Landfill Leachate Mobilizes Arsenic Bound in Aquifer Sediments: Saco, Maine
Type |
- Remediation Performance Monitoring
- Natural Attenuation Evaluation
- Site Characterization
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Location |
Saco, Maine |
Partners |
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Technology |
- Monitored Natural Attenuation
- Impermeable Membrane Landfill Cover
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Contaminants |
- Arsenic
- Landfill Leachate
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Description |
The U.S. Environmental Protection Agency (EPA) is concerned that
leachate plumes at many landfills in the New England area have high
concentrations of arsenic. For example, the Saco Municipal Landfill
near Saco, Maine, a Superfund
Site, has a plume of arsenic with concentrations as high as
700 mg/L (micrograms per liter). USGS studies on the geochemistry
of the leachate plume at the Saco Landfill have shown that the source
of the arsenic is not the landfill, but appears to be the sediments
the plume is moving through. This seemingly contradictory finding
is the result of a partnership between the EPA and the USGS. USGS
scientists:
- Characterized the chemistry of the plume,
- Conducted geochemical and mineralogical investigations to determine
the source of the arsenic, and
- Conducted laboratory and modeling studies to assess the effect
that an impermeable cap on the landfill will have on the arsenic
plume.
USGS scientists characterized geology and water quality of the
site, delineated the extent of contamination in the subsurface,
and developed a conceptual model of the ground-water and surface-water
flow at the site. In addition to standard hydraulic data collection
and water-quality sampling, geophysical surveys were also conducted
to characterize the plume. A surface electromagnetic (EM) terrain-conductivity
survey delineated the leachate plume, and seismic refraction determined
the depth to bedrock. EPA used this information as part of its remedial
investigation of the site.
The results of geochemical and mineralogical studies conducted
by USGS scientists showed that dissolved organic carbon in the leachate
plume is dissolving arsenic from arsenic-containing iron oxides
in the aquifer and bedrock. The dissolution occurs because the degradation
of the dissolved organic carbon in the plume removes oxygen from
the water and creates reducing
conditions that favor the dissolution of iron oxides and the
release of arsenic from the sediments. The dissolution results in
concentrations of arsenic that range up to many hundreds of micrograms
per liter.
To assess the impact of an impermeable membrane that was installed
on top of one of the landfill's cells in 1997, USGS scientists conducted
laboratory and modeling studies to predict the fate of the plume
for up to 60 years after the landfill was covered. The cover was
installed to reduce the production of anaerobic (without oxygen)
leachate from the landfill and allow natural flushing of the aquifer
with oxygenated ground water. The goal of the cover was to decrease
arsenic concentrations in the aquifer to acceptable levels. Results
from laboratory column leaching tests using landfill-contaminated
cores showed that the large reservoir of sorbed organic carbon in
the contaminated sediment would consume dissolved oxygen in uncontaminated
ground water and maintain reducing conditions for several years.
As a result, dissolution of iron oxides and release of arsenic to
ground water would continue. Once oxic (with oxygen) conditions
become reestablished, release of arsenic would become negligible
and natural sorption processes should lower arsenic concentrations
to acceptable levels.
USGS scientists also constructed a one-dimensional reaction-transport
model of a flowpath within the plume with PHREEQC,
a USGS geochemical modeling code. The geochemical model was calibrated
to the laboratory experimental data. The model predicted that after
30 years, arsenic concentrations would slowly decrease from a high
of 650 mg/L to approximately 50 mg/L. After 60 years, the model
predicted that arsenic concentrations would be greater than 10 mg/L,
which is EPA's
current drinking water standard for arsenic. Therefore the intended
effect of the landfill cover might not be seen for many decades.
EPA used this information to help determine the length of time that
sections of the property would need to remain under restriction
regarding future recreational or development use.
These results show that it could take decades for the arsenic levels
to decrease at landfills with similar conditions across New England.
The study also provides information on the natural conditions that
might cause high arsenic concentrations in drinking water in other
locations. Water-resource managers can use this information in their
drinking-water protection programs.
The results also provide insight into the use of monitored natural
attenuation to remediate sites that have been contaminated with
large amounts of organic carbon. Concentrations of reactive sorbed
organic carbon can greatly exceed the concentration of dissolved
oxygen in inflowing ground water, resulting in reducing conditions
for many years.
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More Information |
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Contact |
- Kenneth G. Stollenwerk, USGS, National Research Program, Denver,
CO,
- John A. Colman, USGS, Massachusetts District, Northborough,
MA,
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Publications |
Colman, J.A., and Lyford, F.P., 1999, Sources
and Geochemical Associations of Arsenic in Leachate Plumes From
a Landfill in Saco, Maine: EOS, Transactions of the 1999 Spring
Meeting of American Geophysical Union, Boston, Massachusetts,
June 1-4, 1999.
Stollenwerk, K.G., and Colman, J.A., 2004, Natural remediation of arsenic contaminated ground water associated with landfill leachate: U.S. Geological Survey Fact Sheet 2004-3057, 4 p.
Stollenwerk, K. G., and Colman, J. A., 2002,
Natural Remediation Potential of Arsenic-Contaminated Ground Water, in Welch, A. H., and Stollenwerk, K. G., eds., Arsenic in Groundwater--Geochemistry and Occurrence: Kluwer Academic Press, p. 351-379.
Stollenwerk, K.G., and Colman, J.A., 1999, Natural
Remediation of Arsenic-Contaminated Groundwater: Solute-Transport
Model Predictions: EOS, Transactions of the 1999 Spring Meeting
of American Geophysical Union, Boston, Massachusetts, June 1-4,
1999.
Stollenwerk, K.G., and Colman, J.A., 1998, Natural
Remediation of Arsenic-Contaminated Groundwater: EOS, Transactions
of the 1998 Fall Meeting of American Geophysical Union, San Francisco,
December 6-10, 1998, v. 79. no. 45, p. F314.
Nielsen, M.G., Stone, J.R., Hansen, B.P., and Nielsen, J.P.,
1995, Geohydrology, water quality,
and conceptual model of the hydrologic system, Saco landfill area,
Saco, Maine: U.S. Geological Survey Water-Resources Investigations
Report 95-4027, 94 p.
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Links |
USGS Information on Natural Attenuation
USGS Information on Arsenic
Information on Arsenic
Toxics Landfill Remediation Projects
Related Toxics Projects
Other USGS Landfill Remediation Studies
- Area 6 Landfill, Naval Air Station Whidbey Island, Island County,
Washington
- Surface
Geophysical Investigation of a Chemical Waste Landfill in Northwestern
Arkansas
- Operable Unit 1 (Landfill), Naval Undersea Warfare Center,
Division Keyport, Washington
- Identification
of Potential Water-Bearing Zones by the Use of Borehole Geophysics
in the Vicinity of Keystone Sanitation Superfund Site, Adams County,
Pennsylvania, and Carroll County, Maryland: USGS WRIR 97-4104
(pdf)
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