Evolution of a Ground-Water Sewage Plume after Removal of the 60-Year-Long Source, Cape Cod, Massachusetts: Changes in the Distribution of Dissolved Oxygen, Boron, and Organic Carbon

by Larry B. Barber and Steffanie H. Keefe

ABSTRACT

Sewage effluent consists of a complex mixture of biogenic and synthetic organic compounds with a range of environmental behaviors. Sixty years of disposal of secondary treated sewage effluent into a sand-and-gravel aquifer on Cape Cod, Massachusetts, has resulted in an extensive plume of ground water contaminated by organic and inorganic compounds. In December 1995, the source was discontinued and a monitoring program was initiated that involved collection of about 500 samples below and down-gradient from the infiltration beds every 6 months for 3 years. The samples were analyzed for a variety of constituents including dissolved boron, dissolved oxygen (DO), and dissolved organic carbon (DOC). Removal of the source resulted in rapid flushing of boron and re-distribution of the organic compounds between the ground water and aquifer sediments. Prior to cessation, a portion of the organic carbon in the aquifer existed as DOC with concentrations ranging from 0.1 milligram per liter in the uncontaminated ground water to greater than 5 milligrams per liter in the sewage plume. The DOC was complex and had a range of solubility characteristics resulting in transport rates from conservative to highly retarded. Sediment organic carbon (SOC) concentrations (0.01 to 1.0 percent, or 100 to 10,000 milligrams per kilogram) were significantly higher than DOC concentrations. Thirty months after cessation, concentrations of boron and DOC had decreased beneath the infiltration beds, but DO concentrations remained relatively unchanged. The SOC reservoir appears to undergo desorption into the uncontaminated water moving into the zone of contamination, and considerable time will be required for ground water DOC concentrations to reach background levels. The DOC and SOC function as a pool of oxygen-consumption capacity, which will extend the period of time needed for DO concentrations to return to uncontaminated conditions.