Modeling the Evolution and Natural Remediation of a Ground-Water Sewage Plume

By Kenneth G. Stollenwerk and David L. Parkhurst

Abstract

A one-dimensional reaction-transport model was used to simulate the 60-year chemical development of a plume of sewage-contaminated ground water at the Massachusetts Military Reservation on Cape Cod, Massachusetts. The dominant biogeochemical reactions in the model were oxidation of organic carbon by dissolved oxygen, nitrate, manganese oxides and iron oxyhydroxides. Sorption controlled transport of phosphate, ammonium, potassium, calcium, magnesium, iron(II), and manganese(II). Reaction parameters were chosen to fit the observed concentration profiles of solutes along the longitudinal axis of the core of the plume. The model was then used to predict the evolution of the plume for the first 60 years following cessation of sewage disposal. Based on estimates of the reactivity of sorbed organic carbon, the model predicted that reducing conditions could be maintained for the next 60 years. Sorbed cation concentrations will decrease slowly and move downgradient; pH will remain at pre-cessation levels throughout much of the aquifer. As low ionic strength, uncontaminated ground water elutes through the aquifer, a transient increase in pH is predicted to cause a spike in phosphate concentrations. Phosphate concentrations at the shore of Ashumet Pond could increase for the next 10-15 years.