Agriculture Reference
In-Depth Information
Predicted atrazine BTC for the mixed (sand-residue) column is shown by
the solid curve shown in Figure 9.17. To obtain these predictions the retention
parameters alone (
K
e
, k
1
, k
2
, and
k
irr
) were optimized and
S
max
was derived inde-
pendently from batch kinetic experiments (Ma and Selim, 2005). Therefore,
the SOTS model provided extremely good prediction of the overall BTC as
illustrated by the solid curve in Figure 9.17. Although model calculations pro-
vided some response to the change in concentration due to flow interruption,
the model failed to adequately describe the big increase in atrazine concentra-
tion during the second flow interruption (see Figure 9.17). It is possible that
physical nonequilibrium played an important role during the second flow
interruption, which is not accounted for by the SOTS model described above.
The SOTS model well predicted the peak position of measured atrazine
but slightly underpredicted the concentration maxima for the Sharkey mixed
column (see Figure 9.18). The tailing portion was somewhat overpredicted.
Unlike the sand column, atrazine prediction exhibited stronger retention
than that measured as illustrated by the retardation or delay of the arrival
of the predicted BTC. Moreover, model prediction provided some response
to the flow interruption events indicative of nonequilibrium behavior for
atrazine in Sharkey soil. This model was only capable of responding to
the first interruption event during sorption (see Figure 9.18). It is conceiv-
able that physical nonequilibrium, which is not directly accounted for in
SOTS, is responsible for such behavior during atrazine release. Due to the
large number of parameters considered in mixed soil systems, that is,
K
e
,
k
1
, k
2
, and
k
irr
for both the Sharkey clay soil and the crop residue, nonlinear
optimization to improve model predictions is generally not recommended.
Rather, the use of SOTS as a simplified single rather than a mixed system
was examined. The solid curve in Figure 9.16 represents predicted BTCs
based on a set of “weighted” model parameters. The resulting model calcu-
lations may be regarded as adequate prediction of atrazine behavior in the
mixed Sharkey-crop residue column. Here the set of model parameters were
“weighted averages” of the rate coefficients and were derived based on the
respective contributions of the crop residue (36%) and the Sharkey clay soil
(64%). Therefore, the use of weighted retention parameter values for atrazine
in the SOTS model is conceivable.
References
Barry, D. A., and J. C. Parker 1987. Approximations for solute transport through
porous media with flow transverse to layering.
Transport in Porous Media
, 2:
65-82.
Bosma, W. J. P., and van der Zee S. E. A. T. M. 1992. Analytical approximations for non-
linear adsorbing solute transport in layered soils.
J. Contam. Hydrol
. 10: 99-118.
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