by Denis LeBlanc
This report is available as a pdf below
Over the past decade, the need for understanding the mechanisms of contamination of ground water by toxic wastes has become exceedingly important. Many private and government studies of contaminated aquifers have described the movement, chemical alteration, and dispersal of toxic chemicals in ground-water systems. These studies have also shown, however, that transport of contaminants in aquifers is a very complex process. Many technical questions are yet to be answered about the behavior of specific chemicals under different hydrologic and geologic conditions. Answers to these questions are needed to guide the evaluation and cleanup of contaminated ground-water supplies and to ensure safe use and disposal of toxic chemicals.
The U.S. Geological Survey has begun a nationwide program to study the fate of toxic wastes in ground water. Several sites where ground water is known to be contaminated are being studied by interdisciplinary teams of geohydrologists, chemists, and microbiologists. The objective of these studies is to obtain a thorough quantitative understanding of the physical, chemical, and biological processes of contaminant generation, migration, and attenuation in aquifers. The knowledge obtained from these site-specific studies will contribute greatly to successful evaluation, monitoring, and remedial action at similar sites where contamination by toxic chemicals occurs.
One of the sites being studied by the U.S. Geological Survey under this program is a plume of sewage-contaminated ground water on Cape Cod, Massachusetts. The plume was formed by land disposal of treated sewage to a glacial outwash aquifer since 1936. Although sewage generally is not considered a hazardous waste, even relatively "clean" domestic sewage contains many inorganic and organic chemicals such as sodium, nitrate, detergents, and volatile organic compounds which can be toxic and render a groundwater supply unfit for use. Research on how these sewage-derived contaminants move in the Cape Cod aquifer will add to the understanding of the complex interactions of geohydrologic, chemical, and microbiological processes that affect contaminant migration.
This report summarizes results obtained during the first year of research at the Cape Cod site under the U.S. Geological Survey Toxic-Waste Ground-Water Contamination Program . The seven papers included in this volume were presented at the Toxic Waste Technical Meeting, Tucson, Arizona, in March 1984. They provide an integrated view of the subsurface distribution of contaminants based on the first year of research and discuss hypotheses concerning the transport processes that affect the movement of contaminants in the plume.
A mathematical model of solute transport is used to simulate and evaluate the movement of dissolved constituents within the ground-water-flow system .
The distributions of inorganic and organic chemicals in the plume are described and preliminary conclusions are presented about the behavior of selected contaminants in the aquifer .
A special technique to determine concentrations of semi-volatile organic compounds in the plume at the nanogram per liter level is evaluated and used to identify possible organic tracers of the contaminated ground water.
The distributions of inorganic nitrogen, organic carbon, and bacterial populations in the aquifer are used to show that microbial activity significantly affects the fate of some contaminants.
The abundance and nature of the bacterial population in sediment and water samples are described and preliminary conclusions are presented about the reaction of ground-water bacteria to the subsurface sewage contamination.
Measurements of the denitrification potential of sediment and water samples are used to show that a zone of bacterially mediated denitrification probably has been established in the aquifer in response to the increased quantities of nitrate and organic carbon in the plume.
The authors thank the many persons who have kindly given time, information, and guidance to the project team. Particular thanks are given to Richard Quadri, Stephen Garabedian, Virginia de Lima, Albert Augustine, John Organek, Peter Haeni, Joseph Newell, and other persons in the Geological Survey who assisted with data collection and project logistics. The authors also gratefully acknowledge Col. Philip McNamara and George Sundstrom of Otis Air National Guard Base for their help and cooperation.
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