SECTION C

The Fate of Complex Contaminant Mixtures From Treated Wastewater Discharges

Sewage wastes are sources of ground-water contamination worldwide. The sources range from individual septic systems discharging a few hundred gallons per day to large municipal systems discharging millions of gallons per day. Common to all these sources is the complex mixture of nutrients, trace metals, microbes and other inorganic and organic compounds that compose typical sewage wastewater. Natural physical, chemical, and biological processes interact to transport, transform, and dilute these wastes in the subsurface.

During the past several decades, we have improved our understanding of these subsurface processes by studying specific sites of contamination. The sewage plume at the Massachusetts Military Reservation (formerly Otis Air Force Base) on Cape Cod is one of these sites. Disposal of treated sewage for more than 60 years to the sand and gravel aquifer has formed a plume that is more than 3.5 miles long. The plume is characterized by distinct geochemical zones caused by biodegradation of the contaminants in a aquifer. In 1983, the sewage plume was chosen as a ground-water field site of the U.S. Geological Survey Toxic Substances Hydrology Program.

Since 1983, USGS scientists and their university colleagues have studied the Cape Cod sewage plume to understand how complex mixtures of contaminants interact with the subsurface biota and sediments. The studies initially focused on defining the plume's contaminant distributions, geochemical zones, and microbial populations. From these studies, the scientists formed hypotheses about the physical, chemical, and biological processes affecting the contaminants, and developed field and laboratory methods to study these processes. In the late 1980's, the research team began to use field experiments to examine the factors controlling these processes. The sewage plume and its distinct geochemical zones became a field-sized laboratory where the team has run more than 60 tracer experiments. This experimental work has led recently to a re-examination of the sewage plume. The scientists are using results from the field tests and new observations of the plume to improve models of how complex mixtures of contaminants move and attenuate in the subsurface. In December 1995, sewage disposal ended after 60 years of continuous discharge to the ground-water system. The change has provided the research team with a unique opportunity to observe the natural restoration of the plume and to test the ideas and models developed during the past 16 years.

The papers in this section reflect the diversity of interdisciplinary research at the Cape Cod Toxic Substances Hydrology research site. Five papers describe the first 3 years of the natural restoration of the sewage plume. Several papers describe the fate of specific contaminants, such as phosphorus and zinc, in the plume. Other papers report on research concerning specific physical, chemical, and biological processes, such as dispersion, adsorption, and nitrification, and the simulation of these processes with geochemical models. Tracer experiments are described in several papers, including tests that used trace metals, protozoa, viruses, colloids, and nutrients as tracers. New methods to measure hydrologic and chemical variables are reported in other papers. Three papers describe a remediation experiment being conducted on a nearby plume of solvents on the military base.

The papers in this section reflect the common theme that contaminants in the subsurface are affected by complex interactions of physical, chemical, and biological processes. The studies demonstrate that an understanding of the subsurface fate of complex mixtures of contaminants, such as sewage wastewaters, requires an interdisciplinary investigative approach that links together field observations, field and laboratory experiments, and predictive modeling.

For additional information contact:

Denis R.LeBlanc,
USGS Marlborough, Massachusetts,
(email )

Kathryn M. Hess,
USGS Marlborough, Massachusetts,
(email ), or