Evolution of a Ground-Water Sewage Plume After Removal of the 60-Year-Long
Source, Cape Cod, Massachusetts: Inorganic Nitrogen Species
By Richard L. Smith, Brigid A. Rea Kumler, Thomas R. Peacock, and Daniel
N. Miller
ABSTRACT
High concentrations of nitrate and ammonium in ground water are often associated
with sewage disposal practices. However, little is known about the combined
effect of physical and biological processes upon the long-term fate of inorganic
nitrogen in ground water, particularly after the source of contamination has
been removed. Discharge of treated sewage onto surface infiltration beds at
the Massachusetts Military Reservation on Cape Cod for 60 years created a
ground-water contaminant plume more than 3 miles long. This plume is characterized
by a suboxic to anoxic, ammonium-containing core surrounded by an oxic to
suboxic, nitrate-containing outer zone. Historically, the sewage effluent
contained varying amounts of nitrate and ammonium (200-2000 micromolar (µM)),
oxygen (about 350 µM), and dissolved organic carbon (about 1000 µM). The geochemical
evolution of the up-gradient portion of the plume has been extensively monitored
for 2 years following the cessation of disposal in December 1995. Although
nitrate concentrations in the core of the plume increased initially after
cessation, nitrate levels have gradually decreased in and below the core of
the plume. Nitrate concentrations have remained elevated (about 1 millimolar
(mM)) near the water table, however, well after non-reactive solutes were
flushed from this zone. Ammonium concentrations initially increased after
cessation, then gradually decreased to low levels. These observations indicate
that, while the effluent was being discharged, nitrification of ammonium to
nitrate was occurring in the aquifer under the infiltration beds and was fueled
by the oxygen in the treated sewage. Following the shutoff, nitrification
stopped because oxygen was no longer being introduced with the treated sewage,
and oxygen consumption up-gradient of the ammonium zone prevented oxygen in
uncontaminated water from reaching the ammonium zone. Thus, ammonium moved
down-gradient away from the disposal area, even though ammonium transport
was slowed by cation exchange.