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: )