- M.A. Scholl, 1997.
- Use of a Numerical Model to Estimate the
Effect of Permeability Heterogeneity on Biodegradation Rate Measurements
in an Alluvial Aquifer: EOS, Transactions, American Geophysical Union
1997 Spring Meeting, v. 78, no. 17, p. S140.
A closed landfill near the city of Norman, Oklahoma, is sited on
the floodplain of the Canadian River. Leachate is moving toward
the river, through layered sediments ranging from sand to clay.
A 2-D numerical model that simulates biodegradation processes in
ground water (BIOMOC, Essaid et al., 1995) was used to design a
sampling strategy to measure biodegradation rates of compounds in
the leachate plume. Hydraulic conductivity (K) measurements were
made at one-meter depth intervals through the aquifer along a
transect parallel to flow direction. Sequential Gaussian
simulation was used to generate K distributions, conditioned on
35 data points from the aquifer, over the range log K = -5 to -3.5 m/s
(clean sand to silty sand). Layers of similar K were 1-3
m thick and up to 50 m long. A zero-order biodegradation rate was
set for the contaminant, with no bacterial growth and uniform
bacterial population. A simulation with homogeneous K at the
mean value was used as the control case. Simulations were then
run with different K realizations, and the modeled steady-state
contaminant plume was sampled at various depths and distance
intervals. Apparent biodegradation rates were calculated using
the average flow velocity, the parameter that would be available
in a field situation. For this example (sand-silty sand,
disappearance of compound within 100 m), results of initial
simulations show that apparent rates differ from the control case
by up to 54% if the contaminant plume is sampled every 20 m at
midpoint depth in the aquifer. Apparent biodegradation rates are
slower if flow velocity is higher than average, and faster if
velocity is lower. Estimates can be improved to within 10% of
the control by averaging samples at 3 depths, but there is little
further improvement with samples at 6 depths. Decreasing well
spacing to 10 m apart does not substantially improve the rate
estimates. Further simulations will examine the effects of
different K ranges and bacterial growth on biodegradation rate
measurements.
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