U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2004-3057

May 2004

Natural Remediation of Arsenic Contaminated Ground Water 

Associated With Landfill Leachate

Printed on recycled paper

 Concentrations of arsenic that exceed the drinking 

water standard of 10 μg/L (micrograms per liter) (U.S. Environmental Protection Agency, 2001) are a common problem 

in New England water-supply wells (Zuena and Keane, 1985; 

Marvinney and others, 1994; Ayotte and others, 1999; Peters 

and others, 1999). Much of the arsenic occurs naturally in 

certain rock types of the region and enters ground water 

through weathering (Ayotte and others, 2003).

 Anthropogenically induced changes in ground-water 

chemistry also can cause release of arsenic from the solid 

phase to ground water. Precipitation and ground water seeping through municipal waste in landfills produce leachate 

that is contaminated with various organic and inorganic 

substances from the landfill waste (Baedecker and Back, 

1979; Christensen and others, 2001). Leachate seeping from 

a landfill contaminates the ground water beneath the landfill. 

The ground-water contamination also extends away from 

the landfill in the direction of ground-water flow, forming 

a leachate plume that can result in the release of naturally 

occurring arsenic.

This fact sheet describes results of studies by the 

U.S. Geological Survey at the Saco Municipal Landfill, Saco, 

Maine. The source of arsenic in ground water and effects of 

landfill leachate on arsenic concentration in ground water are 

described.

Photo 1. Drill rig installing monitor well at Saco Area 4 Landfill (landfill 

forms hill in background).

Photo 2. Collection of ground-water samples.

SACO LANDFILL

 The Saco Municipal Landfill (fig. 1) was operated from 

the early 1960s to 1989 and consists of three separate landfill 

areas (Nielsen and others, 1995). Landfill Areas 1 and 2 are 

located on the east side of Sandy Brook and have been capped 

since 1976 and 1989. Natural arsenic concentrations in the 

bedrock and sediments beneath Areas 1 and 2 are relatively 

low [3.0 – 6.9 mg/kg (milligrams per kilogram)] (Stollenwerk 

and Colman, 2003) and the average arsenic concentration in 

leachate-contaminated ground water is 21 μg/L (range <3.0 – 50 

μg/L) (Woodard and Curran, 1998). Arsenic concentrations in 

uncontaminated ground water upgradient from the landfill areas 

range from <3.0 – 28 μg/L (Woodard and Curran, 1998).

 Landfill Area 4 is on the west side of Sandy Brook. This 

landfill has been inactive since 1989 and was covered with an 

impermeable membrane in 1998. The geology of Area 4 is 

distinctly different from Areas 1 and 2, and the bedrock and 

sediments contain greater concentrations of naturally occurring 

arsenic (6.8 – 51.0 mg/kg). Leachate from the Area 4 landfill 

has contaminated surficial alluvium and the upper part of the 

bedrock aquifer (fig. 2). The average arsenic concentration in 

the leachate plume is 326 μg/L (range 52 – 647 μg/L). Essentially all of the arsenic is arsenite [As(III)].

ARSENIC SOURCE IN GROUND WATER

 United States Geological Survey studies on the geochemistry and mineralogy of the leachate-contaminated aquifer at the 

Saco Area 4 landfill have shown that the source of arsenic is not 

the landfill but the sediments the leachate is moving through 

(Stollenwerk and Colman, 2003). Much of the arsenic is pres-
Figure 2. Geohydrologic section line A-A/

 for Area 4. Green arrows indicate modeled flow path of arsenic concentrations (from Stollenwerk, 

2001).

Figure 3. Concentrations of dissolved iron and arsenic measured in 

ground water contaminated by Area 4 landfill leachate.

Figure 1. Site map for the Saco Landfill.

ent in hydrous ferric oxides that coat the aquifer sediments. 

The arsenic in these hydrous ferric oxides may have gradually 

accumulated over time as a result of adsorption of low concentrations of arsenic that occur naturally in the ground water 

of this area. Results from laboratory experiments show that 

dissolved organic carbon in the leachate plume is promoting 

reductive dissolution of these hydrous ferric oxides, releasing 

arsenic to ground water. Reductive 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 hydrous ferric oxides and release 

of arsenic from the sediments. As a result, there is a correlation 

between increasing concentrations of dissolved iron and arsenic 

in the Area 4 leachate plume (fig. 3).

NATURAL REMEDIATION OF ARSENIC

Approach

 The impermeable landfill cover was installed to reduce 

infiltration of anaerobic leachate from the landfill and allow 

natural flushing of the aquifer with oxygenated ground water 

upgradient from the landfill. To assess the effectiveness of this 

natural remediation strategy, laboratory experiments and modeling studies were conducted to predict arsenic concentrations in 

the aquifer for as many as 50 years after the landfill was covered 

(Stollenwerk and Coleman, 2003).

 The laboratory experiments consisted of flushing oxygenated, arsenic-free, uncontaminated ground water from the 

site through cores collected from leachate-contaminated areas 

in the aquifer. An average velocity of 0.2 m/d (meters per day) 

was used for these experiments and is consistent with average 

ground-water velocities in the aquifer. Water eluting from the 

contaminated core was analyzed for arsenic and other selected 

constituents. The data from these experiments were used to: 

1) measure the volume of uncontaminated water required to 

decrease arsenic concentrations to the 10 μg/L drinking water 

standard, 2) identify those reactions that had a significant effect 
Figure 4. Experimental and modeled ferrous iron [Fe(II)] concentrations 

in leachate from one core (from Stollenwerk and Coleman, 2003).

Figure 5. Experimental and modeled arsenite [As(III)] concentrations in 

leachate from one core (from Stollenwerk and Coleman, 2003).

on arsenic concentrations in ground water, 3) determine equilibrium constants for these reactions at this site, and 4) use the 

laboratory experimental data to simulate arsenic concentrations 

in leachate-contaminated ground water after the landfill was 

capped.

 The one-dimensional reaction-transport model PHREEQC 

version 2 (Parkhurst and Appelo, 1999) was used to simulate 

chemical changes resulting from elution of uncontaminated 

ground water through the leachate-contaminated cores. PHREEQC has the capability to model advective transport of water in 

combination with a variety of chemical reactions. After calibration of the model to match the data from the laboratory experiments, the model was used to simulate geochemical changes 

resulting from 50 years of flow of uncontaminated ground water 

through the contaminated aquifer.

Laboratory Results

 The most important reactions in this system are driven by 

organic carbon. In addition to the dissolved organic carbon present in contaminated ground water, significant concentrations of 

organic carbon also have been adsorbed onto the aquifer sediments during the active lifetime of the landfill. In the laboratory 

experiments, as uncontaminated ground water moved through 

the contaminated cores, organic carbon was continually released 

from the solid phase to solution, consuming all dissolved 

oxygen. As a result, reducing conditions within the core were 

maintained and hydrous ferric oxides continued to dissolve.

 Iron concentrations in leachate from a representative core 

are shown in figure 4. Iron concentrations rapidly decreased 

from 50 mg/L (milligrams per liter) to 5 mg/L within a few pore 

volumes as iron-free uncontaminated ground water displaced 

the contaminated pore water. (One pore volume is equal to the 

volume of water within the pore space of the core.) Iron concentrations then stabilized at about 3 mg/L, indicating equilibrium 

between dissolved organic carbon and hydrous ferric oxide 

dissolution. By pore volume 250, organic carbon concentrations 

had decreased to a level where there was sufficient dissolved 

oxygen present in the core to prevent any further reduction of 

hydrous ferric oxides.

 Arsenic concentrations from the same core decreased 

in a manner similar to iron (fig. 5). Arsenic concentrations 

rapidly decreased from 380 μg/L in the initial pore water to 

50 μg/L by pore volume 15. This fraction represents arsenic in 

pore water and the readily desorbable fraction. Dissolution of 

hydrous ferric oxides and release of associated arsenic to solution maintained arsenic concentrations at greater than 10 μg/L 

until conditions in the core became oxic (after organic carbon 

concentration had decreased), at about pore volume 250.

Model Simulations 

 The concentrations of arsenic in the aquifer simulated by 

the model are shown in figure 6. Initial arsenic concentrations 

in 1998 (0 years after capping the landfill) are shown along a 

flow path that extends from beneath the landfill (zero meters) 

to Sandy Brook (300 meters). The simulated flow path encompasses areas underlying the landfill with the highest concentrations of arsenic measured in the ground water. The model 

assumptions are: 1) there is no further contamination of the 

aquifer by leachate from the landfill after 1998, 2) oxygen-containing uncontaminated ground water begins moving through 

the contaminated aquifer in 1998, and 3) ground-water flow is 

one-dimensional with no solute diffusion.

 Modeling results show that arsenic concentrations at 

Sandy Brook could increase in the near term as higher concentrations closer to the landfill migrate through the aquifer. However, concentrations throughout the aquifer should gradually 
Figure 6. Model simulated concentrations of arsenic along flow path 

as a function of time after capping of the landfill. (Zero meters along 

flow path represents contaminated ground water beneath upgradient area of landfill; 300 meters is discharge at Sandy Brook) (from 

Stollenwerk, 2001).

For more information, contact:

Chief, Branch of Regional Research

U.S. Geological Survey

Denver Federal Center, Box 25046, MS 418

Denver, CO 80225

REFERENCES

Ayotte, J.D., Nielsen, M.G., Robinson, G.R., Jr., and Moore, 

R.B., 1999, Relation of arsenic, iron, and manganese in 

ground water to aquifer type, bedrock lithogeochemistry, and 

land use in the New England coastal basin: U.S. Geological 

Survey Water-Resources Investigations Report 99-4162, 60 p. 

Ayotte, J.D., Montgomery, D.L., Flanagan, S.M., and Robinson, 

K.W., 2003, Arsenic in groundwater in eastern New England—Occurrence controls, and human health implications: 

Environmental Science and Technology, v. 37, p. 2075-2083.

Baedecker, M.J., and Back, William, 1979, Hydrogeological 

processes and chemical reactions at a landfill: Ground Water, 

v. 17, p. 429-437.

Christensen, T.H., Kjeldsen, P., Bjerg, P.L., Jensen, D.L., 

Christensen, J.B., Baun, A., Albrechtsen, H.J., and Heron, G., 

2001, Biogeochemistry of landfill leachate plumes: Applied 

Geochemistry, v. 16, p. 659-718.

Marvinney, R.G., Loiselle, M.C., Hopeck, J.T., Braley, David, 

and Krueger, J.A., 1994, Arsenic in Maine ground water—An 

example from Buxton, Maine, in Proceedings of The 1994 

Focus Conference on Eastern Regional Ground Water Issues, 

Burlington, Vt., Oct. 3-5: National Ground Water Association, p. 701-715.

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.

Parkhurst, D.L., and Appelo, C.A.J., 1999, User’s guide to 

PHREEQC (version 2): U.S. Geological Survey WaterResources Investigations Report 99-4259, 312 p.

Peters, S.C., Blum, J.D., Klaue, B., and Karagas, M.R., 1999, 

Arsenic occurrence in New Hampshire drinking water: Environmental Science and Technology, v. 33, p. 1328-1333.

Stollenwerk, K.G., 2001, Natural remediation of arsenic-contaminated groundwater, in Eleventh Annual V.M. Goldschmidt Conference, Hot Springs, Va., May 20-24, Abstract, 

LPI Contribution No. 1088: Houston, Lunar and Planetary 

Institute, CD-ROM.

Stollenwerk, K.G., and Colman, J.A., 2003, Natural remediation potential of arsenic-contaminated ground water, in

Welch, A.H., and Stollenwerk, K.G., eds., Arsenic in ground 

water—Geochemistry and occurrence: p. 351-379, Boston, 

Kluwer.

U.S. Environmental Protection Agency, 2001, National primary 

drinking water regulations—Arsenic and clarifications to 

compliance and new source contaminants monitoring: Federal Register, v. 66, no. 14, p. 6976-7066.

Woodard and Curran, 1998, Final remedial investigation report 

of Saco Municipal Landfill Superfund Site, Saco, Maine: 

Portland, Maine, Woodard and Curran, Inc., 180 p.

Zuena, A.J., and Keane, N.W., 1985, Arsenic contamination in 

private potable well, in U.S. EPA National Conference on 

Environmental Engineering, Northeastern University, Boson, 

Massachusetts, 1985, p. 717-725.

decrease as uncontaminated ground water moves through the 

aquifer. Because of the large concentrations of organic carbon in 

this aquifer, decades may be required for arsenic concentrations 

to decrease to drinking water levels.

IMPLICATIONS

 These results indicate that arsenic contamination of 

ground water could be a problem in other areas of New England 

that have been affected by landfill leachate, provided a source of 

arsenic is present in the bedrock and alluvial materials. Arsenic contamination of ground water may persist for many years 

after closure of a landfill because of the high organic carbon 

content of contaminated sediments, which maintain reducing 

conditions favorable for reductive dissolution of arsenic-containing iron oxides. This study also provides information on the 

geochemical processes that might cause high arsenic concentrations in ground water in other locations not associated with 

landfills. Two key factors are the initial presence of arsenic in 

the geologic materials of an aquifer and a reducing geochemical 

environment.

—Kenneth G. Stollenwerk and John A. Colman