NATURAL RESTORATION OF A TREATED-WASTEWATER PLUME IN THE CAPE COD SAND AND GRAVEL AQUIFER, MASSACHUSETTS

Disposal of secondary-treated wastewater from the Massachusetts Military Reservation to the Cape Cod sand and gravel aquifer by rapid infiltration from 1935 to 1995 resulted in a contaminant plume that is more than 6 kilometers long. The plume is characterized by distinct biogeochemical zones, including a central core in which dissolved oxygen has been completely consumed by the oxidation of organic compounds and ammonium, and nitrate has been removed by denitrification. The central core contains elevated concentrations of ammonium as far as 3 kilometers from the disposal site. A zone of dissolved nitrate-nitrogen at concentrations as high as 30 mg/L (as N) surrounds the ammonium zone. Within 100-500 meters of the disposal site, the anoxic part of the plume also contains elevated concentrations of dissolved iron, manganese, and arsenic derived from the aquifer sediments. Dissolved phosphate at concentrations as high as 5.5 mg/L (as P) has been transported about 600 meters from the disposal site.

In 1995, wastewater disposal was moved to a different location, and natural processes are gradually restoring the water quality in the plume. By 2006, as a result of uncontaminated ground water with low dissolved solids and elevated dissolved oxygen flushing through the contaminated zone, concentrations of conservative wastewater-derived species, such as boron and chloride, have decreased to near-background levels as far as 600 meters from disposal site. However, dissolved oxygen is still absent from the center of the plume because oxygen continues to be consumed by biodegradation of sorbed organic materials and oxidation of ammonium. Because of the persistence of the anoxic zone, dissolved iron and manganese also remain near the disposal site. Phosphate concentrations remain elevated because the phosphate is strongly sorbed to iron oxide coatings on the aquifer sediments and thus is migrating more slowly than the conservative constituents. Field tracer experiments, laboratory measurements, and geochemical modeling indicate that the return of pristine ground-water quality in the area of the plume will take many decades. These findings have implications for the long-term protection of water quality in aquifers that are recharged by wastewater through infiltration beds and septic systems.