United States
Environmental Protection
Agency
Robert S. Kerr Environmental
Research Laboratory
Ada OK 74820
Research and Development
EPA/600/SR-93/022 April 1993
Project Summary
DNAPL Site Evaluation
Robert M. Cohen and James W. Mercer
Dense nonaqueous-phase liquids
(DNAPLs), especially chlorinated sol-
vents, are among the most prevalent
subsurface contaminants identified in
ground-water supplies and at waste dis-
posal sites. There are several site-char-
acterization issues specific to DNAPL
sites including (a) the risk of inducing
DNAPL migration by drilling, pumping
or other field activities; (b) the use of
special sampling and measurement
methods to assess DNAPL presence
and migration potential; and (c) devel-
opment of a cost-effective character-
ization strategy that accounts for
DNAPL chemical transport processes,
the risk of inducing DNAPL movement
during field work, and the data required
to select and implement a realistic rem-
edy. This manual provides information
to address these issues and describes
and evaluates activities that can be
used to determine the presence, fate,
and transport of subsurface DNAPL
contamination. The manual discusses
the scope of the DNAPL problem, the
properties of DNAPLs and subsurface
media affecting DNAPL transport and
fate, objectives and strategies for
DNAPL site characterization, invasive
and non-invasive methods of site char-
acterization, and laboratory methods for
characterizing fluid and media proper-
ties. The manual concludes with sev-
eral case histories illustrating problems
specific to DNAPL sites and priority
research needs for improving DNAPL
site characterization.
This Project Summary was developed
by EPA's Robert S. Kerr Environmental
Research Laboratory, Ada, OK, to an-
nounce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
the back).
Introduction
Dense nonaqueous phase liquids
(DNAPLs), such as some chlorinated sol-
vents, creosote-based wood-treating oils,
coal tar wastes, and pesticides, are im-
miscible fluids with a density greater than
water. As a result of widespread produc-
tion, transportation, utilization, and disposal
of hazardous DNAPLs, particularly since
1940, there are numerous DNAPL con-
tamination sites in North America and Eu-
rope. The potential for serious long-term
contamination of groundwater by some
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 (Figure 1). DNAPL chemicals, es-
pecially chlorinated solvents, are among
the most prevalent ground-water contami-
nants identified in ground-water supplies
and at waste disposal sites.
The subsurface movement of DNAPL is
controlled substantially by the nature of
the release, the DNAPL density, interfa-
cial tension, and viscosity, porous media
capillary properties, and, usually to a lesser
extent, hydraulic forces. Below the water
table, non-wetting DNAPL migrates pref-
erentially through permeable pathways
such as soil and rock fractures, root holes,
and sand layers that provide relatively little
capillary resistance to flow. Visual detec-
tion of DNAPL in soil and ground-water
samples may be difficult where the DNAPL
is transparent, present in low saturation,
or distributed heterogeneously. These fac-
tors confound characterization of the move-
ment and distribution of DNAPL even at
sites with relatively homogenous soil and
a known, uniform DNAPL source. The dif-
ficulty of site characterization is further
compounded by fractured bedrock, het-
erogeneous strata, multiple DNAPL mix-
tures and releases, etc.
Obtaining a detailed delineation of sub-
surface DNAPL, therefore, can be very
costly and may be impractical using con-
ventional site investigation techniques.
Furthermore, the risk of causing DNAPL
migration by drilling or other actions may
be substantial and should be considered
prior to commencing field work. Although
DNAPL can greatly complicate site char-
Printed on Recycled Paper
-------
Dissolved
Contaminant
Plumes
Figure 1. DNA PL chemicals are distributed in several phases: dissolved in groundwater, adsorbed to
soils, volatilized in soil gas, and as residual and mobile immiscible fluids (modified from
Huling and Weaver, 1991;WCGR, 1991).
acterization, failure to adequately define
its presence, fate, and transport can re-
sult in misguided investigation and re-
medial efforts. Large savings and
environmental benefits can be realized
by conducting studies and implementing
remedies in a cost-effective manner.
Cost-effective DNAPL site management
requires an understanding of DNAPL prop-
erties and migration processes, and of the
methods available to investigate and in-
terpret the transport and fate of DNAPL in
the subsurface.
Lighter-than-water NAPLs (LNAPLs)
which do not sink through the saturated
zone, such as petroleum products, are
also present and cause ground-water con-
tamination at numerous sites. Although
many of the same principles and con-
cerns apply at both LNAPL and DNAPL
sites, LNAPL site characterization is not
specifically addressed in this document.
Objectives
This manual is designed to guide inves-
tigators involved in the planning and imple-
mentation of characterization studies at
sites suspected of having subsurface con-
tamination by DNAPLs. Specifically, the
document is intended to
Summarize the current state of
knowledge for characterizing
DNAPL-contaminated sites;
Develop a framework for planning
and implementing DNAPL site char-
acterization activities;
Provide a detailed discussion of the
types of data, tools, and methods
that can be used to identify, char-
acterize, and monitor DNAPL sites,
and an analysis of their utility, limi-
tations, risks, availability, and cost;
Identify and illustrate methods, in-
cluding the development of concep-
tual models, to interpret contaminant
fate and transport at DNAPL sites
based on the data collected;
Assess new and developing site
characterization methodologies that
may be valuable and identify addi-
tional research needs; and,
Review the scope of the DNAPL
contamination problem, the proper-
ties of DNAPLs and media, and
DNAPL transport processes to pro-
vide context for understanding
DNAPL site characterization.
The primary goal of this manual is to
help site managers minimize the risks and
maximize the cost-effectiveness of site in-
vestigation/remediation by providing the
best information available to describe and
evaluate activities that can be used to
determine the presence, fate, and trans-
port of subsurface DNAPL contamination.
Outlook
Remedial activities at a contaminated
site need to account for the possible pres-
ence of DNAPL. If remediation is imple-
mented at a DNAPL site, yet does not
consider the DNAPL, the remedy will un-
derestimate the time and effort required to
achieve remediation goals. Thus, adequate
site characterization is required to under-
stand contaminant behavior and to make
remedial decisions.
There is no practical cookbook approach
to DNAPL site investigation or data analy-
sis. Each site presents variations of con-
taminant transport conditions and issues.
Although there are no certain answers to
many of the DNAPL site evaluation is-
sues, this manual provides a framework
for their evaluation.
Conclusions and
Recommendations
As awareness of DNAPL contamination
increased in the 1980s, research was con-
ducted to better understand the behavior
of DNAPL in the subsurface. Much of this
research was an expansion of the investi-
gations performed by Schwille (1988).
DNAPL research is currently focusing on
remediation (National Center for Ground
Water Research, 1992). Throughout this
progression of DNAPL research, relatively
little effort has been expended on devel-
oping new site characterization tools or
methods for DNAPL sites.
What has generally occurred at DNAPL
sites is that tools and techniques utilized
at contamination sites in general have
been applied with varying degrees of suc-
cess. Additionally, some new tools and
methods have been developed and oth-
ers have been adapted to better satisfy
the requirements of a DNAPL site investi-
gation. Site characterization strategies
have also evolved to more closely match
the special concerns and risks posed by
DNAPL presence.
Despite substantial progress, additional
research on DNAPL site characterization
tools and methods is warranted utilizing a
variety of venues: laboratories, controlled
field sites with emplaced DNAPL, and un-
controlled contamination sites. Additional
research and technology transfer efforts
should focus on
1. Well drilling techniques to demon-
strate the isolation of DNAPL zones
through the use of double-cased
wells or other techniques;
2. Well and boring abandonment tech-
niques to demonstrate the efficacy
of different grouting mixtures and
-------
methods to prevent preferential ver-
tical fluid migration;
3. The utility of surface and borehole
geophysical methods to better char-
acterize DNAPL presence and dis-
tribution, and stratigraphic controls
on DNAPL movement;
4. The utility of soil gas surveying to
better characterize NAPL presence
and related chemical migration;
5. Methods to determine in-situ NAPL
saturation (e.g., borehole geophys-
ics, simple quantitative sample
analysis);
6. Techniques to determine field-scale
constitutive relationships between
saturation, capillary pressure, and
relative permeability;
7. Practical field or laboratory tech-
niques to delineate mobile DNAPL
from DNAPL in stratigraphic traps
from DNAPL at residual saturation;
8. Additional cost-effective methods to
determine NAPL presence, compo-
sition, and properties;
9. Techniques to better define site
stratigraphy, heterogeneity, and
fracture distributions;
10. The long-term capacity of capillary
barriers (e.g., clayey soil layers) to
prevent DNAPL movement, includ-
ing methods for determining barrier
continuity and time-dependent as-
pects of DNAPL-mineral structure
and wettability interactions;
11. Identifying the limited characteriza-
tion efforts required to determine
and implement appropriate reme-
dial measures at DNAPL contami-
nation sites;
12. Further optimization of character-
ization strategies given different
source, hydrogeologic, risk and rem-
edy considerations; and,
13. Refinement of pilot test designs, pro-
tocols, and monitoring requirements
to determine the feasibility and/or
technical impracticali'., f alterna-
tive remedial measure.-,
References
Huling, S.G. and J.W. Weaver, 1991.
Dense nonaqueous phase liquids,
USEPA Ground Water Issue Pa-
per, EPA/540/4-91/002, 21 pp.
National Center for Ground Water Re-
search, 1992. Extended abstracts,
Proceedings of the Subsurface Res-
toration Third Internationa! Confer-
ence on Ground Water Quality
Research, Rice University, Hous-
ton, Texas, 343 pp.
Schwille, F., 1988. Dense Chlorinated
Solvents in Porous and Fractured
Media, Lewis Publishers, Chelsea,
Michigan, 146 pp.
WCGR, 1991. Dense, Immiscible Phase
Liquid Contaminants (DNAPLs) in
Porous and Fractured Media, A
Short Course, DNAPL Short Course
notes, October 7-10, Kitchner
Ontario, Canada, Waterloo Center
for Groundwater Research, Univer-
sity of Waterloo.
-------
Robert M. Cohen and James W. Mercer are with GeoTrans, Inc., Sterling, VA20166.
John E. Matthews is the EPA Project Officer (see below).
The complete report, entitled "DNAPL Site Evaluation," (Order No. PB93-150217;
Cost: $44.50, subject to change), will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Robert S. Kerr Environmental Research Laboratory
U.S. Environmental Protection Agency
Ada, OK 74820
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
EPA/600/SR-93/022
------- |