S C I E N C E ’ S C O M PA S S
gists and statisticians, as well as the goodwill of researchers who have invested
many years of their careers in collecting
detailed data. Our ability to make generalizations about population dynamics will
facilitate conservation efforts, and so benefit future generations.
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
G. M. Mace, R. Lande, Conserv. Biol. 5, 148 (1991).
N. Owen-Smith, J. Anim. Ecol. 59, 893 (1990).
T. Coulson et al., Science 292, 1528 (2001).
B.-E. Sæther, S. Engen, E. Matthysen, Science 295,
2070 (2002).
S. C. Stearns, The Evolution of Life Histories (Oxford
Univ. Press, Oxford, 1992).
B. E. Sæther et al., Proc. R. Soc. London Ser. B 267,
621 (2000).
H. Leirs et al., Nature 389, 176 (1997).
M. Lima, J. E. Keymer, F. M. Jaksic, Am. Nat. 153, 467
(1999).
B. R. Grant, P. R. Grant, Proc. R. Soc. London Ser. B
251, 111 (1993).
Downloaded from www.sciencemag.org on August 28, 2012
in short-lived bird
Sæther et al. demonspecies the population
strate that generalizadynamics are more
tions might exist. The
Image not
strongly influenced by
next step will be to atavailable for
the recruitment of new
tempt to confirm these
individuals into the
generalizations with
online use.
population, whereas in
more data sets from a
long-lived birds the
wider range of populadynamics are more
tions and species.
strongly influenced by the survival rates of
Sæther et al. show that there are prethe adults that are already present in the dictable differences between the populapopulation. Understanding the association tion dynamics of species with different life
between population dynamics and birth, histories. They present a benchmark study
death, and dispersal rates requires that de- in the relatively new field of comparative
tailed data be collected from marked indi- population dynamics. However, the enorviduals. This knowledge is crucial for mous potential of this field can only be rewildlife managers, as it identifies the key alized if data sets of sufficient length and
demographic rate that should be targeted to quality continue to be collected and anareverse population declines or to prevent lyzed. Achieving this goal will require inpopulation eruptions. The f indings of ternational collaboration between ecoloP E R S P E C T I V E S : E N V I R O N M E N TA L C H E M I S T R Y
Tracking Hexavalent Cr
in Groundwater
David Blowes
hroughout the world, groundwater is
used extensively as a source of
drinking water. In the United States,
56% of the population rely on groundwater for their drinking water (1). Much of
the groundwater extracted in North America and elsewhere is supplied by shallow
aquifers, which are susceptible to the release of contaminants from industrial,
agricultural, and domestic activities.
A common groundwater contaminant is
chromium, which is widely used for electroplating, leather tanning, and corrosion
protection (2). Chromium is the second
most abundant inorganic groundwater contaminant at hazardous waste sites (3). On
page 2060 of this issue, Ellis et al. present
a technique for determining the abundance
ratio of stable chromium isotopes and apply it to samples from chromium-contaminated sites (4). The technique provides an
important tool for assessing the migration
of chromium from contaminant sources
and evaluating the effectiveness of chromium removal systems.
The oxidized, hexavalent state of Cr,
Cr(VI), forms chromate (CrO 4 2– ) or
bichromate (HCrO4–). Chromate-containing minerals are very soluble and, because
the chromate ion has a negative charge,
chromate adsorption on aquifer minerals is
T
The author is in the Department of Earth Sciences,
University of Waterloo, Waterloo, Ontario N2L 3G1,
Canada. E-mail: [email protected]
2024
limited (2). As a result, chromate may be
present at concentrations well above water
quality guidelines and may move with the
flowing groundwater in aquifers. In contrast, the reduced state, Cr(III), forms insoluble precipitates under slightly acidic
and neutral conditions, limiting Cr(III) to
very low concentrations in most aquifers
(2). The reduction of Cr(VI) to Cr(III) thus
limits both the concentration and mobility
of dissolved chromium. This difference is
very important because chromate is toxic
and carcinogenic (5–7), whereas Cr(III) is
a nutrient at trace levels.
Variations in the isotopic ratios of light
elements are sensitive indicators of chemical processes that occur in natural systems.
For example, the 34S/32S ratio in dissolved
sulfate increases when bacteria reduce sulfate to sulfide. Ellis et al. (4) now show
that the 53Cr/52Cr ratio also changes during
reduction of Cr(VI) to Cr(III). They show
that abiotic reduction of Cr(VI) resulting
from reaction with the mineral magnetite,
estuarine sediments, and freshwater sediments leads to a consistent 53Cr/52Cr shift.
This observation indicates that 53Cr/52Cr
ratios increase systematically with progressive Cr(VI) reduction in groundwater.
The conventional approach to groundwater remediation is to install a series of
pumping wells, extract the water from the
aquifer, and treat it with techniques previously developed for surface waters. A series of studies in the late 1980s and early
15 MARCH 2002
VOL 295
SCIENCE
A
B
C
D
Groundwater remediation strategies. (A)
No attenuation. (B) Contaminant removal by
natural attenuation processes. (C) Contaminant removal using a permeable reactive barrier. (D) Injection of a reductant to attenuate
the contaminant.
www.sciencemag.org
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1990s indicated, however, that
able reactive barriers. A strong
10,000,000
this “pump-and-treat” approach
reductant such as sodium
9,000,000
was expensive and in many cases
dithionate can be injected di8,000,000
ineffective (8). There are three
rectly into the plume of contamPump and treat
7,000,000
reasons for this. First, the system
inated water (see the first fig6,000,000
extracts and treats clean water
ure) to promote the reduction of
5,000,000
along with the contaminated wachromate and precipitation of
ter. Second, full remediation typchromium hydroxide. Sodium
4,000,000
ically takes longer than predicted
dithionate solutions also can be
3,000,000
because contaminants are often
injected into the path of a chro2,000,000
Permeable reactive barrier (10 year cycle)
retained in fine-grained zones.
mate-containing plume to re1,000,000
Third, contaminants that were
duce natural ferric iron-bearing
Natural attenuation
0
previously precipitated or adminerals, generating a reservoir
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
sorbed to mineral surfaces reof ferrous iron-bearing minerals
Time (years)
lease slowly.
(15). These minerals then reSince the late 1980s, research The costs of different groundwater remediation strategies. Natural duce the chromate as it migrates
has focused on how to overcome attenuation, a permeable reactive barrier, and a pump-and-treat sys- through the aquifer.
the limitations of the convention- tem are considered (14). This comparison focuses on a halogenated hyThe new finding that 53Cr/52Cr
al pump-and-treat approach. One drocarbon plume, but the material used in the reactive barrier was ze- ratios reveal the extent of abiotic
possibility is to use the natural ro-valent iron, the same material used to treat hexavalent chromium. reduction suggests that 53Cr/52Cr
measurements can assist in the
ability of aquifers to attenuate The costs are therefore probably similar for a chromate plume.
evaluation of the effectiveness of
contaminants without intervention (9). This research has resulted in the curred, and there is thus no need to see all these approaches to chromate reduction
development and approval of “monitored whether Cr(VI) mass decreases over time. in groundwater.
natural attenuation” remedies for contami- (ii) The measurement of reduction inteReferences and Notes
nated groundwater. An alternative ap- grates spatially over a flow path, whereas
1. F. van der Leeden, F. L. Troise, D. K. Todd, The Water
proach focuses on the development of in analyses of aquifer solids give information
Encyclopedia (Lewis, Chelsea, MI, 1990).
situ treatment systems that clean the con- on a much smaller spatial scale.
2. C. D. Palmer, P. R. Wittbrodt, Environ. Health Perspect.
53Cr/52Cr measurements can also help
92, 25 (1991).
taminated groundwater within the aquifer
3. National Academy of Sciences Committee on
to evaluate in situ approaches, which have
(10, 11).
Ground Water Cleanup Alternatives, Alternatives for
Ground Water Cleanup (National Academy Press,
Chromate is attenuated naturally be- been developed for sites where natural atWashington, DC, 1994).
cause minerals containing reduced forms tenuation is insufficient to prevent chro4. A. S. Ellis, T. M. Johnson, T. D. Bullen, Science 295,
of iron and sulfur are abundant in many mate migration. In situ approaches include
2060 (2002).
5. E. Nieboer, S. L. Shaw, in Chromium in the Natural
aquifers. These minerals reduce hexavalent permeable reactive barriers, injection of
and Human Environments, vol. 20 of Advances in Enchromium to trivalent chromium and pro- chemical reactants to reduce chromate,
vironmental Science and Technology, J. O. Nriagu, E.
mote the precipitation of insoluble solids and injection of a reductant to react with
Nieboer, Eds. (Wiley, New York, 1988), pp. 399–442.
6. A. Yassi, E. Nieboer, in Chromium in the Natural and
such as chromium hydroxide (9). Organic the aquifer materials to form reduced minHuman Environments, vol. 20 of Advances in Envicarbon-rich materials, such as wetland erals in the aquifer.
ronmental Science and Technology, J. O. Nriagu, E.
Nieboer, Eds. (Wiley, New York, 1988), pp. 443–496.
Permeable reactive barriers are consediments and peat, can also reduce hex7. E. Nieboer, A. A. Jusys, in Chromium in the Natural
avalent chromium. In aquifers where these structed by excavating a portion of the
and Human Environments, vol. 20 of Advances in Enreduced sediments are abundant and the aquifer. The aquifer material is replaced
vironmental Science and Technology, J. O. Nriagu, E.
Nieboer, Eds. (Wiley, New York, 1988), pp. 21–80.
concentrations of hexavalent chromium with a permeable mixture designed to re8. D. M. Mackay, J. A. Cherry, Environ. Sci. Technol. 23,
are low, the attenuation capacity of the act with the contaminant (12). For inor630 (1989).
aquifer may be suff icient to prevent ganic contaminants such as chromium, the
9. C. D. Palmer, R. W. Puls, Natural Attenuation of Hexavalent Chromium in Ground Water and Soils, U.S.
reactive material is selected to retain the
chromium migration (see the first figure).
EPA Ground Water Issue Paper, EPA/540/S-94/505
The appropriateness of monitored natu- contaminant within the barrier (10). Per(U.S. EPA, Office of Research and Development,
Washington, October 1994).
ral attenuation depends on the groundwa- meable reactive barriers containing zeroW. Blowes et al ., J. Contam. Hydrol. 45, 121
ter flow system, the rate of chromium mi- valent iron (iron filings) have been used to 10. D.
(2000).
gration, and the ability of the aquifer ma- treat hexavalent chromium (10, 13).
11. G. Hocking, S. L. Wells, R. I. Ospina, in Chemical Oxidation and Reactive Barriers: Remediation of ChloriLandis (14) has compared the costs asterials to reduce chromate. It can be diffinated and Recalcitrant Compounds, G. B. Wickracult, however, to distinguish measured sociated with three groundwater remediamanyake, A. R. Gavaskar, A. S. C. Chen, Eds. (Battelle,
Columbus, OH, 2000), pp. 307–314.
chromate decreases caused by attenuation tion options for a 400-feet wide and 80permeable reactive barriers are installed in
reactions from those caused by mixing or foot deep contaminant plume: pump-and- 12. Typically
trenches, but barriers also have been constructed by
dispersion in the aquifer. At some sites, it treat, managed natural attenuation, and the
jetting reactive materials into the ground, or by generating fractures within the aquifer and filling the
is therefore useful to test samples of use of a permeable reactive barrier (see the
fracture with reactive materials (11).
aquifer materials for the products of chro- second figure). The comparison suggests 13. D.
W. Blowes et al., An in situ permeable reactive barthat monitored natural attenuation is least
mate reduction.
rier for the treatment of hexavalent chromium and
trichloroethylene in groundwater. Volume 2, PerforThe measurement of chromium isotope expensive, followed by permeable reactive
mance Monitoring, U.S. EPA Report EPA/600/rratios provides a new tool for evaluating barrier systems, particularly if the reactive
99/095b (U.S. EPA, Office of Research and Development, Washington, September 1999).
the extent of chromate natural attenuation barrier is augmented by natural attenuaR. Landis, Section 11 in In Situ Permeable Reactive
and estimating the rate of chromium atten- tion. The pump-and-treat system is much 14. Barriers:
Application and Deployment, U.S. EPA
uation. Relative to conventional methods, more expensive.
Training Manual EPA/542/B-00/001 (U.S. EPA, Solid
53Cr/52Cr
Waste
Management
and Emergency Response, WashInjection of solutions containing
it has two advantages: (i) Each
January 2000).
determination provides a measure of the strong reductants into aquifers provides 15. ington,
J. S. Fruchter et al., Ground Water Monit. Remed.
Spring (no. 2), 66 (2000).
amount of reduction that has already oc- in situ treatment alternatives to permewww.sciencemag.org
SCIENCE
VOL 295
15 MARCH 2002
Downloaded from www.sciencemag.org on August 28, 2012
Cumulative net present cost ($)
CREDIT: ADAPTED FROM (14)
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