The Cape Cod field research site is near the wastewater-treatment facility at the Massachusetts Military Reservation (MMR) on western Cape Cod. A treated-wastewater plume originates from the facility's infiltration beds, which were used from about 1936 to 1995. The plume extends more than 6 kilometers from the disposal site and is about 1 kilometer wide and 23 meters thick. The plume contains a complex mixture of phosphate, nitrate, metal ions, detergents, organic chemicals and microbes. The plume is characterized by distinct geochemical zones and steep, vertical biogeochemical gradients that are attributed to microbiological degradation of organic compounds and limited transverse dispersion during solute transport.

The plume is in a glacial-outwash sand and gravel aquifer that is about 100 meters thick at the research site. The water table slopes to the south toward the ocean at about 1.5 meters per kilometer. Groundwater flows generally southward at about 0.3 to 0.9 meters per day. The plume partially discharges to Ashumet Pond, which is a glacial kettle pond about 0.5 kilometers downgradient from the infiltration beds. The source of water to the aquifer is about 70 centimeters per year of natural recharge from precipitation.

Several well arrays have been installed in the study area to conduct groundwater tracer experiments. The largest array, which includes more than 12,000 sampling points, is about 300 m downgradient from the infiltration beds. In addition, more than 500 wells have been drilled in the treated-wastewater plume to characterize the biogeochemical conditions and investigate processes that affect contaminant transport.

The Massachusetts Military Reservation is a 22,000-acre facility that has been used for military activities since the early 1900s. More than 20 contaminant plumes have been mapped on the reservation and are being investigated and remediated by the U.S. military. The plumes contain contaminants that include industrial solvents, fuel products, landfill leachate, explosive compounds, and propellants.