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1400
1200 Tartre
1000 Initial concentration
Concentrations (in ppm) 800

600
400
200 B
0
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
Time (in minutes)
Figure 19.6. Diffusive flux evaluated from the slope of the rebound at a specific concentration gradient
(initial concentration-“B”)

mass balance approach is that advection may not represent the dominant transport
mechanism.
The effectiveness of biodegradation processes should be evaluated with respect
to reactant consumption or daughter compound generation. The best measure of
these processes is a method that is able to generate data on a small-scale basis so
that a comparison of analyte concentrations in up gradient and plume areas is influ-
enced minimally by spatial and temporal variations in hydrology or geochemistry.
The measurement of in situ flux rates for reactants (e.g., contaminants or nutri-
ents) and products (e.g., daughter compounds or biogenic gases) represents such a
method.
The traditional method for calculating field biodegradation rates requires integra-
tion of conservative tracer results, an estimate of retarded solute migration in the
saturated zone, and hydrogeological calculations (Wiedemeier et al., 1996). An alter-
native method proposes calculation of biodegradation rates using direct monitoring
data (Buscheck and Alcantar, 1995). With either method, the retarded solute velocity
is generally overestimated to account for the uncertainty in model parameters. To our
knowledge, there is no commercial method that directly measures either destruction
rates of contaminants or consumption rates of reactants (Crumbling, 1999; Haggerty
et al., 1998; Lahvis and Baehr, 1996; Reinhard and Hopkins).
The induced in situ sampling technique brings offers of a different approach to
assess the production of daughter compounds. For example, rates of biogenic gas
production from petroleum products (e.g., carbon dioxide and methane) are evaluated
to predict biodegradation rates across a site. These production rates are estimated from
advective flux from active subsurface locations compared to diffusive flux at inactive
locations (e.g., near sources or beyond plume edge). In situ flux measurements are
relatively inexpensive especially for daughter products that partition into the gas
phase from either the groundwater or the vadose zone.