Using the Reactive-Transport Simulator PHAST to Model the Fate of Nitrogen in a Waste-Water Plume, Cape Cod, Massachusetts

Results of reactive-transport modeling are useful in understanding many subsurface hydro-geochemical problems, including the movement of natural and anthropogenic contamination. However, development of a site-specific reactive-transport model is a difficult process involving translation of geographic, hydrologic, and chemical information into the formats required by the simulator. Recent development work on PHAST has focused on (1) simplifying the data translation process by the use of grid-independent spatial data, such as ArcInfo shape files, and (2) providing a comprehensive graphical user interface where users can define and visualize three-dimensional data.
The new features and interface to PHAST have been used to model the transport of ammonium and nitrate in a plume of waste water from a sewage treatment plant on Cape Cod, Massachusetts. The model considers just the ammonium in the sewage effluent, which is assumed to be retarded by surface-sorption reactions and to react with dissolved oxygen from the native ground water to form nitrate.  Results of the modeling indicate that approximately three-fourths of this nitrogen (ammonium and nitrate combined) flows into a kettle pond near the source area and the remainder ultimately discharges to the rivers and estuaries of Cape Cod. In the simulations, the ammonium plume migrates approximately half the distance to the coast before it is completely oxidized to nitrate. The simulated discharge of ammonium-derived nitrate to the rivers and estuaries occurs over a period of nearly 200 years, with maximum loading of about 400 kilograms per year.