Natural Restoration of a Sewage Plume in a Sand and Gravel Aquifer, Cape
Cod, Massachusetts
By Denis R. LeBlanc, Kathryn M. Hess, Douglas B. Kent, Richard L. Smith,
Larry B. Barber, Kenneth G. Stollenwerk, and Kimberly W. Campo
ABSTRACT
Land disposal of treated sewage to infiltration beds at the Massachusetts
Military Reservation on Cape Cod for 60 years has formed a plume of contaminated
ground water in the sand and gravel aquifer that is more than 3.5 miles long.
Sewage disposal ended in December 1995, and no action has been taken to restore
the ground-water quality near the disposal site. In the first 30 months after
disposal ended, the trailing edge of the conservative constituents in the
plume, such as boron, moved more than 800 feet downgradient from the abandoned
beds. Concentrations of dissolved oxygen remained at or near zero near the
disposal beds, however, even though uncontaminated ground water that contains
dissolved oxygen had been flowing into the sewage-contaminated zone from upgradient
areas for 30 months. Biodegradation of organic matter associated with the
sewage-contaminated sediments is probably the primary cause of the continuing
suboxic to anoxic conditions. Nitrate concentrations in the center of the
sewage-contaminated zone decreased to below detectable levels as nitrate moved
away from the abandoned beds along with the ground-water flow or was converted
to nitrogen gas by denitrification. As nitrate levels decreased to zero, the
geochemical environment beneath the beds became more reducing, and dissolved-iron
concentrations increased because insoluble ferric iron oxide coatings on the
sediments were reduced to soluble ferrous iron. Ammonium had been expected
to be oxidized to nitrate as oxygen re-entered the sewage-contaminated zone.
Ammonium concentrations decreased, however, as ammonium desorbed from the
sediments by cation exchange and was transported away from the disposal site
in the reducing geochemical environment. pH did not change significantly because
of the buffering effects of sorption on the sediment surfaces and anaerobic
biodegradation. As a result, the concentrations of sorbed trace metals, such
as zinc and copper, did not change significantly after disposal ended. Phosphorus
concentrations remained elevated in the sewage-contaminated zone because of
slow desorption from the sediments. Results of geochemical modeling of the
natural restoration process indicate that restoration of ground-water quality
to pre-contamination conditions will be slow because of the persistent oxygen
demand in the sewage-contaminated zone.