Infiltration and Distribution of Elemental Mercury DNAPL in Water-Saturated Porous Media: Experimental and Numerical Investigation

Abstract

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.

Full text

Infiltration and Distribution of Elemental Mercury DNAPL
in Water-Saturated Porous Media: Experimental
and Numerical Investigation
Andrea D’Aniello &Niels Hartog &Thomas Sweijen &
Domenico Pianese
Received: 24 July 2017 /Accepted: 26 December 2017
#Springer International Publishing AG, part of Springer Nature 2018
Abstract Liquid elemental mercury occurrence in the
subsurface as dense non-aqueous phase liquid
(DNAPL) is reported worldwide in proximity of several
industrial facilities, such as chlor-alkali plants. Insight
into Hg
0
DNAPL infiltration behavior is lacking and, to
date, there are no experimental observations of its infil-
tration and distribution in water-saturatedporous media,
except for capillary pressure-saturation column experi-
ments. To better understand the processes governing
elemental mercury DNAPL flow behavior, a series of
flow container experiments were performed using mer-
cury DNAPL (in sands and glass beads) and tetrachlo-
roethylene (PCE) (in sands). While liquid Hg
0
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 mea-
surements showed that, in glass beads, liquid Hg
0
pref-
erentially migrated towards higher porosity zones. As
for PCE, infiltration and distribution of Hg
0
DNAPL are
strongly affected by the heterogeneities within the po-
rous formation. However, compared to other DNAPLs,
liquid Hg
0
shows a strong attenuation potential of gam-
ma 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 Hg
0
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.
Keywords Elemental mercury.DNAPL .
Tetrachloroethylene (PCE) .Multiphaseflow .Numerical
modeling .Gamma radiation
1 Introduction
Mercury is recognized worldwide as a hazardous con-
taminant, and the consequences of its release into the
environment can be detrimental for human beings and
ecosystems (US EPA 2007; World Chlorine Council
2011). Mercury can be found in the environment as a
result of natural processes, like volcanic activity, fires,
movement of rivers, lakes, streams, oceanic upwelling,
and biological processes (Risher 2003). However, since
the industrial revolution, anthropogenic activities like
mercury mining (UNEP 2002), mercury amalgamation
for gold and silver mining (Lechler et al. 2000; UNEP
Water Air Soil Pollut (2018) 229:25
https://doi.org/10.1007/s11270-017-3674-0
A. D’Aniello (*):D. Pianese
University of Naples Federico II, Department of Civil,
Architectural and Environmental Engineering, via Claudio 21,
80125 Naples, Italy
e-mail: andrea.daniello@unina.it
N. Hartog
KWR Watercycle Research Institute, Groningenhaven 7,
Nieuwegein, The Netherlands
N. Hartog :T. Sweijen
Utrecht University, Department of Earth Sciences, Environmental
Hydrogeology Group, Princetonplein 9, 3584 CC Utrecht,
The Netherlands

2002; Hylander and Meili 2003), wood preservation
(Bollen et al. 2008), chlor-alkali and thermometer-
manufacturing plants (Biester et al. 2002;Hylander
and Meili 2003; Bernaus et al. 2006; Arbestain et al.
2009; Brooks and Southworth 2011;Milleretal.2013)
considerably contributed to its distribution into the en-
vironment (Risher 2003). In particular, the chlor-alkali
industry is among the principal consumers of mercury as
well as the most important producer of wastes contain-
ing mercury (Pirrone et al. 2010). In 1996, almost 40%
of the mercury world production was due to the chlor-
alkali industry (Sznopek and Goonan 2000). According
to Euro Chlor (1999), most of the chlor-alkali plants
using liquid Hg
0
cells were located in Western Europe,
and the Hg
0
consumption was about 100 tons per year.
According to Lindley (1997), almost 12,000 tons of
liquid Hg
0
was in use as electrodes in European chlor-
alkali plants. Furthermore, the fate of the 90% of liquid
Hg
0
bought by Western European chlor-alkali plants is
not reported (Euro Chlor 1999) and, most probably,
considerable amounts of liquid Hg
0
are still stored in
wastes in proximity of the chlor-alkali facilities
(Hylander and Meili 2003). According to the global
mercury demand by use category, the global chlor-
alkali mercury demand decreased from 23% in 2000 to
15% in 2005 due to the progressive reduction in the use
of mercury cells technology (Pirrone et al. 2010). How-
ever, in 2010, the chlor-alkali production based on mer-
cury cells still represented 21% of the total world capac-
ity (Pirrone et al. 2010).
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;Deebetal.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).
Depending on the site geochemistry, liquid mercury
may also undergo speciation (Leterme and Jacques
2013;González-Fernándezetal.2014; Leterme et al.
2014) and methylation (Compeau and Bartha 1985;
Gilmour et al. 1992), thus allowing for the formation
of organic and inorganic mercury compounds which
may be more toxic, mobile, and soluble than liquid
mercury itself. In addition, Hg
0
may also form in sub-
surface environments or sediments from other mercury
species (Bouffard and Amyot 2009; Richard et al.
2016). However, it may also release contaminants
through volatilization (Walvoord et al. 2008).
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;Ruthetal.2013), only few studies
(Eichholz et al. 1988; Devasena and Nambi 2010;
Sweijen et al. 2014;D'Aniello2017;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;CRC2014), liquid elemental mercury
infiltration behavior deviates with respect to more ex-
tensively studied DNAPLs, such as trichloroethylene
(TCE) and PCE, (Devasena and Nambi 2010;D’Aniello
2017;D’Aniello et al. 2018, Under Review). As a matter
of fact, mercury DNAPL behaves as a nonwetting phase
with respect to both air and water (D’Aniello 2017;
D’Aniello et al. 2018, Under Review); therefore, its
infiltration occurs only when it overcomes an entry
head, both in fully (Devasena and Nambi 2010)and
partially water-saturated porous media (D’Aniello
2017;D’Aniello et al. 208, Under Review). In particu-
lar, in fully water-saturated systems, Hg
0
has a high
capacity to infiltrate (D’Aniello 2017;D’Aniello et al.
2018, Under Review), and its infiltration behavior is
mainly governed by gravitational and capillary forces,
being practically independent of viscous forces
(Devasena and Nambi 2010). Moreover, mercury
DNAPL entrapment in fully water-saturated porous me-
dia results in much lower residual saturation than other
DNAPLs, like TCE and PCE (Devasena and Nambi
2010).
To date, except for capillary pressure-saturation col-
umn experiments (Devasena and Nambi 2010;
D’Aniello 2017;D’Aniello et al. 2018, Under Review),
there are no experimental observations of Hg
0
DNAPL
infiltration and distribution in water-saturated porous
media, where it is expected to have a high capacity to
infiltrate (D’Aniello 2017;D’Aniello et al. 2018, Under
Review). Therefore, to better understand the processes
governing elemental mercury DNAPL flow behavior in
water-saturated porous media under more realistic sce-
narios, and to compare them with those of other
DNAPLs, a series of flow container experiments in fully
water-saturated stratified sands and glass beads is here
described. To allow a quantitative analysis of Hg
0
and
PCE infiltration and distribution behavior, non-intrusive
25 Page 2 of 17 Water Air Soil Pollut (2018) 229:25

measurements of samples porosity, PCE and Hg
0
satu-
rations were performed with a dual gamma ray system.
Finally, numerical simulations were performed to assess
to what extent the conventional multiphase flow model-
ing can allow for the prediction of Hg
0
DNAPL migra-
tion in water-saturated porous media.
2MaterialsandMethods
2.1 Porous Media and Fluids
Two types of glass beads (Glaswarenfabrik Karl Hecht
2013;Retsch2013), and two different sands, namely
Filtersand 0.2–0.5 mm and Silversand S60, (FILCOM-
Sibelco Group 2015), were used. On the basis of Went-
worth classification (1922), the two types of glass beads
are classified as coarse (d
50
= 1 mm; uniform distribu-
tion) and medium (d
50
= 0.375 mm; 0.200–0.500 mm),
while the two sands as medium sands and will be
referred as to medium sand 1 (MS1) and medium sand
2(MS2). The particle size distribution of MS1 (d
50
=
0.39 mm; 0.200–0.500 mm) is wider than MS2 (d
50
=
0.26 mm; 0.125–0.355 mm). For each granular material,
information regarding intrinsic permeability (Table 1)
was obtained experimentally (D’Aniello 2017;
D’Aniello et al. 2018, Under Review) by means of the
falling head method (Bear 1972).
Demineralized water was used as the wetting fluid,
while laboratory grade (≥99%) PCE (Acros Organics)
and 99.9 + % redistilled liquid elemental mercury (Hg
0
)
(Alfa Aesar) were used as the nonwetting fluids
(Table 2).
The capillary pressure-saturation, P
c
(S), data
(Table 3) were obtained with column experiments per-
formed under stable flow behavior (D’Aniello 2017;
D’Aniello et al. 2018, Under Review).
2.2 Flow Container Experiments
Two poly-methyl methacrylate (PMMA) flow con-
tainers (Fig. 1) were used for the DNAPL infiltration
experiments. The one filled with sand was used to
observe PCE and Hg
0
DNAPL infiltration, while the
one filled with glass beads was used only for Hg
0
DNAPL. Therefore, due to the preferential wettability
of PCE to PMMA, the two front walls of the container
filled with sand were layered with a glass plate of
0.5 cm. Conversely, the two side walls, sufficiently far
from the DNAPL source, were not expected to come in
contact with PCE and were not covered with glass. For
the same reason of the front walls, the two PMMA inlet
walls, of 1 cm thickness each, were layered with a glass
plate of 1 mm. More details about the flow containers
can be found in D’Aniello (2017).
The first flow container (Fig. 1, left) was mainly
filled with MS1 and, to study the effects of macro
heterogeneities on PCE and Hg
0
migration, a rectangu-
Tabl e 1 Porous media properties
Porous medium Particle
density
(g/cm
3
)
Porosity Intrinsic
permeability
(10
−7
cm
2
)
Coarse glass beads 2.65 0.410 45.15
Medium glass
beads
2.65 0.440 9.43
Medium sand 1 2.67 0.368 1.61
Medium sand 2 2.65 0.391 1.79
Tabl e 2 Fluids properties
Parameter Unit Hg
0
PCE Water
Density g/cm
3
13.5
a
1.63
b
0.998
Dynamic viscosity 10
−3
Pa s 1.55
c
0.89
b
0.98
Surface tension Dynes/cm 485
d
32.6
e
72
Interfacial tension with water Dynes/cm 375
c
47.8
b
–
a
CRC (2014)
b
Pennel et al. (1996)
c
US DOE (2001) and references therein
d
Adamson and Gast (1997)
e
Mercer and Cohen (1990)
Tabl e 3 P
c
(S) data
Sample System Porosity α
(cm
−1
)
nEntry head
(cm of
NAPL)
Coarse GB Hg
0
-water 0.410 0.0167 5.58 2.84
Medium GB Hg
0
-water 0.440 0.0071 6.83 8.36
MS1 PCE-water 0.353 0.0517 13.41 9.90
MS2 PCE-water 0.375 0.0365 28.99 15.77
MS1 Hg
0
-water 0.354 0.0087 7.97 6.19
MS2 Hg
0
-water 0.375 0.0049 12.83 12.51
αand nare the van Genuchten-Mualem model (Mualem 1976;
van Genuchten 1980)parameters
GB glass beads
Water Air Soil Pollut (2018) 229:25 Page 3 of 17 25

lar lens of 10 cm length by 5 cm height of MS2 was
located in the middle of the container, at 9 cm from the
bottom (Fig. 1, left). The water table was fixed at 1 cm
from the top layer of MS1, located at 25 cm from the
bottom. The two inlet walls were placed 2.5 cm deep
from the top of the MS1 layer, in the middle of the
container, with a spacing of 1.5 cm between each other.
The sand in the container was packed under water-
saturated conditions to avoid any entrapped air between
the sand particles. The flow container was initially filled
halfway with water and then, after having thoroughly
mixed the sandwith water in another container, the sand
was gradually poured into it. To maintain a fully water-
saturated system, a water head over the top layer of the
sand was kept at all times. At each filling step, each sand
layer was tamped in the same manner to compact the
porous medium. To prevent accumulation of sand in the
bottom valve, a nylon filter was put in the bottom hole.
The packing procedure described above was also
used for the PCE experiment. To create visual contrast
between PCE and water, thus allowing for visual obser-
vation of PCE infiltration, the water was dyed with
Bromophenol Blue (Acros Organics), a water soluble
pH indicator. As a result, the water in the container
showed a dark blue color. The water was dyed rather
than the PCE to prevent affecting PCE properties (Ap-
pendix 5.1).
The second sample (Fig. 1, right) consisted of a
water-saturated stratified glass beads system, made of
two layers of coarse glass beads with a medium glass
beads layer of 6 cm height in between, located at 8 cm
from the bottomof the container. The packing procedure
was the same of the sand sample used for the Hg
0
DNAPL experiment, except that glass beads were
poured in the container without any additional compac-
tion besides of the one induced by their own weight. The
two inlet walls, located 2 cm deep from the top of the
upper coarse glass beads layer, had a thickness of
0.5 cm, and the spacing between the two was of 1 cm.
The water table was fixed at 1.5 cm from the top of the
upper coarse glass beads layer, located at 25 cm from the
bottom.
For each experiment, a fixed amount of DNAPL was
carefully added between the inlet walls, thus allowing
the DNAPLs to build enough head to infiltrate and
distribute in the porous samples.
2.3 Dual Gamma System: Setup, Principles,
and Measurements
The Gamma Ray Attenuation (GRA) technique
(Ferrand et al. 1986; Hofstee et al. 1998a;Hofstee
et al. 1998b; Barataud et al. 1999; Johnson et al. 1999;
Baytaşand Akbal 2002; Pires et al. 2005; Pires and
Bereira 2014) is essentially based on the interaction of
radiation with matter. Depending on how the radiation
travels through the sample and, in particular, on how the
radiation gets absorbed, it is possible to determine, in a
Fig. 1 Schematic representation of the flow container samples (dimensions in cm): with sands (left) and glass beads (right)
25 Page 4 of 17 Water Air Soil Pollut (2018) 229:25

non-invasive way, properties related to the structure of
the porous medium and fluids saturations within the
pores.
The dual gamma ray system used consists of two
radioactive sources, namely Americium 241 (
241
Am)
and Cesium 137 (
137
Cs), which emit gamma rays, and
a sodium iodide (NaI) detector (Fig. 2).
The attenuation of
241
Am and
137
Cs gamma rays,
induced by the absorption of a medium, can be de-
scribed by Beer-Lambert law:
Iδ¼Iδ
0⋅e−μδ⋅x
 ð1Þ
where δ= Am, Cs, I
0
is the incident intensity of gamma
rays, Iis the intensity of gamma rays after attenuation
through a medium of length x(path length), and μis the
linear attenuation coefficient of the material. The atten-
uation coefficient, which is dependent on the physical
density of the material, measures the probability per unit
length of a photon to be absorbed or scattered while
interacting with a sample. In the case where the sample
is shielded from the gamma radiation by the presence of
the additional thickness of a container, the incident
intensity is called background intensity and must be
measured in advance. More details regarding the dual
gamma ray measurements can be found in the Appendix
(Section 5.2).
Non-intrusive measurements of the porosity field
followed the packing of the flow container samples
(Section 2.2). These measurements were performed
over the whole samples by means of the dual gamma
ray system, on the basis of a regular grid with 1-cm
spacing. DNAPLs saturation profiles were measured
24 h after the end of the infiltration process, established
when there was no further change of DNAPL level in
the inlet.
2.4 Numerical Modeling: Hg
0
DNAPL Infiltration
and Distribution in Glass Beads
To assess to what extent numerical modeling can de-
scribe Hg
0
DNAPL infiltration and distribution in
water-saturated porous media, the glass beads flow con-
tainer experiment was simulated with the NAPL module
of the Groundwater Dynamics Analysis (GDAn) code
(D’Aniello 2017). GDAn is a two-dimensional finite
element model on unstructured triangular mesh meant
for the analysis of groundwater, multiphase flow, and
solute transport dynamics in porous media. In particular,
the NAPL module solves numerically the governing
equations of multiphase flow of immiscible fluids in
porous media (Abriola and Pinder 1985; Parker et al.
1987;Parker1989), by using the Picard iteration
scheme with an incremental solution procedure (Istok
1989). The model is based on the extended Darcy’slaw
for multiphase flow (Bear 1972); therefore, it describes
NAPLs infiltration and distribution in porous media at
the representative elementary volume scale.
The input parameters of the numerical simulations
were mostly taken from Tables 1,2,and3. In addition
to these data, the irreducible water saturation was set
to 0.05, while the maximum entrapped NAPL satu-
ration to 0.10 (D’Aniello 2017). Specific storage
coefficients were determined assuming a compress-
ibility of 4.6e−10 Pa
−1
for water and of 3.7e−11 Pa
−1
for Hg
0
(Young et al. 2012), and of 1.4e−11 Pa
−1
for
glass beads (Luo et al. 2015), and an average porosity of
0.380 based on dual gamma ray measurements. The
scaling parameter was assumed equal to the ratio be-
tween water surface tension and Hg
0
interfacial tension
with water, on the basis of Lenhard and Parker (1987)
scaling theory, resulting in a value of 0.192. To describe
the P
c
-S and k
r
-S (relative permeability-saturation) rela-
tionships between elemental mercury and water, the van
Genuchten-Mualem model (Mualem 1976;van
Genuchten 1980) was used.
To describe the variability of intrinsic permeability
with porosity and of capillary retention parameters with
intrinsic permeability, the Kozeny-Carman formula
(Kozeny 1927;Carman1938,1956; Bear 1972; Fitts
2002) and the scaled form of the generalized Leverett
function (Leverett 1941; Kueper and Frind 1991;
Dekker and Abriola 2000) were applied. In this way,
each cell of the numerical domain was set to different
values for porosity, intrinsic permeability, and van
Genuchten αparameter. The van Genuchten nparame-
ter was assumed constant within the same glass beads
type. With the information derived from the gamma ray
Fig. 2 Schematic illustration of
the dual gamma ray setup
Water Air Soil Pollut (2018) 229:25 Page 5 of 17 25

measurements, the porosity field (Fig. 7) was deduced
for the whole domain and the Kozeny-Carman formula
(Eq. 2) was used to determine the intrinsic permeability
(k)ineachcell:
k¼d2
50
SF2
ϕ3
1−ϕðÞ
2ð2Þ
with ϕthe porosity, and SF the particles shape factor,
(SF = 20.7), determined as the best fit with the data
(D’Aniello 2017). Then, the scaled form of the general-
ized Leverett function (Eq. 3) was used to determine the
van Genuchten αparameter in each cell. In particular,
for any pair ofpermeability values, the van Genuchtenα
parameter can be scaled as:
α2¼α1
k2
k1

γ
ð3Þ
where α
1
is the van Genuchten parameter and k
1
is the
intrinsic permeability of the reference samples (Tables 1
and 3), α
2
and k
2
are the values to be determined in each
cell, while γis a fitting parameter (equal to 0.5 for the
conventional Leverett function). To verify to what ex-
tent the spatial variability of intrinsic permeability plays
a significant role in the numerical prediction of Hg
0
DNAPL infiltration and distribution, the γparameter
was subjected to a sensitivity analysis, assuming the
value of 0.5 of the conventional Leverett function, and
the very large values of 0.75 and 0.95, thus considering
three different numerical scenarios.
The spatial domain was discretized in 1273 nodes
and 2413 unstructured triangular cells, suitably refined
in proximity of the DNAPL reservoir and of the top of
the medium glass beads layer. Across the whole domain,
the water hydraulic head was set to 26.5 cm, and the
boundary nodes on the top of the domain, outside the
inlet, were constrained to this value assuming a Dirichlet
boundary condition. Between the inlet walls, a volume
of 10 ml of liquid Hg
0
was injected into the domain with
an average infiltration rate of 0.099 ml/s, over a time of
101 s. On all other boundaries, a no-flux-boundary
condition was assumed for both water and elemental
mercury. The solution was advanced over time with a
variable time step ranging from 0.1 to 2 s depending on
convergence history, while convergence of the solution
was ensured by an absolute tolerance of 10
−5
m.
3 Results and Discussion
3.1 Liquid Hg
0
Attenuation Potential of Gamma Rays
Liquid Hg
0
showed a strong attenuation potential of
gamma rays if compared to demineralized water and
PCE (Table 4). As a matter of fact, for all the path
lengths considered (0.1 to 4 cm), liquid Hg
0
shielded
241
Am radiation completely.
The Americium peak practically disappears (Fig. 3)
even with 1-mm thickness of elemental mercury be-
tween the radioactive source and the detector, and only
Cesium radiation is able to pass through liquid Hg
0
.
Even though Cesium has a stronger capacity to pen-
etrate matter than Americium, liquid Hg
0
shows a re-
markably high attenuation coefficient with respect to
this gamma radiation, about 17 times higher than water
(Table 4). According to Eq. 1, this implies that 0.5 cm of
liquid Hg
0
is sufficient to attenuate Cesium radiation of
about 47%, whereas 2 cm can induce an attenuation of
92%. However, according to the thickness (4–5 cm) and
average porosity (0.360–0.380) of the porous samples
used for the Hg
0
DNAPL flow container experiments, at
100% of mercury saturation, liquid Hg
0
can reach a
maximum thickness of about 2 cm within the samples,
thus enabling the use of the Cesium source to determine
its complete range of saturation.
Furthermore, with the current dual gamma ray sys-
tem, elemental mercury saturation can be correctly de-
rived from Eq. 1only when the porous medium is dry or
fully water saturated. As a matter of fact, when a three-
phase system of air, NAPL, and water is encountered
within a porous medium, the determination of the liquid
phases saturation is usually performed by solving simul-
taneously Eq. 1for both radioactive sources, but, in the
case of liquid mercury, this is not possible because of the
complete attenuation of Americium.
Therefore, when analyzing Hg
0
DNAPL flow behav-
ior in porous media by means of non-intrusive techniques
as GRA, attention must be paid to the choice of the
Tabl e 4 Fluids attenuation coefficients
Fluid μ
Am
(cm
−1
)μ
Cs
(cm
−1
)
Demineralized water (DW) 0.186 0.075
DW + Bromophenol 0.185 0.072
PCE 0.633 0.113
Hg
0
–1.280
25 Page 6 of 17 Water Air Soil Pollut (2018) 229:25

sources of radiation. In particular, the sources should be
strong enough to penetrate liquid Hg
0
to at least some
extent, and the sample thickness should not exceed a
certain path length to allow a correct characterization of
Hg
0
DNAPL saturation range within the porous medium.
3.2 PCE and Hg
0
Flow Container Experiments
with Sands
The main difference between the two PCE and Hg
0
infiltration experiments was found in the early stage of
the infiltration process. While PCE infiltrated and dis-
tributed within the water-saturated porous sample, liquid
Hg
0
did not. When elemental mercury was poured be-
tween the inlet walls, the sand particles of the top layer
were displaced and started floating over the top of the
Hg
0
surface, due to their lack of cohesion and lower
density. More mercury was added to build a sufficient
head to overcome the sand entry head but, rather than
infiltrating in the porous sample, Hg
0
found a preferen-
tial pathway between the inlet walls and the front walls
of the container, thus spreading over the top surface of
MS1. Then the experiment was stopped. Conversely,
12 ml of PCE infiltrated over a time of 436 s, under an
initial head of 2.16 cm. Unfortunately, Bromophenol
Blue did not allow the visualization of the PCE infiltra-
tion front within the stratified porous medium; hence, no
track of it over time is available.
Elemental mercury is more prone to infiltrate than
PCE in water-saturated porous media (D’Aniello 2017;
D’Aniello et al. 2018, Under Review) as it shows lower
DNAPL entry heads (Table 3). Therefore, the reason of
such a sharp difference in the infiltration behavior ex-
hibited by the two DNAPLs in the flow container ex-
periments is not immediately clear. Most probably, the
difference in the porosity field of the two flow container
samples played a major role.
Dual gamma ray measurements of the porosity field
(Figs. 4and 5) revealed significant differences among the
two samples. The sample used for the Hg
0
flow container
experiment showed a porosity remarkably lower than the
one used for the infiltration of PCE. Such a strong differ-
ence in porosity likely resulted in a sharp difference in
intrinsic permeability, thus inducing differences in entry
head. In particular, this difference in porosity was more
pronounced in proximity of the inlet, namely where the
infiltration should have occurred. Therefore, Hg
0
did not
infiltrate because it was unable to build a sufficient head
to overcome the higher entry head induced by a locally
lower intrinsic permeability. Hence, the increase in
DNAPL pressure, following the further addition of Hg
0
,
resulted in the overcome of the resistance exerted by
water between the inlet and the front walls rather than
that of water in the sand, thus inducing the Hg
0
escape
through a preferential pathway. To prevent the experi-
ment failure, the inlet walls should have been sealed with
the front walls of the container, thus forcing liquid Hg
0
to
build a head within the inlet space. In addition, whether
the Hg
0
head was higher than the inlet walls height, the
setup could have been easily modified to allow for inlet
walls of different dimensions and shape.
The discontinuity observed in the PCE saturation
profile (Fig. 6) also suggests a dependency of the infil-
tration behavior on the porous medium structure. The
Fig. 3 Typical (gray line) and 1-
mm thickness Hg
0
sample (black
line) dual gamma ray detected
signals
Water Air Soil Pollut (2018) 229:25 Page 7 of 17 25

dual gamma ray detected the presence of PCE every-
where in the lowest part of the sample, with a maximum
saturation of 0.094, while almost no PCE was detected
in the top part. Most likely, for PCE, the effect of micro
heterogeneities was even more pronounced than macro
heterogeneities. As a result, PCE migration probably
developed under an unstable flow regime, enhanced
by the micro heterogeneities present within the sample,
thus resulting in fingers formation (Poulsen and Kueper
1992; Illangasekare 1998; Mayer and Hassanizadeh
2005). This would explain why the dual gamma ray
system was not able to perform a continuous measure-
ment of the PCE front and why PCE was found in the
finer sand. Fingers are expected to be thin, of the order
of the pore sizes; hence, the dual gamma ray beam had
locally either missed the fingers or smeared the satura-
tion value through the whole thickness of the porous
medium, thus resulting in an overall negligible mea-
sured PCE saturation in the top part of the sample.
3.3 Hg
0
Flow Container Experiment with Glass Beads
The porosity field and the elemental mercury saturation
profile of the glass beads flow container experiment are
depicted in Figs. 7and 8.
Fig. 4 PCE flow container
sample: measured porosity field
Fig. 5 Hg
0
flow container
sample: measured porosity field
25 Page 8 of 17 Water Air Soil Pollut (2018) 229:25

The final distribution of elemental mercury (Fig.
8) in the highest porosity zone (Fig. 7) shows that its
infiltration and distribution were affected by the het-
erogeneities present within the sample. The infiltra-
tion front was fast and reached the top of the medium
glass beads layer in 31 s. After 101 s, the elemental
mercury source was depleted, and the distribution
occurred. No elemental mercury was able to pene-
trate the medium glass beads layer, and a flat pool, of
about 1-cm height, formed on top of this layer. Static
equilibrium was visually observed after 112 s from
the beginning of the infiltration process. The
maximum DNAPL saturation measured at the end
of the experiment was of 0.368, where the Hg
0
pool
was formed.
3.4 Numerical Modeling of the Hg
0
Flow Container
Experiment with Glass Beads
Numerical simulations, performed with GDAn
(Section 2.4), showed an overall good agreement with
the experimental results. The main features of Hg
0
DNAPL migration, such as the magnitude of the infil-
tration time, the stretched shape of the infiltration front
Fig. 6 Measured PCE saturation
profile
Fig. 7 Glass beads flow
container sample: measured
porosity field
Water Air Soil Pollut (2018) 229:25 Page 9 of 17 25

along the vertical axis as a result of the predominance of
gravity over capillary forces, and the spreading above
the medium glass beads layer without penetrating it,
were well captured.
The inspection of Figs. 9,10,and11,wherethe
progression of the Hg
0
infiltration front is reported
over time for each permeability scenario, shows that
themoretheγparameter (Eq. 3) increases, the sharper
becomes the variation of the van Genuchten αparam-
eter with intrinsic permeability, thus resulting in a
better prediction of the infiltration and distribution
phenomena. With increasing γ, the shape of the infil-
tration front becomes less symmetric, moving to-
wards areas with higher porosity and intrinsic perme-
ability, as observed in the experiment (Fig. 8), where
Hg
0
mainly migrated in the left part of the container
once the medium glass beads layer was reached.
However, numerical simulations were not able to
perfectly replicate the experimental observations. As
a matter of fact, the simulated infiltration front was
Fig. 8 Measured Hg
0
saturation
profile in glass beads
Fig. 9 γ=0.50—Simulated Hg
0
infiltration front (Hg
0
saturation= 0.03) over time (10–
100 s) every 10 s
25 Page 10 of 17 Water Air Soil Pollut (2018) 229:25

slower than the observed one, reaching the medium
glass beads layer in about 40 s rather than 31 s, and
achieving the static equilibrium configuration in
about 180 s rather than 112 s.
Furthermore, due to the more pronounced lateral
spreading of the simulated Hg
0
infiltration front above
the medium glass beads layer, the resulting maximum
saturations (Figs. 12,13,and14) were lower than the
measured ones, ranging between 0.114 and 0.128 with γ
ranging from 0.95 to 0.5.
However, to numerically reproduce the final distri-
bution of elemental mercury observed in the experiment
(Fig. 8), it was essential to take into account the spatial
variability of both intrinsic permeability and retention
properties (Eqs. 2and 3) across the whole simulation
domain. As a matter of fact, with increasing γ,elemental
Fig. 10 γ=0.75—Simulated
Hg
0
infiltration front (Hg
0
saturation= 0.03) over time (10–
100 s) every 10 s
Fig. 11 γ=0.95—Simulated
Hg
0
infiltration front (Hg
0
saturation= 0.03) over time (10–
100 s) every 10 s
Water Air Soil Pollut (2018) 229:25 Page 11 of 17 25

mercury was found at higher saturations in the left part
of the domain, while it reached lower saturations in the
right part, where the Hg
0
pool became thinner (Figs. 12,
13,and14).
The discrepancies found between the experimental
results andthe numerical predictions are probably due to
the lack of knowledge of the Hg
0
inflow rate over time
and to the low agreement of the correlation between the
measured porosity field with the estimated intrinsic
permeability and retention properties. As a matter of
fact, considering an average value of the inflow rate
makes it practically impossible to predict the position
of the infiltration front over time, and it may lead to
substantial overestimations of the extent of Hg
0
migra-
tion. In absence of any measurement, the prediction of
the inflow rate in a specific porous system requires a
detailed knowledge of the locations of varying perme-
ability lenses in the proximity of the DNAPL source
(Kueper and Gerhard 1995). Even though the non-
intrusive measurements performed with the dual gamma
ray system allowed a detailed description of the porosity
field, the relationship of porosity with intrinsic perme-
ability, as well as with retention properties, would have
required more experimental determinations to guarantee
a more reliable estimation. The information based on
Kozeny-Carman formula and on the modified Leverett
Fig. 12 γ=0.50—Simulated
Hg
0
saturation at 180 s
Fig. 13 γ=0.75—Simulated
Hg
0
saturation at 180 s
25 Page 12 of 17 Water Air Soil Pollut (2018) 229:25

function proved to be not sufficient outside the porosity
range defined with the hydraulic conductivity and P
c
(S)
experiments on glass beads. Nevertheless, the three
scenarios showed improvements with increasing γ,thus
confirming numerical predictions dependency on the
correlation between intrinsic permeability and retention
properties, as found for organic DNAPLs by Dekker and
Abriola (2000).
4 Conclusions
As for other DNAPLs, infiltration and distribution of
Hg
0
DNAPL in water-saturated porous media are affect-
ed by the heterogeneities present within the porous
formation. In the flow container experiment with sands,
liquid Hg
0
would have required to build a higher head
than PCE to infiltrate due to the presence of a lower
permeability zone beneath the inlet location. However,
the experiment was not successful due to a defect of the
setup that allowed liquid mercury to find a preferential
pathway through the inlet walls and the front walls of
the container. As a result, liquid Hg
0
spread over the top
of the sample, and no infiltration occurred. Conversely,
the Hg
0
flow container experiment with glass beads was
successful. In this experiment, visual observations and
dual gamma ray measurements showed that liquid Hg
0
preferentially migrated towards higher porosity zones.
As porosity is in relation with intrinsic permeability,
hence with retention properties, elemental mercury
DNAPL infiltration in water-saturated porous media is
likely to occur through higher permeability lenses,
where the entry heads are expected to be lower.
Numerical simulations of the glass beads experiment
showed an overall good agreement with the experimen-
tal results as the main features of Hg
0
DNAPL infiltra-
tion and distribution in water-saturated porous media
were well captured. However, numerical simulations
were not able to perfectly replicate the experimental
observations, despite the detailed description of the
porosity field performed with the dual gamma ray sys-
tem. The knowledge of the inflow rate, the reliable
estimation of the porous formation permeability, and
the accurate representation of the correlation between
P
c
(S) curves and intrinsic permeability proved to be
critical for the prediction of liquid Hg
0
infiltration and
distribution in water-saturated porous media as for or-
ganic DNAPLs, like PCE.
However, if compared to other DNAPLs, such as
PCE, liquid Hg
0
shows a strong attenuation potential
of gamma rays. Therefore, when analyzing Hg
0
DNAPL flow behavior in porous media with non-
intrusive techniques as GRA, the sources of radiation
should be strong enough to penetrate liquid Hg
0
to at
least some extent, and the sample thickness should not
exceed a certain path length to allow a correct charac-
terization of Hg
0
DNAPL saturation range within the
porous medium.
Acknowledgements The authors wish to thank the two anony-
mous reviewers for their constructive comments which helped to
improve this manuscript.
Fig. 14 γ=0.95—Simulated
Hg
0
saturation at 180 s
Water Air Soil Pollut (2018) 229:25 Page 13 of 17 25

Funding information This research was financially supported
by the P.O.R. Campania FSE 2007/2013-2014/2020.
Appendix
PCE Experiment with Sands: Bromophenol Blue
Addition to Water
Bromophenol Blue was used in a concentration of 1 g/l,
equal to 1/4 of bromophenol solubility in water (O’Neil
2006). To avoid any presence of bromophenol granular
residuals, after several rounds of mechanical mixing, the
water was filtered using a 10-μm mesh nylon filter.
The choice of dying water rather than PCE was
dictated by the fact that oil soluble dyes like Sudan III,
Sudan IV, and Oil Red, generally used to dye PCE
(Schwille 1988; Hofstee et al. 1998a; Hofstee et al.
1998b), proved to influence PCE interfacial properties
(Tuck et al. 2003), reducing its entry pressure and af-
fecting its spreading behavior, thus enhancing its mobil-
ity, and, in the case ofSudan IV, to leave residuals on the
sediments as a result of PCE dilution (Hartog et al.
2010), thus affecting the porous medium characteristics
and, potentially, PCE flow behavior. Therefore, water
was dyed with Bromophenol Blue, generally used for
conservative tracer tests (Cirpka et al. 2006;Rockhold
et al. 2007; Chiogna 2010). Furthermore, as shown in
Table 4(Section 3.1), adding bromophenol to
demineralized water does not result in any appreciable
change in the water phase gamma ray attenuation.
Dual Gamma Ray Measurements: Additional Details
The NaI detector measures an average value, over the
beam cross section (6-mm diameter) and the path length,
of the number of arriving photons (counts) over their
energy, rather than intensities. Intensities are determined
by first summing up the count rates of the peak areas
(Fig. 3), from 40.4 to 82 keV for
241
Am, and from 623.5
to 740.9 keV for
137
Cs, and then by dividing them by the
measurement time. The measurement time was chosen
according to liquid Hg
0
high attenuation potential
(Section 3.1) and was set to 120 s (D’Aniello 2017).
The attenuation coefficients were measured by plac-
ing different amounts of cuvettes in a row, thus leading
to different thicknesses (path lengths, Eq. 1). To estab-
lish the background intensity, the cuvettes were mea-
sured empty first, then, with the aim of determining the
intensity measurements, each cuvette was filled with the
substance/material to be analyzed. Then, on the basis of
Beer-Lambert equation (Eq. 1), the attenuation coeffi-
cients were determined by means of a least square fitting
technique over the measurement points.
Polystyrene (PS) squared cuvettes, of 1-cm thickness
and 4-ml volume, were used for most of the measure-
ments, while PMMA was used for tetrachloroethylene,
due to the almost null resistance of PS to PCE. Due to
the high attenuation potential showed by liquid Hg
0
(Section 3.1), to completely investigate its behavior with
respect to gamma radiation, further measurements were
carried out for path lengths of 0.5 and 0.1 cm (1 mm).
For the half-centimeter path length, a squared cuvette, of
half-centimeter thickness, was used. Instead, for the 1-
mm path length, the desired Hg
0
thickness was obtained
by joining together three square glass plates, of 1-mm
thickness each, and creating a circular hole of 2-cm
diameter into the middle plate. The hole was connected
to the top by a channel of 2-mm width to allow the
filling with Hg
0
, after which the top port was sealed with
super glue. More details regarding the dual gamma ray
setup, its calibration, and operating principles can be
found in D’Aniello (2017).
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