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.