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.