3050
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
2.1.1
Water uptake model
Water uptake was calculated by
Hydrus-1D
, through a
macroscopic approach determining the sink term in
Richards
equation
(Feddes et al., 1978; Vogel, 1988). In this study water
stress was considered according to Feddes’ formulation (Feddes
et al., 1978).
The root water uptake was calculated directly by
Hydrus
code, through a macroscopic approach determining the sink
term in the Richards equation. This term,
s
[T
-1
], was calculated
from the equation
s
(
h,h
φ
,x,t
)
=
α
(
h,h
φ
,
x,t
)
b
(
x,t
)
T
p
(
t
)
(1)
where
T
p
(
t
) [LT
-1
] was the normalized root distribution [L
-1
],
a function of space and time (in the case of root growth). The
function
α
[-] represented the response to plant stress (0
≤ α ≤
1), by varying the hydraulic and osmotic head.
2.1.2
Contaminant uptake model
Roots contaminant uptake, when present, was calculated
with models defined as passive and active. The first assume that
the solute uptake is locally proportional to root water uptake and
the concentration of the solute dissolved in water:
p(x,t) = s(x,t) c(x,t)
(2)
The active root solute uptake
a(x,t)
[ML
-3
T
-1
] was calculated
using
Michaelis-Ment
en kinetics (Jungk, 2002). The theoretical
maximum uptake value was called as potential active solute
uptake
A
p
(
t
) [ML
-2
T
-1
], characteristic of the pair plant-solute and
function of time (Šim
ů
nek and Hopmans, 2009).
, = ,
+ , ,
(3)
K
m
was defined as
Michaelis-Menten constant
[ML
-3
].
Applied values were
K
m
= 1,32
µ
g/cm
3
and
A
p
= 0,4757
µ
g
·
cm
-2
/day for Pb
2+
.
2.1.3
Soil
The soil analyzed is a
Halpic Gleysol
. In the simulations
only the unsaturated zone was modeled. The water table was
assumed to have a fixed depth (90 cm). Two horizons were
considered:
A
, clay and
C
, sandy clay loam. The parameters of
van Genuchten - Mualem hydraulic model
(van Genuchten,
1980) were estimated for each horizon using pedotransfert
functions proposed by Tomasella et al. (2003).
Table 1. Hydraulic parameters of the
van Genuchten - Mualem model
(van Genuchten, 1980) relative to Halpic Gleysol
horizon
θ
r
[cm
3
/cm
3
]
θ
s
[cm
3
/cm
3
]
α
[cm
-1
]
n
[-]
K
s
[cm/day]
A
0.1555
0.5688
0.0654
1.1910
61.66
C
0.0900
0.4265
0.0450
1.3154
68.02
2.1.4
Boundary conditions
The top boundary conditions were imposed using daily
values of precipitation, potential evaporation and transpiration.
The reference evapotranspiration was determined using the
equation of
Penman-Monteith
(Allen et al., 1989; Allen et al.,
1998).
Pressure head
h
was considered constant and equal to zero at
the bottom of the profile.
2.1.5
Soil-contaminant interaction
Ion sorption in soil solid phase was considered using a linear
model for both horizons. The distribution coefficients (
K
d
) were
inferred from a study (Soares, 2004) about tropical soils with
similar characteristics, using 1500 cm
3
/g and 70 cm
3
/g,
respectively, for Pb
2+
and Zn
2+
. Standard values for the
diffusion coefficients in free water were applied (Shackelford
and Daniel, 1991).
2.1.6
Simulation phases
The numerical simulations were organized in three phases:
pre-contamination, contamination and remediation. The first
phase (one year) was necessary to fix average pressure head
profile. The presence of shrubby vegetation was included in the
top
BC
.
During the contamination phase, the presence of containers,
leaching metal ions in presence of rain, was simulated. It was
also estimated that the vegetation suffered degradation due to
toxicity of the contaminants. Therefore, transpiration values
were considered equivalent to 20% of the reference. In this
phase, no contaminant uptake was considered. The simulated
period was of five years.
For remediating the soil, original vegetation was substituted
by
Chrysopogon zizanioides
. According to the experimental
study by Tavares (2009), this variety doesn’t suffer any toxicity
effect at considered concentrations. The
Feddes
´ parameters
where estimated by analogy with similar plants from
Poaceae
family (Wesseling, 1991). The solute uptake model selection
and the determination of the relative parameters were performed
in Lugli (2011). An active model was used for Pb
2+
and a
passive model for Zn
2+
. The process was considered complete
when the soil concentration of each contaminants would be
punctually lower than the Brazilian standard values for
industrial areas (CONAMA, 2009) respectively 900 mg/kg and
2000 mg/kg for Pb
2+
and Zn
2+
.
2.2
Previous studies
Lugli and Mahler (2012) showed that, by increasing the
fraction of contaminant sorbed on the solid phase, the phyto-
extraction process became less effective. No relevant
consequences were observed in the remediation of Pb
2+
. For
contaminants characterized by low retardation factors (e.g.
Zn
2+
), the remediation process resulted more efficient.
Moreover, water stress partially inhibited contaminant
uptake, but prevented plume migration towards water table. The
results also showed that phyto-extraction process becomes more
efficient by increasing the amount of transpiration at the
expense of the portion of evaporation (e.g. increased crop
density).
In the present study, some results form Lugli and Mahler
(2012) were revisited analyzing the influence of root depth. The
focus was to study if an engineered choice of plant species in
terms of root distribution could promote (or not) an
improvement in remediation process. For both Pb
2+
and Zn
2+
three different root models were analyzed and compared with
the reference (root depth = 40 cm). All of them where static and
linear interpolated with the lowest value corresponding to zero;
the depth was determined multiplying the plume depth for each
contaminant by 2/3, 1 and 1.5.
