its biochemical and morphological characteristics were studied. Plants were exposed to various concentrations of Se: 1 mg Se L−1, 2 mg Se L−1 and 5 mg Se L−1 in sodium selenite and sodium selenate solutions and in a combination of selenite (2 mg Se L−1) and selenate (2 mg Se L−1). When the Se was added in the form of selenate, plants accumulated higher amounts of Se compared to plants exposed to selenite. Comparisons of the combined addition of selenite and selenate (2 + 2 mg Se L−1) with their individual applications (2 and 5 mg Se L−1) showed that for the combination, the L. minor fronds accumulated more Se than in selenite alone. Plants exposed to any of the concentrations of sodium selenate or sodium selenite, or the combination of selenite and selenate, showed inferior physiological performances, and lower concentrations of photosynthetic pigments, compared to control plants. Consequently, growth was also suppressed under the stress conditions caused by higher concentrations of Se in any form. The efficient absorption of Se from the water by L. minor indicates the potential use of this species in phytoremediation processes for waters polluted with Se.
As a trace element, selenium can cause serious harm to organisms when the concentration is too high. Coal-fired power plants are the main source of man-made selenium emissions. How to control the selenium pollution of coal-fired power plants to realize the renewable selenium and the sustainability of coal has not attracted enough attention from the whole world. This article outlines the conversion and occurrence of selenium in coal-fired power plants. A small part of the selenium produced by combustion can be removed by selective catalytic reduction (SCR) and electrostatic precipitator (ESP) after the gas phase undergoes physical condensation and chemical adsorption to combine with the particulate matter in the flue gas.Because the chemical precipitation method has poor selenium removal effect, the remaining part enters the flue gas desulfurization absorption tower and can be enriched in the desulfurization slurry. The occurrence situation and conversion pathway of selenium in desulfurization slurry are introduced subsequently, the research progress of selenium removal from wet desulfurization wastewater is reviewed from three aspects: physics, biology and chemistry. We believe that the coupling application of oxidation-reduction potential (ORP) and pH can optimize selenium removal in the desulfurization system by improving the oxidation control. As a technology for wet desulfurization system to treat selenium pollution, it has a good development prospect in near future. Implication Statement Selenium is a trace element present in coal. It is not only of great significance to the life activities of organisms, but also a kind of rare resource. As the most important source of man-made emissions, coal-fired power plants will cause waste of selenium resources and selenium pollution in the surrounding environment. In this study, the occurrence, conversion and control of selenium in coal-fired power plants were systematically sorted out and analyzed. It is helpful for scholars to study the selenium transformation process more deeply. It is of great significance for policy formulation of recommended control technologies and emission limits. It is of great value for the formulation of recommended control technology and the in-depth study of the selenium transformation process.
Selenium (Se) has potentially deleterious impacts on flora and fauna of aquatic ecosystems. As Se moves through a wetlands system, various processes such as sorption onto sediments, plant uptake, and volatilization into the atmosphere can attenuate Se resulting in its storage in the wetlands. A comparison of inlet and outlet Se fluxes can be used to determine the mass of Se stored in a wetlands system. Inlet and outlet total Se concentrations and water discharge were measured at the Pariette Wetlands, UT, and used to calculate Se fluxes. The difference between inputs and outputs or fluxes gave great insight into how much Se was being retained or stored in the wetlands. The average influx of Se was 1530 kg year⁻¹ and outflux was 380 kg year⁻¹. On average, 75% (1150 kg year⁻¹) of Se entering the wetlands was retained or stored by some biogeochemical process. Processes associated with Se retention included bioaccumulation into the biota, volatilization by plants and sediments, precipitation of insoluble phases, and sorption to sediments, which accounted for most of the attenuated Se. Water movement through the Pariette Wetlands system did not appreciably alter annual Se attenuation rates. Input, output, storage, and fate of Se for four other wetlands were compared with Pariette Wetlands with Se storage being similar among three of the wetlands: Tulare Lake (65%), Imperial (46%), Brawley (72%), and Pariette (75%).
Selenium (Se) mobilization in the soils of the Pariette Draw and subsequent accumulation into the Pariette Wetlands threaten wildlife. To gain a better understanding of the soil biogeochemistry of Se in the draw, the physical and chemical properties for two soils in the Pariette Draw located in eastern Utah were compared: one a formerly arid soil (elevation 1448 m) that was flooded following the creation of the wetlands in 1975, and the other an arid soil (elevation 1467 m). The soils were analyzed, and the influence of inundation by irrigation return water of Pariette Draw of Utah was assessed to obtain a better understanding of soil Se and salts in this system. It appears that Se mobility, especially in the wetlands soil, is associated with the distribution of soluble salts in the soil profile. Due to a fluctuating water table caused by a deluge of irrigation return water and high evapotranspiration rates coupled with low precipitation, capillary migration is the dominant mechanism driving the distribution and accumulation of salts and soluble Se in the upper horizons (Byz) of the wetlands soil. The distribution of soluble salts and Se in the upland soil is typical of a downward gravity-driven hydrology process. Gypsum solubility regulated sulfate levels within the Byz horizons of the wetlands soil. We surmise that soluble Se is regulated by the solubility of a sodium selenate sulfate coprecipitate. It appears that the relatively low concentration of Se in the local Pariette Wetlands soil is not the source and cause of Se responsible for adversely affecting wildlife in the wetlands.
In October 1985 the U.S. Department of the Interior (DOI), through the National Irrigation Water Quality Program (NIWQP), began a series of field investigations at 26 areas in the Western United States to determine whether irrigation drainage has had harmful effects on fish, wildlife, and humans or has reduced beneficial uses of water. In 1992 NIWQP initiated the Data Synthesis Project to evaluate data collected during the field investigations. Geologic, climatologic, and hydrologic data were evaluated and water, sediment, and biota from the 26 areas were analyzed to identify commonalities and dominant factors that result in irrigation-induced contamination of water and biota. Data collected for the 26 area investigations have been compiled and merged into a common data base. The structure of the data base is designed to enable assessment of relations between contaminant concentrations in water, sediment, and biota. The data base is available to the scientific community through the World Wide Web at URL <>. Analysis of the data base for the Data Synthesis included use of summary statistics, factor analysis, and logistic regression.
Leaf material from selenium accumulating plants was collected and analyzed for selenium to obtain a relative indication of selenium concentrations in soils and identify sites suitable for further soil study. Selenium concentrations of 14 samples of leaf material from Astragalus oxyphysus ranged from 0.08 to 3.5 microg/g dry weight and had a median concentration of 0.25 microg/g. Five replicate samples of A. oxyphysus had a mean selenium concentration of 0.22 microg/g and a standard deviation of 0.07. Selenium concentrations of 17 samples of leaf material from Atriplex lentiformis ranged from 0.08 to 7.5 microg/g and had a median concentration of 0.35 microg/g. As a general guideline, the National Academy of Sciences recommends a maximum safe tolerance level of 2 microg/g of selenium in animal feeds. One sample of A. oxyphysus, collected in the Panoche Creek drainage, exceeded 2 microg/g. Three samples of A. lentiformis, collected in Klipstein Canyon, Tumey Fan, and Panoche Fan, equaled or exceeded 2 microg/g. These sites may be suitable. 34 refs., 5 figs., 2 tabs.
A method for determining Se and As in plant and animal tissues by hydride generation inductively coupled plasma mass spectrometry was developed, based on an evaluation of sample digestion, chemical interference and isobaric interference. Plant or animal tissue samples were digested with HNO3+ HCIO4. SeVI was reduced to SeIV by heating the samples in 4 mol l–1 HCI, but As remained as AsV. Digestion with HNO3+ HClO4 was superior to digestion with HNO3+ H2O2, because the former was efficient, complete, and had less isobaric interference from bromine. The observed chemical interference might be caused by Cu in animal tissues forming precipitates with the hydrides during hydride generation. This chemical interference was removed by raising the sample acidity to 4 mol l–1 HCl. The isobaric interference of 81BrH with 82Se was removed by evaporating the samples at 200 °C and by isobaric correction. Se could be measured by using 77Se, 78Se or 82Se. The limit of detection was about 0.02 µg l–1 for both Se and As, which is less than the background equivalent concentration of 0.05 µg l–1 for Se and As. Based on a dilution factor of 50, the limit of detection for both Se and As in tissue samples was 0.001 mg kg–1(0.001 ppm). The measured Se and As contents in five NIST SRMs were in good agreement with the certified values.
Phytoremediation of toxic inorganic selenium compounds by accumulation, assimilation, and volatilization is an ideal way to rid contaminated soils and sediments of these molecules. In this context, salt marsh cordgrass (Spartina alterniflora) was investigated for its potential to produce dimethylselenoniopropionate (DMSeP), which as we have shown can serve as a precursor for the enzymatic volatilization of the relatively nontoxic gas, dimethylselenide (DMSe). Plants grown in sand culture, under varying saline conditions amended with the environmentally toxic form of selenium (selenate) were analyzed for organoselenonium compounds. DMSeP was positively identified in plant tissue and partially purified plant extracts by alkaline degradation to DMSe, 1H and 77Se NMR, and by enzymatic cleavage by DMSP lyase to DMSe (and acrylate). DMSeP levels were highest in plants grown in high salt (full-strength seawater) and high selenium. Preliminary evidence suggests that cordgrass may also produce Se-methyl selenomethionine, the putative precursor of DMSeP. This appears to be the first report for the biological assimilation of selenate into DMSeP by a plant species. These findings suggest a possible mechanism for the volatilization of selenium, as DMSe, analogous to that of dimethylsulfide (DMS) production by the salt tolerant cordgrass, Spartina alterniflora.
Selenium-laden effluents from oil refineries are polluting San Francisco Bay, California. One environmentally friendly way of cleaning up selenium (Se) from effluents is by plant and microbial Se volatilization using constructed wetlands. Using mesocosms, we investigated the role of biological Se volatilization in a 36-ha constructed wetland located adjacent to San Francisco Bay. The constructed wetland was highly effective in removing Se from selenite-contaminated oil refinery wastewater:  89% of the Se was removed. Inflow Se concentrations of 20−30 μg L-1 decreased to <5 μg L-1 in the outflow. Most of the Se was removed by immobilization into sediments and plant tissues where Se concentrations reached 5 and 15 mg kg-1, respectively. Biological volatilization may have accounted for as much as 10−30% of the Se removed. The highest mean rates of Se volatilization for vegetated sites were 190, 180, and 150 μg of Se m-2 day-1 (rabbitfoot grass, cattail, and saltmarsh bulrush, respectively). Rates for the most dominant species, saltmarsh bulrush, varied during the year:  the mean rates were 150, 70, and 25 μg of Se m-2 day-1 in February, June, and October, respectively. We conclude that biological Se volatilization is a significant pathway of Se removal in wetlands.
Ecological impacts of water-quality problems have developed in the western United States resulting from the disposal of seleniferous agricultural wastewater in wetland areas. Overt effects of selenium toxicosis occurred at five areas where deformities of wild aquatic birds were similar to those first observed at Kesterson National Wildlife Refuge in the west-central San Joaquin Valley of California. These areas are: Tulare Lake Bed Area, California, Middle Green River Basin, Utah, Kendrick Reclamation Project Area, Wyoming, Sun River Basin, Montana, and Stillwater Wildlife Management Area, Nevada. Potential for ecological damage is indicated at six more sites in Oregon, Colorado, the Colorado/Kansas border, and South Dakota out of 16 areas in 11 states where biological tissue data were collected. This conclusion is based on the fact that selenium bioaccumulated in bird livers to median levels that had exceeded or were in the range associated with adverse reproductive effects. Selenium concentrations in samples of fish and bird eggs support these conclusions at a majority of these areas. Reason for concern is also given for the lower Colorado River Valley, although this is not exclusively a conclusion from these reconnaissance data. Biogeochemical conditions and the extent of selenium contamination of water, bottom sediment, and biota from which this assessment was made are given here. In a companion paper, the biogeochemical pathway postulated for selenium contamination to take place from natural geologic sources to aquatic wildlife is defined.
The Cienega de Santa Clara, on the east side of the Colorado River delta, is a brackish wetland supported by agricultural drainage water from the United States that provides habitat for endangered fish and bird species. Bioaccumulation of selenium has created toxicity problems for wildlife in similar wetlands in the United States. This is the first selenium survey in the Cienega de Santa Clara. Ten sites were selected to collect water (dissolved), sediments (total), plants, invertebrates, and fish. Samples were collected from October 1996 to March 1997. Selenium was detected in all samples. Concentrations in water ranged from 5 to 19 microg/L and increased along a salinity gradient. Although water levels of selenium exceeded EPA criterion for protection of wildlife, levels in sediments (0.8-1.8 mg/kg), aquatic plants (0.03-0.17 mg/kg), and fish (2.5-5.1 mg/kg whole body, dry wt) did not exceed USFWS recommended levels. It is concluded from this study that the levels of selenium in water did not affect the overall health of the fish sampled. Therefore, it is important to maintain or improve the water quality entering this wetland to continue to have normal levels of Se in the food chain components.
The consequences of elevated Se accumulation at the Kesterson Reservoir National Wildlife Refuge in the Central Valley of California created adverse effects on wildlife and led to extensive research on the behavior of Se in both the wetland and upland ecosystems. Selenium concentrations in water entering the Kesterson Reservoir averaged 300 microg L(-1). In pond waters 20-30% of the Se was selenate, while only 2% was selenite in the drainage water entering the reservoir. Submerged rooted aquatic plants fed on by water birds were found to contain 18-390 mg Se kg(-1) dry weight. Mosquitofish collected from the San Luis Drain contained 332 mg Se kg(-1), and those collected from the ponds ranged from 339 to 380 mg kg(-1). Livers of water birds had Se concentrations ranging from 19.9 to 127 mg kg(-1). The high concentrations of Se accumulation in the food chain of the wetland strongly suggest that Se bioaccumulation was the cause of death and deformity of embryos of the waterfowl nesting at the wetland habitat. In June 1986, the Kesterson Reservoir was closed to drain-water inputs, and the wetland was transformed to an upland grassland. New remedial plans were proposed. These new plans involved soil, water, and vegetation management to dissipate Se by bioaccumulation and volatilization through soil microorganisms and plants. The investigations of the potential transfer of Se from farm land into the crop and vegetables in the Central Valley indicated that plant tissue Se concentrations generally fall in a nonseleniferous category, except that the highest Se concentration of cotton was at a threshold where toxicity in animals could occur at a relatively low frequency. At the Kesterson upland grassland habitat, average total Se concentrations ranged from 500 to 8000 microg kg(-1) and water-extractable Se ranged from 10 to 700 microg kg(-1) in the top 15 cm of soil and varied greatly, by a factor greater than 100, among soil samples. Uptake of Se by the plants was profoundly affected by the soil available Se concentration, soil moisture, pH, soil salinity, soil sulfate concentration, soil reoxidation condition, kind of plant species, and soil-management practices. The rate of soil Se dissipation at the Kesterson grassland system was from 1% (low methylation rate) to 5% (high methylation rate) Se inventory per year and it will take from 46 to 230 years to bring the soil Se down to a normal level, 4 mg Se kg(-1) soil. However, the Kesterson upland grassland habitat had Se bioaccumulation values less than 10% of those of the previous wetland. The potential food-chain contamination at the existing Kesterson grassland is much less problematic. No negative impact on wildlife has been reported for the upland habitat. Plants may contribute to the Se reoxidation process and be able to reduce the movement of Se in the soil. At the Kesterson grassland, the distribution of soil Se is extremely uneven; high levels of soil Se concentrated only in isolated spots. Therefore, leaching of soil Se is not at an area level. It is unlikely that problems of transport of Se from the Kesterson soil to the adjacent uncontaminated environment by leaching can occur.
In many environmental contaminant situations selenium has become the primary element of concern because of its bioaccumulative nature in food webs. Initial concerns about selenium were related to fish kills at Belews Lake, NC, Martin Lake, TX, and Kesterson Reservoir, CA, and to bird deformities at Kesterson Reservoir. Additional concerns were identified under the National Irrigation Water Quality Program at Salton Sea, CA, Kendrick, WY, Stewart Lake, UT, and Grand Valley and Uncompahgre Valley, CO. Recent studies have raised concerns about selenium impacts on aquatic resources in Southeastern Idaho and British Columbia. The growing discomfort among the scientific community with a waterborne criterion has lead the US Environment Protection Agency to consider a tissue-based criterion for selenium. Some aquatic ecosystems have been slow to recover from selenium contamination episodes. In recent years, non-governmental researchers have been proposing relatively high selenium thresholds in diet and tissue relative to those proposed by governmental researchers. This difference in opinions is due in part to the selection of datasets and caveats in selecting scientific literature. In spite of the growing selenium literature, there are needs for additional research on neglected organisms. This review also discusses the interaction of selenium with other elements, inconsistent effects of selenium on survival and growth of fish, and differences in depuration rates and sensitivity among species.