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Toxics Program Remediation Activities

Landfill Leachate Mobilizes Arsenic Bound in Aquifer Sediments: Saco, Maine

  • Remediation Performance Monitoring
  • Natural Attenuation Evaluation
  • Site Characterization
Location Saco, Maine
  • Monitored Natural Attenuation
  • Impermeable Membrane Landfill Cover
  • Arsenic
  • Landfill Leachate

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.

More Information
  • Kenneth G. Stollenwerk, USGS, National Research Program, Denver, CO,
  • John A. Colman, USGS, Massachusetts Water Science Center, Northborough, MA,

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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