Section E
Geochemical and Microbiological Processes in Ground Water and Surface Water
Affected by Municipal Landfill Leachate
Landfilling is the most common method of disposal of municipal waste. The
United States produces more than 150 million tons of solid waste each year,
over 70 percent of which is disposed of in landfills. According to the U.S.
Environmental Protection Agency, 3,581 landfills were operating in the United
States in 1995, down from 7,683 in 1982. Many of the now-closed landfills
were unlined and sited on alluvial deposits because the land had little economic
value and excavations were conveniently available from sand and gravel operations.
Whereas Superfund and other hazardous-waste sites have received much attention,
little is known about the hazards to ground-water resources and environmental
receptors posed by a typical municipal landfill.
In 1995, the U.S. Geological Survey began an intensive investigation as part
of the Toxic Substances Hydrology Program at a closed municipal landfill in
Norman, Oklahoma. The Norman Landfill Research Site is located on the Canadian
River alluvial plain in central Oklahoma. Due to the prevalence of landfills
in this type of hydrogeologic setting, an increased understanding of the hydrologic
and geochemical processes controlling the migration of organic and inorganic
contaminants from the Norman landfill is applicable to many sites across the
United States. The contamination of the shallow alluvial aquifer at the Norman
landfill provides an excellent opportunity to study the spatial variability
of biogeochemical processes and the resulting effects on the fate of degradable
contaminants in the leachate plume. The existence of zones with differing
anaerobic microbial processes facilitates investigations of the role of electron
acceptors on the fate of organic compounds in situ. The influence of
surface-water and ground-water interactions on contaminant degradation can
be studied where the slough, a small stream and wetland, overlie the leachate
plume. The shallow water table provides a setting to study how recharge and
seasonal surface-water inputs affect aquifer chemistry and interaction with
a leachate plume. The heterogeneous permeability structure of the alluvium
provides an opportunity to study the effects of physical heterogeneities on
the fate and transport of contaminants in the aquifer.
The landfill accepted solid waste from the City of Norman, Oklahoma, from
1922 to 1985, when the landfill was closed and covered with a vegetated earthen
cap. The landfill never utilized liners or leachate collection systems, and
a leachate plume has developed that extends at least 225 meters from the edge
of the landfill. The shape and size of the plume is influenced by physical
heterogeneities and changes in hydrologic conditions at the site. Several
transport and reaction zones have been identified along a transect from the
edge of the landfill, under the slough, and toward the Canadian River (fig.
1). The observed spatial variability of the leachate plume is due to the complex
interaction between biogeochemical and hydrogeological processes, including
biodegradation, sorption, dispersion, and dilution. The heterogeneity of available
electron acceptors and the mixing of anoxic plume water with oxygenated recharge
water have resulted in a wide range of microbial degradation rates.
The research products from the studies at the Norman landfill will include
a conceptual model describing the flow system and the nature and magnitude
of biogeochemical processes that occur as landfill leachate reacts with native
ground water and aquifer solids. Additionally, quantitative analysis of the
flow system and the biogeochemical processes affecting the fate and transport
of landfill-derived contaminants will facilitate the development of a geochemical
budget for iron, sulfur, and carbon in the contaminated aquifer and surface
water, and will further our understanding of the evolution of the contaminant
plume. The emphasis of this research project is on developing an understanding
of the processes controlling contaminant migration and attenuation. The knowledge
gained from studying the Norman landfill will provide insight into natural
biogeochemical and hydrogeologic processes that cause intrinsic bioremediation
and will be of use in the design of effective bioremediation technologies.
For additional information contact:
Scott C. Christenson,
USGS, Oklahoma City, Oklahoma
(email: ), or
Isabelle M. Cozzarelli,
USGS, Reston, Virginia
(email: )