United States R.S. Publication: 9355.4-16FS
Environmental Environmental and Emergency EPA/540/F-94/049
Protection Agency Research Laboratory Response PB94-963317
September 1994
xvEPA DNAPL SITE CHARACTERIZATION
Office of Emergency and Remedial Response
Hazardous Site Control Division (5203G) Quick Reference Fact Sheet
INTRODUCTION
Dense nonaqueous phase liquids (DNAPLs), such as some chlorinated solvents, coal tar wastes, creosote based wood-
treating oils, and some pesticides, are immiscible fluids with a density greater than water. As a result of widespread
production, transportation, use, and disposal of hazardous DNAPLs, particularly since 1940, there are numerous
DNAPL contamination sites in North America. The potential for significant long-term groundwater contamination by
DNAPL chemicals at many sites is high due to their toxicity, limited solubility (but much higher than drinking water
limits), and significant migration potential in soil gas, groundwater, and/or as a separate phase liquid. DNAPL
chemicals, particularly chlorinated solvents, are among the most prevalent groundwater contaminants identified in
groundwater supplies and at waste disposal sites. For these and other reasons, it is important to investigate for DNAPLs
(Table 1).
Table 1. Why Investigate DNAPL?
To facilitate early removal action (e.g., if the DNAPL zone is still expanding)
To adequately account for it in performing a risk assessment
To evaluate possible cosolvency effects (e.g., NAPL mobilizing immobile chemicals)
Because DNAPL transport principles ana properties differ from those associated with solutes
To adequately characterize the source area
To characterize mass loadings and mass-in-place (e.g., most mass will be in the DNAPL)
To adequately account for the dissolution of DNAPL (e.g., can persist for decades; effluent changes over
time leaving behind a residual that may be more difficult to remediate)
To avoid spreading contamination via characterization and/or remediation
INVESTIGATION STRATEGY
Site characterization, a process following the scientific method, is performed in phases (Figure 1). During the initial
phase, a hypothesis or conceptual model of chemical presence, transport and fate is formulated using available site
information and an understanding of the processes that control chemical distribution. Based on the initial hypothesis,
a data collection program is designed in the second phase to test and improve the site conceptual model and thereby
facilitate risk and remedy assessment. After analyzing the newly acquired data within the context of the initial
conceptual model, an iterative step of refining the hypothesis is performed using the results of the analysis, and
additional data may be collected. As knowledge of the site increases and becomes more complex, the working hypothesis
may take the form of either a numerical or analytical model. Data collection continues until the hypothesis is proven
sufficiently. During implementation of a remedy, the subsurface system is stressed. This provides an opportunity to
monitor and not only learn about the effectiveness of the remediation, but to learn more about the subsurface. Therefore,
remediation (especially pilot studies) should be considered part of site characterization, yielding data that may allow
improvements to be made in the conduct of remediation.
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NOTE:
Characterization should bt conducted in a
phased, evolutionary manner starting with
review of available data forty fitU work
should focus on areas beyond the DNAPL
zone and use nontnvastve methods in the
DNAPL zone. Each characterization
activity should be designed to test the
conceptual model in a manner that will
increase (At capacity to per/arm ris*
and remedy analysis.
To limit (he potential far promoting
contaminant migration, avoid, (t) conducting
unecessary field work- (2) driUtna through
capillary 6amers btnealh DNAPL: (3) pumping
from beneath DNAPL tones: and. (4) using
invasive characterization or remediation
methods without due consideration /or the
potential consequences.
REVIEW EXISTING DATA
(CHAPTER 7)
Industry type and processes used
Documented DM or aaposal ol ONAPL
AvofeMe lite, total, or ream*
invettioabon reports
Corporate/Choi records
Government records
Unmmly. library, historical society records
Interviews with key personnel
Aerial photographs
UNDERSTANDING OF THE DNAPL PROBLEM,
DNAPL AND MEDIA PROPERTIES, AND
TRANSPORT PROCESSES
(CHAPTERS 3. 4. t 5)
| DEVELOP INITIAL CONCEPTUAL MODEL
1
SITE CHARACTERIZATION ACTIVITIES
NONINVASIYE METHODS INVASIVE METHODS LABORATORY METHODS
(CHAPTER 8) (CHAPTER 9) (CHAPTER 10)
Surface geophysics
Soil gos anoJvM
(CHAPTER 9)
Test pits
Borinas
«MI
rtyoVautc tests
DELINEATE
DNAPL
SOURCE
AREAS
EXAMINE
SUSPECT AREAS
now drains and
Catch bows
Pit*, ponds. baooM
Septic tank!
leach Mai
Franc! drain.
Sews
USTs
ttttveo/tuM ion
•mical itoreai <
Chemkal i-amler i
•one storoae areas
Discolored soils
OsriMtad earth
DatyrterJ kw-ryiea. araai
DETERMINE
DNAPL
ZONE
own
estimate
satumiont
am
DETERMINE
FLUID
PROPERTIES
Density
Mertocio! tension
SoluMily
Sarptivc properties
Vapor transport properties
Chemical composition
Coeotxncy and reactivity
DETERMINE
MEDIA
PROPERTIES
Intrinsic
permeaMHiei
Porosities
Organic carton
content
Heteroaeneiliit
MOKŁ DATA IS NEEDtD
REFINE THE CONCEPTUAL MODEL
1
—| RISK ASSESSMENT)
. *
-| REMEDY ASSESSMENT |»
JTREATABIUTY AND
^ PILOT STUDIES
SITE REMEDIATION
AND LONG-TERM
MONITORING
DETERMINE THE
NATURE, EXTENT.
MIGRATION RATE.
AND FATE OF
CONTAMINANTS
NATURE AND EXTENT
MWL
iraunnMlar cantaminotiM
Adsorbed «ol and
met canlamir
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SITE CONCEPTUALIZATION
The subsurface movement of DNAPL is controlled substantially by the nature of the release, the DNAPL density,
interfacial tension, and viscosity, porous media and capillary properties, and usually to a lesser extent, hydraulic forces.
Below the water table, non-wetting DNAPL migrates preferentially through permeable pathways such as soil and rock
fractures, root holes, and sand layers that provide relatively little capillary resistance to flow. There are several
conceptualizations that can be made for DNAPL sites; two are illustrated in Figure 2. As shown, DNAPL chemicals can
migrate in the subsurface as volatiles in soil gas, dissolved in groundwater, and as a mobile, separate phase liquid.
Depending on how they are released, DNAPLs may be fairly widespread throughout the subsurface (Figure 2a) or they
may have a limited distribution (Figure 2b), making detection difficult. Several relatively simple quantitative methods
for examining DNAPL presence, migration, containment, dissolution, and mobilization within the context of various
conceptual models are described by Cohen and Mercer (1993).
DNAPL SITE CHARACTERIZATION OBJECTIVES
The objectives of DNAPL site characterization include: (1) to determine DNAPL properties, (2) to identify DNAPL
release/source areas, (3) to define stratigraphy, (4) to delineate DNAPL distribution, and (5) to minimize investigation
risk. DNAPL and media properties control DNAPL migration in the subsurface; Table 2 lists these properties. It is also
important to identify stratigraphic barriers and traps, and to determine migration pathways, including man-made
features. As part of the DNAPL distribution, a distinction needs to be made between mobile versus DNAPL at residual.
Visual detection of DNAPL in soil and groundwater samples may be difficult where the DNAPL is colorless, present in
low saturation, or distributed heterogeneously. These factors confound characterization of the movement and
distribution of DNAPL even at sites with relatively homogeneous soils and a known, uniform DNAPL source. The
difficulty of characterization is further compounded by fractured bedrock, heterogeneous strata, multiple DNAPL
mixtures and releases, and other complicating factors.
NONINVASIVE METHODS
Noninvasive methods can often be used during the early phases of field work to optimize the cost-effectiveness of
a DNAPL site characterization program. Specifically, surface geophysical surveys, soil gas analysis, and
photointerpretation can facilitate characterization of contaminant source areas, geologic controls on contaminant
movement (i.e., stratigraphy and utilities), and the extent of subsurface contamination. Conceptual model
refinements derived using these methods reduce the risk of spreading contaminants during subsequent invasive
field work.
Surface Geophysics
At contamination sites, geophysical surveys are usually conducted to: (1) assess stratigraphic and hydrogeologic
conditions; (2) detect and map electrically conductive contaminants; (3) locate and delineate buried wastes and
utilities; (4) optimize the location and spacings of test pits, borings, and wells; and (5) facilitate interpolation of
subsurface conditions among boring locations. Surface geophysical methods that potentially may be used at
DNAPL sites are identified in Table 3. Subsurface DNAPL is generally a poor target for conventional geophysical
methods. Although ground-penetrating radar, EM conductivity, and complex resistivity have been used to infer
NAPL presence at a very limited number of sites, direct detection and mapping of non-conductive subsurface
DNAPL using surface geophysical techniques is an unclear, and apparently limited, emerging technology. The
value of surface geophysics at most DNAPL sites will be to aid characterization of waste disposal areas,
stratigraphic conditions, and potential routes of DNAPL migration.
Soil Gas Analysis
Many DNAPLs, including most halogenated solvents, have high vapor pressures and will volatilize in the vadose
zone to form a vapor plume around a DNAPL source. Volatile organic compounds (VOCs) dissolved in
groundwater can also volatilize at the capillary fringe into soil gas. Thus, soil gas surveys may be used to help
identify contaminated zones for further study. Table 4 presents characteristics of various contaminants and their
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s t/i^^^r ^
X Gaseous Vapors
Dissolved
Contaminant
Plumes
(a) DNAPL chemcials are distributed in several phases: dissolved in
groundwater, adsorbed to soils, volatilized in soil gas, and as residual and
mobile immiscible fluids (modified from Ruling and Weaver, 1991; WCGR,
1991).
Finger
Pool
(b) DNAPL chemicals in fingers and pools (modified from Anderson et al.,
1992).
Figure 2. DNAPL site conceptualizations.
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Table 2. Properties of DNAPL and Media that Influence Contaminant
Migration (from Mercer and Waddell, 1992).
PROPERTY
Saturation
Residual saturation
Interfacial tension
Wettability
Capillary pressure
Relative permeability
Solubility
Volatilization
Density
Viscosity
DEFINITION
The volume fraction of the total void volume occupied by that
fluid at a point.
Saturation at which NAPL becomes discontinuous and is
immobilized by capillary forces.
The free surface energy at the interface formed between two
immiscible or nearly immiscible liquids. Surface tension is the
interfacial tension between a liquid and its own vapor.
Describes the preferential spreading of one fluid over solid
surfaces in a two-fluid system; it depends on interfacial tension.
The difference between the non-wetting fluid pressure and the
wetting fluid pressure. It is a measure of the tendency of a
porous medium to attract the wetting fluid and repel the
non-wetting fluid.
The ratio of the effective permeability of a fluid at a fixed
saturation to the intrinsic permeability.
The maximum concentration of the chemical that will dissolve
under a set of chemical and physical conditions.
The transfer of matter from liquid and soil to the gaseous phase.
Mass per unit volume of a substance. Specific gravity is the
ratio of a substance's density to that of some standard substance,
usually water.
The internal friction within a fluid that causes it to resist flow.
RANGE OF VALUES
0-1
Approximately 0.1 -0.5.
Values of interfacial and surface tensions
for NAPL-forming chemicals generally
range between 0.01 5-0.5 N/m.
Determined through contact-angle
studies. Commonly, NAPL is the wetting
fluid in the vadose zone and the
non-wetting fluid in the saturated zone.
Depends on the interfacial tension,
contact angle, and pore size.
0.0-1 .0, depending on the fluid
saturation.
Varies widely depending on chemical
and aquifer conditions.
Depends on organic partitioning between
water and air, and NAPL and air.
Specific gravities of petroleum products
may be as low as 0.7, whereas
chlorinated aliphatic compounds can be
as high as 1.2 to 1.5.
Varies depending on fluid and
temperature.
relation to soil gas surveys. Several chemical characteristics indicate whether a measurable vapor concentration can be
detected. Ideally, compounds such as VOCs monitored using soil gas analysis will: (1) be subject to little retardation
in groundwater; (2) partition significantly from water to soil gas (Henry's Law constant >0.0005 atm-m3/mole); (3) have
sufficient vapor pressure to diffuse significantly upward in the vadose zone (>0.0013 atm @ 20o C); (4) be persistent;
and (5) be susceptible to detection and quantitation by affordable analytical techniques.
Studies during the 1980s generally indicated the utility of soil gas surveying for delineating VOC source areas and VOC-
contaminated groundwater (e.g., Marrin and Thompson, 1987; Thompson and Marrin, 1987). More recently, well-
documented field experiments were conducted at the Borden, Ontario DNAPL research site to investigate the behavior
and distributions of TCE in soil gas caused by (1) vapor transport from a DNAPL source in the vadose zone and (2)
dissolved transport with groundwater from a DNAPL source below the water table (Hughes et al., 1990a; Rivett and
Cherry, 1991). The extent and magnitude of soil gas contamination derived from the vadose zone source was much
greater than that derived from the groundwater source. Rivett and Cherry (1991) attribute the limited upward diffusion
of groundwater contaminants to the low vertical transverse dispersivities (mm range) which are observed in tracer
studies. Soil gas contamination, therefore, is not a reliable indicator of the distribution of DNAPL or groundwater
contamination at depth below the water table. Although the vadose zone source produced TCE concentrations in soil
gas and groundwater over a much wider area than the groundwater source site, the TCE in groundwater derived from the
vadose zone source was less concentrated than at the groundwater source site and was restricted to the upper 5 ft of the
saturated zone. Overall, these studies suggest that soil gas contamination will usually be dominated by volatilization
and vapor transport from contaminant sources in the vadose zone rather than from groundwater, and that the upward
transport of VOCs to the vadose zone from groundwater is probably limited to dissolved contaminants that are very near
to the water table.
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Air Photo Interpretation
Historic aerial photographs can be analyzed to document waste disposal practices and locations, geologic conditions,
drainage patterns, pooled fluids, site development including excavations for pipelines and underground storage tanks,
signs of vegetative stress, soil staining, and other factors relevant to assessing subsurface chemical migration. The Earth
Science Information Center (ESIC) at the U.S. Geological Survey in Reston, Virginia catalogs and disseminates
information about aerial photographs and satellite images available from public and private sources. ESIC will provide
a listing of available aerial photographs for any location in the United States and order forms for their purchase.
Another use of air photos is to perform fracture trace analysis. Fracture trace analysis involves stereoscopic study of
aerial photographs to identify surface expressions of vertical or nearly-vertical subsurface zones of fracture concentration.
In fractured rock terrain, particularly in karst areas, groundwater flow and chemical transport are usually concentrated
in fractures.
Table 3. Surface Geophysical Methods for Evaluating DNAPL Site
Contamination (from Cohen and Mercer, 1993).
OBJECTIVE
Delineate limits of waste disposal areas.
Delineate buried utility corridors.
Map stratigraphy, particularly permeable pathways
and the surfaces of fine-grained capillary barrier
layers and bedrock, to determine potential routes of
DNAPL migration and stratigraphic traps.
Delineate conductive inorganic contaminant
plumes that may be associated with DNAPL
contamination.
Delineate geophysical anomalies that may result
from an accumulation of DNAPL.
METHODS
Following review of historical documentation, interviews, aerial
photographs, and available site data, consider use of ground-penetrating radar
(GPR), electromagnetic (EM) conductivity, magnetometer, and metal
detection surveys.
Following review of historical documentation, utility records, interviews,
aerial photographs, and available site data, consider use of GPR, EM
conductivity, magnetometer, and metal detection surveys.
Following review of aerial photographs and available site, local, and regional
hydrogeologic data, consider use of GPR, EM conductivity, electrical
resistivity, and high resolution seismic surveys.
Following review of available data on waste disposal practices, chemical
migration, and hydrogeologic conditions, consider use of EM conductivity,
electrical resistivity surveys.
Consider using GPR to infer DNAPL accumulations at shallow depths in low
conductivity soils because DNAPL presence will probably alter the dielectric
properties of the subsurface (Olhoeft, 1986; WCGR, 1991). Also consider
using electrical methods to infer DNAPL accumulation based on the presence
of low conductivity anomalies (Davis, 1991). Note that the use of surface
geophysical methods for direct detection of DNAPL presence is an emerging,
but limited, technology that may not be cost-effective. Very few
geophysicists are experienced in the application of surface survey methods
for DNAPL detection. These applications, therefore, should be treated with
caution.
Note: Survey results depend on site-specific conditions. At some sites, an alternate method may provide better results.
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Table 4. Characteristics of Contaminants in Relation to Soil Gas Surveying
(modified from Marrin, 1987).
GROUP/CONTAMINANTS
APPLICABILITY OF SOIL-GAS SURVEY
TECHNIQUES
Group A: Halogenated methanes, ethanes, and ethenes
Chloroform, vinyl chloride, carbon tetrachloride,
trichlorofluoromethane, TCA, EDB, TCE
Detectable in soil gas over a wide range of environmental
conditions. Dense nonaqueous phase liquid (DNAPL) will sink in
aquifer if present as pure liquid.
Group B: Halogenated propanes, propenes, and benzenes
Chlorobenzene, trichlorobenzene, 1 ,2-dichloropropane
Limited value; detectable by soil-gas techniques only where probes
can sample near contaminated soil or groundwater. DNAPL
Group C: Halogenated polycyclic aromatics
Aldrin, DDT, chlordane, heptachlor, PCBs
Do not partition into the gas phase adequately to be detected in soil
gas under normal circumstances. DNAPL
Group D: C, - Cg petroleum hydrocarbons
Benzene, toluene, xylene isomers, methane, ethane,
cyclohexane, gasoline, JP-4
Most predictably detected in shallow aquifers or leaking
underground storage tanks where probes can be driven near the
source of contamination. Light nonaqueous phase liquids
(LNAPLs) float as thin film on the water table. Can act as a
solvent for DNAPLs, keeping them nearer the ground surface.
Group E: C9 - C,2 petroleum hydrocarbons
Trimethylbenzene, naphthalene, decane, diesel and jet A
fuels
Limited value; detectable by soil gas techniques only where probes
can sample near contaminted soil or groundwater. DNAPL
Group F: Polycyclic aromatic hydrocarbons
Anthracene, benzopyrene, fluoranthene, chrysene, motor
oils, coal tars
Do not partition adequately into the gas phase to be detected in soil
gas under normal circumstances. DNAPL
INVASIVE METHODS
Following development of the site conceptual model based on available information and noninvasive field methods,
invasive techniques will generally be required to advance site characterization and enable the conduct of risk and remedy
assessments. Generally, these invasive activities include: (a) drilling and test pit excavation to characterize subsurface
solids and liquids; and (b) monitor well installation to sample fluids, and to conduct fluid level surveys, hydraulic tests,
and borehole geophysical surveys. Invasive field methods should be used in a phased manner to test and improve the
site conceptual model based on careful consideration of site-specific conditions and DNAPL transport processes.
Invasive Method Risks
The risk of enlarging the zone of chemical contamination by use of invasive methods is an important consideration that
must be evaluated during a DNAPL site investigation. Drilling, well installation, and pumping activities typically
present the greatest risk of promoting DNAPL migration during site investigation. Drilling and well installations may
create vertical pathways for DNAPL movement. In the absence of adequate sampling and monitoring as drilling
progresses, it is possible to drill through a DNAPL zone without detecting the presence of DNAPL (USEPA, 1992).
Increased hydraulic gradients caused by pumping may mobilize stagnant DNAPL. Pumping from beneath or adjacent
to the DNAPL zone can induce downward or lateral movement of DNAPL, particularly in fractured media due to the
development of relatively high fluid velocities. In general, groundwater should not be pumped from an uncontaminated
aquifer directly beneath a capillary barrier and overlying DNAPL zone. If the risks cannot be adequately minimized,
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alternate methods should be used, if possible, to achieve the characterization objective; or the objective should be
waived.
Drilling Methods
Conventional drilling methods have a high potential for promoting downward DNAPL migration (USEPA, 1992).
Specific conditions that may result in downward DNAPL migration include: an open borehole during drilling and prior
to well installation; an unsealed or inadequately sealed borehole; a well screen that spans a barrier layer and connects
an overlying zone with perched DNAPL to a lower transmissive zone; an inadequately sealed well annulus that allows
DNAPL to migrate through the well-grout interface, the grout, the grout-formation interface, or vertically-connected
fractures in the disturbed zone adjacent to the well; and, structural degradation of bentonite or grout sealant, or well
casing, due to chemical effects of DNAPL or the groundwater environment.
To minimize the risk of inducing DNAPL migration as a result of drilling, site investigators should: (1) avoid
unnecessary drilling within the DNAPL zone; (2) minimize the time during which a boring is open; (3) minimize the
length of hole which is open at any time; (4) use telescoped casing drilling techniques to isolate shallow contaminated
zones from deeper zones; (5) use high-quality continuous sampling procedures (e.g., coring or split-spoon sampling) in
a potential DNAPL zone and carefully examine subsurface materials brought to the surface as drilling progresses to
avoid drilling through a barrier layer beneath DNAPL (i.e., stop drilling at the top of the barrier layer); (6) consider using
a dense drilling mud (i.e., with barium sulfate additives, also known as barite) or maintaining a high hydrostatic head
with water to prevent DNAPL from sinking down the borehole during drilling; (7) use less-invasive direct-push
sampling methods where appropriate (e.g., cone penetrometer, Geoprobe®, and HydroPunch techniques); (8) select
optimum well materials and grouting methods based on consideration of site-specific chemical compatibility; and (9)
if the long-term integrity of a particular grout sealant is questionable, consider placing alternating layers of different
grout types and sealing the entire distance between the well screen and surface to minimize the potential for vertical
migration of DNAPL.
The risk of causing DNAPL migration generally increases where there are fractured media, heterogeneous strata,
multiple release locations, large DNAPL release volumes, and barrier layers that are subtle (e.g., a thin silt layer beneath
sand) rather than obvious (e.g., a thick soft clay layer beneath sand). At many sites, the DNAPL zone can be adequately
characterized by limiting drilling to shallow depths. Characterization of deeper units can be accomplished by deeper
borings and wells beyond the edge of the DNAPL zone. The "outside-in" approach whereby invasive activities are
conducted beyond suspected DNAPL areas to improve the conceptual model before drilling in the DNAPL zone also is
used to reduce risks associated with DNAPL site characterization (USEPA, 1992).
Well Construction
The design and construction of wells at DNAPL sites require special consideration of (1) the effect of well design and
location on immiscible fluid movement and distribution in the well and near-well environment; (2) the compatibility of
well materials with NAPLs and dissolved chemicals; and (3) well development options. Based on experiments, field
experience, and the principles of DNAPL movement, it is apparent that the following factors may cause the elevation
and thickness of DNAPL in a well to differ from that in formation and/or lead to vertical DNAPL migration.
(1) If the well screen or casing extends below the top of a DNAPL barrier layer, a measured DNAPL pool thickness may
exceed that in the formation by the length of the well below the barrier layer surface.
(2) If the well bottom is set above the top of a DNAPL barrier layer, the DNAPL thickness in the well may be less than
the formation thickness.
(3) If the well connects a DNAPL pool above a barrier layer to a deeper permeable formation, the DNAPL elevation and
thickness in the well are likely to be erroneous and the well will cause DNAPL to short-circuit the barrier layer and
contaminate the lower permeable formation. The height of the DNAPL column at the well bottom will tend to equal
or be less than the critical DNAPL height required to overcome the capillary resistance offered by the sandpack and/
or formation.
(4) DNAPL which enters a coarse sandpack may sink to the bottom of the sandpack rather than flow through the well
screen. Small quantities of DNAPL may elude detection by sinking down the sandpack and accumulating below the
base of the well screen.
(5) Similarly, if the bottom of the well screen is set above the bottom of the sandpack and there is no casing beneath the
screen, small quantities of DNAPL may elude detection by sinking out the base of the screen and into the underlying
sandpack.
8
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(6) Hydrophilic (e.g., quartz) sandpacks generally should be coarser than the surrounding media to ensure that mobile
DNAPL can enter the well. Screen or sandpack openings that are too small may act as a capillary barrier to DNAPL
flow. Laboratory experiments (Hampton et al., 1991; Hampton and Heuvelhorst, 1990) indicate that (a) NAPL flows
more readily into wells with uniform coarse hydrophilic sandpacks than wells with finer-grained and/or nonuniform
hydrophilic sandpacks; and (b) NAPL recovery can be optimized using a hydrophobic sandpack material (such as
teflon chips or made by applying a water-repellent coating to sand) with angular grains and a nonuniform grain size
distribution.
(7) If the well screen is located entirely within a DNAPL pool and water is pumped from the well, DNAPL will upcone
in the well to maintain hydrostatic equilibrium causing the DNAPL thickness in the well to exceed that in the
formation.
(8) The elevation of DNAPL in a well may exceed that in the adjacent formation by a length equivalent to the DNAPL-
water capillary fringe height where the top of the pool is under drainage conditions.
(9) DNAPL will not flow into a well where it is present at or below residual saturation or at negative pressure.
A well that is completed to the top of a capillary barrier and screened from the capillary barrier surface to above the
DNAPL-water interface is most likely to provide DNAPL thickness and elevation data that are representative of
formation conditions. Well development should be limited in wells containing DNAPL to gentle pumping and removal
of fine particles to minimize DNAPL redistribution. Measurements should be made of immiscible fluid stratification
in the well prior to and after development.
Fluid Thickness, Elevation, and Sampling Surveys
Fluid elevation and thickness measurements are made in wells to assist determination of fluid potentials, flow directions,
and immiscible fluid distributions. With knowledge of DNAPL entry areas, the surface slope of capillary barriers,
hydraulic data, and other observations of DNAPL presence, well data can be evaluated to infer the directions of DNAPL
migration. Interpretation of fluid data from wells containing NAPL may be complicated by several factors related to the
measurement method, fluid properties, well design, or well location. DNAPL in wells, therefore, should be evaluated
in conjunction with evidence of geologic conditions and DNAPL presence obtained during drilling.
While conducting immiscible fluid level and thickness measurements, care should be taken to slowly lower and raise the
measuring device within the well to avoid disturbing the immiscible fluid equilibrium and creating emulsions. Similarly,
measurements should be made prior to purging and sampling activities. The cost of purchase and decontamination of
the measuring device should be considered when selecting a measurement method, particularly given uncertainties
involved in interpreting NAPL thickness and elevation data. Measurements are typically made using interface probes,
bailers, hydrocarbon-detection paste, or other methods (e.g., with a weighted cotton string).
Fluid sampling surveys should be conducted at potential DNAPL sites to examine wells for the presence of LNAPLs and
DNAPLs. NAPL samples can be tested for physical properties and chemical composition. Various sampling devices
can be employed to acquire fluid samples from the top and bottom of the well fluid column. Villaume (1985)
recommends use of a bottom-loading bailer or mechanical discrete-depth sampler for collecting DNAPL samples.
Huling and Weaver (1991) suggest that the best DNAPL sampler is a double check valve bailer which should be slowly
lowered to the well bottom and then slowly raised to provide the most reliable results. DNAPL can be sampled from wells
with a shallow water table (<25 ft deep) with a peristaltic pump and from depths to approximately 300 ft using a simple
inertia! pump. An advantage of the peristaltic and inertia! pumps is that fluid contact is confined to inexpensive tubing
(and a foot-valve with the inertial pump). The cost to decontaminate or replace DNAPL-contaminated equipment is
usually a major factor in selecting a sampling method.
Determining DNAPL Presence
DNAPL presence can be: (1) determined directly by visual examination of subsurface samples; (2) inferred by
interpretation of chemical analyses of subsurface samples; and/or (3) suspected based on interpretation of anomalous
chemical distribution and hydrogeologic data. Methods to visually detect DNAPL in subsurface samples are identified
in Table 5. Indirect methods for assessing the presence of DNAPL in the subsurface rely on comparing measured
chemical concentrations to effective solubility limits for groundwater and to calculated equilibrium partitioning
concentrations for soil and groundwater. Chemical data indicative and/or suggestive of DNAPL presence are siven in
Table 5.
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Table 5. Determinant, Inferential, and Suggestive Indications of DNAPL
Presence (modified from Cohen and Mercer, 1993, Cherry and Feenstra, 1991;
Newell and Ross, 1992; and Cohen et al., 1992).
DETERMINING DNAPL PRESENCE
BY VISUAL EXAMINATION OF
SUBSURFACE SAMPLES
INFERRING DNAPL PRESENCE
BY INTERPRETING
CHEMICAL ANALYSES
SUSPECTING DNAPL PRESENCE
BASED ON ANOMALOUS FIELD
CONDITIONS
Methods to detect DNAPL in wells:
* NAPL/water interface probe detection of
immiscible phase at base of fluid column
* Pump from bottom of fluid column and
inspect retrieved sample
* Retrieve a transparent, bottom-loading bailer
from the bottom of a well and inspect the fluid
sample
* Inspect fluid retrieved from the bottom of a
well using a mechanical discrete-depth sampler
* Inspect fluid retained ona weighted cotton
string that was lowered down a well
Methods to enhance inspection of fluid
sample for DNAPL presence:
* Centrifuge sample and look for phase
separation
* Add hydrophobia dye (such as Sudan IV or
Red Oil) to sample, shake, and look for
coloration of DNAPL fraction
* Examine UV fluorescence of sample (many
DNAPLs will fluoresce)
* Assess density of NAPL relative to water
(sinkers or floaters) by shaking solution or by
using a syringe needle to inject NAPL globules
into the water column
Methods to detect DNAPL in sou and rock
samples:
* Examine UV fluorescence of sample (many
DNAPLs will fluoresce)
* Add hydrophobic dye and water to soil sample
in polybag or jar, shake, and examine for
coloration of the NAPL fraction
* Conduct a soil-water shake test without
hydrophobic dye (can be effective for NAPLs
that are neither colorless nor the color of the soil
* Centrifuge sample with water and look for
phase separtion
* Perform a paint filter test, in which soil is
placed in a filter funnel, water is added, and the
filter is examined for separate phases
Chemical analysis results from which
DNAPL presence ca be inferred (with
more or less certainty depending on the
strenth of the overall data):
* Concentrations of DNAPL chemicals in
groundwater are greater than 1 % of the
pure phase solubility on effective
solubility
* Concentrations of DNAPL chemicals
on soils are greater than 10,000 mg/kg
(equal to 1 % soil mass)
* Concentrations of DNAPL chemicals in
groundwater calculated from water/soil
partitioning relationships and soil samples
are greater than pure phase solubility or
effective solubility
* Organic vapor concentrations detected
in soil gas exceeds 100-1000 ppm
Field conditions that suggest DNAPL
presence:
* Concentrations of DNAPL chemicals increase
with depth in a pattern that cannot be explained
by advective transport
* Concentrations of DNAPL chemicals increase
up the hydraulic gradient from the contaminant
release area (apparently due to containated soil
gas migration and/or, DNAPL movement along
capillary and/or permeability interfaces that
slope counter to the hydraulic gradient)
* Erratic patterns of dissolved concentrations of
DNAPL chemicals in groundwater which are
typical of DNAPL sites due to heterogeneity of
(I) the DNAPL distribution, (2) the porous
media, (3) well construction details, and (4)
sampling protocols
* Erratic, localized, very high contaminant
concentrations in soil gas, particularly located
just above the water table (where dense gas
derived from DNAPL in the vadose zone will
tend to accumulate)
* Dissolved DNAPL chemical concentrations in
recovered groundwater that decrease with time
during a pump-and-treal operation, but then
increase significantly after the pumps are turned
off (although complexities of contaminant
desorption, formation heterogeneity, and
temporal and spatial variations of the
contaminant source strength can produce similar
results)
* The presence of dissolved DNAPL chemicals
in groundwater that is older than potential
contamiant releases (using age dating) suggests
DNAPL migration
* Deterioration of wells and pumps (can be
caused by DNAPL; i.e., chlorinated solvents
degrade PVC)
* Patterns of dissolved chemicals that may be
indicative of pulsed releases associated with
recharge events through a DNAPL zone
* Plume behavior where most of the plume mass
is still near the source area, even after adequate
time has passed to allow dissolved transport
away from the source area
* Mass removed during remediation far exceeds
original calculation of dissolved and adsorbed
mass-in-place
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Where present as a separate phase, DNAPL compounds are generally detected at <10% of their aqueous solubility limit
in groundwater. This is due to the effects of non-uniform groundwater flow, variable DNAPL distribution, the mixing
of groundwater in a well, and the reduced effective solubility of individual compounds in a multi-liquid NAPL mixture.
Typically, dissolved contaminant concentrations >1% of the aqueous solubility limit are highly suggestive of NAPL
presence. Concentrations <1 %, however, do not preclude the presence of NAPL. In soil, contaminant concentrations
in the percent range are generally indicative of NAPL presence. However, NAPL may also be present at much lower soil
concentrations. Feenstra et al. (1991) detail an equilibrium partitioning method for assessing the presence of NAPL in
soil samples based on determining total chemical concentrations, soil moisture content, porosity, organic carbon
content, approximate composition of the possible NAPL, sorption parameters, and solubilities.
INTEGRATED DATA ANALYSIS
There is no practical cookbook approach to site investigation or data analysis. In addition to the site characterization
techniques described herein, many additional methods (i.e., using tracers, interpreting chemical distributions and ratios,
and conducting hydraulic tests) can be used to enhance site evaluation. Each site presents variations of contaminant
transport conditions and issues. Site characterization, data analysis, and conceptual model refinement are iterative
activities which should satisfy the characterization objectives outlined in Figure 1 as needed to converge to a final
remedy. During the process, acquired data should be utilized to guide ongoing investigations. For example, careful
examination of soil, rock, and fluid samples obtained as drilling progresses should be made to identify DNAPL presence
and potential barrier layers and thereby guide decisions regarding continued drilling, well construction, and/or borehole
abandonment. Geologic, fluid elevation, and chemical distribution data should be organized (preferably using database,
CAD, and/or GIS programs) and displayed on maps that are updated periodically to help determine the worth of
additional data collection activities. With continued refinement of the site conceptual model, the benefit, cost, and risk
of additional work can and should be evaluated with improved accuracy. This is the advantage of a flexible, phased
approach to site characterization.
REFERENCES
Anderson, M.R., R.L. Johnson, and J.F. Pankow, 1992. Dissolution of dense chlorinated solvents into groundwater: 3.
Modeling contaminant plumes from fingers and pools of solvent, Environmental Science and Technology, 26(5):901 -
908.
Cherry, J.A. and S. Feenstra, 1991. Identification of DNAPL sites: An eleven point approach, draft document in Dense
Immiscible Phase Liquid Contaminants in Porous and Fractured Media, short course notes, Waterloo Centre for
Ground Water Research, Kitchener, Ontario.
Cohen, R.M., A.P. Bryda, S.T. Shaw, and C.P. Spalding, 1992. Evaluation of visual methods to detect NAPL in soil and
water, Ground Water Monitoring Review, 12(4): 132-141.
Cohen, R.M. and J.W. Mercer, 1993. DNAPL Site Evaluation, Lewis Publishers, Chelsea, MI.
Davis, J.O., 1991. Depth zoning and specializing processing methods for electromagnetic geophysical surveys to remote
sense hydrocarbon type groundwater contaminants, Proceedings of the Fifth National Outdoor Action Conference
on Aquifer Restoration, Ground Water Monitoring, and Geophysical Methods, Las Vegas, NV, pp. 905-913.
Ruling, S.G. and J.W. Weaver, 1991. Dense nonaqueous phase liquids, USEPA Groundwater Issue Paper, EPA/540/4-
91, 21 pp.
Marrin, D.L., 1987. Soil gas sampling strategies: Deep vs. shallow aquifers, Proceedings of 1st National Outdoor
Action Conference on Aquifer Restoration, Ground Water Monitoring and Geophysical Methods, National Water
Well Association, Dublin, OH, pp. 437-454.
Mercer, J.W. and R.K. Waddell, 1993. Contaminant transport in groundwater (Chapter 16) in Handbook of Hydrology,
D.R. Maidment (Editor), McGraw-Hill, New York.
Newell, C.J. and R.R. Ross, 1992. Estimating potential for occurrence of DNAPL at Superfund sites, USEPA Quick
Reference Fact Sheet, R.S. Kerr Environmental Research Laboratory, Ada, OK.
Olhoeft, G.R., 1986. Direct detection of hydrocarbons and organic chemicals with ground penetrating radar and
complex resistivity, Proceedings of Petroleum Hydrocarbons and Organic Chemicals in Ground Water: Prevention,
Detection, and Restoration, National Water Well Association, American Petroleum Institute, Houston, TX, pp. 284-
305.
USEPA, 1992. Dense nonaqueous phase liquids - A workshop summary, Dallas, TX, April 17-18, 1991, EPA/600/R-
92/030, Robert S. Kerr Environmental Research Laboratory, Ada, OK.
Villaume, J.F., 1985. Investigations at sites contaminated with DNAPLs, Ground Water Monitoring Review, 5(2):60-
74.
WCGR, 1991. DNAPL short course notes, Dense Immiscible Phase Liquid Contaminants (DNAPLs) in Porous and
Fractured Media, A short course, October 7-10, Kitchner Ontario, Canada, Waterloo Centre for Ground Water
Research, University of Waterloo.
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NOTICE: The policies and procedures set out in this document are intended solely as guidance. They are not
intended, nor can they be relied upon, to create any rights enforceable by any party in litigation with the United'1
States. EPA officials may decide to follow the guidance provided in this memorandum, or to act at variance with
the guidance, based on an analysis of specific site circumstances. The Agency also reserves the right to change
this guidance at any time without public notice.
For more information, contact:
Randall Breeden
U.S. Environmental Protection Agency
Hazardous Site Control Division (5203G)
401 M Street, S.W.
Washington, DC 20460
Authors:
Office of Emergency and Remedial Response/Office of Research and Development
401 M Street, S.W.
Washington, DC 20460
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