Evolution of a Ground-Water Sewage Plume After Removal of the 60-Year-Long Source, Cape Cod, Massachusetts: pH and the Fate of Phosphate and Metals

By Douglas B. Kent and Valerie Maeder

ABSTRACT

The distribution of copper (Cu), phosphorus (P), and zinc (Zn), whose fate and transport are controlled by pH-dependent adsorption, was examined as part of a multidisciplinary study of the evolution of ground-water quality in a sewage-contaminated aquifer following source cessation. Prior to source cessation, Cu and Zn were observed 10 to 20 meters below the water table under the sewage-disposal beds; most of the dissolved Cu was likely present as strongly complexed species. Down gradient of the beds, these metal ions were only observed in a narrow region near the upper boundary of the sewage plume. The initial distribution of P under the disposal beds showed considerable spatial variability and was similar to that of boron, which is assumed to be non-reactive. Examination along a flow path showed a region where dissolved P apparently was absent even though it was present both up and down gradient. After source cessation, it was anticipated that ground water under the disposal beds would become more acidic, which would lead to major changes in the mobility of these contaminants. However, pH values under the disposal beds exhibited only minor fluctuations, between 5.8 and 6.2 in the core of the contaminated region, during the 2.5 years since source cessation. This is likely related to the continued importance of anaerobic biodegradation reactions under the disposal beds. Minor changes in the distribution of Cu resulted from transport of weakly adsorbing Cu complexes away from the contaminated zone under the disposal beds. Small but consistent decreases in P concentrations suggest that P was slowly being transported away from the contaminated zone under the beds. The distribution and concentrations of both free Zn and Cu under the disposal beds did not change significantly over the 2.5 years of the study, consistent with extensive adsorption of these metal ions onto the aquifer sediments at the observed pH values.