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.