There are several methods that may be used to
estimate biodegradation rates of organic compounds dissolved in groundwater
(Wiedemeier et al., 1999). Two commonly used methods are presented below.
Conservative tracers found commingled with a contaminant
plume can be useful for estimating the biodegradation rates of petroleum hydrocarbons
and chlorinated solvents (Wiedemeier et al., 1996). For example, the isomers
of trimethylbenzene have been found to be biologically recalcitrant under some
anaerobic conditions. Comparing the loss of trimethylbenzene to the loss of
BTEX along the groundwater flow path can allow estimation of biodegradation
rates. In a similar manner to petroleum hydrocarbons, the biodegradation rate
for chlorinated hydrocarbons may be estimated using a conservative tracer.
At sites where commingled petroleum hydrocarbon and chlorinated solvent plumes
are present, the trimethylbenzene isomers can be used to estimate biodegradation
rates for BTEX and chlorinated solvents.
Buscheck and Alcantar (1995) derive a relationship
that allows calculation of approximate biodegradation rate constants. An important
assumption that must be made when using this method is that the contaminant
plume has reached a steady-state configuration. This method involves coupling
the regression of contaminant concentration (plotted on a logarithmic scale)
versus distance downgradient (plotted on a linear scale) to an analytical solution
for one-dimensional, steady-state, contaminant transport that includes advection,
dispersion, sorption, and biodegradation. The effects of volatilization are
assumed to be negligible. When used with accurate estimates of dispersivity
and groundwater flow and solute transport velocity, this method gives reasonable
first-order biodegradation rates. Examples of how to apply this method are
given in Buscheck and Alcantar (1995) and Wiedemeier et al. (1996).
This method can also be used to estimate biodegradation rates for chlorinated
solvents dissolved in groundwater. Chapelle et al. (1996) present a similar
method for estimating biodegradation rate constants.
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