Non-aqueous-phase liquid (NAPL) contaminants introduced into the unsaturated zone spread as a liquid phase;
however, they can also vaporize and migrate in a gaseous state. Vapor plumes
migrate easily and thus pose a potential threat to underlying aquifers.
Large-scale column experiments were performed to quantify partitioning
processes responsible for the retardation of carbon disulfide (CS
Subsurface contamination is a major concern in industrialized as well as in
developing and emerging countries. Non-aqueous-phase liquid (NAPL) contaminants introduced into the
unsaturated zone spread as a liquid phase; however, they can also vaporize
and migrate in a gaseous state. In particular, vapor (gas) plumes migrate
easily in the unsaturated zone
Carbon disulfide (CS
Experimental studies
The component- and porous-medium-dependent behavior of gas-phase retention
emphasizes the need for a thorough investigation into retardation of carbon
disulfide (CS
The experiments were conducted in vertical, stainless steel columns of 2 m
length packed with two different types of porous media
(Fig.
Flowchart of the vapor-retardation experiment showing column, injection, and saturation/drainage setup.
The ports along the column at a distance of 25 cm allowed for the installation of tensiometers to monitor capillary pressures. At the bottom of the column, the injection section with a base plate was installed. Into this base plate, a porous plate made of recrystallized silicon carbide was glued to act as a suction plate for the water drainage. The bottom of the column was realized as a constant-mass-flux boundary while the top was open to the surroundings, hence, at constant pressure.
Two different types of porous media (Table
Their grain-size distributions as well as capillary pressure–water
saturation relationships are shown in Fig.
The columns were packed by dry pluviation using a sand rainer
The injection section at the bottom of the column allowed for the injection
of a gas-mixture slug at a predefined mass flux and, in addition, for a
controlled upward flow stabilizing the vapor front. The CS
Characteristic properties of the porous media used for the experiments.
Physicochemical properties of contaminant carbon disulfide (CS
Grains-size distribution, capillary pressure–saturation relationship, and relative permeabilities (for the wetting phase) of materials used in experiments.
Scanning electron microscopy images of fine glass beads (left panel)
and Geba fine sand (right panel) at a scale of
100
Table
In the case of moist experiments, the gas-mixture slug was humidified with ultra-pure water (RH of 100 %) to avoid a drying up of the moist porous medium. For the preparation of the gas-mixture slug, a custom-built miniature vaporizer (ICTV; University of Stuttgart, Germany) with an ultra-low volume pump (M6; VICI AG International, Schenkon, Switzerland) was used. The nitrogen used for the chase was bubbled through a gas scrubber filled with ultra-pure water. The inlet steel capillary loop (length of 4 m) and the scrubber were placed in a temperature-controlled water bath (Ministat 125, Huber Kältemaschinenbau GmbH, Germany) to minimize temperature-induced fluctuations during the experiments. The mass balance was closed based on the flow rate measured at the inflow, and the injection and effluent concentrations.
In the column outflow, CS
Various experiment series were conducted in two different porous media (fine
glass beads and Geba fine sand) under both dry and partially saturated
(moist) conditions. Within each series, the columns were not repacked and no
saturation-and-drainage cycle (SD) was carried out since first tests proved
that the partitioning processes were fully reversible. The water saturation
or total amount of water was monitored throughout the experiment. The slug of
the gas mixture was injected with a predefined mass flux into the bottom of
the 2 m long column such that it resulted in the designated seepage velocity.
In each series, experiments were performed with different velocities
including 25, 50, 100, and 200 cm h
The experiments were conducted in four steps. In the first step, the flow rates (slug and chase) were adjusted to match the target seepage velocity. In the second step, the column was flushed with nitrogen. While maintaining constant flux, the inflow was switched to the slug injection of the gas mixture in the third step. After injecting 3.5 PV, it was switched back to the nitrogen chase (fourth step).
The quantification of retardation was based on gas concentration measurements
of CS
TMA was applied to obtain transport parameters (seepage velocity,
Eq.
Retardation of a component can be estimated using analytical solutions based
on experimental parameters and component-dependent coefficients for the
different partitioning domains.
The coefficient
Column experiments were conducted with dry and moist porous media to
characterize retardation of CS
Experimental conditions of vapor-retardation experiments in fine glass beads and Geba fine sand in dry and moist conditions (series).
Several series of experiments were performed in each porous medium to
quantify retardation. Series 1 refers to the experiments conducted in dry
porous media while Series 2 to 4 refer to the experiments in moist
conditions. A saturation-and-drainage cycle was performed prior to each moist
series. A slug of 3.5 PV of the gas mixture was injected ensuring,
even for high flow rates, a sufficient residence time to reach equilibrium in
the 2 m long column. Different velocities (25, 50, 100, and
200 cm h
The moist porous medium required for this investigation was obtained by
saturation and subsequent drainage via a suction plate. The capillary
pressure was measured with tensiometers installed at the column ports to
derive water saturations along the column
(
Figure
Initial water saturation.
Unfortunately, no tensiometer measurement data were available for Series 2 and 3
in fine glass beads. Thus, the missing profiles have to be considered an
element of uncertainty when evaluating retardation of CS
A constant water-saturation profile in the porous medium was ensured by the humidification of all gases (gas-mixture slug and nitrogen chase) prior to injection. This was confirmed by a continuous weight measurement of the entire column throughout all experiments conducted within a series. Hence, the initial water-saturation profile could be maintained during the experiments.
Breakthrough curves of CS
Different migration velocities were applied to study their impact on the
transport of argon and CS
Dispersion coefficients were determined from temporal-moment analysis (see Sect.
Theoretical and experimental effective binary diffusion
coefficient
Breakthrough curves of CS
The experimental coefficients
The dispersivity
Different series of experiments were conducted to quantify retardation of
CS
Breakthrough curves of CS
The BTCs of argon showed excellent reproducibility in repetition experiments
in both materials with the same conditions (
The BTCs of CS
The retardation coefficients of CS
Retardation coefficients of CS
In fine glass beads, a nonlinear increase in the retardation coefficient
from
In Geba fine sand, higher retardation coefficients compared to fine glass
beads were measured in the experiments. These ranged between
An evaluation of the processes responsible for retardation is possible when
utilizing the analytical solution (Eq.
Adsorption to the solid phase is governed by the partitioning
coefficient
Adsorption on the air–water interface in a partially water-saturated porous
medium depends on the air–water partitioning coefficient
The theoretical retardation coefficient (Eq.
Attributing the observed discrepancy in Geba fine sand to air–water
interfacial adsorption of CS
The lower graph in Fig.
Mass balance analyses were performed to obtain mass recovery (
The results discussed above excluded Series 3 in Geba fine sand. Series 3
referred to the second saturation and drainage cycle which was carried out to
establish a static water saturation different than in Series 2. However,
significant CS
A direct comparison with reported degradation rate constants determined from batch experiments could not be achieved due to the only availability of effluent gas concentrations in our experiments. Nonetheless, they showed that biodegradation may have a considerable potential for mitigating the contaminant mass transfer by vapor migration to the underlying aquifer, provided that favorable conditions for the specific microbes can be ensured, for instance, via soil venting. A detailed investigation of biodegradation was beyond the scope of this work but should be addressed in future research.
The retardation of CS The experimental retardation coefficients were compared to an analytical
solution considering accumulation in the gas phase, partitioning to the
aqueous phase, and adsorption to the air–water interface. Adsorption to the
solid phase was neglected due to negligible fractions of organic matter in
the porous media used. The analytic solution compared very well with the
experimental results in fine glass beads, identifying dissolution as the main
contribution to retardation. However, it underpredicted retardation in Geba
fine sand. The discrepancy was ascribed to an increased relative contribution
of air–water interfacial adsorption in Geba fine sand as a result of a
significant underestimation of the interfacial area. They were estimated
using a correlation derived from microtomography measurements of glass beads
and natural soils and utilizing the smooth-sphere assumption
Clear evidence of the biodegradation of CS
The experimental data used to produce the results and graphs presented in
this paper are available at
The Chapman–Enskog formula is used to estimate the binary diffusion
coefficient of component
Porous media affect diffusion of gases since space is occupied by grains and
possibly by additional fluid phases. Therefore, Fick's law is often modified
by the factor
Flow of fluids in a porous medium may vary significantly on a microscale due
to the velocity field in pores, irregularities of the pore size, flow
restrictions, or dead-end pores resulting in additional spreading denoted as
dispersion. These influences have to be taken into account in analytical or
numerical solutions of flow in porous media. This is done by introducing the
longitudinal dispersion coefficient
The measured BTC data had to be prepared to allow for
the usage of the TMA generally applied to
responses from dirac input. The breakthrough curves of the step-input
boundary condition (1) were transformed to a dirac-input boundary condition (2)
Tables
Experimental conditions of vapor-retardation experiments in fine
glass beads: series, experiment, theoretical seepage velocity, injection
duration, and injected mass and recovery of CS
Experimental conditions of vapor-retardation experiments in Geba
fine sand: series, experiment, theoretical seepage velocity, injection
duration, and injected mass and recovery of CS
Simon M. Kleinknecht designed and conducted this experimental study. Holger Class and Jürgen Braun were responsible for the scientific and experimental supervision. Simon M. Kleinknecht prepared the paper with contributions from both co-authors.
The authors declare that they have no conflict of interest.
The authors thank the reviewers whose insightful comments and suggestions improved the manuscript. Edited by: B. Berkowitz Reviewed by: two anonymous referees