Toxic chemicals and other wastes pose a serious threat to the Nation's groundwater resources. Domestic and municipal wastewater, the most common waste, is discharged to aquifers nationwide through septic systems and community treatment plants.

Wastewater-contaminated groundwater typically contains many types of toxic organic and inorganic chemicals of global concern, such as nitrogen, metals, organic compounds, and pathogens. The restoration and protection of groundwater quality depend on knowledge of the physical, chemical, and microbiological processes that affect the movement and fate of these contaminants in aquifers. Hydrologists are working to improve understanding of the interactions among subsurface-transport processes and the additional complexity caused by the natural variability of aquifer materials. This understanding is needed to develop practical methods to clean up and protect groundwater resources.

The objectives of this study are to (1) improve understanding of the physical, chemical, and microbiological processes that affect the movement and fate of contaminants of global concern through naturally heterogeneous aquifers, (2) develop and test predictive models of the complex interactions among these processes, (3) examine the processes by which toxic chemicals and pathogens are removed naturally from aquifers, and (4) develop methods to enhance the natural cleanup process.

The treated-wastewater plume at the Massachusetts Military Reservation on Cape Cod is used as a field laboratory by a team of government and university scientists to examine the processes that affect toxic chemicals, pathogens, and other contaminants in groundwater. The plume was created by disposal of treated wastewater to the sand and gravel aquifer beneath the reservation from 1936 to 1995. Drilling and sampling are used to determine the distributions of chemicals and microbes in the plume, and hypotheses are developed to explain these distributions.

Field experiments, such as tracer studies; detailed characterization of the variability of the aquifer's physical and chemical properties; laboratory studies; and computer modeling are used to test these hypotheses. The understanding of the movement and fate of the contaminants will be used to predict the rate at which groundwater quality can be restored naturally if disposal is stopped and to develop methods to artificially enhance the rate of restoration.