ArticlePDF Available

Numerical simulations of the flow and transport of dense non-aqueous phase liquids (DNAPLs) in naturally heterogeneous porous media

Authors:
Janet Whittaker
Janet Whittaker
Janet Whittaker
  • This person is not on ResearchGate, or hasn't claimed this research yet.
Peter Grathwohl at University of Tuebingen
Peter Grathwohl
Georg Teutsch
Georg Teutsch
Georg Teutsch
  • This person is not on ResearchGate, or hasn't claimed this research yet.
Edward Sudicky at Aquanty
Edward Sudicky
Download full-text PDF
Read full-text
Download citation

Abstract

The migration and fate of a dense non-aqueous phase liquid (DNAPL) in heterogeneous sand and gravel deposits is simulated using the compositional program CompFlow. Spatial distributions of lithofacies mapped in outcrops are the source of hydraulic conductivity fields, thus closely resembling natural heterogeneity. The simulations illustrate the role of the geological structures and their hydrogeological properties in determining the distribution and saturation of a DNAPL following its release, and hence its subsequent dissolution.
Content uploaded by Peter Grathwohl
Author content
All content in this area was uploaded by Peter Grathwohl on Mar 02, 2015
Content may be subject to copyright.
Groundwater Quality: Remediation and Protection (Proceedings of the GQ'98 Conference held at
Tubingen, Germany, September 1998). IAHS Publ. no. 250, 1998.
Numerical simulations of the flow and transport
of dense non-aqueous phase liquids (DNAPLs) in
naturally heterogeneous porous media
JANET WHITTAKER, PETER GRATHWOHL,
GEORG TEUTSCH
Department of Applied
Geology,
Geological
Institute,
University
of
Tubingen,
Sigwartstrasse 10, D-72076
Tubingen,
Germany
EDWARD SUDICKY
Waterloo Centre for
Groundwater
Research,
University
of
Waterloo,
Waterloo, Ontario,
Canada
N2L 3G1
Abstract The migration and fate of a dense non-aqueous phase liquid
(DNAPL) in heterogeneous sand and gravel deposits is simulated using the
compositional program CompFlow. Spatial distributions of lithofacies
mapped in outcrops are the source of hydraulic conductivity fields, thus
closely resembling natural heterogeneity. The simulations illustrate the role
of the geological structures and their hydrogeological properties in
determining the distribution and saturation of a DNAPL following its
release, and hence its subsequent dissolution.
INTRODUCTION
Many groundwater contaminants originating from industrial sites, waste disposal
sites and accidental spillage have been released into the subsurface as dense non-
aqueous phase liquids (DNAPLs). The subsurface flow and transport of such
contaminants is highly sensitive to spatial variability in hydraulic properties. In the
initial stages after a release, a DNAPL migrates in its own phase under the influence
of gravity and capillary forces. As the DNAPL travels through the subsurface, a
residual saturation remains trapped in pores, and pools form above regions of low
permeability. Regions of high permeability may constitute fast pathways or, if entry
pressures of surrounding regions are much lower, areas where DNAPLs can
accumulate at saturations much higher than the residual saturation. Dissolution of a
compound from the DNAPL phase takes place over a much longer time scale. Mass
transfer rates have been shown to depend, for example, on the water and DNAPL
saturations, the local water velocity and the mean grain size (e.g. Mayer & Miller,
1996),
thus they are highly influenced by heterogeneities in the hydrogeological
properties and the location of preferential flow pathways.
Both (a) the location and saturation of the non aqueous phase and (b) the
dissolution rates from the DNAPL have important implications for the cleanup of
contaminated sites: the rate at which the processes occur determines the efficiency
and duration of remediation efforts. However, the prediction of the fate of DNAPLs
in the subsurface is difficult due to the challenges of parameter identification in real
aquifer systems.
Several studies have used Gaussian simulations of heterogeneous conductivity
fields as a basis for simulations of DNAPL flow and transport, however such
Numerical simulations of the flow and transport ofDNAPLs in naturally heterogeneous media 191
statistically-generated conductivity fields may not necessarily be able to represent
features of natural geological deposits such as abrupt transitions in properties or
patterns of connectivity of extreme values (Koltermann & Gorelick, 1996).
Therefore, there is also a need to study the flow and transport of DNAPLs using data
from naturally heterogeneous porous media.
Outcrops, as analogues of aquifers deposited in similar environments, are an
(a)
Om
Om 11.5 m
2.5 m
Hydraulic conductivity (K ms'1
-10.0 -9.0 -8.0 -7.0 -6.0 -5.0 -4.0 -3.0 -2.0
(b)
Groundwater flowlines before spi Flow direction
(c)
TCE saturation (s)
(d) 0.00 0.06 0.12 0.18 0.24 0.30 0.36 0.42
>0.48
i. •..».>£ : \ti
1*-.
rV'-f.
->>
TCE molar fraction in water (X) log (X/Xs "gPf
-6.00 -5.25 -4.50 -3.75 -3.00 -2.25 -1.50 -0.75 -0.01
Fig. 1 Numerical simulation of a TCE spill in heterogeneous sand and gravel
deposits: (a) hydraulic conductivity distribution, (b) groundwater flow lines before
the spill, (c) TCE saturation after 30 days, (d) TCE molar fraction in water after
30 days.
192 Janet VMttaker et al.
extensive source of data: mapping of lithofacies provides two dimensional
information on geometries and spatial characteristics. Material is also easily
accessible for sampling and laboratory analysis of hydrogeochemical properties.
Thus comprehensive, realistic data sets may be formed at a level of detail not
possible for a subsurface investigation.
METHOD
The release of a DNAPL and its migration in highly heterogeneous sand and gravel
aquifers are simulated for data sets derived from outcrop studies using CompFlow
(Unger et al, 1995), a comprehensive compositional modelling system. Flow, mass
transfer and transport processes are considered; solutions for phase pressures and
contaminant species molar fractions are found using robust numerical techniques. As
an illustration, a spill of trichloroethene (TCE) is simulated for a two dimensional
section measuring 11.5 x 2.5 m, with a computational cell size of 5 x 5 cm. The
hydraulic conductivity of the section is very heterogeneous due to the presence of
lenses of well-sorted gravels in otherwise poorly-sorted sands and gravels (Fig. 1(a)).
The lenses determine the pattern of groundwater flow: Fig. 1(b) shows the flow lines
corresponding to fully-saturated groundwater flow from left to right (the base is
assumed to be impermeable). The TCE is introduced into the porous medium at the
top boundary at a volumetric flow rate of 10
1
day"1 over an area of 0.5 m2 (both per
metre width of aquifer). The average groundwater velocity is approximately
0.2 m day"1.
RESULTS
The TCE saturation and molar fraction in water 30 days after the start of the TCE
release are displayed in Figs 1(c) and 1(d), respectively. The high permeability
lenses cause, in the first instance, a faster spreading of the TCE, but are later areas
where the TCE accumulates at higher saturations (up to 90%) due to the higher pore
entry pressures needed for the TCE to penetrate further into the surrounding lower
permeability sands and gravels. On reaching the base, a pool of TCE forms, such
that the non-aqueous phase occupies around 30% of the pore space. These regions,
where TCE is trapped at high saturations, have low water velocities, meaning that
dissolution rates are slow and the TCE is very persistent. In Fig. 1(d) the dissolved
TCE is seen to be transported along the water flow paths, leading to a much greater
mixing over the depth of the aquifer (i.e. a wider plume) than would be expected
under homogeneous conditions.
CONCLUSIONS
Outcrop information of heterogeneous sand and gravel deposits has been used for the
simulation of flow and transport of a DNAPL in the subsurface. Such detailed data
sets allow the investigation of the influence of geological structures and their
Numerical simulations of the flow and transport ofDNAPLs in naturally heterogeneous media 193
hydrogeological properties on the migration and long-term dissolution of DNAPLs.
REFERENCES
Koltermann, C. E. & Gorelick, S. M. (1996) Heterogeneity in sedimentary deposits: A review of structure-imitating,
process-imitating, and descriptive approaches.
Wat.
Resour.
Res. 32(9), 2617-2658.
Mayer, A. S. & Miller, C. T. (1996) The influence of mass transfer characteristics and porous media heterogeneity on
nonaqueous phase dissolution.
Wat.
Resour.
Res. 32(6), 1551-1567.
Unger, A. 1. A., Sudicky, E. A. & Forsyth, P. E. (1995) Mechanisms controlling air sparging for remediation of
heterogeneous formations contaminated by dense non-aqueous phase liquids.
Wat.
Resour. Res. 31(8), 1913-1925.
... For illustration, Figure 1 shows the computer simulation of a DNAPL (dense non-aqueous phase liquids) spill (TCE) on top of a heterogeneous sand and gravel aquifer ( Figure 1a) (Whittaker et al. 1998). Based on a realistic hydraulic conductivity distribution from outcrop data (Klingbeil et al. 1999), the spill is simulated (shown in Figure 1b) using a multi-phase numerical simulator (Unger et al. 1995). ...
... As seen in Figure 1c, the Figure 1. Computer simulation of a DNAPL spill in a heterogeneous aquifer (Whittaker et al. 1998) distribution of the DNAPL in the subsurface is strongly controlled by the subsurface heterogeneity. 'Pools' develop at the interfaces between high and low conductivity layers (e.g. the base of gravel lenses). ...
Source Remediation vs. Plume Management: Critical Factors Affecting the Cost-Efficiency
Conference Paper
Full-text available
  • Jan 2001
  • Land Contam Reclamat
This summary paper reviews just some of the extensive scientific literature from the past 20 years on the various aspects of contaminant source remediation and plume management. Some of the major findings of the numerous research projects are presented. The first section outlines the controlling mass transfer mechanisms which need to be considered for any in situ source remediation technology. It then outlines the innovative technologies, such as the application of surfactants, use of co-solvents, thermal technologies or bio-enhancement, which try to overcome some of the physico-chemical difficulties for in situ source remediation. In the second part of the paper, various plume management approaches are discussed as an alternative to in situ source remediation. Due to the longevity of the contaminant emission, the long-term cost-effectiveness of the various plume management approaches becomes the key issue. Therefore, a combination of physico-chemical and economical modelling is needed to provide a quantitative basis for decision-making. An example of a combined modelling approach for a reactive barrier is given. Finally, the various factors in the determination of whether source remediation or plume management is likely to be a preferable, i.e. more cost-effective, option, are summarised.
View
Mechanisms Controlling Vacuum Extraction Coupled With Air Sparging for Remediation of Heterogeneous Formations Contaminated by Dense Nonaqueous Phase Liquids
Article
Full-text available
  • Aug 1995
The numerical model CompFlow is used to study the mechanisms controlling vacuum extraction, coupled with air sparging, as a means for remediation of heterogeneous formations contaminated with dense nonaqueous phase liquids (DNAPLs). Two dominant mechanisms are demonstrated to control this remediation technology. First, at early times, the gas phase directly contacts the DNAPL, particularly in the unsaturated zone, causing relatively rapid transfer of contaminant from the nonaqueous phase to the gas phase and subsequent removal by the vacuum extractor. Second, at later times, remediation is controlled by the transfer of contaminant from the nonaqueous phase to the aqueous phase below the water table. During this time the vacuum extractor pumps both liquid and vaporized water in the aqueous and gas phases. This causes the contaminant that is dissolved in the aqueous phase to migrate vertically upward across the permeability layers toward the vacuum extractor where it is removed. This intermediate to late time removal mechanism is shown to be controlled by contaminant dissolution, which is a slower transfer process than the direct DNAPL vaporization that occurs at early time. Our analysis indicates that as long as both air and water are actively flushed through the DNAPL zone, both early-time vaporization and intermediate- to late-time dissolution are effective mechanisms leading to the removal of the DNAPL. We show that it may be possible to design the remedial system so as to reduce its performance sensitivity to geologic heterogeneity. A lack of sensitivity of a remedial design to heterogeneity is highly desirable because a robust design implies that the degree of site characterization required for reasonable success will be less than that needed for a less robust scheme.
View
Efficiency of air sparging for remediation of heterogeneous formations contaminated by dense non­ aqueous phase liquids
Article
Full-text available
  • Jan 1995
A numerical model is used to study the efficiency of air sparging, coupled with vacuum extraction, as a means of remediating heterogenous formations contaminated by dense non-aqueous phase liquids. Two dominant mechanisms were previously demonstrated to control this remediation technology. First, at early times, the gas phase directly contacts the DNAPL, particularly in the unsaturated zone, causing rapid vaporization of contaminant and subsequent removal by the vacuum extractor. Second, at later times, pumping of liquid and vaporiz­ ed water by the vacuum extractor effectively acts as a vertical pump-and- treat technology. This late-time removal mechanism is shown to be controlled by contaminant dissolution which is a slower transfer process than direct DNAPL vaporization that occurs at early time. The efficiency of this technology is examined by comparing its contaminant mass removal rates with that by other remediation technologies. In particular, dewatering combined with vacuum extraction as well as water flooding combined with pump-and-treat are used as remediation alternatives. The former technology is chosen to test whether enlarging the unsaturated zone improves the contact between the gas and non-aqueous phases. This should then extend the operation time of the early-time mechanism controlling air sparging and possibly enhance the removal efficiency. The latter technology is chosen to determine whether the second late-time mechanism controlling air sparging can be more efficiently and economically implemented using a traditional pump and treat approach.
View
Heterogeneity in Sedimentary Deposits: A Review of Structure-Imitating, Process-Imitating, and Descriptive Approaches
Article
Full-text available
  • Apr 1996
Numerical models that solve governing equations for subsurface fluid flow and transport are commonly applied to analyze quantitatively the effects of heterogeneity. These models require maps of spatially variable hydraulic properties. Because complete three-dimensional information about hydraulic properties is never obtainable, numerous methods have been developed to interpolate between data values and use geologic, hydrogeologic, and geophysical information to create images of aquifer properties. Image creation approaches fall into three general categories: structure-imitating, process-imitating, and descriptive. Structure-imitating methods rely on one or more of the following to constrain the geometry of spatial patterns in geologic media: correlated random fields, probabilistic rules, and deterministic constraints developed from facies relations. Structure-imitating methods include spatial statistical algorithms and geologically based sedimentation pattern-matching approaches. Process-imitating models include aquifer model calibration methods and geologic process models. Aquifer model calibration methods use governing equations for subsurface fluid flow and transport to relate hydraulic properties to heads and solute information through history and steady state data matching. Geologic process models combine fundamental laws of conservation of mass and momentum with sediment transport equations to simulate spatial patterns in grain size distributions. At the sedimentary basin scale, multiprocess models include thermomechanical mechanisms of basin subsidence. Descriptive methods couple geologic observations with facies relations to divide an aquifer into zones of characteristic hydraulic properties. All approaches are capable of reproducing heterogeneity over a range of scales and considering some types of geologic information. Some approaches are strictly spatial while some are linked to the time evolution of sedimentation. Some approaches can be conditioned on measurements. Recent advances aimed at infusing geologic information into images of the subsurface include extracting more information from sedimentological facies models, incorporating qualitative geologic information into random field generators and simulating depositional processes. Classes of research missing from the literature include multiprocess models that incorporate diagenesis and three-dimensional surface water flow, hybrid methods that combine features of existing approaches, and approaches that can make use of all available geologic, geophysical, and hydrologic data. Contents
View
The influence of mass transfer characteristics and porous media heterogeneity on nonaqueous phase dissolution
Article
A two-dimensional multiphase flow and species transport model was developed and applied to the case of nonaqueous phase liquid (NAPL) emplacement and dissolution in both homogeneous and heterogeneous porous media systems. Simulations were performed to observe dissolution rate variations and the degree of NAPL-aqueous phase nonequilibrium as a function of two aqueous phase velocities and five forms of the NAPL-aqueous phase mass transfer formulation. An integrated form of the Damkohler number was introduced to analyze the degree of NAPL-aqueous phase nonequilibrium. Mass removal rates for homogeneous media were insensitive to the form of the NAPL-aqueous phase mass transfer formulation, yielding results similar to a local equilibrium approach for all but one mass transfer formulation. This latter formulation was most sensitive to NAPL saturation and yielded significant nonequilibrium behavior, which was manifested as a decrease in NAPL dissolution rates as the NAPL volume fraction decreased. Variations in mass elution rates between homogeneous and heterogeneous media were observed, with more significant variations found for variances in porous media properties than for horizontal correlation lengths. In heterogeneous media, decreases in dissolution rates were attributed to the existence of relatively immobile regions of NAPL with saturations greater than the residual saturation of the media, so-called NAPL pools. These results illustrate the importance of the statistical characteristics of heterogeneous porous media on NAPL distribution and dissolution processes.
View