THE CAPE COD WASTEWATER PLUME AS A FIELD LABORATORY FOR THE MULTIDISCIPLINARY STUDY OF PROCESSES AFFECTING CONTAMINANT TRANSPORT IN GROUND WATER

A treated-wastewater plume in a sand and gravel aquifer is the subject of multidisciplinary research at the U.S. Geological Survey Toxic Substances Hydrology Cape Cod Research Site. The plume was formed by disposal to infiltration beds of effluent from the Massachusetts Military Reservation wastewater-treatment facility from about 1936 to 1995. The plume contains many contaminants of global concern, including nitrate, phosphate, trace elements, organic compounds, and microbes. Early studies described the plume's geometry and extent, spatial distributions of contaminants, and steep biogeochemical gradients. The absence of dissolved oxygen and nitrate in the plume's core was attributed to microbiological degradation of organic compounds and to limited transverse dispersion. These descriptive observations were followed by experimental and numerical-modeling studies, which continue today and demonstrate how physical, chemical, and microbiological processes interact at different transport scales to produce the plume's physicochemical characteristics. Tracer experiments, in which chemicals and microbes of interest are injected into different biogeochemical zones, have been an important part of these studies; to date, more than 100 experiments have been conducted. The largest experiment, in which nonreactive and reactive tracers were monitored as they moved more than 300 m in the aquifer, confirmed that transverse dispersion is limited. Other experiments demonstrated how variability in chemical conditions controls rates of sequestration and mobility of bacteria, viruses, protozoa, redox-sensitive solutes, and strongly sorbing metals. During the past decade, results of the process-oriented studies have been applied to predict how long wastewater-derived contaminants will persist in the aquifer. Field observations, laboratory experiments, and numerical modeling indicate that complete restoration of pristine water quality will take many decades because of limited physical dispersion and the persistence of sorbed inorganic and organic contaminants on sediment grains. These findings may have important implications for restoration of ground-water quality at other sites.