Agriculture Reference
In-Depth Information
bio-uptake. This reduction in bioavailabil-
ity, however, also slows down biodegrad-
ation, which leads to increased persistence
of organic compounds, especially in top-
soils. The effect of sorption on the kinetics
of biodegradation and advective pollutant
transport in infiltrating soil pore water can
be derived from a simple mass balance
equation that assumes the aqueous concen-
tration as the rate-limiting factor for biodeg-
radation (first-order kinetics):
assumes, as above, that adsorbed pollutants
are not only immobile but also inaccessible
to soil microorganisms, and describes the
removal of water-dissolved, mobile pollu-
tants by first-order rate biodegradation kin-
etics, then the sorption will have no net
long-term benefit in terms of pollutant at-
tenuation during soil passage. This can, for
instance, be seen, by considering a hypo-
thetical scenario with a fixed, constant pol-
lutant concentration in soil pore water,
C
w,top
(g cm
−3
), at the soil-atmosphere interface
and a constant pore-water flow velocity,
u
z
.
For this hypothetical scenario, the pollutant
concentration in soil pore water will tend
towards a steady state (d
C
w
/d
t
= 0), in which
the concentration profile no longer changes
with time but is simply a function of dis-
tance from the soil surface, described by the
following exponential decay:
d
d
C
t
d
d
C
t
Cu
C
z
¶
¶
(12.1)
q
w
+
r
s
= −
lq
−
q
w
w
Z
q
(−) and
r
(g cm
−3
) represent the volumetric
water content and the dry bulk density in
soil.
C
w
(g cm
−3
) is the pollutant concentra-
tion in soil pore water, and
C
s
(g g
−1
) the pol-
lutant concentration associated with the
solid soil matrix. The parameter
l
is the
first-order rate constant for biodegradation
(s
−1
),
u
z
(cm s
−1
) is the soil pore water flow
velocity in the
z
direction, and
z
(cm) is the
vertical distance from the soil surface to-
wards the groundwater table,
t
(s) the time.
Assuming local equilibrium conditions be-
tween soil and aqueous concentrations
characterized by a distribution coefficient
K
d
(=
C
s
/
C
w
) [cm
3
g
−
1
]
results in:
d
d
−
l
.
(12.3)
Z
()
=
w,top
.
e
u
CzC
z
w
The exponential decay of the pollutant
concentration with increasing downward
distance from the soil surface can be inter-
preted as a measure for the sustainable,
long-term pollutant attenuation in soil. The
sustainable pollution attenuation depends
on the ratio between the first-order biodeg-
radation rate,
l
, and the soil pore-water flow
velocity,
u
z
, but not on the model parameter,
K
d
, which accounts for the pollutant sorp-
tion. Enhanced sorption will delay the mi-
gration of the pollutant, and hence the time
needed for the system to reach the steady
state described by Eqn 12.3. Equally, sorp-
tion would delay the disappearance of the
pollutant from the soil once concentrations
at the soil surface return to zero. If we as-
sume that sorbed pollutants are inaccessible
to soil microorganisms, then the water-
filtration benefits of pollutant sorption by
soil organic carbon will be only temporary.
Sorption initially reduces pollutant leach-
ing when soil is freshly exposed to a new
source of contamination, but this apparent
benefit is annulled by the disadvantage of
prolonged pollutant leaching once the con-
tamination input has ceased. This analysis
is only valid for the model assumptions out-
lined above, which paint a rather simple
C
t
l
u
¶
¶
C
z
w
=−
C
−
z
w
(
)
(
)
w
r
r
1
+
K
1
+
K
q
d
q
d
(12.2)
(
)
Thus, the term
1+
K
d
r
q
slows down the or-
ganic pollutant biodegradation and also ad-
vective pollutant transport. In this model,
rates are slowed down by sorption (i.e.
K
d
),
but also depend on the solid-to-liquid ratio
(here:
r
/
q
), i.e. decrease with decreasing
water content. For many organic pollutants,
the magnitude of
K
d
increases with soil or-
ganic matter content. Therefore, water con-
tents and organic matter may be major
reasons for the persistence and retention of
strongly hydrophobic organic pollutants by
organic matter in soils.
In terms of the soil's water filtration
capacity, the most important long-term con-
sideration is the attenuation of pollution
over a certain distance (i.e. between the soil
surface and the groundwater table). If one
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