Re-greening of Murdock wetlands is a joint effort
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ARGONNE, Ill. (March 21, 2008) — In Murdock, Neb., a small village south
of Omaha, Argonne planted more than 2,000 trees in 2005. These new flora
not only improve the wildlife habitat and provide aesthetic recreational value,
but they also play a key role in an integrated plan to take up and degrade
the carbon tetrachloride that has contaminated the community's water.
During the 1950s and ‘60s, the U.S. Department
of Agriculture (USDA) applied
fumigants containing carbon tetrachloride to grain in storage facilities at
Murdock.
Carbon tetrachloride is a toxic pollutant that is a probable human carcinogen.
The clear liquid is moderately soluble in water and evaporates readily from
surface water. It is moderately mobile in most soil and can seep into groundwater.
The U.S. Environmental Protection
Agency (EPA) determined that the fumigants
containing carbon tetrachloride had migrated through the near-surface and unsaturated
vadose zone soils into groundwater at Murdock, because of either spillage or
application methods.
In 1991, the USDA's Commodity
Credit Corporation directed Argonne to perform
a series of technical investigations to determine the distribution and monitor
the extent of contamination caused by the carbon tetrachloride. Sampling in
2004 found a maximum concentration of 281 µg/L at the headwaters of
Murdock's tributary creek. Under Nebraska regulations, the limit for carbon
tetrachloride in surface water is 44.2 µg/L. However, the findings did
not indicate any unacceptable health risks associated with vapor intrusion
or direct exposure to near-surface soils at Murdock.
Prior to the recent work at Murdock, no active measures had been taken to
reduce the toxicity or volume of the existing carbon tetrachloride. However,
since it was determined that this passive approach would take more than 80
years to be effective, an integrated remediation system was designed and installed
in 2005 to boost the reduction of the pollutant. Researchers used computer
flow-and-transport modeling to develop the integrated system, which minimizes
interference from clean surface water and soil water while maximizing benefits
to the community and ecosystem.
The integrated remediation system involves three components:
- Hot-spot control through extraction of water from the contaminated
aquifer and treatment with a modified spray irrigation system;
- Phytotechnology, which uses plants to reduce contaminant concentrations;
and
- Supplemental treatment by engineered wetlands downgradient
of the contaminated zone.
The first components of the system, the groundwater extraction well and modified
spray apparatus, apply a light spray to the local school's athletic fields
near the contaminant source. Since carbon tetrachloride is highly volatile,
exposure of the fine mist to air and sunlight results in an almost immediate
reduction in contaminant levels. Consequently, this system successfully treats
an area of elevated carbon tetrachloride levels within the existing plume and
captures some of the contaminant emanating from the source area.
Second, the phytotechnology portion of the treatment system addresses the
portion of the contaminant plume that has migrated toward the creek in the
area. The more than 2,000 trees (six species) planted were selected for disease
tolerance and rapid growth. The deep-rooting species were planted in wells
to focus their roots toward direct uptake of contaminated groundwater. Native
prairie plants were planted for surface cover between the trees to prevent
clean surface water from mixing with the contaminated groundwater. A vegetation
buffer along the creek banks controls erosion.
Third, researchers set up engineered wetlands where the contaminated groundwater
flows from the phytoremediation area. The overall basin design incorporates
an undulating bottom that increases residence time of water and fosters biodiversity
in the wetlands. The basin increases water surface area and contaminant evaporation
and decreases the risk of flooding.
Baseline sampling of the integrated system in July 2005 indicated that the
project's goals are achievable. Carbon tetrachloride levels had already risen
in the tree tissues, and the spray system had reduced contamination to a more
acceptable level.
Evaluation of the system's performance through the end of 2006 showed that
- The groundwater extraction-spray irrigation component is effectively
decreasing contaminant concentrations and improving the quality of the
school's athletic fields;
- The planted trees have flourished and weathered a temporary
outbreak of a fungal infection, and the effectiveness of the phytoremediation
zone has increased;
- The carbon tetrachloride concentrations in surface water of
the wetlands decrease dramatically along the flow path, so that the final
discharge has been free from the contaminant or has contained only a trace;
- The air in the phytoremediation zone remains safe for visitors
and workers.
Researchers also prepared and installed a set of 19 informational signs, including
technical illustrations and photographs, along a nature trail that winds through
the phytoremediation and wetlands areas at Murdock. In March 2007, these signs
and a companion visitor guide won a distinguished award in the annual Society
for Technical Communication competition for the Chicago chapter. In June 2007,
the signs and visitor guide won a distinguished award in the technical art
category of the STC international competition.
Collaborators include L.M. LaFreniere, M.C. Negri, R.A. Sedivy and Y.E. Yan
of Argonne; D. Steck and S.M. Gilmore of the U.S. Department of Agriculture;
and P. Kulakow, S. Hutchinson and L. Erickson of Kansas
State University.
Argonne National Laboratory seeks solutions to pressing national problems in science and technology.
The nation's first national laboratory, Argonne conducts leading-edge basic
and applied scientific research in virtually every scientific discipline. Argonne
researchers work closely with researchers from hundreds of companies, universities,
and federal, state and municipal agencies to help them solve their specific
problems, advance America 's scientific leadership and prepare the nation for
a better future. With employees from more than 60 nations, Argonne is managed
by UChicago
Argonne, LLC for
the U.S.
Department of Energy's Office
of Science.
By Jenny deAngelis.
For more information, please
contact Steve McGregor (630/252-5580 or media@anl.gov)
at Argonne.
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