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Perchlorate uptake by salt cedar (Tamarbc ramosissima}
in the Las Vegas Wash riparian ecosystem
Edward T. Urbansky*, Matthew L. Magnuson, Catherine A. Kelty,
Stephanie K. Brown
United States Environmental Protection Agency, National Risk Management Research Laboratory, Water Supply and Water
Resources Division, 26 West Martin Luther King Drive, Cincinnati, OH 45268, USA
Received 24 March 2000; accepted 25 March 2000
Abstract
Perchlorate ion (C1OJ) has been identified in samples of dormant salt cedar (Tamarix ramosissima) growing in the
Las Vegas Wash. Perchlorate is an oxidant, but its reduction is kinetically hindered. Concern over thyroid effects
caused the Environmental Protection Agency (EPA) to add perchlorate to the drinking water Contaminant
Candidate List (CCL). Beginning in 2001, utilities will look for perchlorate under the Unregulated Contaminants
Monitoring Rule (UCMR). In wood samples acquired from the same plant growing in a contaminated stream,
perchlorate concentrations were found as follows: 5-6 u-g g"1 in dry twigs extending above the water and 300 |xg g~'
in stalks immersed in the stream. Perchlorate was leached from samples of wood, and the resulting solutions were
analyzed by ion chromatography after clean-up. The identification was confirmed by electrospray ionization mass
spectrometry after complexation of perchlorate with decyltrimethylammonium cation. Because salt cedar is regarded
as an invasive species, there are large scale programs aimed at eliminating it. However, this work suggests that salt
cedar might play a role in the ecological distribution of perchlorate as an environmental contaminant. Consequently,
a thorough investigation of the fate and transport of perchlorate in tamarisks is required to assess the effects that
eradication might have on perchlorate-tainted riparian ecosystems, such as the Las Vegas Wash. This is especially
important since water from the wash enters Lake Mead and the Colorado River and has the potential to affect the
potable water source of tens of millions of people as well as irrigation water used on a variety of crops, including
much of the lettuce produced in the USA. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Perchlorate; Salt cedar; Tamarix; Riparian ecosystem; Drinking water; 1C; ESI-MS; Eradication
* Corresponding author. Tel.: +1-513-569-7655; fax: + 1-513-569-7658.
E-mail address: urbansky.edwardlS'epa.gov. (E.T. Urbansky).
0048-9697/00/S - see front matter 2000 Elsevier Science B.V. All rights reserved.
PII: 30048-9697(00)00489-7
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E.T. Urbansky et al. / The Science of the Total Environment 256 (2000) 227-232
1. Introduction
Salt cedar (Tamarix ramosissima) is a woody
plant, indigenous to Asia now found as an inva-
sive species in western riparian (streambank)
ecosystems (Westbrooks, 1998). Southwestern
states experience the greatest effects due to the
favorable climate (Muzika and Swearingen, 1999)
with more than a million acres of land occupied
(Deuser et al., 1998). T. ramosissima accumulates
salt, which it then secretes through its leaves. This
brackifies the nearby soil to the extent that native
genera (e.g. Salix) cannot grow (Muzika and
Swearingen, 1999). Because of the serious impact
on native flora, projects to eradicate salt cedar
have been undertaken by the Natural Park Ser-
vice (Deuser et al., 1998). Tamarisks have colo-
nized much of the land surrounding the Las Ve-
gas Wash and can be found growing in shallow
surface waterways, such as creeks or springs. The
plant consumes and transpires incredible amounts
of water so that it can bring about fluctuations in
the local water table when it becomes active,
roughly from March to December.
Perchlorate has been identified as a contami-
nant in the soil, surface water, and ground water
of the Las Vegas Wash (Urbansky, 1998; Renner,
1999). The contamination in this region has been
attributed to the production of ammonium per-
chlorate by defense and aero-space contractors
dating back several decades (Damian and Pon-
tius, 1999). Perchlorate's nature makes both
potable water treatment and site remediation
difficult (Urbansky and Schock, 1999; Espenson
2000). Perchlorate affects the thyroid (Wolff, 1998;
Clark, 2000), and was placed on the Contaminant
Candidate List (CCL) (Environmental Protection
Agency, 1998; Perciasepe, 1998) and the Unregu-
lated Contaminants Monitoring Rule (UCMR)
(Browner, 1999). Water from the Las Vegas Wash
flows into Lake Mead and becomes part of the
Colorado River. Lake Mead and the downstream
portion of Colorado River serve as the major
source of potable water for southern California
(including Los Angeles), southern Nevada (greater
Las Vegas), and parts of Arizona. Consequently,
the drinking water of tens of millions of people is
potentially at risk. Moreover, Colorado River
water and other contaminated waters are used to
irrigate farms that supply significant portions of
the produce consumed in the USA. Therefore,
this contamination has the potential to affect the
nation's food supply if perchlorate is absorbed by
food plants.
Because salt cedar influences the hydrology of
the Las Vegas Wash and perchlorate salts are
very water-soluble, it seems reasonable to con-
sider whether salt cedar can take up perchlorate
along with other salts.
2. Experimental section
Sections of tamarisk twigs, branches, and stalks
ranging from 2 to 12 mm were collected from
plants growing in the Las Vegas Wash. Samples
were taken from sections of plant submerged in
the water and from sections extending above the
surface of the water. A mass of 80 g (wet mass) of
the submerged stalks (3-12 mm diameter) was
soaked in running deionized (DI) water for 2 min
and then rinsed with a spray of DI water. A mass
of 20 g (diy mass) of the exposed twigs (2-7 mm
diameter) was similarly cleaned. Branch sections
were shredded in a blender; and leached at 5°C
for ~ 60 h; the plant matter to water ratio was 20
g dl~'. The mixture was then suction filtered
through Corning (Corning, NY, USA) 0.45-(jum
cellulose acetate with a glass fiber pre-filter.
Ion chromatography (1C) standards were pre-
pared in DI water using sodium perchlorate (GFS
Chemicals, Columbus, OH, USA). A stock solu-
tion containing 1000 jig ml"' was diluted as
needed for spiking, constructing a calibration
curve, and determining retention time. The col-
lected shredded wood filtrates prepared at 20 g
df1 were diluted at a ratio of 1:6 v/v with DI
water. The diluted solution from the submerged
branches was cleaned up by passing a 5.0-ml
aliquot through either Supelco (Bellefonte, PA,
USA) Envi-Carb (carbon), Envi-Chrom P (poly-
styrene), or Waters (Milford, MA, USA) Sep-Pak
CliS cartridge.
A Dionex DX300 (Sunnyvale. CA, USA) 1C
(500-(jil loop) equipped with AG11 and ASH
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E.T. Urbansky et al. / The Science of the Total Environment 256 (2000) 227-232
229
columns (4 mm diameter) was used with isocratic
0.10 M NaOH (aq.) at 1.00 ml min~' with sup-
pressed conductivity detection (Jackson et al.,
1999; Wirt et al., 1998).
To prepare a sample for electrospray ionization
mass spectrometry (ESI-MS), a 5.0-ml aliquot of
dry twig 20 g dl~' filtrate was run through six C1S
cartridges. The 1C was operated in preparative
mode (2.00-ml loop). The eluate was collected
from 7.1 to 9.0 min. This collected fraction of
eluate was reinjected in its entirety using the
same sample loop, and the peak at 8 min (C1O^~)
was collected.
Complexation ESI-MS has been applied to
water samples (Magnuson et al., 2000a,b; Urban-
sky and Magnuson, 2000; Urbansky et al., 2000).
Perchlorate forms a stable ion pair with a quater-
nary ammonium cation. This complex is extracted
into CH2C12 and injected without separation. The
following were added to the collected fraction: 50
(jul of 0.20 M C10H21N(CH3)3Br(aq.) solution
(Fluka, Buchs, Switzerland) and 300 fjul of CH,C1,
(J.T. Baker Ultra Resi-analyzed, Phillipsburg, NJ,
USA). After extraction, the CH2C12 phase was
drawn off with a syringe. Aliquots of 25 JJL! were
injected into the ESI-MS system as described
(Magnuson et al., 2000a,b)
3. Results and discussion
Chromatograms for preparative and analytical
1C are shown in Fig. 1. Negative ion ESI-MS
signals at m/z 380 and 400 jjum correspond
to C10H21NMe3(Br)(ClO4r and C]0H21NMe3-
(C1O4)2, respectively. Peaks for the two ions are
shown in Fig. 2. The average signal-to-blank ratio
(S/B) using the sum, (AX(}u + A4mj, was 4.2.
Even separately, (5/6)380u = 4.9 and (S/B\Wu =
2.1, either of which confirms the identification.
The 1:6 v/v dilution of the 20-g dl~' filtrate of
the submerged stalks was found to contain 10 jxg
ml ~' by 1C, while that of the dry twigs was found
to contain 200 ng ml"1. These concentrations
should be viewed as minima. There is no standard
material by which to gauge the effectiveness of
the leaching procedure; therefore, this prelimi-
nary finding is limited by the state of the science.
a
retention time, min
perchiorata
456
retention time, m!n
Fig. 1. (a) Preparative ion chromatograph of the 20 g dl~
filtrate of dormant, dry (above water level) Taniarix mmosis-
sinia twigs. Peak 5 is perchlorate. Injection was made using a
2-ml sample loop. The fraction from 7 to 9 min was collected
and reinjected for analysis, (b) Analytical ion chromatograph
of the 7-9-min collected fraction of eluate from Fig. 1. Peak 4
is the perchlorate peak. Injection was made using a 2-ml
sample loop. The fraction from 7.9 to 9.2 min was collected.
The identification of perchlorate was confirmed by complexa-
tion electrospray ionization mass spectrometry (cESI-MS). The
perchlorate was extracted into dichloromethane using decyl-
trimethylammonium cation (see Fig. 2 and text for more
details).
No difference was observed between the clean-up
cartridges, suggesting that perchlorate is not re-
tained.
Submerged samples contained 300 pug g"1,
while exposed samples contained 5-6 (Jig g" .It
is difficult to draw a conclusion about rate and
selectivity of uptake in T. ramosissima, but it is
clear that immersed stalks can absorb/adsorb
significant amounts. Perhaps this could be ex-
ploited for remediation. Perchlorate-reducing
microorganisms, especially facultative anaerobic
bacteria are readily cultured in the laboratory
(Coates et al., 2000; Giblin et al., 2000; Logan,
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E. T. Urbansky et al. / Tlie Science of the Total Environment 256 (2000) 227-232
m/z: 380
so -
40 ~
20 -
0 -J
9.59
HH 608
HA 16658
11.51
MH 584
MA 20324
13.87
MH 631
MA 17321
10.0
m/z: 400
»!,'
100
80 -
60 -
40 •
20 -
0-
9.54
MH 230
MA 5654
11.46
HK 225
HA 6991
13.97
HH 214
HA 7946
10.0
15.0
23.77
,, „- HH 8674
MH 6194 «* S8908
HA 78964
25.93
MH 875B
MA 99S15
2B.20
HH 7578
MA 89003
22.09 23.77
MH 1270 MH 1073
MA 16367 HA 15292
25.93
HH 998
HA 14860
28.20
KB 919
HA 14167
30.11
KH 6634
HA 80230
30.0
30.36
MX 740
HA 130B1
30.0
Fig. 2. Confirmation of perchlorate by negative ion complexation ESI mass spectrum obtained by extracting peak 4, which was
collected from eluate in Fig. 2, with a cationic surfactant (decyltrimethylammonium bromide) into dichloromethane. Both the
bromoperchiorato and bis(perchlorato) complex anions are observed in the mass spectrum. Ci0H2iNMe3(BrXClO4)~ has m/z - 380
u; C10H21NMe3(ClO4)f has m/z = 400 u. The first three injections are the blank (extract of 1C eluate of just eluent after going
through the suppressor). The last five injections are the dichloromethane extract of peak 4. Injection volume was 25 jxl.
1998). Phytoremediation of perchlorate by willows
(genus Salix) has been reported by Nzengung and
Wang (1998). Because significant incorporation of
perchlorate would be expected with long-term
rather than short-term or one-time exposure, diel
fluctuations in concentrations may be averaged
out, providing a more uniform measure of expo-
sure as a biological indicator.
4. Conclusions
For better or worse, salt cedar must now be
regarded as a major part of the ecosystem in the
American Southwest and certainly in the Las
Vegas Wash. Accordingly, plans to eradicate
Tamarix must be evaluated in terms of total eco-
logical impact, including effects on aqueous phase
pollutants, such as perchlorate. It is not possible
at this time to state what effect Tamarix has on
perchlorate distribution in the Las Vegas Wash.
Only two things are known for certain: Tamarix
consumes an enormous amount of water and
Tamarix can take up perchlorate.
Several lines of research are suggested by these
facts. It may be possible to use Tamarix to con-
centrate the highly soluble perchlorate salts as
part of a remediative strategy at some sites or to
accumulate it in the wood. On the other hand,
high rates of transpiration in salt cedar may make
the perchlorate concentrations in ground water
higher if perchlorate ion is selectively excluded
when water is taken into the root system. Conse-
quently, it will be important to answer the fol-
lowing questions: (1) Do tamarisks absorb all of
the perchlorate along with the water they take in?
(2) Do tamarisks secrete perchlorate salts on their
leaves or retain them within the plant? (3) Can
tamarisks harbor microbes in the rhizosphere that
are capable of reducing perchlorate? (4) Do
tamarisks metabolize (reduce) perchlorate to
other oxyanions of chlorine or to chloride? (5)
Can perchlorate levels found in tamarisk wood be
used to assess long-term contamination of ripar-
ian ecosystems?
The nature, rate, and selectivity of perchlorate
uptake and processing by salt cedar are all un-
known, therefore, we cannot say whether this
plant is improving or reducing water quality. Fur-
thermore, without accurate information on the
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E. T. Urbansky et al. / The Science of the Total Environment 256 (2000) 227-232
231
transport and fate of perchlorate in Tamarix, the
impact of eradication programs on perchlorate
cannot be evaluated. Many programs rely on herb-
icides; however, any perchlorate in the plant is
likely to be returned to the soil and ground water
as the plant deteriorates and decomposes. This
could potentially compound a problem if the salt
cedar acts as a reservoir and the perchlorate is
released upon the death of the plant. In order to
ensure the protection of native species and the
waterways and sound environmental management
practices, it is necessary to completely understand
the relationship among source water, salt cedar,
and perchlorate in ecosystems such as the Las
Vegas Wash.
Acknowledgements
We acknowledge Professor Jacimaria R. Batista
(University of Nevada-Las Vegas) for assistance
in procuring samples of salt cedar and Brenda
Pohlmann (Nevada Division of Environmental
Protection) for helpful discussions. Mention of
specific brand names or models should not be
construed to suggest endorsement or recommen-
dation by the US government. This paper was
produced by US government employees in the
course of their official duties and is, therefore,
exempt from copyright.
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