Geoscience Reference
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similar to that in the sterile soil. After four days, the effect of microbial activity on
decomposition is evident, and the distribution pattern is significantly different.
After seven days, the parathion is almost completely decomposed. This example
emphasizes the necessity to consider additional processes, such as degradation, in
analyses of pollutant transport.
An equation that accounts for parathion transport, subject to both diffusion and
microbial decomposition, can be written as
ot
¼ D
o
2
c
o
c
ox
2
R
xt
;
ð
12
:
1
Þ
where c is the concentration, D is the coefficient of molecular diffusion, x is
distance, t is time, and R
xt
denotes the rate of microbial decomposition. Because
parathion is converted quantitatively, mole for mole, its decomposition product is
oc
0
ot
¼ D
0
o
2
c
0
ox
2
R
xt
;
ð
12
:
2
Þ
where c
0
is the concentration of the decomposition product. Clearly, the rate of
decomposition at any distance and time depends on the local concentration of
parathion and on microbial activity.
The transport of a contaminant from a point source into the subsurface was
examined for two herbicides (napropamide and bromacil), by introducing them
through a single-drip irrigation source. Lateral and vertical sampling was per-
formed to determine the spatial distribution, over time, of water and contaminants;
results for one-time and one-application rate are shown in Fig.
12.40
. In general,
napropamide was moderately adsorbed by the soils and concentrated around the
emitter. At a high application rate, the compound moved laterally more than
vertically, and at a lower application rate, it penetrated to a greater depth. After
several cycles of wetting and drying, napropamide was partially leached out of the
emitter zone. The amount leached is related to the soil hydraulic properties:
Bromacil, which is weakly adsorbed by the soil, was more uniformly distributed in
the soil but did exhibit slight retardation. The effect of application rate was similar
to that observed for napropamide, and after several cycles of wetting and drying,
bromacil was completely leached from vicinity of the emitter.
Preferential flow of bromacil and napropamide through undisturbed cores of
structured Evsham clay soil (Aquic Eutrochrept) from Oxfordshire, England, under
continuous and discontinuous watering regimes and with different initial moisture
contents, was reported by White et al. (
1986
). A conservative solute, chloride, was
used as to trace water movement. Adsorption coefficients range between 1.7 and
2.0 L/kg for bromacil and between 17.8 and 22.7 L/kg for napropamide. In a dry
soil, bromacil attains equilibrium with the solid phase almost instantaneously,
while napropamide shows a delay of 2-3 h. In an initially wet soil, napropamide
does not reach equilibrium with the solid phase even after 48 h.
