Environmental Engineering Reference
In-Depth Information
5.5.5 Evaluation of system performance
5.5.5.1 Electron donor evaluation
The production of acetate and hydrogen gas is an indication that the bio-
degradation of ethanol is occurring at the site. The hydrogen produced by
the oxidation of ethanol can be used for the dechlorination of PCE (Equation
5.1). With incomplete oxidation of ethanol (Equation 5.2), 2 moles of ethanol
would be required to produce enough hydrogen for complete dechlorination
of PCE. With complete oxidation of ethanol (Equation 5.3), only 1 mole of
ethanol would be required.
The Surfer contour plots of PCE and ethanol groundwater concentrations
were used to estimate the mass of each compound in the dissolved phase.
Calculations were made for the groundwater concentration data collected
immediately following the solvent extraction test and after 2
1
/
2
years of
groundwater monitoring (Table 5.5). The estimated residual PCE DNAPL
was obtained from the partitioning tracer test data. Ethanol mass was also
estimated based on concentrations measured in the recovery wells during
the solvent extraction test, which was subtracted from the total mass injected
(Table 5.5).
These estimates of compound mass were used to calculate the ratio of
ethanol to PCE to evaluate the relative amount of electron donor available
for the reductive declorination of PCE (Table 5.5). The estimate of ethanol
mass from the recovery well data (47,234 moles) is 10-fold higher than that
from the measured dissolved phase concentrations (5119 moles). The total
mass of PCE in the dissolved phase (11 moles) is also much lower than the
estimate of the DNAPL residual from the partitioning tracer test (254 moles),
as would be expected. The data indicate that excess electron donors (ethanol)
are present to drive the reductive dechlorination of PCE to completion. The
ratio of ethanol to PCE is approximately 180:1 when the estimate of ethanol
from the recovery wells is used, compared to 19:1 when the estimate of
ethanol from the groundwater concentrations is used. If we assume it takes
2 moles of ethanol for complete dechlorination and there are no competing
reactions, then there is 9 to 90 times the amount of ethanol to remove the
265 moles of PCE remaining in the subsurface.
The dissolved phase concentrations were used to estimate the masses
of ethanol and PCE immediately following the cosolvent flush and after
2
1
/
2
years of groundwater monitoring. Although these estimates do not
take into account the residual PCE DNAPL that is present in the system,
they do show that excess ethanol remains in the dissolved phase. The ratios
from these estimates range from 465:1 immediately following the cosolvent
extraction to 482:1 after 2
1
/
2
years of groundwater monitoring. If we assume
there are no competing reactions and there is complete oxidation of ethanol,
these ratios indicate that there is more than 200 times the amount of ethanol
present in solution available for the complete dechlorination of PCE. Even
though the concentrations of both ethanol and PCE have decreased over
time, their ratio has remained constant.
