1U.S. Geological Survey

2 Oklahoma Water Resources Board

U.S. Department of the Interior USGS Fact Sheet 081-00

U.S. Geological Survey June 2000

DO LANDFILLS LEAK?

We call it ìgarbageî or ìtrashî but it is ìmunicipal solid wasteî

to your city government and the waste industry. Municipal solid

waste is a combination of non-hazardous wastes from households, commercial properties, and industries. The U.S. Environmental Protection Agency (USEPA) reports that the United States

produced about 230 million tons of solid waste in 1999, about 57

percent of which is disposed of in landfills (U.S. Environmental

Protection Agency, 1999).

Disposal of municipal solid waste in landfills was largely unregulated prior to the 1970s. Most solid waste was deposited in

unlined pits. Precipitation and ground water seeping through

this waste produces leachate, which is water contaminated from

the various organic and inorganic substances with which it comes

in contact as it migrates through the waste. Leachate seeping

from a landfill contaminates the ground water beneath the landfill, and this contaminated ground water is known as a plume.

The normal movement of ground water causes the leachate plume

to extend away from a landfill, in some cases for many hundreds

of meters. Many studies have shown leachate plumes emanating from old unlined landfills. Estimates for the number of closed

landfills in the United States are as high as 100,000 (Suflita and

others, 1992).

Federal and state regulations were passed in the 1980s and

1990s to manage disposal of solid waste. Those regulations require that most landfills use liners and leachate collection systems to minimize the seepage of leachate to ground water. Although liners and leachate collection systems minimize leakage,

liners can fail and leachate collection systems may not collect all

the leachate that escapes from a landfill. Leachate collection systems require maintenance of pipes, and pipes can fail because

they crack, collapse, or fill with sediment. The USEPA has concluded that all landfills eventually will leak into the environment

(U.S. Environmental Protection Agency, 1988). Thus, the fate and

transport of leachate in the environment, from both old and modern landfills, is a potentially serious environmental problem.

STUDYING LEACHATE PLUMES AT A MUNICIPAL

SOLID-WASTE LANDFILL

The Norman Landfill Environmental Research Site

The Norman Landfill (fig. 1) is a closed municipal solid waste

landfill, formerly operated by the city of Norman, Oklahoma. The

landfill is sited directly on the Canadian River alluvial aquifer

and has no liner or leachate collection system, so a leachate

plume has developed in ground water in the aquifer. The ground

water and leachate plume flow away from the landfill toward

the Canadian River, a large tributary of the Arkansas River that

drains into the Mississippi River.

The Norman Landfill was designated a research site by the

U.S. Geological Survey (USGS) through its Toxic Substances

Hydrology Research Program. Monitoring wells and instruments have been installed in and adjacent to the leachate plume.

A small stream and wetland overlie the leachate plume, and

studies are in progress to determine the fate of leachate compounds that enter the wetland from the ground water. USGS

hydrologists and technicians have accomplished comprehensive site characterization, which provides a wealth of information about the site hydrogeology and geochemistry. This site

characterization provides essential information to the scientists conducting research about the chemical, biological, and

hydrologic processes in ground water and surface water affected by landfill leachate. Research is in progress at the site

by scientists from the University of Oklahoma, Oklahoma State

University, other universities, the USEPA, and the USGS.

In addition to providing a laboratory for studies of ground

and surface water contaminated by landfill leachate, the

Norman Landfill Environmental Research Site is used to study

other types of contaminant problems. The plume can be used

to study microbiological and geochemical processes that are

not specific to landfills.

All research at the Norman Landfill Environmental Research

Figure 1. Map of the Norman Landfill Environmental Research Site.

The Norman Landfill Environmental Research Site:

What Happens to the Waste in Landfills?

By Scott C. Christenson and Isabelle M. Cozzarelli

U.S. Department of the Interior USGS Fact Sheet FS-040-03

U.S. Geological Survey August 2003
2

Site is designed to investigate problems and processes that have

a high transfer value to other subsurface contamination problems. Comprehensive physical, chemical, and microbial characterizations at this and other USGS Toxic Substances Hydrology

Program sites provide fundamental knowledge of the processes

that control important types of contamination problems. This

knowledge of fundamental processes can be generalized to a

wide range of field conditions by comparing results to field and

laboratory experiments at other sites with differing conditions

and properties. The resulting knowledge and methods improve

the effectiveness and reduce the cost of characterization and

remediation at contaminated sites across the nation.

NATURAL ATTENUATION

Can Microorganisms Reduce Landfill Contaminants?

Scientists who investigate environmental contamination problems are interested in an environmental cleanup approach known

as natural attenuation. Natural attenuation refers to naturally

occurring physical, chemical, and biological processes that can

reduce concentrations of contaminants. In most contaminated

aquifers, one aspect of natural attenuation involves the degradation of contaminants by microorganisms, which in some instances prevents contaminant migration. These microorganisms

are naturally present in aquifers, even deep below the surface of

the earth.

Natural attenuation can be a better alternative for

remediation of certain types of contaminants compared to other

methods of remediating contaminated ground water. Natural

attenuation can be less expensive and more effective than other

methods, such as extracting contaminated ground water with

wells and treating it at water treatment plants. Much of the research at the Norman Landfill Environmental Research Site investigates different aspects of natural attenuation.

BIOGEOCHEMICAL ZONES

Where Does Natural Attenuation Occur?

University of Oklahoma and USGS scientists used a combined

microbiological and geochemical approach to identify the important processes occurring in the aquifer contaminated by leachate

from the Norman Landfill (Cozzarelli and others, 2000, Harris

and others, 1999). The combined sciences of microbiology and

geochemistry are called biogeochemistry. The Norman Landfill

researchers identified zones in the leachate plume at Norman

Landfill where different biogeochemical processes are occurring.

One method to identify different biogeochemical zones is to measure the concentration of certain chemicals and minerals, those

that are involved in biogeochemical processes, in the ground water

and in the aquifer. Electron acceptors are minerals or chemicals

that can occur naturally in aquifer solids or ground water, such as

iron oxides in the sediments or sulfate dissolved in ground water.

These chemicals are called electron acceptors because microorganisms transfer electrons to them during respiration, which is part

of the process the microorganisms use to obtain energy. During respiration, an electron is transferred from an electron donor, such as

an organic contaminant compound, to an electron acceptor. This

electron transfer occurs when microorganisms break down organic

contaminant compounds. The availability of electron acceptors in

an aquifer is therefore an important factor for evaluating the effectiveness and sustainability of natural attenuation in contaminated

aquifers.

Some evidence of natural attenuation at Norman Landfill is

shown in figure 2 (Cozzarelli and others, 2000), illustrated as

generalized hydrogeologic sections through the leachate plume

in the aquifer. The three illustrations within figure 2 show chemical concentrations of important indicators of natural attenuation processes along the same vertical slice of the aquifer. These

illustrations demonstrate that the leachate plume begins near

the surface (on the left side of the figure), where the solid waste

is stored in the landfill. Ground water flows to the south toward

the Canadian River. The leachate plume migrates toward the

bottom of the aquifer as it flows toward the river.

Sulfate occurs naturally in ground water in the Canadian

River alluvial aquifer. Sulfate is depleted in the center of the

Figure 2. Concentrations of electron acceptors and donors in

the Norman Landfill leachate plume.
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leachate plume (fig. 2A) because the microorganisms use sulfate as an electron acceptor. When microorganisms transfer electrons to sulfate, sulfate changes chemically to form dissolved

sulfide or hydrogen sulfide gas. The highest rates of sulfate reduction have been measured at the plume boundaries, such as

where fresh water from rainfall infiltrating the aquifer mixes

with the contaminant plume (Cozzarelli and others, 2000), and

causes the sharp concentration gradients observed in figure 2A.

The degradation of organic contaminants occurs most rapidly

at the plume boundaries.

Iron occurs naturally as mineral coatings on sediments in

the Canadian River alluvial aquifer. Dissolved iron concentrations increase in the leachate plume (fig. 2B) because microorganisms transfer electrons to the iron on the mineral coatings,

which contain insoluble ferric iron, while degrading the organic

contaminants. With the addition of an electron, the iron is reduced to ferrous iron, which dissolves in water. Although the

solid-phase electron acceptor (ferric iron) cannot be measured

in the ground water, the detection of the end product of the reaction (ferrous iron) in water provides evidence that iron reduction has occurred.

The concentration of non-volatile dissolved organic carbon

(NVDOC) is shown in figure 2C. NVDOC is a measure of the

organic contaminant compounds in the landfill. In the center of

the plume, the concentration of NVDOC shows little change with

distance, indicating that NVDOC is not efficiently degraded in

this zone.

Researchers at Norman Landfill have learned that most of

the natural attenuation occurs at the boundaries of the plume

where electron acceptors are available. Sulfate concentrations

are low in the center of the plume, as are measured rates of

iron and sulfate reduction. The natural attenuation capacity of

the aquifer, that is, its ability to attenuate contaminants, is depleted in the center of the leachate plume because microorganisms have used all the electron acceptors during migration

of the leachate plume.

VOLATILE ORGANIC COMPOUNDS

Evidence for Natural Attenuation

USGS scientists have been investigating volatile organic

compounds (VOCs) in the leachate at Norman Landfill

(Eganhouse and others, 2001). VOCs are organic compounds

that tend to vaporize at room temperature and pressure. Examples of VOCs include some of the compounds in gasoline,

lubricants, paints, and solvents. Some VOCs are highly toxic or

carcinogenic. VOCs end up in landfills in many ways, including

the disposal of ordinary household items such as cleaners or

marking pens. Although VOCs make up less than 0.1 percent of

the mass of organic carbon in the leachate plume, they are

useful indicators of natural attenuation.

At Norman Landfill, USGS scientists compared concentrations of two different alkylbenzene isomers, n-propylbenzene

and i-propylbenzene, in landfill leachate. Isomers of

alkylbenzene have the same number and type of atoms, but

the molecules have slightly different chemical structures. These

different isomers have similar physical properties, so they

should be affected by volatilization, dilution, and sorption in a

similar manner. The concentration of n-propylbenzene decreases much faster as leachate flows away from the landfill

than does the concentration of i-propylbenzene (fig. 3). This

decrease in concentration of n-propylbenzene is caused by biological degradation, indicating that biologically mediated natural attenuation is decreasing the concentrations of some contaminants at Norman Landfill. This technique of comparing

alkylbenzene isomers as indicators of biological processes can

be applied at sites with contaminants other than landfill

leachate.

INVESTIGATING THE SUBSURFACE TO REVEAL THE

RATE OF NATURAL ATTENUATION

Field experiments are being carried out at Norman Landfill to investigate how the rate of natural attenuation may

vary with aquifer permeability (permeability is a measure

of the ability of a material to transmit fluid). These experiments use push-pull or single-well injection-withdrawal tests

(Istok and others, 1997). During the injection phase of the

test, a solution consisting of ground water amended with

tracers, electron donors, or electron acceptors is injected or

ìpushedî through a well into the aquifer. During the extraction phase, the test solution is pumped or ìpulledî from the

same well. Concentrations of tracers, reactants, and possible reaction products are measured as a function of time

in order to construct breakthrough curves, measure reaction rates, and to compute mass balances for each solute.

Figure 3

Figure 3. Distribution of the alkylbenzene isomers (a) npropylbenzene and (b) i-propylbenzene in the leachate plume

at Norman Landfill. Concentrations in micrograms per liter (Fg/

L) are proportional to bubble diameter. Maximum concentration:

n-propylbenzene = 0.80 Fg/L, i-propylbenzene = 1.24 Fg/L

(from Eganhouse and others, 2001).
These tests can be conducted anywhere in the aquifer,

making it possible to investigate processes and rates in different geologic textures and geochemical environments.

Push-pull tests were conducted at Norman Landfill to measure biodegradation rates of simple organic acids in the leachate

plume (Scholl and others, 2001). Wells were drilled into layers of

three different types of sediments (medium sand, silt/clay lenses

in sand, and poorly sorted gravel), each with a different permeability. Biodegradation rates of two simple organic acids, formate and lactate, were compared in the three different zones in

the anoxic leachate plume at the site. These organic acids were

used as microbial process indicators because they degrade at

different rates depending on the dominant microbial processes.

A conservative tracer (bromide) and the two organic acids were

added to 50 or 100 liters of contaminated ground water pumped

from each test well. The mixture was then re-injected and allowed to mix with the natural ground water. Daily samples were

taken from the injection well until organic acids could no longer

be detected. Although complete disappearance of the formate

and lactate occurred within 7-9 days in all the wells, there were

differences in degradation patterns. The results of the test show

that the loss of lactate was due to natural attenuation and that

there are differences in the rate of natural attenuation in areas

of different permeability. These variable degradation rates may

be related to microbial community structure, sediment chemistry, and water flow regime.

IMPLICATIONS

Research at the Norman Landfill Environmental Research Site

has shown that chemicals leaching from old unlined landfills are

contaminating ground water, but that some of the contaminant

concentrations are being reduced by natural attenuation. Modern

landfills are designed to minimize contamination of ground water,

but modern landfills eventually may leak contaminants into the

environment. Research results from Norman Landfill will be useful to scientists and regulators trying to determine the effects of

landfill leachate on the environment.

4

REFERENCES

Cozzarelli, I. M., Suflita, J. M., Ulrich, G. A., Harris, S. H., Scholl, M. A., Schlottmann, J. L., and Christenson, Scott, 2000,

Geochemical and microbiological methods for evaluating anaerobic processes in an aquifer contaminated by landfill

leachate, Environmental Science and Technology, v. 34, p. 4025-4033.

Eganhouse, R.P., Cozzarelli, I.M., Scholl, M.A., and Matthews, L.L, 2001, Natural attenuation of volatile organic compounds

(VOCs) in the leachate plume of a municipal landfill: Using alkylbenzenes as a process probe: Ground Water, v. 39, no. 2,

p. 192-202.

Harris, S.H., Ulrich, G.A., and Suflita, J.M., 1999, Dominant terminal electron accepting processes occurring at a landfill

leachate-impacted site as indicated by field and laboratory measures: in Morganwalp, D.W., and Buxton, H.T., eds., 1999,

U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Charleston, South

Carolina, March 8-12, 1999--Volume 3 -- Subsurface Contamination from Point Sources: U.S. Geological Survey WaterResources Investigations Report 99-4018C, pp. 541-548.

Istok, J.D., Humphrey, M.D., Schroth, M.H., Hyman, M.R., and OíReilly, K.T., 1997, Single-well, ìpush-pullî test for in situ

determination of microbial activities: Ground Water, v. 35, no. 4, p. 619-631.

Scholl, M.A., Cozzarelli, I.M., Christenson, S.C., Istok, J., Jaeschke, J., Ferree, D.M., and Senko, J., 2001, Measuring variability of in-situ biodegradation rates in a heterogeneous aquifer contaminated by landfill leachate: EOS, Transactions,

American Geophysical Union, v. 82, no. 20, May 15, 2001, p. 146.

Suflita, J.M., Gerba, C.P., Ham, R.K., Palmisano, A.C., and Robinson, J.A., 1992, The worldís largest landfill: Environmental

Science and Technology, v. 26, no. 8, p. 1486-1495.

U.S. Environmental Protection Agency, 1988, Federal Register, v. 53, no. 168, August 30, 1988, p. 33345.

U.S. Environmental Protection Agency, 1999, Municipal solid waste in the United States: 1999 Facts and Figures ñ EPA530-R01-014.

For information about the Norman Landfill site, contact:

Scott C. Christenson

U.S. Geological Survey

202 NW 66th Street (Building 7)

Oklahoma City, OK 73116

(405) 810-4409

Email: schris@usgs.gov

For information about USGS research at Norman Landfill, contact:

Dr. Isabelle Cozzarelli

U.S. Geological Survey

National Research Program (MS 431)

12201 Sunrise Valley Dr.Reston, VA 20192

Telephone: (703) 648-5899

Email: icozzare@usgs.gov

USGS Norman Landfill World Wide Web Sites:

Oklahoma District: http://ok.water.usgs.gov/norlan/

National Research Program: http://water.usgs.gov/nrp/organic/norman.htm