The transport behaviour of elemental mercury DNAPL in
saturated porous media: Analysis of ﬁeld observations and
two-phase ﬂow modelling
⁎, Niels Hartog
, Annemieke Marsman
, Thomas J.S. Keijzer
Deltares, Princetonlaan 6, 3584 CB Utrecht, The Netherlands
Utrecht University, Department of Earth Sciences, Environmental Hydrogeology Group, Budapestlaan 4, 3584 CD Utrecht, The Netherlands
KWR Watercycle Research Institute, Nieuwegein, The Netherlands
Philips Innovation Services, Eindhoven, The Netherlands
article info abstract
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.
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) 24–34
⁎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).
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
(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
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 water–sand 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 DNAPL–water 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
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) 24–34
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
Summary of fluid properties of mercury DNAPL, PCE and water at 20° to 25 °C
Parameter Unit Mercury HgO PCE Water
Surface tension Dynes·cm
Interfacial tension with water Dynes·cm
Vapour-pressure Pa 0.07
5.3 · 10
2.00 · 10
U.S. DOE (2001) and references therein.
At 10 °C.
At 10 °C, Sanemasa (1975).
Wallschläger (1996) as referred in Schroeder and Munthe (1997).
Soil properties used in this study.
Parameter Unit Sand Silty sand Silty clay Medium sand Fine sand Crozier loam
Hydraulic conductivity cm·h
120 80 0.21
van Genuchten (α)cm
van Genuchten (n) 2.68
Irreducible water content 0.045
saturation 0.10 0.10 0.10 0.08
Residual PCE saturation –– – 0.275
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
Carsel and Parrish (1988).
Devasena and Nambi (2010).
Ippisch et al. (2006).
Busby et al. (1995).
26 T. Sweijen et al. / Journal of Contaminant Hydrology 161 (2014) 24–34
) 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,
denotes the entry-pressure induced by capillary
pressure between two fluids within a pore throat [Pa], σdenotes
the interfacial tension with water [N·m
]. Next, assume no
flow and a hydrostatic fluid pressure, such that P
gis the gravitational constant [m·s
], ρthe density [kg m
and hthe entry head [m]. Rewrite Eq. (1):
Using the parameters from Table 1, Eq. (2) yields that
the required entry-heads for mercury and PCE are similar
). 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 ﬂow model
To model the two-phase flow processes of mercury DNAPL–
water and PCE–water 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
Newton–Raphson iteration method, including a convergence
criterion of 10
, 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 fluid–fluid 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 ﬁeld-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.
Fitting results for semi-variograms.
Cone-pressure Mercury DNAPL
Sill 0.9166 20.80 0.00125
Range [m] 0.1003 1.097 0.07423
Nugget effect 3.39·10
28 T. Sweijen et al. / Journal of Contaminant Hydrology 161 (2014) 24–34
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
of surface area, with a volume of 0.24 m
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
DNAPL of either PCE or mercury DNAPL, over a 2 m
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
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
spill and 70 h: A) mercury DNAPL and B) PCE.
29T. Sweijen et al. / Journal of Contaminant Hydrology 161 (2014) 24–34
3.1. Characterization and modelling of mercury DNAPL at a
3.1.1. Image-derived mercury DNAPL morphology and vertical
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 (2–3mm
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 (1–8%; 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
surface area. The average mercury DNAPL content for this 9 m
investigated sediment profile, was therefore 1.4 L m
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
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
mercury DNAPL after 10 h and B) 1.48 m
PCE after 10 h.
30 T. Sweijen et al. / Journal of Contaminant Hydrology 161 (2014) 24–34
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,
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 ﬁeld-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
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
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
3.2.1. Mercury transport in saturated homogeneous
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
that of mercury (1.36 m
) 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
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
). This caused the infiltration depth to decrease
substantially, while increasing the horizontal variance from
to 3.55 m
. 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) 24–34
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
Pa·s to 0.89 · 10
Pa·s) the infiltration
depth did not change substantially, although entrapment
occured slightly faster (Fig. 4). The horizontal variance
decreased from 1.37 m
to 0.92 m
. 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
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
which was substantially larger than for PCE (3.40 m
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 ﬁeld 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
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 (water–air–mercury) 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) 24–34
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
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
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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