Agriculture Reference
In-Depth Information
Crop Residue + Sharkey soil
Mixed Column
1.0
0.8
0.6
Measured
Predicted - SOTS
0.4
0.2
0.0
0
5
10
15
20
25
30
Pore Volume (V/V
o
)
FIGURE 9.18
Measured and predicted atrazine breakthrough curves from a mixed-soil column of crop resi-
due plus Sharkey clay soil.
with sugarcane harvest residue. In Figure 9.18, the crop residue was mixed
with Sharkey clay soil (very fine, montmorillonitic nonacid, thermic Vertic
Haplaquept), which was obtained from the St. Gabriel Research Station,
Iberville Parish, Louisiana. The sand (acid-washed) material was used as a
reference matrix where no clay or organic matter was present, where adsorp-
tion of atrazine is not expected. Furthermore, an estimate for
S
max
based on
the amount of mulch incorporated into each column was made. Since it was
assumed that the sand material was inert and atrazine could only be retained
by the crop residue, a value of
S
max
of 98.77 μg/g was estimated for the mixed
sand-crop residue column. For the Sharkey-crop residue column, estimated
S
max
of 278.83 μg/g was used. Specifically, the contributions of the crop resi-
due to the sorptive capacity accounted for 36% (i.e.,
f
= 0.36) of the total
S
max
.
These estimates were based on the respective amount of crop residue mixed
with the sand and Sharkey soil mixed columns.
The crop residue caused delay in atrazine breakthrough and extensive tail-
ing as seen in Figure 9.17 for the sand-crop residue mixed column. Measured
results indicated very little response in atrazine concentration to the first
flow interruption at 3.1 pore volumes. In contrast, a jump in atrazine concen-
tration in the leachate solution was observed for the second flow interruption
at 9.5 pore volumes (Ma and Selim, 2005). A response to flow interruption is
indicative of nonequilibrium behavior during transport caused by chemical
and/or physical processes, for example, interparticle diffusion and kinetic
retention (Murali and Aylmore, 1980; Reedy et al., 1996).
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