DETERMINATION OF THE DISCHARGE AREA AND NUTRIENT FLUX FROM A SEWAGE PLUME TO A GLACIAL-KETTLE POND ON CAPE COD, MASSACHUSETTS

The discharge area of a plume of secondarily treated sewage emanating from the Massachusetts Military Reservation to Ashumet Pond, Cape Cod, Mass., was determined on three occasions (June 1999, August 2001, June 2003) by using temporary drive points to sample shallow ground water beneath the pond. Ashumet Pond is a glacial-kettle, ground-water-flow-through pond about 500 meters downgradient from the sewage disposal site. Decreases over time in the size and location of the discharge area and in the concentrations of sewage-plume indicators, such as specific conductance and boron, reflect the natural flushing of the sewage plume from the aquifer since sewage disposal stopped in 1995. Dissolved phosphorus concentrations as great as 3 mg/L as P and dissolved nitrogen concentrations as great as 13 mg/L as N were measured in samples from along about 300 meters of shoreline, with the greatest concentrations detected in samples from within 10 meters of the shoreline. Calculations of phosphorus and nitrogen flux through the discharge area into Ashumet Pond, as determined from the product of water-flux estimates from a steady-state ground-water-flow model and linearly interpolated concentrations from the drive points and near shore multilevel wells, indicate that phosphorus and nitrogen discharge into the pond with the inflowing ground water at rates as high as about 110 and 940 kg/yr, respectively. The estimated phosphorus and nitrogen loads have decreased over the 4-year period because of the flushing of the sewage plume from the aquifer since sewage disposal ceased, although the phosphorus flux decreases more slowly than nitrogen because of significant retardation of phosphorus in the aquifer. The flux estimates were sensitive to changes in the spatial discretization scheme used to interpolate the chemical data and changes in the discretization of the ground-water-flow model.