Geochemical and Hydrologic Controls on Phosphorus Transport in a Sewage-Contaminated Sand and Gravel Aquifer Near Ashumet Pond, Cape Cod, Massachusetts

Walter, Donald; Rea, Brigid; Stollenwerk, Kenneth; Savoie, Jennifer

The disposal of secondarily treated sewage onto rapid infiltration sand beds at the Massachusetts Military Reservation, Cape Cod, Massachusetts, has created a plume of sewagecontaminated ground water in the underlying sand and gravel aquifer; a part of the sewage plume that contains dissolved phosphorus extends about 2,500 feet downgradient of the sewage-disposal beds. A part of the plume that contains nearly 2 milligrams per liter of phosphorus currently (1993) discharges into Ashumet Pond along about 700 feet of shoreline. The sewage plume discharges from about 150 to about 195 kilograms of phosphorus per year into the pond. Hydraulichead measurements indicate that the north end of Ashumet Pond is a ground-water sink and an increased component of ground-water discharge and phosphorus flux into the pond occurs at higher water levels. Phosphorus was mobile in ground water in two distinct geochemical environments- an anoxic zone that contains no dissolved oxygen and as much as 25 milligrams per liter of dissolved iron, and a more areally extensive suboxic zone that contains little or no iron, low but detectable dissolved oxygen, and as much as 12 milligrams per liter of dissolved manganese. Dissolved phosphorus is mobile in the suboxic geochemical environment because continued phosphorus loading has filled available sorption sites in the aquifer. Continued disposal of sewage since 1936 has created a large reservoir of sorbed phosphorus that is much greater than the mass of dissolved phosphorus in the ground water; the average ratios of sorbed to dissolved phosphorus in the anoxic and suboxic parts of the sewage plume were 3 1 : 1 and 155: 1, respectively. Column experiments indicate that phosphorus in the anoxic core of the plume containing dissolved iron may be immobilized within 17 years by sorption and coprecipitation with iron oxyhydroxides following the cessation of sewage disposal and the introduction of uncontaminated, oxygenated ground water into the aquifer. Residual oxygen demand associated with sorbed organic compounds and ammonia could retard the movement of oxygenated water into the aquifer. Sorbed phosphorus in the suboxic part of the aquifer will continue to desorb into the ground water and will remain mobile for perhaps hundreds of years. Also, the introduction of uncontaminated water into the aquifer may cause dissolved-phosphorus concentrations in the suboxic zone of the aquifer to increase sharply and remain higher than precessation levels for many years due to the desorption of loosely bound phosphorus. Data from three sampling sites, located along the eastern and western boundaries of the sewage plume and downgradient of abandoned sewage-disposal beds, indicate that ground-water mixing and phosphorus desorption may already be occurring in the aquifer in response to the introduction of uncontaminated recharge water into previously contaminated parts of the aquifer.