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Natural Restoration of a Treated-Wastewater Plume

Natural Aquifer Restoration

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Overview

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Natural restoration, also called natural attenuation, is the cleanup of groundwater contamination by natural physical, chemical, and microbiological processes. The rate and effectiveness of natural restoration of groundwater quality is affected by many factors, including the type and amount of contamination, and the physical and geochemical nature of the aquifer. Prediction of the rate and effectiveness of cleanup requires a sound scientific understanding of the complex interactions of these processes and factors. The USGS is adding to this understanding through a detailed study of a treated-wastewater plume at the Massachusetts Military Reservation on Cape Cod. The plume has been undergoing natural restoration for 10 years since 60 years of land disposal of treated sewage ended in 1995. The study includes monitoring of groundwater quality, tracer tests and laboratory experiments to identify and characterize the cleanup processes, and numerical modeling to integrate the understanding of these processes.


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Background

Natural restoration, also called natural attenuation, is the approach in which water quality is allowed to restore itself naturally without any intervention other than the removal of the source of contamination. It has been used increasingly over the past 10-15 years as a cost-effective means to deal with many types of contaminants, including hydrocarbons from fuel spills, industrial solvents, and leachate from landfills and municipal wastewater-disposal facilities. The effectiveness of natural restoration depends on many factors, such as the type and amount of contaminants and the physical, chemical, and microbiological processes that affect the transport, fate, and attenuation of the contaminants.

photo The reliance on natural attenuation requires an understanding of these processes so that predictions can be made of the effectiveness and speed of the cleanup. Because of the uncertainties of these predictions, reliance on natural attenuation also requires monitoring to characterize the important processes and track the progress of the cleanup. There is a need for detailed field studies to increase understanding of the natural-restoration processes and develop methods for improved prediction and monitoring of the cleanup process.

Disposal of secondary-treated wastewater from the Massachusetts Military Reservation to the Cape Cod sand and gravel aquifer by rapid infiltration from 1935 to 1995 resulted in a contaminant plume that is more than 6 kilometers long. In 1995, wastewater disposal was moved to a different location, and natural processes are gradually restoring the water quality in the plume. An extensive network of monitoring wells is in place to monitor the progress of the restoration.


Objectives and Approach


The objectives of this study are to (1) increase the understanding of the physical, chemical, and biological processes that result in natural restoration of contaminated aquifers, (2) develop and test methods to monitor the progress of natural restoration, and (3) develop methods for the prediction of the nature and rates of the chemical and microbiological changes in water quality during natural restoration.

The natural-restoration study includes detailed monitoring of groundwater quality near the abandoned wastewater-infiltration beds and field tracer experiments, laboratory measurements, and geochemical modeling to examine specific restoration processes. Groundwater samples are collected at least annually from an array of monitoring wells and multilevel samplers in and within about 2,000 feet downgradient from the infiltration beds. The samples are analyzed for cations, anions, nitrate, ammonium, and organic and inorganic carbon species, and ultraviolet/visible absorbance. Tracer experiments and laboratory experiments with sediment and water from the aquifer are conducted to examine the rates and controls on specific restoration processes. Numerical reactive-transport groundwater models integrate the field and laboratory to increase understanding of and ability to predict the overall restoration process.


Sources of Additional Information


National Research Council, 2000, Natural Attenuation for Groundwater Remediation:
Washington, D.C., National Academy Press, 274 p.
http://newton.nap.edu/catalog/9792.html


U.S. Geological Survey, 2006, Natural Attenuation:
http://toxics.usgs.gov/topics/attenuation.html


U.S. Environmental Protection Agency, 2001, A citizen’s guide to monitored natural attenuation:
Office of Solid Waste and Emergency Response, EPA-542-F-01-004, April 2001, 2 p.
http://www.epa.gov/tio/download/citizens/mna.pdf


U.S. Geological Survey, 2007, Decades Required for Natural Processes to Clean Wastewater-Contaminated Groundwater:
http://toxics.usgs.gov/highlights/gw_cessation.html


Selected Publications

Link to Full Bibliography

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USGS Data Series 198
Ground-water-quality data for a treated-wastewater plume undergoing natural restoration, Ashumet Valley, Cape Cod, Massachusetts, 1994-2004     Report


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International Association of Hydrological Sciences Publication - December 2007
Fate of consumer-product chemicals in the subsurface environment—Twenty-five years of research on Cape Cod, Massachusetts, USA     Abstract


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Environmental Science and Technology - January 2006
Long-term natural attenuation of carbon and nitrogen within a groundwater plume after removal of the treated wastewater source.     Abstract


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USGS Water-Resources Investigations Report 03-4017
Reactive-transport simulation of phosphorus in the sewage plume at the Massachusetts Military Reservation, Cape Cod, Massachusetts.     Abstract


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USGS Water-Resources Investigations Report 99-4018C
Natural restoration of a sewage plume in a sand and gravel aquifer, Cape Cod, Massachusetts.     Abstract


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USGS Water-Resources Investigations Report 99-4018C
Evolution of a groundwater sewage plume after removal of the 60-year-long source, Cape Cod, Massachusetts--pH and the fate of phosphate and metals.     Abstract


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USGS Water-Resources Investigations Report 99-4018C
Evolution of a groundwater sewage plume after removal of the 60-year-long source, Cape Cod, Massachusetts -- Changes in the distribution of dissolved oxygen, boron, and organic carbon.     Abstract


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USGS Water-Resources Investigations Report 99-4018C
Modeling the evolution and natural remediation of a groundwater sewage plume.     Abstract



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