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.