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.