• Remediation Performance Monitoring
• Natural Attenuation Evaluation
• Site Characterization

• U.S. Environmental Protection Agency, Region I
• Maine Department of Environmental Protection
• City of 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.

• Ground-Water Solute-Transport Simulation
• Superfund Fact Sheet, Saco Municipal Landfill, Saco, Maine, U.S. Environmental Protection Agency

• Kenneth G. Stollenwerk, USGS, National Research Program, Denver, CO,
• John A. Colman, USGS, Massachusetts District, Northborough, MA,

Colman, J.A., and Lyford, F.P., 2001,

Bedrock and overburden sources of arsenic in leachate plumes from a landfill in Saco, Maine [Extended Abstract], in USGS Workshop on Arsenic in the Environment, February 21-22, 2001, Denver, CO [web publication only].

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., 2001,

Natural remediation of arsenic-contaminated ground water--solute-transport model predictions [Extended Abstract], in USGS Workshop on Arsenic in the Environment, February 21-22, 2001, Denver, CO [web publication only].

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 Grouundwater: Kluwer Publishers, in press.

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.

 

USGS Information on Natural Attenuation

• A Unique Approach to Evaluating Natural Attenuation is Applied Worldwide
• Relying on Nature to Clean Up Contaminated Ground Water
  
• Topical Information on Natural Attenuation, Toxics Program
  
• Listing of Toxics Program Projects Evaluating Natural Attenuation
  
• Listing of Other USGS Projects Evaluating Natural Attenuation

USGS Information on Arsenic

• USGS Arsenic Studies Group
• Arsenic in ground water of the United States, National Analysis of Trace Elements, National Water-Quality Assessment
• Southeast Michigan Drinking Water Initiative (Arsenic is the main contaminant of concern)
• Arsenic Statistics and Information, USGS Minerals Information

Information on Arsenic

• Arsenic in Drinking Water, U.S. Environmental Protection Agency
• ToxFAQsTM for Arsenic, Agency for Toxic Substances and Disease Registry

Toxics Landfill Remediation Projects

• Alternative Landfill Cover Assessment, Fort Carson, CO
• Solvent Plume Source Removal Test (Fenton's Reagent), (the source was in a landfill)
• Quantifying Subsurface Biodegradation, Norman Municipal Landfill, Norman, OK

Related Toxics Projects

• Waste Disposal and Contaminant Migration in the Arid Southwest, Amargosa Desert Research Site, Nevada

Other USGS Landfill Remediation Studies

• Area 6 Landfill, Naval Air Station Whidbey Island, Island County, Washington
  • Simulation of Ground-Water Flow and Potential Contaminant Transport at Area 6 Landfill, Naval Air Station Whidbey Island, Island County, Washington: USGS WRIR 01-4252
  • Technical Oversight and Scientific Investigations at U.S. Navy Facilities in Washington and Alaska, Project Description
• Surface Geophysical Investigation of a Chemical Waste Landfill in Northwestern Arkansas
• Operable Unit 1 (Landfill), Naval Undersea Warfare Center, Division Keyport, Washington
  • Natural Attenuation of Chlorinated Volatile Organic Compounds in Ground Water at Operable Unit 1, Naval Undersea Warfare Center, Division Keyport, Washington: USGS WRIR 02-4119
  • Technical Oversight and Scientific Investigations at U.S. Navy Facilities in Washington and Alaska, Project Description
• 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)