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.