PLUME-SCALE ASSESSMENT OF INORGANIC NITROGEN TRANSPORT AND ATTENUATION IN A TREATED-WASTEWATER PLUME, CAPE COD, MASSACHUSETTS

Land disposal of treated wastewater by infiltration to a sand and gravel aquifer over a 60-year period (1936 to 1995) has created a plume of contaminated groundwater that is migrating toward coastal discharge areas. To develop a better understanding of the potential impact of the plume on the discharge areas, data from two extensive groundwater sampling events in 1994 and 2007 were used to map the size and location of the plume, calculate the masses of nitrate, ammonium, boron, and chloride in the plume, and calculate the change in mass between sampling events to assess the occurrence and extent of nitrogen reactivity. Plume-mapping results indicate that the treated-wastewater plume was approximately 1,200 m wide, 30 m thick, and 7,700 m long in 2007 and contained 87,000 kg nitrate-N and 31,600 kg total ammonium-N. Because of adsorption, ammonium migrates at a relatively slow rate, and the ammonium plume was 3,300 m long in 2007. Despite the size of the dataset (1,160 samples), the boundaries of the wastewater plume were difficult to locate in some areas, particularly near the leading edge, where data are sparse and the plume is geochemically similar to ambient groundwater. Between 1994 and 2007, the mass of nitrate in the plume changed relatively little, whereas the mass of ammonium declined by nearly 40 percent. The reactions responsible for the apparent ammonium mass loss were not identified. A comparison of the mass of nitrogen in the plume in 2007 with the estimated total mass of nitrogen in the treated wastewater discharged to the aquifer during operation of the treatment plant indicated that approximately 85 percent of the total mass had been removed from the aquifer by biogeochemical reactions, pumping withdrawals, or discharge to hydrologic boundaries over the 71-year period. However, the total mass estimate, which is based on an assumed loading history, is highly uncertain. The results of this plume-scale investigation indicate that biologically reactive nitrogen from treated wastewater disposal has persisted in the aquifer for decades, forming a long plume, and will affect coastal discharge areas for decades in the future. The uncertainties and data gaps encountered in the analysis of this well-studied plume have important implications for assessments of natural restoration at other wastewater-impacted sites.