International Conference on WAter POllution in natural POrous media at different scales.
Assessment of fate, impact and indicators. WAPO2
TRANSPORT MODELING OF TRIBUTYLTIN COUPLED TO
VARIABLE SALINITY
N.-O. Kitterød1, C.E. Amundsen2, P. Snilsberg2, R. Aasen2, and T. Eggen2
(1) Norwegian Institute for Agricultural and Environmental Research, 1432 Ås Norway,
email: nils-otto.kitterod@bioforsk.no
(2) Norwegian Institute for Agricultural and Environmental Research, 1432 Ås Norway
ABSTRACT
Leakage of tributyltin (TBT) to coastal environment is documented globally and represents environmental
hazards because of long half lives in anaerobic environment and accumulation in biota. Concentrations below
1 ng/l influence marine organisms. Hence, pollution control authorities request abatements to mitigate toxic
concentration levels. Dredging of contaminated sediments is one abatement strategy, but this approach
requires safe landfill repositories. An alternative strategy is to cover contaminated sea bottom areas by cap
layers of favourable composition and thereby reduce TBT exposure to the environment. Both abatement
strategies require understanding of chemical and physical processes involved at the pore scale and at the field
scale. In the present study pore scale processes are investigated by laboratory experiments on dredged
sediments from contaminated sea bottom and integrated to field scale by numerical simulations.
The distribution of TBT between solid and water phase is a function of several parameters viz texture;
composition of clay minerals; content of organic matter; pH; and salinity in the pore water. The influences of
these parameters on the mobility of TBT in sediments are studied with emphasize on variable salinity flux
through the porous media. In our data the mobility of TBT increases as a function of decreasing salinity.
Long term leakage of TBT is simulated in a typified near shore landfill with initial marine salt water
concentration in the pore water. Initial TBT concentration in pore water is estimated to 30 ng/l, which is
corresponding to a chemical equilibrium of 1 mg/kg of TBT in the sediments. Because sorption of TBT varies
as a function of salinity, the transport of TBT has to be coupled to concentration of NaCl in the pore water.
The pore water flow depends not only on the relation between infiltration of meteoric water and permeability
of the sediments, but also on the concentration of NaCl, thus Darcy law has to be coupled to transport of
NaCl. In this way transport of TBT require a coupling of three types of physics. First is Darcy pore water
flow coupled to concentration of NaCl, and then concentration of NaCl is coupled to desorption of TBT.
Based on experimental results we estimated partitioning coefficients which indicate retardation factors in the
order of 1·109 to 1·1011. These factors imply very low liquid transport of TBT. Calculations indicate that in a
time span of 10,000 years less than 1‰ of the initial TBT mass in sediments will be mobilized in the liquid
phase and transported to the environment.
Key words: Tributyltin (TBT), saltwater intrusion, coupled transport modelling
1. INTRODUCTION
1.1. Background
Organotin compounds (OTC) are used for a number of different industrial proposes. In this context
the most important use is antifouling agents for ships, but OCT are also applied in fungicides, and as
stabilizing additives in polymers (Hoch and Schwesig, 2004). OCT in general is toxic to aquatic life,
but especially tributyltin (TBT) is reported to have negative effects as imposex and abnormal growth,
at extreemly low concentrations. Concentrations in the liquid phase in the order of 1 ng/l have
detectable impact on some marine species (Fent, 1996). The Pollution Control Authorities has
documented critical concentration levels of TBT in fjord sediments in many locations in Norway, and
there is a call for remediation technology. One mitigation strategy is covering of contaminated fjord
sediments with favourable capping sediments. Another abatement possibility is to remove
contaminated sediments and design safe repositories on land. In this paper we focus on land
repositories, but the results and modelling approach are transferable also for evaluation and
optimization of capping abatement.
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BARCELONA (SPAIN)
APRIL 11th – 13th, 2007
International Conference on WAter POllution in natural POrous media at different scales.
Assessment of fate, impact and indicators. WAPO2
1.2. Purpose
The purpose of this study is two fold: First, we investigate adsorption and desorption of TBT for
different porous materials included core samples form contaminated sediments. Adsorption and
desorption is performed with variable pH and salinity conditions. Secondly, we use the estimated
relations between salinity and sorption capacity to simulate long term leakage from a typified land
repository. In this paper we summarize the most important result of the sorption studies, and
emphasize what is important for modelling of TBT. More details are available in Amundsen et al.
(2006).
2. METHOD AND MATERIALS
2.1. Modelling
Leakage of TBT from dredged sediments deposited in a repository on land takes place in two
separate phases. First phase happens during deposition where the sediments are compacted due to
gravity. Reduction of porevolume causes outflux of contaminated porewater. This dewatering of the
sediments is a short term process and has to be met by specific abatements that ensure remediation and
degradation of TBT, for example by exposure of water to ultraviolet light that is proven to be an
effective degradation method of TBT. The second phase takes place after compaction has ceased. This
is a long term process that is driven by advection and molecular diffusion. In this study we model the
second phase of TBT leakage out of the repository. This is a problem that is coupled twice to the
concentration of NaCl. First, the poreflow depends on the density gradients and not only on infiltration
velocities and permeability which is the case if porewater density is constant. Secondly, mobilization
of TBT is connected to the partitioning coefficient Kd, which is a function of the NaCl concentration.
The first coupling is done through the density function:
ρ = ρ f + γc1 ,
(1)
where ρf is the freshwater density, and c1 is the NaCl concentration. The ratio γ is given by:
γ =
ρs − ρ f
cs − c f
,
(2)
where ρs is saltwater density (M/L3), ρf is freshwater density, cs is concentration of NaCl in
saltwater (M/L3) and cf is NaCl in freshwater concentration.
The concentration of NaCl c1, is solved by the advection-diffusion equation:
φ
∂ (ρc1 )
+
∂t
q
∂ (ρc1 )
=
∂xi
∂
∂xi
⎛
∂c ⎞
⎜⎜ ρDm 1 ⎟⎟ ,
∂xi ⎠
⎝
(3)
where φ is porosity, t is time, Dm is molecular diffusion and x is locations within the repository.
The density ρ is given in (1), and q is given by Darcy’s law expressed in terms of pressure p (Pa):
q=
κ
∇( p + ρgz ) ,
η
(4)
where κ is permeablity (L2), η is dynamic viscosity (M/L/T), g is gravity (L/T) and z (L) is the
elevation above the base of the repository.
The pressure p, is solved by inserting (4) into the equation for conservation of mass, which yields:
∂ρ ∂c1
∂p
+ ∇ ⋅ (ρ q ) = 0 ,
(5)
+ φ
∂c1 ∂t
∂t
here, the compressibility ζ, is included which make it possible to include compaction. In the
present study compaction is disregarded, thus ζ = 0.
ρ (ζ (1 − φ ) + ζφ )
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APRIL 11th – 13th, 2007
International Conference on WAter POllution in natural POrous media at different scales.
Assessment of fate, impact and indicators. WAPO2
The second coupling is made through the portioning coefficient Kd of TBT which determines the
concentration of TBT in the liquid phase c2 and the solid phase c3. In this study we apply a linear
sorption isoterm:
c3 = K d c 2 ,
(6)
where Kd vary as a linear function of NaCl concentration c1 in the porewater:
K d = Ac1 + B,
(7)
where A and B is constants that is estimated by the desorption experiments described below.
Finally, the concentration of TBT in the liquid phase c2, is solved by the advection diffusion
equation for TBT:
φ
∂ (ρc 2 )
+
∂t
q
∂ (ρc 2 )
=
∂xi
∂
∂xi
⎛
∂c
⎜⎜ ρDm 2
∂xi
⎝
⎞
⎟⎟ ,
⎠
(8)
where q is the porewater flow velocity which is derived from (4) and (5). The partitioning of TBT
between solid and liquid phase is given by (6).
To solve the equations above, parameters; boundary conditions; and initial conditions has to be
specified. This is presented in the case study below. For this project the partial differential equations
(pde) above was solved with the finite element code Comsol Multiphysics (2006) where coupling of
pde is build in as an option.
2.2. Materials
Transport of TBT in contaminated sediments involves both adsorption and desorption. Lapse of
adsorption and desorption is usually very different due to hysteresis effects (Hoch et al. 2002).
Significant less desorption than adsorption is typical for a lot of organic compunds, and TBT is not an
exception to this. Hence, in this context the limiting process is desorption. Of that reason the most
important material in this context is core samples of TBT contaminated sediments. Thus the
parameters A and B in (7) is derived from desorption (or mobilization) analysis of TBT for typical
Norwegian fjord sediments.
The core material is taken from two different fjord locations close to the town Grimstad in the
southern part of Norway. One location has sediments with low concentrations of TBT. This location is
close to a pleasure boat harbour (Vessøynesset). From another location which is close to a
shipbuilding yard (Vikkilen), we got samples with high TBT concentration (Tab.1). The high content
of fine grained material imply that the hydraulic conductivity is too low for feasible mobilization
experiments, thus some coarse sand had to be mixed into the samples. Three different salinities of
influent fluid were used: distilled water; brackish water; and marine water with corresponding
salinities of 0‰, 8‰ and 33‰. Recharge of the percolating water was 0.25 ml/min. Totally 9 liter of
water was eluted through each column. This amount of water corresponds to about 3500 mm of
precipitation. Recalculated to the ratio between liquid and solid mass (LS), 9 liter of elution
corresponds to a LS = 17. All elution experiments were done at temperature of 10°C.
In addition to the mobilization experiments on core samples, standard batch experiments were done
on different porous material (olivin; gneis; medium sand; medium clay; organic rich fresh water
sediments). For more details c.f. Amundsen et al. 2006.
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APRIL 11th – 13th, 2007
International Conference on WAter POllution in natural POrous media at different scales.
Assessment of fate, impact and indicators. WAPO2
Table 1. Physical and chemical characterization of core samples and admixture sand used for desorption analysis.
Vessøynesset
Vikkilen
Sand
0.3470
0.011
-
TOC (%)
0.6
1.0
0.1
Fraction solid phase (%)
80.1
64.6
100
2-0,6 mm
62.8
74.8
85.9
0,6-0,2 mm
6.2
9.7
10.9
0,2-0,06 mm
6.4
8.2
1.9
0,06-0,02 mm
12.3
1.9
0.6
0,02-0,006 mm
4.4
0.6
0
0,006-0,002 mm
1.7
1.3
0.2
<0,002 mm
6.2
3.6
0.6
TBT (mg/kg)
Sand (%)
Silt (%)
Clay (%)
3. RESULTS
3.1. Batch experiments
Before presenting the desorption results, it should be mention that the batch experiments confirms
previous reported results (Hoch et al. 2002; Hoch and Schwesig 2004; Hoch, 2004; Burton et al.
2005), namely that important factors for sorption of TBT in porous media is the Cation Exchange
Capacity CEC, and Total Organic Content TOC (Fig. 1). The medium sand and medium clay samples
used for this study had ten and thirty times higher TOC than gneis and olivin, and more than twice as
high CEC. It is also reported clear relation between pH and sorption that is consistent to our analysis.
Because TBT is a weak basic (pKa = 6.25), pH is very important because it affects the electrical
charge of TBT and the surface properties of the adsorbing/desorbing medium. For medium sand and
clay the batch experiments gave adsorption maximum between pH 6 and 7 and also an absorption
maximum at pH around 8 (Tab. 2).
Figure 1. Adsorption isoterms for TBT for different test material for pH=6 and salinity = 0 ‰
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BARCELONA (SPAIN)
APRIL 11th – 13th, 2007
International Conference on WAter POllution in natural POrous media at different scales.
Assessment of fate, impact and indicators. WAPO2
The subject of this study is marine sediments, therefore batch experiments were done at pH=7.8
which is close to the pH conditions for the marine environment. At this pH, batch experiments were
done with variable salinity of liquid phase (Tab.3). With salinity varying from 0 to 33‰, Kd is varying
between 700 and 1000 l/kg except at zero salinity where Kd is above 1800 l/kg for medium sand.
Corresponding values for medium clay Kd is between 500 and 700 l/kg (Tab. 3).
Table 2. Partitioning coefficients (Kd [l/kg]) at different pH for medium sand (TOC = 1.3%, CEC = 217 mmolc/kg) and
medium clay (TOC = 3.8 %, CEC = 209 mmolc/kg) for salinity = 0 ‰.
Kd (l/kg)
pH = 5.0
pH = 5.5
pH = 6.0
pH = 6.5
pH = 7.0
pH = 8.0
medium sand
323
486
1260
2531
352
580
medium clay
206
837
771
259
339
1380
Table 3. Partitioning coefficients (Kd [l/kg]) at variable salinity for medium sand (TOC = 1.3%, CEC = 217 mmolc/kg) and
medium clay (TOC = 3.8 %, CEC = 209 mmolc/kg) for pH = 7.8.
Kd (l/kg)
[NaCl]=0.0‰
[NaCl]=8.25‰
[NaCl]=16.5‰
[NaCl]=24.75‰
[NaCl]=33.0‰
medium sand
1854.3
1018.4
890.6
882.7
752.6
medium clay
709.6
859.6
765.6
499.6
679.6
3.2. Mobilization experiments
The results of the mobilization experiments show a general decreasing mobilization of TBT with
increasing salinity (Fig. 2). This trend is independent of the initial TBT concentrations in solid phase.
This is probably due to an increase of colloidal transport with decreasing salinity. pH in effluent fluid
is generally decreasing as volume of influent fluid are increasing (Fig. 2). This trend is true
independent of influent salinity except for one case. For distilled influent water into core sample with
low initial concentration of TBT, the pH was increasing. Numerical results of the mobilization
experiments are given in Tab. 4. Based on this, we estimated the most likely values for the constants in
(7). To get consistent units as input to the numerical experiments, Kd has to be given in m3/kg. With
this unit, we got A = 8.3·105 (m3/kg)2. For B an interval was estimated: 2·105 < B < 5·106 m3/kg.
Figure 2. Elution of TBT from core samples with low initial concentration of TBT in soild phase (Vikkilen) and high initial
concentration (Vessøynesset). Mobilization of TBT decrease as a function of increasing salinity of influent water. pH of
effluent water is generally decreasing.
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International Conference on WAter POllution in natural POrous media at different scales.
Assessment of fate, impact and indicators. WAPO2
Table 4. Mobilized mass of TBT for different salinity of the influent fluids. Estimated partitioning coefficient
based on mobilized mass and concentration of TBT in effluent fluid.
Mobilized TBT
Kd (l/kg) is
Kd (l/kg)
Core sample
Salinity (‰)
µg/kg
% av total
Average
Min-Max
Vikkilen
0
1.83
0.53
5249
918-15219
8
0.37
0.11
18411
12218-27109
33
0.03
0.01
505320
31835-694000
0
0.12
1.07
4914
791-22000
8
0.09
0.79
5045
218-11000
33
0.02
0.19
18055
1719-22000
Vessøynesset
3.3. Numerical experiments
First, the density dependent flow simulations were tested against bench mark solutions of Henry’s
problem and Elder’s problem. Secondly, a 2d cross section of a typified land repository with
horizontal extension of 30 m and vertical thickness of 10 m was simulated. Freshwater influx was
introduced on top of the repository. Outflux was allowed to occur on one side at the upper 0.5 m on
the vertical end of the repository. Results are presented with the following parameters: fresh water
influx of 75 mm/year (2.4·10−9 m/s); fresh water hydraulic conductivity of 1·10−6 m/s; corresponding
to a permeability κ of 1·10−13 m2; for dynamic viscosityη equal to 0.001 kg/(m·s); gravity g equal to
9.81 m/s; and a freshwater density ρf of 1000 kg/m3. For the advective diffusion equations, porosity φ
is equal to 0.35. Because flow velocities are low, transport by molecular diffusion is important, thus
Dm was varied from 2.4·10−8 to 2.4·10−9 m2/s. Saltwater density ρs is 1025 kg/ m3 corresponding to a
saltwater concentration cs, of 35 kg/m3. Based on observations above, initial concentration of TBT in
solid phase was 1 mg/kg. From estimation of A and B in (7) this gave initial concentration for liquid
TBT between 2.9·10−11 and 3.4·10−11 kg/m3. If initial saltwater concentration decrease, the liquid TBT
will increase (Fig. 3).
Figure 3. Estimation of initial concentration of liquid TBT.
The initial flow condition for the flow field is critical in this kind of simulations. After compaction
has ceased it’s most likely to have vertical flow components upward. The magnitude of the vertical
flux is difficult to estimate, and no observations were available to constrain this uncertainty.
Simulations were done with initial hydrostatic pressure i.e. no vertical flux and with steady state
infiltration with liquid concentrations corresponding to saltwater and liquid concentrations of TBT
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APRIL 11th – 13th, 2007
International Conference on WAter POllution in natural POrous media at different scales.
Assessment of fate, impact and indicators. WAPO2
consistent to figure 3. The flow simulations demonstrate the importance of including density effects in
the simulations (Fig. 4). Inflowing freshwater will not affect deeper part of the repository until NaCl is
transported out of the sediments. This process is determined by molecular diffusion, thus Dm is a
sensitive parameter in this kind of simulations. Most important however, is the partitioning coefficient
which explains the extremely low mobilization of TBT. Calculations of TBT leakage in 10,000 years
indicate leakage of less than 1‰ of initial TBT mass in sediments.
Figure 4. Simulated flow field with initial steady state flow velocities, NaCl concentration of 35 kg/m3 (red color in lower
part of model), and initial TBT consistent to figure 3. After 1 year of freshwater infiltration with 75 mm/year, most of the
influent water is flowing at top of the repository because of the density contrast. Fresh water with salinity (0 kg/m3) is
indicated with blue color in upper part of model. The streamlines at top of the model indicate this situation.
4. DISCUSSION AND CONCLUSION
The laboratory experiments show extremely strong sorption of TBT. This is important since TBT
has serious negative impact on aquatic environment even at concentrations as low as 1 ng/l in liquid
phase. Numerical simulations of a highly coupled problem like this, is a challenge because of the
significant different time scales that is involved. In this case we have three time scales: advective time
scale; diffusive time scale; and chemical time scale. These three time scales are linked together. For
the flow field indicated in figure 4, the advective time scale (Tadv) is equal to the horizontal length
divided by the poreflow velocity. With the parameters introduced above, we get Tadv = 150 years. The
corresponding diffusive timescale (Tdiff) is equal to the vertical length squared, divided by the
molecular diffusion constant. In this case Tdiff varies between 130 years for molecular diffusion equal
to 2.4·10−8 to 1300 years for molecular diffusion equal to 2.4·10−9 m2/s. If Tadv and Tdiff are on the same
order of magnitude, it means that the two transports mechanism is equally important. In reality the
effective molecular diffusion is probably one or two order of magnitude lower than we have applied
for this study. The most important time scale however, is the chemical time scale, which in this case is
related to retardation. With a linear adsorption isotherm, the retardation factor is constant and straight
forward to calculate. For stable freshwater conditions, the retardation factor R is 1·109. For stable
saltwater conditions, the retardation factor R is 2·1011. If R in practice is infinite, and chemical
equilibrium between TBT in solid and liquid phase is faster than Tadv, it means that concentration of
liquid TBT out of the repository is constant. In this case the mass flux of liquid TBT is a function of
the infiltration (or precipitation) rate only. Given a repository of 1 m width, with bulk density of 2000
kg/m3, and initial solid concentration of TBT equal to 1 mg/kg, we have totally 0.6 kg of TBT in the
repository. In a time span of 10,000 years a constant infiltration rate of 75 mm/year yield a leakage of
less than 1 ‰ of initial TBT mass in sediments for a fresh water system and 1 ppm for a saltwater
system.
Acknowledgements
Thanks to David Johansson and other staff at Comsol AB for prompt and profound hints on numerical
problems. Thanks also to Ingunn Helgeland for doing necessary batch analysis. This project was
funded by the Norwegian Pollution Control Authority, and by the Norwegian Research Council
through the project “Fixation and mobilization of organic contaminants”.
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BARCELONA (SPAIN)
APRIL 11th – 13th, 2007
International Conference on WAter POllution in natural POrous media at different scales.
Assessment of fate, impact and indicators. WAPO2
REFERENCES
Amundsen, C.E., Snilsberg, P., Kitterød, N.-O., Aasen, R., and Eggen, T. (2006): Transport of tributyltin from near shore repositorys (in Norwegian), Norwegian Institute for Agricultural and
Environmental Research, Report 139.
Burton, E.D., Phillips, I.R. og Hawker, D.W. (2005): In-situ partitioning of butyltin compounds in
estuarine sediments. Chemosphere 59, 585-592.
Comsol Multiphysics (2006): Comsol Multiphysics Earth Science Module, Comsol Version 3.3.0.405.
Fent, K. (1996): Ecotoxicity of organotin compounds. Crit. Rev. Toxicol. 26, 1-117.
Hoch, M. (2004): Asessment of salinity variations in TBT adsorption onto kaolinite and
montmorillonite at different pH levels. Water, Air, and soil Pollution, 152, 349-362.
Hoch, M., and Schwesig, D., 2004, Parameters controlling the partitioning of tributylin (TBT) in
aquatic systems, Applied Geochem. 19, 323-334.
Hoch, M., Alonso-Azcarate, J. og Lishick, M. (2002): Adsorption behaviour of toxic tributyltin to
clay-rich sediments under various environmental conditions. Environmental Toxicology and
chemistry 21, 1390-1397.
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