Phosphorus Transport in Sewage-Contaminated Ground Water, Massachusetts Military Reservation, Cape Cod, Massachusetts

By Donald A. Walter, Denis R. LeBlanc, Kenneth G. Stollenwerk, and Kimberly W. Campo

ABSTRACT

The disposal of secondarily treated sewage effluent at the Massachusetts Military Reservation on western Cape Cod, Massachusetts, between 1936 and 1995 has created a plume of contaminated ground water in the underlying sand and gravel aquifer in which dissolved phosphorus concentrations can exceed 10 mg/L (milligrams per liter). Ground water with phosphorus concentrations as high as 2 mg/L is currently (1998) discharging into nearby Ashumet Pond. Phosphorus is transported in two geochemical environments in the plume--an anoxic environment in which phosphorus is closely associated with dissolved iron and no dissolved oxygen, and a more extensive suboxic environment in which there is low, but detectable, dissolved oxygen and no dissolved iron. The adsorption of phosphorus onto iron and aluminum oxides has greatly retarded the movement of phosphorus relative to ground-water velocities. Continued loading of phosphorus onto the sediments, however, has created a large reservoir of sorbed phosphorus and allowed for the breakthrough and significant transport of dissolved phosphorus. Concentrations of phosphorus in ground water in the center of the plume have remained generally unchanged since 1993, whereas phosphorus concentrations along the eastern edge of the plume, where the highest concentrations are observed, have changed significantly. High concentrations of dissolved phosphorus along the eastern and western edges of the plume are associated with low specific conductances. This suggests that phosphorus desorption may be occurring in the aquifer in response to an influx of clean water and that phosphorus could remain in solution for long periods of time after other plume constituents have been flushed from the aquifer.