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.

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.

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.

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