United States
Environmental Protection
Agency
Solid Waste and
Emergency Response
(5102W)
EPA-542-N-95-004
July 1995
Issue No. 12
Ground Water Currents
Developments in innovative ground water treatment
TRACERS DETECT AQUIFER CONTAMINATION
Bv Carl Enfield, EPA's National Risk Management Research Laboratory, Ada, Oklahoma
The EPAs National Risk
Management Laboratory
(NRMRL) at Ada, Oklahoma,
along with the University of
Florida and the University of
Texas, have developed a tracer
procedure to detect the
amount of contamination in
aquifer formations. The tracer
procedure has been successfully
applied in a highly controlled
field experiment in a contami-
nated cobbly sandy gravel
aquifer at Hill Air Force Base
in Layton, Utah. The tracer
procedure should substantially
improve our ability to
remediate aquifers and is an
improvement over the tradi-
tional reliance upon water
samples from monitoring wells
and core samples. To date, the
extent and distribution of
ground water contaminants
have been decided by chemical
analysis performed on water
samples from monitoring wells.
The hydraulic gradient and
direction of water flow is
determined by carefully
analyzing water depth data
from the monitoring wells.
Although monitoring well data
effectively show the existence
and the extent of the contami-
nation, they provide little of
the data needed to design
remedial alternatives. Core
samples have been used to
determine the amount of
contamination in the sediment
at the measi rement points s).
However, due to heterogene-
ities, cores are not effective in
deciding tht total amount of
contamination without taking
many samples and doing many
expensive chemical analyses.
This Ls where the develop-
ment of the tracer fits in.
Tracer principles build upon
the work of the past two
decades in which researchers
have learned how organic
chemicals move in aquifers.
Chemicals, dissolved in water,
move at speeds equal to, or less
than, the speed of water. The
speed of those chemicals that
move slowei than water is
directly proportional to the
amount of c>rganic matter
associated with the sediments.
Most uncontaminated geologic-
sediments v hich are low in
organic matter do not retard
the movement of solubili/ed
chemical contaminants like
benzene or trichloroethen -.
However, wien soils are
contaminated with nonaqueous
phase liquids (NAPLs), like
gasoline or luel oil, the amount
of organic matter increases
dramatically NAPLs, like
naturally occurring organic
matter, retard the movement of
chemicals ai id also serve a.- a
source of contamination.
The tracer procedure involves
injecting non-retarded tracers
(like bromide) that move with
the water along with tracers
retarded by the NAPL (but
which are not a component of
the NAPL) making it possible
to compute the amount of
NAPL in the formation. The
computation is based on the
ratio of the travel times of the
retarded and non-retarded
tracer. Travel time is deter-
mined by measuring the tracers
concentration as a function of
time at the monitoring well.
The travel time is the time
required for the tracers center
of mass to move from the
injection well to the monitor-
ing well (the larger the travel
time ratio, the more NAPL in
the formation). The computed
amount NAPL present is based
on a water to NAPL partition-
ing for the selected tracer(s).
Analysis of known tracer
compounds in water is easy,
compared to the measurement
of all the different contami-
nants in soil core samples.
Knowing the NAPL distribu-
tion in the subsurface will
reduce the amount of remedial
fluid/energy needed to achieve
remedial objectives by directing
remedial activities only at
contaminated locations.
Current approaches treat an
aquifer as a homogeneously
contaminated region.
On the test site at Hill Air
Force Base, interpretation of
tracer data provided logical
descriptions of the contami-
nant distribution which were
reproducible. In contrast, the
interpretation of the core data
is extremely difficult because
representative core samples
cannot be obtained due to the
size and amount of cobbles in
the formation. The researchers
hope to provide a proven, cost-
effective method of characteriz-
ing the amount and distribu-
tion of NAPL in a contami-
nated site. Additional tests will
be run over the next year to
perfect the procedure.
For more information, call Carl
Enfold at EPAs NRMRL at 405-
436-8530.
THIS MONTH IN CURRENTS
TRACER
p. 1
CHROMIUM NATURAL ATTENUATION p. 2
WELLHEAD MODEL
p. 2
ABIOTIC DEGRADATION
p. 3
Recycled/Recyclable
Printed with Soy/Canola ink on paper that contains at least 50% recycled fiber
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RESEARCH NEWS
NATURAL ATTENUATION OF HEXAVALENT CHROMIUM
By Robert W. Puls, EPA National Risk Management Research Laboratory, Ada, Oklahoma
Recent research findings support
that there is natural attenuation of
hexavalent chromium in ground
water and soil. Chromium is an
important industrial metal used in
diverse products and processes. At
many locations chromium has
been released to the environment
via leakage, poor storage or
improper disposal practices.
Concerns about the impact of
chromium on human health and
the environment require an eval-
uation of the potential risk of
chromium entering the ground
water flow system and being
transported beyond compliance
boundaries. At such sites where
such potential exists, active reme-
dial measures such as excavation
or pump-and-treat have been
undertaken. Experience at sites
where pump-and-treat reme-
diation of chromium -contami-
nated ground water is currently
under way suggests that, although
it is feasible to remove high levels
of chromium from the subsur-
face, as concentrations decrease it
becomes more difficult to remove
the remaining chromium. While
several new remedial technologies
are being investigated, there is still
concern atx »ut the cost of .such
remediation technology; and, at
many sites, :here is a debati • about
the need for i^xpensive remediation.
Researchers have identified
natural redi ctants that can trans-
form the more toxic hexavalent
form of chromium [Cr(VI>] to
the less toxic trivalent form
[Cr(IH)]. Under alkaline to
slightly acidic conditions, t lis
Cr(III) precipitates as a fair!y
insoluble hydroxide, thereby
immobilizii ig it within the soil.
Such "natural attenuation" of
hexavalent chromium is of great
interest bee .use it suggests hat
strict water quality standards do
not have to be attained every-
where within and beneath the
site. If natural attenuation does
occur, pump-and-treat
remediation could desist after the
most contaminated ground water
has been removed, even if the
maximum contaminant level
(MCL) has not been achieved.
Under certain circumstances,
expensive remedial measures may
not even be necessary. However,
before the natural attenuation
option is adopted, it should be
demonstrated that natural atten-
uation is likely to occur under the
(continued on page 4)
VISITOR'S DAY
The United States Depart-
ment of Energy's (DOE)
Ames Laboratory, teaming up
with EPA and state regulators
and Wesringhouse Savannah
River Company, will host envi-
ronmental experts from around
the nation when they gather in
Augusta, Georgia, August 29,
1995 to evaluate an innovative
methodology for streamlining
the characterization of contami-
nated waste sites. The new
method, Expedited Site Char-
acterization (ESC), will be
used as part of the regulatory-
NEW FOR THE BOOKSHELF
NEW WELLHEAD ANALYTIC MODEL
A new computer program has
been developed to determine
timeof-travel capture zones in
relatively simple geohydrological
settings. The Wellhead Analytic
Element Model (TOAEM)
contains an advanced algorithm
for determining capture zones
for any well at any time based on
precise knowledge of the
locations of all stagnation points
and dividing streamlines. No
models currently in use for
wellhead protection contain such
an algorithm. WMEM differs
from existing analytical models
in that it can handle fairly
realistic boi indary conditi< >ns
such as streams, lake's and iqinfer
recharge due to precipitation.
It has features that rnaki rriL'
inclusion ot open or close* 1,
head-specified boundaries
possible (for example to model
streams). 7 he newly developed
model will serve ground water
professionals who wish to
determine < apture zones in
relatively sir nple geohydro
logical settings. WMEM
consists of two executable^: the
preprocessor GAEP and the flow
model CZAiM.
The preprocessor GAEP is
designed ro simplify the proce-
dures for getting data into a
ground water model; specifically
it facilitates the interactive
process of ground water flow
modeling that precedes capture
zone delineation. GAEP
separates the time consuming
(but routine) task of digitizing
hydrography data from the
creation of conceptual models
and subsequent analytic element
input data files. With GAEP, the
modeler is free to concentrate on
interpretation of modeling
results rather than the details of
data modification and entry into
approved remedial investigation
at the D-Area Oil Seepage Ba-
sin on the Savannah River Site.
The ESC brings geophysical,
geotechnical, hydrologic, ana-
lytical and computer software
technologies together at the sire
for concurrent analysis. If you
want more details and/or
wish to register for the August
29 Visitors Day, call Beth
Weiser, ESC Logistics Coordi-
nator at 515-294-4731 or email
to weiser@ameslab.gov, or
FAX to 515-294-6963.
CZAEM. The new algorithms
developed for the accurate
delineation of capture zone
boundaries are implemented in
the computer code CZAEM,
which accurately defines capture
zone boundaries. The W/AEM
package is documented in
various ways. The primary
documentation is contained in a
program manual, which includes
installation instructions, program
descriptions and a tutorial for the
integrated use of GAEP and
CZAEM. Reference manuals
for both GAEP and CZAEM
continued on page 3
Ground Water Currents
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SITE SUBJECTS
METAL-ENHANCED ABIOTIC DEGRADATION OF VOCS
By Chien T. Chen, EPA's National Risk Management Research Laboratory Edison, N.J.
EnviroMeral Technologies,
Inc. (ETI) of Canada has
developed the meral-enhanced
abiotic degradation technol-
ogy to treat halogenated
volatile organic compounds
(VOCs) in water. The ETI
technology can be installed
and operated above ground in
a reactor or in situ as a
"funnel and gate" system.
The above ground reactor can
be an alternative to the air
stripper and activated carbon
as methods of remediating
halogenated organic com-
pounds. The Superfund
Innovative Technology
Evaluation (SITE) Program
evaluated an above ground
reactor for remediation of
ground water from the SGL
Printed Circuits site in
Wayne, New Jersey for three
months during 1994 and
1995. The technology was
designed to lower chlorinated
VOC concentrations below
regulatory limits after one
pass through the reactive iron
medium. A reactive, zero-
valent, granular iron medium
conducts reductive
dehalogenation of the VOCs,
yielding simple organic
compounds and halogen salts
as bv-products.
The above ground reactor
design may be used to
simulate the treatment process
at pilot scale, allowing for
measurement, control,
modification and optimiza-
tion of design and operating
parameters, or may be
operated a^ a stand-alone-
treatment init. Contami-
nated ground water enters rhe
reactor through an air dim -
nator, 5-micron water filter
(to remove suspended solids
that may inhibit water f-ow
through the reactive iron
medium), and a flow m-:ter.
Water flows by gravi ry
through the reactive iron
medium and into the ex'Hec-
tor line at :he bottom of the
reactor and then exirs through
the effluent line. The effluent
line is plumbed so that ,,bout
one foot of influent war T
remains ponded above the
surface of \ he reactive in >n
medium. \ passive gas /ent
in the top )f the reactor
prevenrs a< cumulation of
excess pressure. A manhole
with a sight glass allows
observation of the reactive
iron surface and access to the
tank interior. Reactor
materials include the reactive
iron medium, pea gravei or
well sand, i reactor tank for
above ground installations
and appropriate ancillar/
pumps and piping, whe e
necessary.
For the SITE evaluation,
about 60,833 gallons of
ground water were treated
during 13 weeks of the
demonstra:ion. The reactor
maintained a flow rate of
about 0.5 gallons per m nute
throughout most of the
demonstration. The primary
objectives of the SITE
demonstration were to
(1) determine whether or not
effluent met New Jersey
Department of Environmen-
tal Protection (NJDEP) and
federal maximum contami-
nant level (MCL) require-
ments for all chlorinated
VOCs detected and
(2) determine the removal
efficiency of tetrachloroethene
(PCE) and other chlorinated
VOCs. Initial comaminant
concentrations were: 4,100
to 16,000 micrograms per
liter (ug/L) PCE, 54 to 590
ug/L trichloroethene (TCE),
up to 1,200 ug/L cis- 1,2-
dichloroethene (cDCE).
Removal efficiency for both
TCE and PCE exceeded
99.98%. Vinyl chloride and
cDCE were not detected until
week 6 of the demonstration;
and, they occasionally ex-
ceeded NJDEP regulatory
limits, which may be attrib-
uted to unexpectedly high
levels of PCE in the influent.
The SITE Program exam-
ined only chlorinated sol-
vents, primarily TCE and
PCE. However, the developer
claims that its system is
applicable to all halogenated
VOCs in water.
For more information about
the SITE Demonstration, call
Chien Chen at EPA's National
Risk Management Research
Laboratory at 908-906-6985.
(continuedfrom page 2)
are provided in the W/?AEM
manual. A tutorial for stand-
alone use of rhe program
CZAEM ;s available as a
separate document. Finally,
both GAEP and CZAEM codes
support on-line help,
The WM.EM was developed
under a ax>perarive agreement
between EPA and Indiana
University and the University of
Minnesota. A Project Summary
is available' by calling CERI at
513-569-"7562 and requesting
Document Number EPA/600/
SR-94/210. For a copy of the
reports (while EPA stock lasts)
and associated software please send
a letter of request and one format-
ted 3.5 inch high density PC
diskette (specify DOS or Windows)
to: CSMoS, Robert S. Kerr
Environmental Research Labora-
tory, PO. Box 1198, Ada, OK
74821, Ann: WMEM. The two
volume report only (Order No.
PB95-194189, cost: $19.50 and
Order No. PB95-167375, cost
$27.00 ) are also available from:
National Technical Information
Service, 5285 Port Royal Road,
Springfield, VA 22161 (telephone
number: 703-487-4650). The
cost is subject to change.
GROUND WATER SAMPLING WORKSHOP
A summary of the EPA
sponsored Ground Water
Sampling Workshop, held in
Dallas, Texas, November 30-
December 2, 1993, is now
available. The primary
objective of the workshop
{continued on page 4)
Ground Water Currents
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(continuedfrom page 2)
specific conditions ar the site
being investigated.
If natural attenuation is to be
considered a viable option, then
— ideally — it must be
demonstrated that: (1) there
are natural reductants present
within the aquifer; (2) the
amount of Cr(VI) and other
reactive constituents does not
exceed the capacity of the
aquifer to reduce them; (3) the
time scale required to achieve
the reduction of Cr(Vl) to the
target concentration is less than
the rime scale for the transport
of the aqueous Cr(VI) from
source area to the point of
compliance; (4) the Cr(III) will
remain immobile; and (5) there
is no net oxidation of Cr(lII) to
Cr( VI). The most difficult
types of information to obtain
are the time scales for the
reduction and oxidation of
chromium in the soil.
Demonstrating Cr(VI)
reduction in aquifer by mass
balances that rely primarily on
the aqueous concentrations
from monitoring well networks
are valid only if it is demon-
strated thai Cr(VI) precipitates
are not forming in the aquifer.
The moni oring network must
be sufficiently dense thai
estimates of Cr(VI) are jixurate.
In the research rinding,1.,
several soil tests are descr bed
that are useful in determining
the mass o'Cr(VI) and ( >(III)
in the source areas and the
reduction ,ind oxidation
capacities of die aquifer materi-
als. Some simple conceptual
models art presented wh,>re:">y
this information, combined
with knowledge of the residence
time o! the chromium between
the source and the point of
compliant:'_', can be used to
determine the feasibility >f
nature attenuation of Ct(Vl).
The major limitation to rhb
approach is the lack of ir forma-
tion about the rate of oxidation
and reduction of chromi un
under conditions likely to be
encountered by plumes emanat-
ing from chromium sources.
Without better information
about these rate processes under
a wider range of conditions with
respect to pH, the use of the
natural attenuation option for
contaminated soils will continue
to be a highly debated issue.
The findings are presented in
GROUND WATER ISSUE;
NATURAL ATTENUATION
OF HEXAVALENT CHRO-
MIUM IN GROUND WATER
AND SOILS (Document No.
EPA/540/S-94/505). The
document can beoidered from
the EPA Center for Environmen-
tal Research Information (CERI)
by calling 513-569-7562.
For arty additional information,
call Bob Puts atEPAs National Risk
Management Research Laboratory at
405-436-8543.
(continuedfrom page 3)
was to provide a forum for
the presentation and discussion
of recent research findings on
ground water sampling in the
context of continuing advances
in environmental monitoring
technologies and changing
environmental regulatory
requirements. Secondary
objectives of the workshop were
to improve communication
and the transfer of information
between diverse groups, to
encourage consistency in
ground water sampling
programs where appropriate
and to identify research and
technology transfer needs.
Participants included research-
ers, practitioners, regulators and
policy makers from govern-
mental and non-governmental
sectors, including universities,
private industry and environ-
mental consultants.
A copy of the workshop
summary, "Ground Water
Sampling -— A Workshop
Summary," which includes
papers presented, can be
ordered by calling CERI at
513-569-7562. Please refer to
Document Number
EPA/600/R-94/205 when
ordering.
To order additional copies of Crowd W
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