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.