Modeling the Influence of Adsorption on the Fate and Transport of Metals
in Shallow Ground Water: Zinc Contamination in the Sewage Plume on Cape Cod,
Massachusetts
By Douglas B. Kent, Robert H. Abrams, James A. Davis, and Jennifer A. Coston
ABSTRACT
Land disposal of sewage effluent at the Massachusetts Military Reservation
on Cape Cod, Massachusetts, for nearly 60 years resulted in contamination
of the aquifer with zinc (Zn). In contrast to non-reactive constituents of
the sewage plume, which were observed greater than 5,500 meters down gradient
of the source, Zn contamination was observed only within approximately 400
meters of the source. Under the disposal beds, Zn contamination was observed
12-25 meters below the water table. Greater than 200 meters down gradient
of the source, Zn contamination was only observed in a 2-4 meter-thick region
near the top of the sewage plume. The fate and transport of Zn in the aquifer
most likely was controlled by pH-dependent adsorption onto aquifer sediments.
A model that coupled flow, transport, and equilibrium adsorption was used
to examine the influence of variable pH on the fate and transport of Zn. Adsorption
was described using a semi-empirical surface complexation modeling approach;
model parameters were determined from laboratory batch experiments. Model
results captured the principal features of the distribution of Zn contamination
in the aquifer after nearly 60-years of disposal. The model also was used
to predict that acidification of ground water under the disposal beds, which
should occur as a natural consequence of source cessation, would cause significant
increases in Zn concentration and mobility.