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The transport behaviour of elemental mercury DNAPL in saturated porous media: Analysis of field observations and two-phase flow modelling

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Mercury is a contaminant of global concern. The use of elemental mercury in various (former) industrial processes, such as chlorine production at chlor-alkali plants, is known to have resulted in soil and groundwater contaminations worldwide. However, the subsurface transport behaviour of elemental mercury as an immiscible dense non-aqueous phase liquid (DNAPL) in porous media has received minimal attention to date. Even though, such insight would aid in the remediation effort of mercury contaminated sites. Therefore, in this study a detailed field characterization of elemental mercury DNAPL distribution with depth was performed together with two-phase flow modelling, using STOMP. This is to evaluate the dynamics of mercury DNAPL migration and the controls on its distribution in saturated porous media. Using a CPT-probe mounted with a digital camera, in-situ mercury DNAPL depth distribution was obtained at a former chlor-alkali-plant, down to 9m below ground surface. Images revealing the presence of silvery mercury DNAPL droplets were used to quantify its distribution, characteristics and saturation, using an image analysis method. These field-observations with depth were compared with results from a one-dimensional two-phase flow model simulation for the same transect. Considering the limitations of this approach, simulations reasonably reflected the variability and range of the mercury DNAPL distribution. To further explore the impact of mercury's physical properties in comparison with more common DNAPLs, the migration of mercury and PCE DNAPL in several typical hydrological scenarios was simulated. Comparison of the simulations suggest that mercury's higher density is the overall controlling factor in controlling its penetration in saturated porous media, despite its higher resistance to flow due to its higher viscosity. Based on these results the hazard of spilled mercury DNAPL to cause deep contamination of groundwater systems seems larger than for any other DNAPL.
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The transport behaviour of elemental mercury DNAPL in
saturated porous media: Analysis of eld observations and
two-phase ow modelling
Thomas Sweijen
a,b,
, Niels Hartog
b,c
, Annemieke Marsman
a
, Thomas J.S. Keijzer
d
a
Deltares, Princetonlaan 6, 3584 CB Utrecht, The Netherlands
b
Utrecht University, Department of Earth Sciences, Environmental Hydrogeology Group, Budapestlaan 4, 3584 CD Utrecht, The Netherlands
c
KWR Watercycle Research Institute, Nieuwegein, The Netherlands
d
Philips Innovation Services, Eindhoven, The Netherlands
article info abstract
Article history:
Received 15 November 2013
Received in revised form 23 March 2014
Accepted 24 March 2014
Available online 30 March 2014
Mercury is a contaminant of global concern. The use of elemental mercury in various (former)
industrial processes, such as chlorine production at chlor-alkali plants, is known to have resulted in
soil and groundwater contaminations worldwide. However, the subsurface transport behaviour of
elemental mercury as an immiscible dense non-aqueous phase liquid (DNAPL) in porous media
has received minimal attention to date. Even though, such insight would aid in the remediation
effort of mercury contaminated sites. Therefore, in this study a detailed field characterization of
elemental mercury DNAPL distribution with depth was performed together with two-phase flow
modelling, using STOMP. This is to evaluate the dynamics of mercury DNAPL migration and the
controls on its distribution in saturated porous media. Using a CPT-probe mounted with a digital
camera, in-situ mercury DNAPL depth distribution was obtained at a former chlor-alkali-plant,
down to 9 m below ground surface. Images revealing the presence of silvery mercury DNAPL
droplets were used to quantify its distribution, characteristics and saturation, using an image
analysis method. These field-observations with depth were compared with results from a
one-dimensional two-phase flow model simulation for the same transect. Considering the
limitations of this approach, simulations reasonably reflected the variability and range of the
mercury DNAPL distribution. To further explore the impact of mercury's physical properties
in comparison with more common DNAPLs, the migration of mercury and PCE DNAPL in
several typical hydrological scenarios was simulated. Comparison of the simulations suggest
that mercury's higher density is the overall controlling factor in controlling its penetration in
saturated porous media, despite its higher resistance to flow due to its higher viscosity. Based
on these results the hazard of spilled mercury DNAPL to cause deep contamination of
groundwater systems seems larger than for any other DNAPL.
© 2014 Elsevier B.V. All rights reserved.
Keywords:
Mercury
PCE
DNAPL
Multi-phase flow
Camera push-probe
STOMP
1. Introduction
The historical use of elemental and ionic forms of mercury
in various industrial proce'sses has caused mercury to become
a contaminant of global concern. This is due to its long-range
transport in the atmosphere, its persistence in the environment,
its ability to bioaccumulate in ecosystems and its substantial
negative effect on human health and the environment (Horvat
et al., 2003; Ullrich et al., 2001; Walvoord et al., 2008; World
Chlorine Council, 2011). Contaminated industrial sites include
former wood-preservation and mining sites as well as (former)
chlor-alkali plants (e.g. Arbestain et al., 2009; Bernaus et al.,
Journal of Contaminant Hydrology 161 (2014) 2434
Corresponding author at: Utrecht University, Department of Earth
Sciences, Environmental Hydrogeology Group, Budapestlaan 4, 3584 CD
Utrecht, The Netherlands. Tel.: +31 302 532 497.
E-mail address: T.Sweijen@uu.nl (T. Sweijen).
http://dx.doi.org/10.1016/j.jconhyd.2014.03.001
0169-7722/© 2014 Elsevier B.V. All rights reserved.
Contents lists available at ScienceDirect
Journal of Contaminant Hydrology
journal homepage: www.elsevier.com/locate/jconhyd
2006; Biester et al., 2002a, 2002b; Brooks and Southworth,
2011; Miller et al., 2013).
To date, the scientific research on mercury contamination in
wastewater, soil, and groundwater systems has focussed largely
on the behaviour and ecotoxicological effects of ionic and
methylated Hg forms (Gabriel and Williamson, 2004; Horvat et
al., 2003; Jackson, 1998; Johannesson and Neumann, 2012; Lopes
et al., 2013; Schuster, 1991). Mercury species have contaminated
groundwater due to their use in industrial processes. For
example through the use of HgCl
2
in wood-preservation
(Bollen et al., 2008). Other mercury species may result from
the redox transformation of elemental mercury to mercury(II)
compounds by oxidation or to methylmercury by subsequent
methylation under reducing conditions. The extent to which
mercury undergoes these redox transformations strongly de-
pends on the soil and groundwater redox chemistry (Hu et al.,
2013; Schuster, 1991). Under sufficiently reducing groundwater
conditions, reduction of ionic mercury species may result in the
formation of elemental mercury phase (Bollen et al., 2008).
Nowadays, chlor-alkali plants are still responsible for the
largest industrial use of elemental mercury, although its global
use is rapidly decreasing (World Chlorine Council, 2011)and
the European chlorine producers are phasing out its usage by
2020 (EuroChlor, 2010). At numerous (former) industrial sites,
liquid elemental mercury in the subsurface has been observed,
particularly at (former) chlor-alkali plants. For example, liquid
mercury wasfound at a former chlor-alkali plant near Onodaga
Lake, New York, at depths down to 17 m below surface (Deeb
et al., 2011; ITRC, 2012). Also, liquid mercury was observed
in soil samples, and wells in Lavaca Bay (Texas), surrounding a
former chlor-alkali plant (Scanlon et al., 2005). Moreover, at
the Oak Ridge Y-12 National Security Complex, 11 million kg
mercury was used during 1950 to 1963 from which an estimated
193,000 kg was lost to the soil (Brooks and Southworth, 2011).
At such sites exposure to mercury DNAPL during handling is a
keydifficulty(Deeb et al., 2011).
Being the heaviest known liquid, with a density of
13.5 kg L
1
at standard conditions and immiscible with
water, liquid elemental mercury can be considered as a dense
non-aqueous phase liquid (DNAPL). Analogue to better known
DNAPLs, sites contaminated with mercury DNAPL are likely to
act as long-lasting sources for more mobile and toxic mercury
compounds, either as mercury vapouror ionic mercury species
in groundwater. However, in contrast with more common
DNAPL types (e.g. creosote, carbon tetrachloride, trichloroeth-
ylene (TCE) and perchloroethylene: PCE), scientific studies on
the infiltration behaviour of mercury DNAPL into soils and
aquifers are still very limited (Devasena and Nambi, 2010).
Contrasting behaviour for mercury DNAPL is however expected
as liquid mercury is not only an order of magnitude denser
than more commonly studied DNAPLs, but also its viscosity
and particularly its surface tension deviate (Table 1). To date
however, it is unknown how these different properties affect
the transport and distribution behaviour of elemental mercury
infiltrating the subsurface. Improved insight in the infiltra-
tion behaviour of mercury DNAPL, and how it differs from
more common DNAPLs, will aid the characterisation, risk-
assessments and evaluation of remediation options of mercury
DNAPL contaminated sites.
Only recently, the saturation behaviour of mercury DNAPL
in water-saturated sands has been studied by using short
column experiments (Devasena and Nambi, 2010). As known
for other DNAPLs, their results indicated that mercury DNAPL
flow is governed by gravitational and capillary forces, hence its
high density and surface tension, respectively. Moreover, they
showed that for the watersand system, mercury DNAPL can be
considered as a non-wetting liquid because an entry-pressure
was required for mercury to infiltrate the water-saturated sands.
Following Cohen and Mercer (1990), this implies that water or
air preferentially wets the surface of sand grains rather than
mercury. In this respect, mercury acts as a non-wetting DNAPL
similar to PCE (e.g. Schwille, 1988). However as shown in the
study of Devasena and Nambi (2010), the distinctively different
fluid properties may result in much lower residual saturations
for mercury (0.04) than for TCE (0.14) and PCE (0.17).
The recent pioneering study by Devasena and Nambi (2010)
provides valuable insight in the characteristics of the two-phase
mercury DNAPLwater system. However the dynamic aspects
of mercury infiltration and distribution remain unclear, partic-
ularly at the field scale. Therefore, in this study we combined
field characterisation data of mercury DNAPL contaminated
soil at a former chlor-alkali site in The Netherlands with a
multi-phase flow model. Goals of this study were 1) to assess
the field scale characteristics of mercury DNAPL infiltration and
distribution, 2) to assess the ability of the multi-phase flow
model for mercury to reproduce field observations using
literature-derived input values and 3) to assess the differ-
ences and similarities in the behaviour of liquid elemental
mercury and a common DNAPL (PCE).
2. Materials and methods
2.1. Field-site
The field study was performed at a former chlor-alkali
plant in the Netherlands. During a previous study at this site,
detailed geochemical analysis of shallow (b30 cm depth) soil
samples confirmed contamination by various mercury spe-
cies in the unsaturated zone (Bernaus et al., 2006). The site is
located close to a river in an alluvial plain. Consequently the
aquifer consists of river sediments, such as sandy and clayey
materials. The groundwater level at the site is approximately
1 m below ground level. In this study, a cone penetration test
(CPT) was performed at the site.
2.1.1. CPT-probe characterisation
Cone penetration tests are frequently used for describing
the lithology of the subsurface. Measured parameters include
the cone-pressure, which is the pressure at the tip of the cone,
and the resistance of pushing the probe into the soil. The latter
parameter was normalized to the cone-pressure, to obtain the
friction number. The CPT-cone used was equipped (in-house)
with a camera to acquire in-situ images to investigate the
depth distribution of the mercury DNAPL contamination. The
CPT-cone had a diameter of 36 mm, the overlying tube had a
diameter of 56 mm and the length of the CPT-cone with
camera was 1.2 m. In the CPT-cone a sapphire glass window
(12 × 18 mm) was present, located 76 cm above the CPT-cone
tip. Behind the glass window a mirror construction enabled a
2MP camera (1200 × 1600 pixels) equipped with LED-lights
to visualize the subsurface, using a resolution of 10 μmby
11 μmper pixel. The camera visualized the soil profile including
25T. Sweijen et al. / Journal of Contaminant Hydrology 161 (2014) 2434
the vertical mercury distribution at a 1 cm resolution over a
9 m vertical transect. The presence of elemental mercury was
visually apparent in the appearance of bright silvery blobs. A
description of the lithological characteristics was made using the
visual observations, as well as the conventional cone-pressure
and friction data from the CPT probe. Following e.g. Nohl and De
Boer (2008), low friction numbers and high cone pressures
indicate sandy lithologies, whereas high friction numbers and
low cone pressures indicate clay lithologies.
2.1.2. Image analysis method
Image analysis was performed to determine the vertical
mercury DNAPL distribution, as observed through the CPT-
mounted camera. No smearing of DNAPL on the sapphire
glass-window was observed, indicating that the observed
DNAPL presence was relatively undisturbed by the probing.
The image analysis was performed similar to the methods
used previously in laboratory studies, to visualize DNAPL
distribution and to determine saturations (Darnault et al.,
1996; Kechavarzi et al., 2000; Luciano et al., 2010). Here, we
assumed the percentage of pixels representing mercury in
the in-situ images to represent the mercury saturation.
First step in the image processing was to manually colour
the observed mercury droplets. Mercury DNAPL was recog-
nized by its silvery reflection and rounded droplet shapes.
Mercury recognition could not be done automatically, because
image-processing software was not able to recognize mercury
droplets, due to wide colour range of the background and
subtle colour difference in the mercury droplets depending on
light orientation. Images were then opened in the open source
software R, including package: EBimage (Pau et al., 2010).
Photos were written in a grey-scaled matrix and filtered, such
that pixels containing mercury were given a value of 1 and the
other pixels a value of 0. Where mercury DNAPL was observed,
it filled up the space between the sediment grains and the
sapphire-glass window, therefore the ratio of pixels containing
mercury was taken as a measure of the mercury saturation.
The representativeness of the derived values for pore
saturation was verified using the concept of Representative
Elementary Volume (REV). The REV was visualized by changing
the area of interest on a photo from the maximum size,
1200 × 1600 pixels (i.e. one whole photo) to a minimum size,
12 by 16 pixels over 100 steps. As a consequence the mercury
pore saturation was a function of the area of interest. With
increasing areas, the saturation stabilized, indicating that the
determined mercury saturation was a representative average
value for the scale of the image (12 × 18 mm). The spatial
correlation of the image-derived mercury saturations was
analysed using one-dimensional variogram analysis, using a
spherical model. These results were compared to the variogram
analysis of the cone-pressure and friction number.
2.2. Comparison of mercury and PCE DNAPL properties
Mercury DNAPL has a high potential to infiltrate to
substantial depths due to its exceptionally high density
Table 1
Summary of fluid properties of mercury DNAPL, PCE and water at 20° to 25 °C
a
.
Parameter Unit Mercury HgO PCE Water
Density kg·m
3
13,500
a
1630
b
1000
Viscosity 10
3
Pa·s 1.55
c
0.89
b
1.00
Surface tension Dynes·cm
1
485
c
32.6
b
72
Interfacial tension with water Dynes·cm
1
375
c
47.8
Vapour-pressure Pa 0.07
a,d
1867
b
Solubility μg·L
1
27.4
e
5.3 · 10
4f
2.00 · 10
5b
a
CRC (2014).
b
Schwille (1988).
c
U.S. DOE (2001) and references therein.
d
At 10 °C.
e
At 10 °C, Sanemasa (1975).
f
Wallschläger (1996) as referred in Schroeder and Munthe (1997).
Table 2
Soil properties used in this study.
Parameter Unit Sand Silty sand Silty clay Medium sand Fine sand Crozier loam
Porosity 0.43
a
0.46
a
0.36
a
0.33
b
0.43
a
0.41
a
Hydraulic conductivity cm·h
1
29.70
a
0.25
a
0.02
a
120 80 0.21
van Genuchten (α)cm
1
0.145
a
0.016
a
0.005
a
0.32
b
0.024
c
0.009
d
van Genuchten (n) 2.68
a
1.37
a
1.09
a
4.3
b
4.4
c
1.38
d
Irreducible water content 0.045
a
0.034
a
0.070
a
0.1/0.288
b
0.045
a
0.095
a
Residual Hg
0
saturation 0.10 0.10 0.10 0.08
b
0.049
e
0.014
e
Residual PCE saturation –– 0.275
b
0.17 0.13
Scaling factor (β) mercury 0.192 0.192 0.192 0.192 0.192 0.26
Scaling factor (β) PCE –– 1.6 1.6 2.2
a
Carsel and Parrish (1988).
b
Devasena and Nambi (2010).
c
Ippisch et al. (2006).
d
Busby et al. (1995).
e
Scaled values.
26 T. Sweijen et al. / Journal of Contaminant Hydrology 161 (2014) 2434
Fig. 1. Analysis of in-situ images, from left to right: in-situ image, in-situ image with black coloured mercury, and the volumetric mercury DNAPL content as function of the area of interest: A) 4.00 m depth and B) 3.53 m depth.
27T. Sweijen et al. / Journal of Contaminant Hydrology 161 (2014) 2434
(13.5 kg·L
1
) and low maximum residual saturation in
sands (Devasena and Nambi, 2010). For any DNAPL to infiltrate
a lithological sequence, an entry-pressure has to be overcome.
Both PCE and mercury DNAPL are non-wetting, thus water has to
be pushed out of the pores before infiltration occurs. Assuming
Leverett scaling to apply (Devasena and Nambi, 2010)the
relation between required entry-head pressures for PCE and
mercury DNAPL can be estimated as follows (e.g. Lenhard,
1994):
PHg
e¼PPCE
e
σHg σPCE
 ð1Þ
where P
e
denotes the entry-pressure induced by capillary
pressure between two fluids within a pore throat [Pa], σdenotes
the interfacial tension with water [N·m
1
]. Next, assume no
flow and a hydrostatic fluid pressure, such that P
e
=gρh.Where
gis the gravitational constant [m·s
2
], ρthe density [kg m
3
]
and hthe entry head [m]. Rewrite Eq. (1):
hHg
e¼hPCE
e
ρPCE ρHg

σHg σPCE

:ð2Þ
Using the parameters from Table 1, Eq. (2) yields that
the required entry-heads for mercury and PCE are similar
(h
e
Hg
= 1.03h
e
PCE
). The analysis thus indicate that the effect of
the higher density and surface tension on the entry-head of
mercury DNAPL balance each other, such that the entry-head
is similar to that of PCE. Consequently, the ability to infiltrate
low permeable layers is expected to be similar for mercury
DNAPL and PCE under hydrostatic conditions.
2.3. Numerical multi-phase ow model
To model the two-phase flow processes of mercury DNAPL
water and PCEwater in the saturated zone, the module water
oil in STOMP (Subsurface Transport Over Multiple Phases) was
used (White and Oostrom, 2006). This model numerically
solves differential equations for multi-phase flow by using
NewtonRaphson iteration method, including a convergence
criterion of 10
6
, maximum number of iteration of 20 and an
acceleration factor of 1.25. The model is based on the extended
Darcy's equation for multi-phase flow. It, therefore, assumes a
continuous flow on the Darcy scale, which implies process
descriptions to be at the macro-scale and not at the pore-scale.
The relative permeability, which is a function of saturation,
was described by using the van Genuchten and Mualem's
model (Lenhard and Parker, 1987a,b; Parker and Lenhard,
1987; van Genuchten, 1980). Following Devasena and Nambi
(2010) the van Genuchtenfunction was used for simulatingthe
capillary-saturation behaviour. The capillary saturation func-
tion was scaled for different fluidfluid systems, or different
interfacial tensions, by using Leverett-scaling (Leverett, 1941).
For common NAPLs, Leverett-scaling is proven to work well in
permeable layers and reasonable well for impermeable layers
(Busby et al., 1995, Lenhard and Parker, 1987b). Following the
applicability of Leverett-scaling on common NAPLs, mercury
capillary saturation functions were obtained by using Leverett-
scaling, which seemed to apply on mercury DNAPL in permeable
saturated porous media, based on limited experimental litera-
ture (Devasena and Nambi, 2010).
2.3.1. Model scenario of eld-investigation
Using literature-derived input values for the two-phase flow
of mercury DNAPL, and the soil properties such as hydraulic
conductivity (Table 2), a one-dimensional two-phase flow
Fig. 2. Soil profiles at a former chlor-alkali plant. A) Observed friction number. B) Observed cone-pressure. C) Interpretation of cone-pressure and friction number in
terms of soil classification. D) Mercury DNAPL saturation profiles for field-observations by the CPT-probe (solid line) and model results for a simplified case model
(dotted line). (1) Indicates mercury DNAPL accounted for by the model, (2) indicates mercury DNAPL accumulations which were not accounted for by the model.
Table 3
Fitting results for semi-variograms.
Friction
number
Cone-pressure Mercury DNAPL
fraction
Sill 0.9166 20.80 0.00125
Range [m] 0.1003 1.097 0.07423
Nugget effect 3.39·10
-11
7.36·10
-10
2.66E·10
-14
28 T. Sweijen et al. / Journal of Contaminant Hydrology 161 (2014) 2434
model scenario was designed based on the lithological informa-
tion from the CPT log measurements. The model included the
first 10 m of the subsurface, discretized by 1000 cells, and the
lithology according to the cone-pressure and friction number
(Fig. 2). For the focus of this study the whole model domain was
initially assumed fully water-saturated. Attempts for a two-
dimensional simulation did not yield numerical convergence
due to size of the model and the high resolution used to describe
the lithological heterogeneities. The hydraulic conductivity,
porosity, pressure-saturation curve in terms of van Genuchten
parameters, and the irreducible water content, the fraction of
irreplaceable water within the pores, were derived from typical
parameters (Carsel and Parrish, 1988), for the sand, silty sand
(silt) and silty clay, identified in the CPT-cone results. The
residual mercury saturation found at the former chlor-alkali-
plant was known, as described before. At the site, the location,
volumes and rates of elemental mercury spills were unknown.
Therefore, we assumed one mercury spill right above the
characterised soil section, by using Neumann's boundary
condition. We assumed, that the infiltration area was limited
to 1 m
2
of surface area, with a volume of 0.24 m
3
spilled over
8 h. The initial and boundary conditions maintained a fully-
saturated aquifer with a hydrostatic groundwater distribution.
The modelled mercury DNAPL infiltration was considered for
the depths below the approximate groundwater level of 1 m,
below ground surface. The distribution obtained by the multi-
phase flow modelling was compared to the distribution observed
in the field.
2.3.2. Model comparison of PCE and mercury DNAPL
In addition to the one-dimensional case scenario for the
field site, we simulated various two-dimensional multi-phase
flow scenarios to compare the transport and distribution for
the infiltration of mercury and PCE DNAPL, in homogeneous
(sand) and heterogeneous (with Crozier loam) aquifers. The
model dimensions used were X = 25 m, Y = 1 m and Z =
50 m with a discretization of 25 × 1 × 50 cells, respectively.
For the initial and boundary conditions a hydrostatic
groundwater distribution in a saturated aquifer was main-
tained, by using a constant hydraulic pressure at Z = 0 (water
pressure was assumed to be 0.59 MPa) and hydraulic pressure
distributions at the west and east side of the model domain. A
Dirichlet boundary condition simulated a pulse injection of
1.5 m
3
DNAPL of either PCE or mercury DNAPL, over a 2 m
2
injection area and assuming an 0.21 MPa infiltration pressure,
equivalent to 90 cm mercury head. This volume approximates
the average release during 8 major mercury spill events at the
Oak Ridge Y-12 National Security Complex (~1.8 m
3
per spill), as
derived from the total estimated mercury release of 193,000 kg
(Brooks and Southworth, 2011).
The soil- and fluid parameters used (Table 2)werederived
from literature, for medium sand, fine sand and Crozier loam
(Busby et al., 1995; Carsel and Parrish, 1988; Ippisch et al.,
2006). To obtain a set of parameters as realistic as possible for
the modelling of mercury DNAPL the residual saturation of
mercury was linearly extrapolated for fine sand and Crozier
loam by using known residual saturation for mercury in
medium sand and using known data for PCE in medium sand,
fine sand and Crozier loam (Parker et al., 1995).
The following three scenarios were used to compare the
main characteristics of mercury and PCE migration and final
distribution in a saturated porous media: 1) mercury and PCE
spill in homogeneous medium sand, 2) sensitivity analyses on
the influence of density and viscosity on mercury migration
and 3) mercury and PCE spill in fine sand including a 2 m thick
loam layer at 5 m depth. Spatial moments were used to
characterise the DNAPL bodies and to allow a quantitative
comparison (Freyberg, 1986; Kueper and Frind, 1991).
3. Results and discussion
In Section 3.1 results are shown of field-observations of a
soil section surrounding a former chlor-alkali plant, and the
according two-phase flow simulations of infiltrating mercury
DNAPL. Whereas in Section 3.2 a model comparison between
mercury DNAPL and PCE is made.
Fig. 3. Model results for a saturated homogeneous medium sand after a 1.35 m
3
spill and 70 h: A) mercury DNAPL and B) PCE.
29T. Sweijen et al. / Journal of Contaminant Hydrology 161 (2014) 2434
3.1. Characterization and modelling of mercury DNAPL at a
eld site
3.1.1. Image-derived mercury DNAPL morphology and vertical
distribution
The in-situ images obtained with the CPT mounted
camera typically revealed mercury DNAPL present in small
and discontinuous droplets (b2 mm in diameter),representing
ganglia of interconnected DNAPL between multiple grains.
These ganglia were considerably larger than the estimated
average blob sizes (b10 μm) in field studies on PCE DNAPL
zones (Hartog et al., 2010). In some occasions, mercury DNAPL
droplets were interconnected forming larger ganglia (23mm
in diameter), with elongated shapes spanning N6mm, sug-
gesting spatially continuous mercury DNAPL accumulations,
particularly in the vertical direction (Fig. 1A). However, since
the distribution of ganglia is 3-dimensional, the 2-dimensional
images under estimated their interconnectedness. Therefore,
elemental mercury that appears in the images as isolated
droplets might also be horizontally linked ganglia.
The vertical distribution of image-derived mercury pore
saturations is shown in Fig. 2, along with CPT-derived cone-
pressures and friction numbers. As the calculated pore satura-
tions stabilized with increasing area within the image (Fig. 1),
the obtained values were representative averages for the scale
of the images. The vertical mercury DNAPL distribution was
characterised by generally low saturations (b0.8%) and intervals
of relative large saturations up to 13.2%. Theoretically, the lowest
detectable mercury saturation would be that of one pixel (10 μm
by 11 μm). However, the image analysis method was dependent
on the ability to recognize blobs visually. Therefore, based on the
smallest mercury droplet (60 by 66 μm) identified in the images,
an operational detection limit of 0.002% was derived for an
image with a single droplet that size. However, the determined
saturations for images with observable mercury were at least
400 times larger, all in excess of 0.8%, suggesting that mercury
DNAPL was not present at lower saturations. Moreover, the
observed range of the image-derived saturations was in keeping
with the range of the residual saturations (18%; Table 2)
observed in experiments by Devasena and Nambi (2010).
Integration of the image-derived pore-saturations over
depth yielded a total amount of 13 L mercury DNAPL per m
2
surface area. The average mercury DNAPL content for this 9 m
investigated sediment profile, was therefore 1.4 L m
3
for
mercury DNAPL, or an estimated average pore saturation of
0.35% (assuming a porosity of 0.40).
3.1.2. Lithological heterogeneity and spatial correlation with
mercury DNAPL
The CPT-based lithological characterisation down to 9 m
below ground surface indicated a dominantly sandy stratigra-
phy with alternating silty clay layers of up to 0.5 m thickness
(Fig. 2). These lithological variations were revealed by the
Fig. 4. Centre of mass for the sensitivity analyses of mercury DNAPL in
saturated homogeneous medium sand.
Fig. 5. Model results for saturated fine sand including a Crozier Loam layer at 5 m depth. A) 1.48 m
3
mercury DNAPL after 10 h and B) 1.48 m
3
PCE after 10 h.
30 T. Sweijen et al. / Journal of Contaminant Hydrology 161 (2014) 2434
in-situ images, whereas minor heterogeneities present in the
images were not always included in the CPT-data.
Within this investigated aquifer section, the mercury
DNAPL appeared to have accumulated on top of coarse to fine
stratigraphic transitions, some of which were minor as indicated
by subtle changes in friction number and cone-pressure (Fig. 2).
All of these intervals were thin (b9 cm), spanning at maximum
only several times the vertical image resolution (18 mm). For
example the thickness of the mercury accumulation interval at
3.6m, 4m, and 6.7m depth, where 2cm, 4cm and 8cm,
respectively.
To evaluate the overall spatial correlation of mercury DNAPL
with depth in relation with the lithological heterogeneity
observed by the CPT-probe, variogram analysis was performed.
The CPT-derived friction number and cone-pressures, both
reflect variation in lithological properties. But as can be seen
from Fig. 2, the friction number reflected more strongly
lithological boundaries whereas the cone-pressure variation
reflected various lithological types. Consequently, this resulted
in a more irregular and heterogeneous pattern for the friction
number, which was reflected in the fitted variogram values,
using a spherical model with a nugget effect (Table 3). The
higher resolution of measurements for the friction number and
cone-pressure resulted in smoother variograms, whereas the
variogram for mercury DNAPL was more angular. However,
the variograms could be fitted with a relative low intrinsic
(nugget) uncertainty (Table 3). The range fitted for the vertical
mercury distribution was 7 cm, similar to the range fitted for
the friction number (10 cm) and considerably lower than for
the cone-pressure (110 cm), reflecting the sensitivity of the
mercury DNAPL distribution to lithological transitions.
3.1.3. Modelling analysis of eld-observations
The simulations with the one-dimensional model that
included the major lithological units, resulted in mercury
DNAPL accumulations on top of the clay-layers at 5.5 m,
6.35 m and 8.37 m, in keeping with the image-derived field
observations (Fig. 2D, indicated by 1). However, the model
did not reflect all accumulations of mercury directly beneath
clay-layers (indicated by 2). This could be related to minor
lithological changes not included in the model, but which
were reflected by small variations in the friction number and
cone-pressure. The magnitude of simulated mercury pore
saturations was in general within range of the saturations
found in the field investigation. Although, overestimations of
mercury DNAPL pore saturation occurred on top of silty clay
layers. Overall the modelling results suggested that the input
parameters used in the mercury flow model (Table 2) were
reasonable.
Generally, however, the model overestimated the thick-
ness of DNAPL accumulations, also some accumulations were
predicted by the model but not observed in the field. This
could be due to the one-dimensional model assumption
that the mercury DNAPL infiltration occurred directly above
the modelled stratigraphic sequence and thus represents a
maximum of mercury DNAPL retention capacity. Likely, mercury
DNAPL infiltration occurred elsewhere, where the observed
mercury DNAPL accumulations resulted from lateral spreading.
In addition, the field DNAPL saturations may have decreased
from initial values to some extent by ageing through volatiliza-
tion and dissolution, as has been described for chlorinated
solvent DNAPL source zones (Parker et al., 2003). However,
since both the vapour pressure and the solubility of elemental
mercury are very low, e.g. four orders of magnitude lower than
PCE (Table 1), mercury DNAPL zones seem considerably less
prone to ageing. Nevertheless, the extent to which these physical
as well as chemical processes affect DNAPL distribution charac-
teristics over time should be addressed in future studies.
3.2. Mercury DNAPL and PCE: a comparison of
transport behaviours
The lack of other mercury DNAPL field studies precludes
the evaluation of site-specific conditions on mercury DNAPL
transport and source zone evolution in the saturated zone.
However, in dealing with mercury DNAPL contaminated sites
one major question is to what extent mercury DNAPL migration
and distribution behaviour differs from more commonly studied
DNAPL phases, such as perchloroethylene (PCE). To explore
the differences in transport behaviour of mercury DNAPL
and PCE their contrasting properties (Table 1) were tested in
several simulations under homogeneous and heterogeneous
hydrogeological conditions.
3.2.1. Mercury transport in saturated homogeneous
porous media
Mercury DNAPL and PCE infiltration was modelled in a
homogeneous sandy aquifer consisting of medium sand, using
equal infiltration pressure and volume for both DNAPLs.
Results indicated distinctively different transport behaviours
in terms of both transport rate and distribution.
The simulations showed that mercury infiltrated 3 times
deeper than PCE (Fig. 3), mainly due to its lower residual
saturation (Table 2). Mercury DNAPL distribution stabilized
after 20 h, compared to 5 h for PCE. The final vertical mercury
front (saturation N0.01%) reached a depth of 32 m below
groundwater, with the centre of mass of the mercury DNAPL at
11.65 m depth. In contrast, the PCE DNAPL front only reached a
depth of 11 m, with a centre of mass at 3.63 m depth. The
relative large horizontal variance of PCE (2.33 m
2
)comparedto
that of mercury (1.36 m
2
) was due to the high infiltration-
pressure (90 cm mercury head). Consequently, mercury infil-
trated deeper than PCE due to the low residual saturation and
limited horizontal spreading. Therefore, another PCE spill was
simulated with an infiltration pressure equivalent to 90 cm
PCE, which resulted in an infiltration depth of 16 m with a
reduced horizontal variance of 1.37 m
2
and a centre of mass at
5.6 m. Steady-state was achieved after 15 h, which was closer
to the 20 h for mercury DNAPL. Clearly, the infiltration rate and
final distribution of both PCE and mercury was dependent
on infiltration conditions, however under all scenarios tested
elemental mercury migrated substantially deeper than PCE.
The observed differences in the infiltration behaviours
between mercury and PCE DNAPL can be attributed to
different properties of both DNAPLs. To evaluate the impor-
tance of density and viscosity on mercury DNAPL migration, a
sensitivity analysis was performed (Fig. 4). First, the density
of mercury was changed to that of PCE (i.e. 13,500 kg·m
3
to
1650 kg·m
3
). This caused the infiltration depth to decrease
substantially, while increasing the horizontal variance from
1.36 m
2
to 3.55 m
2
. The centre of mass was shifted from the
upper half of the DNAPL zone to the lower half, from 11.65 m
31T. Sweijen et al. / Journal of Contaminant Hydrology 161 (2014) 2434
to 5.60 m depth. Moreover, the time to reach entrapment
was increased from 20 to 25 h. These observations indicated
that the density is the major driving force for mercury DNAPL
distribution during infiltration. In a second model run where
only the viscosity of mercury was changed to that of PCE
(from 1.55 · 10
3
Pa·s to 0.89 · 10
3
Pa·s) the infiltration
depth did not change substantially, although entrapment
occured slightly faster (Fig. 4). The horizontal variance
decreased from 1.37 m
2
to 0.92 m
2
. The time required for
reaching entrapment decreased from 20 to 15 h, which is in
keeping with a decrease in viscosity, i.e. lower resistance to
flow. Though all these factors impacted on mercury DNAPL
migration, its final distribution was mainly controlled by its
high density, while its higher penetration potential is due to its
low residual saturation.
3.2.2. Mercury transport in saturated heterogeneous
porous media
To assess the influence of heterogeneities within the soil
system on mercury DNAPL migration in comparison to PCE,
a spill was simulated in a fine sand aquifer containing
continuous 5 m thick loam layer at 5 m depth. Although
both DNAPLs infiltrated into the loam layer the extent of the
infiltration depth differed substantially. Mercury DNAPL
permeated through the entire loam layer, whereas PCE only
infiltrated the upper part of the loam layer (Fig. 5). As a result
mercury reached a far greater depth (20.5 m) than PCE
(6 m). Strikingly, the horizontal spreading of mercury DNAPL
on top of the loam layer resulted in a variance of 6.91 m
2
,
which was substantially larger than for PCE (3.40 m
2
). In
contrast, the height of PCE accumulation was higher, resulting
in the centreof mass located at a moreshallow depth of 3.34 m,
compared to 6.83 m for mercury. Both DNAPLs required 10 h
to reach entrapment, even though both DNAPLs migrated
differently. Overall, the two-phase flow modelling results
indicated the higher penetration potential of mercury DNAPL
compared to PCE, which was further emphasized in the presence
of layered heterogeneities.
3.3. Image-derived in-situ characterization of DNAPL distributions
To the best of our knowledge, our study is the first to
present the use of in-situ derived images to determine DNAPL
distribution in the field. This allowed the characterisation of
mercury DNAPL droplets at the mm scale (Fig. 1), which could
be up-scaled using the concept of representative elementary
volume (REV) to represent the variation of mercury DNAPL
saturation at a cm scale along a depth of 9 m (Fig. 2). In
principle, other DNAPL phases, such as creosote can be
characterised by this method, as long as they present a phase
that provides sufficient contrast with the sedimentary back-
ground to allow visual detection. This dependence on visibility
impedes detection of colourless DNAPL types, such as PCE,
unless they could be high-lighted in-situ (e.g. Sudan dye) prior
to camera observations.
Compared to detailed core-sampling and analysis, as con-
ducted previously for chlorinated solvent DNAPL characteriza-
tion (e.g. Hartog et al., 2010; Parker et al., 2003), the advantages
of the optical in-situ DNAPL characterization method are the
rate of data collection and not having to bring these heavily
contaminated sediments to surface for analysis. In addition, the
method allows for determination of the pore saturation along
with morphological characteristics directly from visual obser-
vation. However, since morphology affects visibility, absolute
quantification through calibration of the measurements is
challenging and derived pore saturation values should be
considered approximations. In this study for example, the
operational detection limit was determined at 0.002% per
image, based on the smallest detected mercury droplet (60 by
66 μm). Since pore saturations below 0.8% were not observed,
it is unlikely that significant mercury accumulations were
missed. However, other field-specific conditions, such as DNAPL
type, lithological characteristics or significant ageing, might
have resulted in finely dispersed DNAPL morphologies with
dominant dimensions smaller than 10's of micrometres, that
would have been too small for visible detection.
3.4. Considerations for the characterisation of mercury
DNAPL eld sites
With this study we aimed to provide a first-order under-
standing of the behaviour and distribution of elemental
mercury DNAPL in the saturated zone at field conditions and
scales, taking into account the experience and insight in more
commonly studied DNAPL types, such as PCE. In comparison
with PCE, the low residual saturation that results from the
combined properties of mercury DNAPL, result in a significantly
stronger vertical penetration potential under relatively homog-
enous conditions. Under heterogeneous conditions with hori-
zontal layering, the vertical penetration potential of infiltrating
mercury DNAPL is further increased compared to PCE by its
high density.
The current study confirmed the presence of mercury
DNAPL to at least 9 m depth, while at another former chlor-
alkali plant mercury DNAPL was found at 17 m depth below
surface (Deeb et al., 2011; ITRC, 2012). However, it is likely that
in some locations mercury DNAPL has penetrated considerably
deeper, since chlorinated solvent DNAPL have been found at
greater depths (N50 m), and our comparison indicates that the
penetration potential of mercury DNAPL in the saturated zone
is significantly stronger than that of PCE DNAPL.
In contrast to PCE, the high surface tension of elemental
mercury with respect to air (Table 1) is likely to cause mercury
to behave as a non-wetting phase with respect to dry soil in
the unsaturated zone, which would require some pooling of
mercury to overcome entry pressures (Sweijen, 2013). Wheth-
er, where and to what extent mercury DNAPL will reach
the saturated zone at a particular site, will, therefore, largely
depend on theability of mercury DNAPL to permeate or bypass
(e.g. via surface water or sewer system) the unsaturated zone.
The behaviour of elemental mercury and the distribution of
mercury DNAPL in the unsaturated zone are further compli-
cated by mercury vapour transport and dynamic volatilization
condensation processes (e.g. Walvoord et al., 2008). To further
increase the understanding of elemental mercury transport
behaviour at field sites, studies focussing on the fundamental
principles governing three-phase flow (waterairmercury) in
conjunction with the dynamic aspects of elemental mercury
vapour transport in the unsaturated zone are critically important.
In addition to the physical factors influencing the mercury
DNAPL behaviour at field sites, elemental mercury in the
saturated zone is affected by chemical processes. As indicated
32 T. Sweijen et al. / Journal of Contaminant Hydrology 161 (2014) 2434
earlier, dissolution and volatilization of elemental mercury
may result in source zone ageing reducing overall pore
saturations. Although, based on the vapour pressure and
solubility of elemental mercury (Table 1), the potential of
these processes for the reduction of mercury DNAPL seems
limited compared to chlorinate solvent DNAPLs (Parker et al.,
2003), redox transformations may result in more soluble
and volatile species. For instance, elemental mercury can be
oxidized to result in mercury-oxide, which has a substantially
higher solubility than pure phase mercury (Table 1), or mercury
can be transformed into methyl-bound mercury species under
reducing conditions (e.g. Gabriel and Williamson, 2004). To
which extent and under which conditions these redox trans-
formation processes may significantly enhance the ageing of
mercury DNAPL zones is a pressing research topic considering
the toxicity of these transformation products and the persis-
tence with which mercury DNAPL can act as a long-term source
of contamination.
4. Conclusion
Through analysis of both field data and two-phase flow
modelling, we present in this paper a first assessment of
mercury DNAPL transport and distribution behaviour in
saturated porous media. Also, we present a novel method-
ology of interpreting in-situ DNAPL saturations by the
processing of images obtained with a CPT-probe. Using this
method, a soil section surrounding a former chlor-alkali plant
was studied, revealing mercury DNAPL to be present in
saturations up to 13.2%, which were located at lithological
interfaces. Using the multi-phase flow model STOMP (Subsur-
face Transport Over Multiple Phases), the results from a one-
dimensional mercury infiltration model representing this
characterised soil section, suggested that the input parameters
used in the mercury flow model (Table 2) were reasonable. A
model comparison of the transport behaviour of mercury
DNAPL and PCE in the saturated zone indicated that mercury
DNAPL had a higher potential to penetrate to greater depths than
PCE DNAPL. In the presence of horizontally layered heteroge-
neity the penetration potential of mercury DNAPL was further
enhanced relative to PCE DNAPL. Compared to commonly
known DNAPLs (e.g. PCE) elemental mercury is an extraordi-
nary DNAPL, with a high density, surface tension and a low
residual saturation. Due to its low maximum residual satura-
tion and its high density mercury DNAPL may migrate to
substantial depths.
Acknowledgements
We would like to thank Prof. Dr. S. M. Hassanizadeh for
valuable discussions on this research. Also, we appreciate the
constructive comments and suggestions from three anony-
mous reviewers that helped improve the manuscript.
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... The occurrence of liquid elemental mercury in the subsurface as Dense NonAqueous Phase Liquid (DNAPL) has been reported worldwide in proximity of several chlor-alkali plants like in Lavaca Bay, Texas (USA), (Scanlon et al., 2005), near the Onondaga Lake, New York (USA), where it was found 17 m below the ground level (Deeb et al., 2011;ITRC, 2012), at the Oak Ridge Y-12 National Security Complex, Tennessee (USA), where the estimated loss of liquid Hg 0 to the soil is about 193,000 kg (Brooks and Southworth, 2011), at the Botany Industrial Park, Sydney (Australia), (Golder Associates, 2011), and at a former plant in the Netherlands (Sweijen et al., 2014). Once Hg 0 DNAPL infiltrates in the subsurface and immobilizes, it acts as a long lasting source of mercury contamination (Davis et al., 1997). ...
... Despite the worldwide importance and presence of liquid elemental mercury in the subsurface, only recently its DNAPL behaviour in porous media has been addressed. As for more extensively studied DNAPLs, such as creosote, carbon tetrachloride, trichloroethylene (TCE), and tetrachloroethylene (PCE), liquid elemental mercury requires to overcome an entry head to infiltrate in fully water saturated porous media (Devasena and Nambi, 2010), and it has a relatively high capacity to infiltrate as illustrated by numerical simulations (Sweijen et al., 2014;D'Aniello et al., 2018b) and by capillary pressure-saturation, P c (S), experiments under stable flow behaviour (D'Aniello et al., 2018a). However, in contrast with other DNAPLs, liquid elemental mercury was found to behave as a nonwetting phase with respect to both air and water, and therefore it requires to overcome an entry head to infiltrate in partially water saturated porous media (D'Aniello et al., 2018a). ...
Article
Industrial use has led to the presence of liquid elemental mercury (Hg0) worldwide in the subsurface as Dense NonAqueous Phase Liquid (DNAPL), resulting in long lasting sources of contamination, which cause harmful effects on human health and detrimental consequences on ecosystems. However, to date, insight into the infiltration behaviour of elemental mercury DNAPL is lacking. In this study, a two-stage flow container experiment of elemental mercury DNAPL infiltration into a variably water saturated stratified sand is described. During the first stage of the experiment, 16.3 ml of liquid Hg0 infiltrated and distributed into an initially partially water saturated system. Afterwards, during the second stage of the experiment, consisting of the simulation of a “rain event” to assess whether the elemental mercury already infiltrated could be mobilized due to local increases in water saturation, a significant additional infiltration of 4.7 ml of liquid mercury occurred from the remaining DNAPL source. The experiment showed that, under conditions similar to those found in the field, Hg0 DNAPL infiltration is likely to occur via fingers and is strongly controlled by porous medium structure and water saturation. Heterogeneities within the porous medium likely determined preferential pathways for liquid Hg0 infiltration and distribution, as also suggested by dual gamma ray measurements. Overall, this study highlights that the infiltration behaviour of mercury DNAPL is strongly impacted by water saturation. In the field, this may result in a preferential infiltration of Hg0 DNAPL in wetter areas or in its mobilization due to wetting during a rain event, as indicated by this study, or a groundwater table rise. This should be considered when assessing the likely distribution pathways of historic mercury DNAPL contamination as well as the remediation efforts.
... Liquid elemental mercury has been found worldwide in the subsurface as dense non-aqueous phase liquid (DNAPL) in several chlor-alkali sites (Scanlon et al. 2005;Di Molfetta and Fracassi 2008;Deeb et al. 2011;ITRC 2012;Brooks and Southworth 2011;Golder Associates 2011;Sweijen et al. 2014). Once in the subsurface, Hg 0 DNAPL acts as a long-lasting source of contamination (Davis et al. 1997), thanks to its low aqueous solubility, 0.07 mg/l (Eichholz et al. 1988). ...
... Despite its extensive use in petroleum engineering for mercury porosimetry (Wardlaw and Taylor 1976;Vavra et al. 1992;Pittman 1992;Smith et al. 2002;Newsham et al. 2004;Ruth et al. 2013), only few studies (Eichholz et al. 1988;Devasena and Nambi 2010;Sweijen et al. 2014;D'Aniello 2017;D'Aniello et al. 2018, Under Review) are available on liquid Hg 0 flow behavior in porous media as a DNAPL. Thanks to its extraordinary density and surface tension, of about 8 and 15 times higher than tetrachloroethylene (PCE) (Schwille 1988;CRC 2014), liquid elemental mercury infiltration behavior deviates with respect to more extensively studied DNAPLs, such as trichloroethylene (TCE) and PCE, (Devasena and Nambi 2010;D'Aniello 2017;D'Aniello et al. 2018, Under Review). ...
Article
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Liquid elemental mercury occurrence in the subsurface as DNAPL is reported worldwide in proximity of several industrial facilities, such as chlor-alkali plants. Insight into Hg0 DNAPL infiltration behaviour is lacking and, to date, there are no experimental observations of its infiltration and distribution in water saturated porous media, except for capillary pressure-saturation column experiments. To better understand the processes governing elemental mercury DNAPL flow behaviour, a series of flow container experiments were performed using mercury DNAPL (in sands and glass beads) and PCE (in sands). While liquid Hg0 was not able to infiltrate in the sand filled container due to an overall lower permeability of the sample and a defect of the setup, in the glass beads experiment mercury DNAPL infiltration occurred. Dual gamma ray measurements showed that, in glass beads, liquid Hg0 preferentially migrated towards higher porosity zones. As for PCE, infiltration and distribution of Hg0 DNAPL are strongly affected by the heterogeneities within the porous formation. However, compared to other DNAPLs, liquid Hg0 shows a strong attenuation potential of gamma rays. Finally, numerical simulations of the glass beads experiment showed an overall good agreement with the experimental results, highlighting that, among the factors influencing the prediction of liquid Hg0 migration in water saturated porous media, the most critical are (i) the knowledge of the inflow rate, (ii) the reliable estimation of the porous formation permeability, and (iii) the accurate representation of the correlation between retention properties and intrinsic permeability.
... Insight into the infiltration and flow behaviour of elemental mercury in the subsurface is critical for effective characterization and remediation approaches. Although in petroleum engineering liquid elemental mercury is extensively used as a nonwetting fluid for the analysis of the pore structure (mercury porosimetry) of geological formations under vacuum conditions (Wardlaw and Taylor, 1976;Vavra et al., 1992;Pittman, 1992;Smith et al., 2002;Newsham et al., 2004;Ruth et al., 2013), only recently a few studies (Devasena and Nambi, 2010;Sweijen et al., 2014;D'Aniello, 2017;D'Aniello et al., 2018) have addressed liquid Hg 0 infiltration behaviour in water saturated porous media under environmental conditions. Devasena and Nambi (2010) confirmed that liquid Hg 0 acts as a nonwetting fluid, thus requiring to overcome an entry head to infiltrate into water saturated granular porous media, and that residual mercury entrapment in the two-phase Hg 0 -water system was much lower (0.04) than in the TCE-water (0.14) and PCE-water (0.17) systems. ...
... They argued that this significant variation in residual saturation was probably due to the high interfacial tension of mercury with water, and to its high density and viscosity, and that mercury DNAPL flow behaviour is governed by gravitational and capillary forces, and is practically independent of viscous forces. Supported by field site characterization of mercury DNAPL distribution, Sweijen et al. (2014) made a first attempt for the modelling of the infiltration and distribution behaviour of elemental mercury in a sandy aquifer for water saturated conditions. In particular, the numerical comparison between Hg 0 and PCE DNAPLs infiltration behaviour indicated that Hg 0 had a higher potential to penetrate to greater depths than PCE, a difference which was further enhanced in the presence of horizontally layered heterogeneities. ...
Article
Mercury is a contaminant of global concern due to its harmful effects on human health and for the detrimental consequences of its release in the environment. Sources of liquid elemental mercury are usually anthropogenic, such as chlor-alkali plants. To date insight into the infiltration behaviour of liquid elemental mercury in the subsurface is lacking, although this is critical for assessing both characterization and remediation approaches for mercury DNAPL contaminated sites. Therefore, in this study the infiltration behaviour of elemental mercury in fully and partially water saturated systems was investigated using column experiments. The properties affecting the constitutive relations governing the infiltration behaviour of liquid Hg⁠0, and PCE for comparison, were determined using P⁠c(S) experiments with different granular porous media (glass beads and sands) for different two- and three-phase configurations. Results showed that, in water saturated porous media, elemental mercury, as PCE, acted as a non-wetting fluid. The required entry head for elemental mercury was higher (from about 5 to 7 times). However, due to the almost tenfold higher density of mercury, the required NAPL entry heads of 6.19 cm and 12.51 cm for mercury to infiltrate were 37.5% to 20.7% lower than for PCE for the same porous media. Although Leverett scaling was able to reproduce the natural tendency of Hg⁠0 to be more prone than PCE to infiltrate in water saturated porous media, it considerably underestimated Hg⁠0 infiltration capacity in comparison with the experimental results. In the partially water saturated system, in contrast with PCE, elemental mercury also acted as a nonwetting fluid, therefore having to overcome an entry head to infiltrate. The required Hg⁠0 entry heads (10.45 and 15.74 cm) were considerably higher (68.9% and 25.8%) than for the water saturated porous systems. Furthermore, in the partially water saturated systems, experiments showed that elemental mercury displaced both air and water, depending on the initial water distribution within the pores. This indicates that the conventional wettability hierarchy, in which the NAPL has an intermediate wetting state between the air and the water phases, is not valid for liquid elemental mercury. Therefore, for future modelling of elemental mercury DNAPL infiltration behaviour in variably water saturated porous media, a different formulation of the governing constitutive relations will be required.
... 1.3.1), metallic mercury can reach greater depths, like near the Onodaga Lake, where it was found 17 m below the ground level (Deeb et al., 2011;ITRC, 2012), or in the site described in Sweijen et al. (2014), ...
... The governing equations of multiphase flow of immiscible fluids in porous media can be used to describe Hg 0 DNAPL flow behaviour (Sweijen et al., 2014), as also confirmed by the experimental and numerical results described in Chapters 4 and 5. Hence, the hypothesis of isothermal flow of incompressible fluids into a water wet nondeforming medium, with continuity of the air phase within the porous medium and negligibility of its pressure field, hold true. In the same manner, the k r -S constitutive relations , and the extensions made by Kaluarachchi and Parker (1992) to consider hysteresis due to NAPL entrapment, Type-II hysteresis (Lenhard et al. 1989), can be used. ...
Thesis
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This thesis focuses on the characterization of the flow behaviour of elemental mercury (Hg0) DNAPL (Dense NonAqueous Phase Liquid) in porous media. In the subsurface, Hg0 DNAPL can act as a long lasting source of contamination, causing detrimental consequences for the human health and the environment. Therefore, insight into the flow behaviour of elemental mercury in porous media is needed and is critical in assessing the control of contaminant spreading as well as remediation approaches. However, the scientific literature on Hg0 DNAPL is still very limited and, to date, it remains unclear to what extent the validity of the classical constitutive relations, used to describe DNAPLs flow behaviour, as well as the scaling theory, holds for elemental mercury. These issues become crucial in partially water saturated porous media, where liquid Hg0 is likely to behave as a nonwetting phase with respect to both air and water. To address these knowledge gaps, experimental and numerical analysis were performed. In particular, the properties affecting the constitutive relations governing liquid Hg0 infiltration behaviour were explored first, using capillary pressure-saturation, Pc(S), experiments in different granular porous media, and in two- and three-phase fluid systems. Then, the infiltration and (re)distribution behaviour of Hg0 DNAPL was studied in variably water saturated stratified porous media with flow container experiments and dual gamma ray measurements of porosity and fluid saturations. Experimental results indicated that elemental mercury infiltration is strongly controlled by the porous medium water content and can be triggered by its changes due to, for example, rain events. Finally, a new theoretical formulation of elemental mercury retention properties in variably water saturated porous media was proposed and, to assess to what extent numerical modelling can predict elemental mercury migration in porous media, the flow container experiments were simulated using GDAn, the code developed by the Author.
... Mobility of Hg in groundwater can be facilitated in areas where the groundwater redox conditions promote the conversion of oxidized Hg species to Hg 0 or MeHg, which are less prone to solid phase sorption (Lamborg et al., 2013). In addition, areas contaminated by Hg 0 spills, can result in Hg present as an immiscible dense non-aqueous phase liquid (DNAPL) that can migrate deep below the surface due to its high density, surface tension, and low residual saturation (Sweijen et al., 2014). ...
Article
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Remediation of mercury (Hg) contaminated sites has long relied on traditional approaches, such as removal and containment/capping. Here we review contemporary practices in the assessment and remediation of industrial-scale Hg contaminated sites and discuss recent advances. Significant improvements have been made in site assessment, including the use of XRF to rapidly identify the spatial extent of contamination, Hg stable isotope fractionation to identify sources and transformation processes, and solid-phase characterization (XAFS) to evaluate Hg forms. The understanding of Hg bioavailability for methylation has been improved by methods such as sequential chemical extractions and porewater measurements, including the use of diffuse gradient in thin-film (DGT) samplers. These approaches have shown varying success in identifying bioavailable Hg fractions and further study and field applications are needed. The downstream accumulation of methylmercury (MeHg) in biota is a concern at many contaminated sites. Identifying the variables limiting/controlling MeHg production-such as bioavailable inorganic Hg, organic carbon, and/or terminal electron acceptors (e.g. sulfate, iron) is critical. Mercury can be released from contaminated sites to the air and water, both of which are influenced by meteorological and hydrological conditions. Mercury mobilized from contaminated sites is predominantly bound to particles, highly correlated with total sediment solids (TSS), and elevated during stormflow. Remediation techniques to address Hg contamination can include the removal or containment of Hg contaminated materials, the application of amendments to reduce mobility and bioavailability, landscape/waterbody manipulations to reduce MeHg production, and food web manipulations through stocking or extirpation to reduce MeHg accumulated in desired species. These approaches often rely on knowledge of the Hg forms/speciation at the site, and utilize physical, chemical, thermal and biological methods to achieve remediation goals. Overall, the complexity of Hg cycling allows many different opportunities to reduce/mitigate impacts, which creates flexibility in determining suitable and logistically feasible remedies.
... NAPLs can be categorized into light non-aqueous phase liquids (LNAPLs) and dense non-aqueous phase liquids (DNAPLs) depending on their density relative to that of water. Chlorinated solvents such as trichloroethylene and tetrachloroethylene are the most common examples of DNAPL, whereas petroleum compounds such as benzene, toluene and xylene (BTEX) are examples of LNAPL (Sweijen et al. 2014;Karadag et al. 2016). ...
Article
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The influence of infiltration on the migration of a light non-aqueous phase liquid (LNAPL) in double-porosity soil using the light transmission visualization (LTV) technique is investigated. Two LNAPL volumes (low and high volumes) were exposed to two rainfall intensities (light and heavy infiltration). For comparison purposes, the experiments were also repeated by compacting the flow chamber with silica sand only to represent the single-porosity medium and to investigate the influence of double-porosity on LNAPL migration. High-resolution LTV images of the flow chamber during LNAPL injection and subsequent water infiltration events were collected. Results show that the LNAPL migration depth during injection and its migration velocity were both correlated to the LNAPL volume. Subsequent water infiltration events caused the LNAPL that was entrapped in the porous media to be pushed further downward in all the experiments. The LNAPL migration velocity was 1.1 and 1.6 cm/h for the low and high LNAPL spillage volumes for double-porosity experiments, respectively, a reduction rate of 64.7 and 70% compared to the LNAPL migration velocity during LNAPL injection, respectively. However, for single-porosity experiments, the LNAPL migration velocity was 0.7 and 1.2 cm/h for the low and high LNAPL volumes, respectively. Furthermore, it was observed that the capillary fringe level was depressed in the saturated zone due to the influence of both infiltration and LNAPL volume. This study demonstrates that the LTV technique is an accurate and cost-effective laboratory tool for the visualization of the time-dependent influence of infiltration on LNAPL migration in porous media.
... Generally, NAPLs are divided into two main types based on its density; dense non-aqueous phase liquid (DNAPL) which is denser than water and light non-aqueous phase liquid (LNAPL) which has density less than water. Chlorinated solvents such as trichloroethylene and tetrachloroethylene are the most spread examples of DNAPL, whereas benzene, toluene, ethyl-benzene and xylene (BTEX) are examples of LNAPL (Sweijen et al., 2014). When LNAPL is released into the subsurface, it will migrate downward until it reaches the water table (Mercer and Cohen, 1990). ...
Article
Over the last few decades, contamination of groundwater and soil by non-aqueous phase liquids (NAPLs) has become a serious and wide-spread problem for the environment In this research, a light transmission visualization (LTV) method was used to observe the migration of dense non-aqueous phase liquid (DNAPL) and light non-aqueous phase liquid (LNAPL) in double-porosity soil within a three-fluid phase system (air-NAPL-water). The double-porosity characteristics of the soil were created using a composition made up of local sand and sintered kaolin clay spheres arranged in a periodic manner. Toluene was used to simulate LNAPL while tetrachloroethylene (PCE) represented the DNAPL. Both NAPLs were dyed using Oil-Red-O for better visualization. For comparison purposes, the same experiments were carried out using just local silica sand acting as a type of single-porosity soil. A significant difference in the migration of the toluene and PCE was observed as both the NAPL migration rates in the double-porosity medium were much faster compared to the migration rates found in the single-porosity medium. This result is most likely due to the occurrence of inter-aggregate pores in the double-porosity soil that contribute to increasing velocity of fluids migration through porous media. Other factors such as the wettability of fluids and capillary pressure characteristics that exist in the soil pores were found to be influential factors in fluid migration within porous media. In addition, the results show that chemical properties have a significant influence on the NAPL migration in porous media. It was found that the migration velocity of toluene was much faster compared to the migration velocity of the PCE. This observation is most likely caused by the fact that the distribution coefficient of toluene was higher than that of PCE which in turn means that the retardation factor of toluene is lower than that of PCE in the same porous media. This paper proved that the LTV provides a non-intrusive and non-destructive technique for studying multiphase flow in double-porosity soil media where rapid changes in fluid distribution in the entire flow domain is not easy to measure using conventional tools.
... Furthermore, it is important to measure the concentration and saturation of the contaminants to assess the hazards that affect the environment and human health (Darnault et al., 2001). Although, an in situ study on DNAPL migration was conducted by Sweijen et al. (2014) using a traditional method (e.g., cone penetration test probe), field studies are still not easy to conduct on NAPL due to the toxic and hazardous nature of the chemicals involved, as well as the high cost of the equipment, so it is replaced by laboratory and numerical simulations. Over the last decades, more information on laboratory and numerical simulation of NAPL has become available (Kamaruddin et al., 2011a ). ...
Article
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Over the last decades and among numerous techniques, image analysis techniques occupy a noticeable place in monitoring non-aqueous phase liquid (NAPL) migration in porous media. In recent years, photographic methods have been shown to be valuable and effective tools for measuring NAPL migration and characterization. This study aims to provide an overview of NAPL fate and behavior in subsurface systems. Furthermore, a review of recent literature published on using photographic methods in NAPL migration in one and two dimensions is summarized and presented in this paper. Besides the discussion of the research efforts, recommendations for future research in using photographic methods are provided. This study concluded that, although photographic methods have some limitations and drawbacks, photographic methods are still promising and valuable tools for measuring NAPL migration.
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