United States
Environmental Protection
Agency
Solid Waste and
Emergency Response
(OS-110W)
EPA/542/N-93/006
June 1993
Ground Water Currents
Developments in innovative ground water treatment.
Funnel and Gate System Directs Plumes
to In Situ Treatment
By Robert C.Starr and John C.Cherry, Waterloo Centre for Groundwater Research
I he Waterloo Centre for
Groundwater Research has de-
veloped Funnel-and-Gate sys-
tems that isolate contaminant
plumes in ground water and
funnel the plumes through in
situ bioreactors. The Funnel-
and-Gate consists of low hy-
draulic conductivity cutoff
walls with gaps that contain in
situ reactors (such as reactive
porous media), which remove
contaminants by abiotic or bi-
ological processes. The cutoff
walls (the funnel) modify flow
patterns so that ground water
flows primarily through high
conductivity gaps (the gates).
Ground water plumes are thus
directed through the in situ
reactors in the gates where
physical, chemical or biologi-
cal processes remove contami-
nants from ground water.
Remediated ground water exits
the downgradient side of the
reactor. Funnel-and-Gate sys-
tems can be installed at the
front of plumes, to prevent
further plume growth, or im-
mediately downgradient of
contaminant source zones to
prevent contaminants from de-
veloping into plumes.
This approach is largely pas-
sive in that after installation,
in situ reactors are intended to
function with little or no
maintenance for long periods.
This contrasts with the energy
and maintenance-intensive
character of pump-and-treat
systems. Additionally, the
Funnel-and-Gate system can
overcome limitations of the
pump-and-treat method,
which is usually not effective
for restoring aquifers, particu-
larly if lighter than water
nonaqueous-phase liquids
(LNAPLs) and dense non-
aqueous-phase liquids
(DNAPL s) are present.
Funnel-and-Gate systems
can be built in several config-
urations. They include straight
walls with one or more gates,
V-shaped funnels with the
gate at the point and wide
U-shaped funnels with one or
more gates along the bottom
of the U. The sides of the U
extend upstream and can
partially surround a contami-
nant source. A contaminant
source zone can be complete-
ly surrounded by cutoff walls,
except for a gate in the wall
on the downstream side.
With this configuration, the
cutoff wall on the upstream
side deflects clean ground wa-
ter around the contaminant
source. Any water that infil-
trates into the enclosure or
flows through the cutoff walls
then flows through the gate
and out of the enclosure.
This configuration minimizes
the amount of water that
flows through the contami-
nant source zone and hence
the amount of contaminated
ground water that must be
treated. This configuration
also maximizes the residence
time of ground water in the
gate, which leads to a more
complete treatment.
A variety of plume configu-
rations and contaminants can
be treated. An arrangement of
multiple gates in parallel can
be used to intercept an excep-
tionally wide plume. Or, a
complex plume containing a
number of different contami-
nants can be treated by pass-
ing through a series of gates
aligned in sequence, each
containing a different reactive
porous medium. For example,
a plume at an electroplating
facility that contains both
chlorinated hydrocarbons and
metals can be treated using
one reactor to degrade the
organics and a second to pre-
cipitate the metals. Multiple
parallel treatment gates and
gates in series can also be
combined.
Funnel-and-Gate systems
can be constructed through
the entire thickness of an
aquifer if ground water plumes
extend from top to bottom of
the aquifer as mightbe the case
for DNAPL contamination. If
a ground water plume occu-
pies only the uppermost por-
tion of an aquifer (e.g., if the
contaminant source is a LNAPL
or a volatile liquid in the va-
dose zone), then an installa-
tion that extends only through
the upper portion of the aqui-
fer will be sufficient.
The Waterloo modelling
analysis is intended to ad-
vance the general understand-
ing of funnel-and-gate systems.
It provides insight into factors
that influence plume contain-
ment using these systems and
the residence time of contam-
inants in the gate. Therefore,
the system can be located so
that all of the water that flows
through a contaminant source
zone subsequently flows
(SEE FUNNEL AND GATE. PAGE 4)
This Month in Currents
This month s Currents features news and events
from our friends North of the Border.
Funnel and Gate System
Chromium Remediation
Waterloo Barrier
DNAPL Site Evaluation
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POINTS OF INTEREST
Innovative Remediation of Chromium
By Robert W. Puls, Robert S. Kerr Environmental Research Laboratory
Imesearchers at EPA's Rob-
ert S. Kerr Environmental Re-
search Laboratory (RSKERL).
in cooperation with research-
ers at the University of Okla-
homa, are pursuing several
different innovative technolo-
gies for the remediation of
chromium-contaminated
ground water and soil. Reme-
diation techniques being eval-
uated at both the laboratory
and field scale include the fol-
lowing: (1) in situ application
of a geochemical barrier to
chromium transport in ground
water using elemental iron
mixed with site aquifer mate-
rials; (2) in situ immobilization
of chromium in ground water
through stimulation of indige-
nous microbial populations to
promote biotic reduction of
chromate [Cr(VI)]; (3) mobili-
zation of Cr(VI) from highly
contaminated source area soils
where a solubility-controlling
equilibrium limits the effec-
tiveness of chromate removal
via traditional pump-and-treat
technology: (4) use of above-
ground poly electrolyte ultra-
filtration technology to remove
Cr(VI) from contaminated
ground water pumped to the
surface; and (5) assessment of
the natural attenuation capac-
ity of the system for chromium
immobilization through adsorp-
tion and reduction processes.
The field site used for pilot
field-scale studies and the
source of soils and aquifer
sediments for laboratory-based
work is located at the U.S.
Coast Guard (USCG) Support
Center near Elizabeth City,
North Carolina. The USCG
is also cooperating in the
research.
The first two technologies
are in situ immobilization
techniques which take advan-
tage of the fact that chromium
exists in either the +3 (re-
duced) or +6 (oxidized) oxida-
tion states in natural systems.
In the oxidized form, as chro-
mate or bichromate, chromi-
um is toxic and mobile,
whereas in the reduced form
as chromium+3, it is non-
toxic (actually a micronutri-
ent) and immobile. The latter
species is extremely insoluble
and adsorbs to mineral surfac-
es almost irreversibly. In most
subsurface environments, the
trivalent reduced species forms
an insoluble mixed chromi-
um-iron hydroxide solid phase.
The incorporation of elemen-
tal iron into aquifer sediment
mixtures creates highly reduc-
ing conditions which reduce
the chromium to the +3 state
and result in the formation of
the insoluble hydroxide. Labo-
ratory batch and column stud-
ies using site materials have
demonstrated this to be a very
efficient and promising tech-
nology. Microbial stimulation
in laboratory batch and col-
umn studies have likewise
demonmated reduction of
chromium to the insoluble
non-toxic formusing benzoate
as an electron donor/carbon
source. The benzoate micro-
cosms have effectively re-
duced Cr(VI) levels from
6 parts per million (ppm) to
less than detection (0.1 ppm).
Column experiments have
likewise shown similar results.
The third technology has
also been laboratory-based to
this point. Soil washing stud-
ies using various anionic sur-
factants have shown promise
in desorbing and dissolving
sediment-bound and precipi-
tated forms of chromate in
source area soils. It is expected
that soil washing of source
area soils will enhance the ef-
ficiency of pump-and-treat of
chromium-contaminated
ground waters in the source
area, which otherwise may be
in equilibrium with these
chromate-laden solid phases
at aqueous concentrations ex-
ceeding ground water cleanup
standards.
The fourth technology was
recently successfully field-
tested. A polyelectrolyte ul-
trafiltration (PEUF) pilot
plant was constructed for the
selective removal of chromate
(toxic hexavalent chromium)
from ground water at the
USCG site. The system was
installed during March, 1993.
Ground water from three wells
having the highest chromium
concentrations (3-5 mg/L) was
pumped directly and continu-
ously through the PEUF sys-
tem. PEUF effluent chromate
concentrations, monitored on-
site using ion chromatogra
phy, were less than 0.07 mg/L.
The PEUF system produces
significantly less waste mass
for ultimate disposal compared
to other conventional treat-
ment systems; and, processed
ground water may be reinject-
ed into the aquifer.
The final approach involves
an Investigation of the
precipitation-dissolution, ad-
sorption-desorption and oxi-
dation-reduction processes
which govern chromium
transport and transformation
in subsurface systems. In a
sense this research comprises
the baseline data against
which the other treatments
are compared and evaluated.
In many natural systems, these
chemical interfacial processes
can naturally attenuate inputs
of hexavalent chromium to
the subsurface. In particular,
reduction of the toxic hexava-
lent form to the reduced tri-
valent form often occurs to
significant extent due to the
presence of organic material
and iron-bearing minerals in
soil and aquifer sediments.
Future research will hope-
fully include full-scale field
evaluation of approaches 1-3.
Plans are underway to scale up
the above-ground PEUF sys-
tem and use it to reduce chro-
mium concentrations in the
most contaminated portions of
the aquifer at the USCG site
For more information, call
Bob Puls at RSKERL at
405-436-8543.
of HP* Woocrte fecewvr
Ground Water Currents
-------
Waterloo Barrier-Containment Wall for In Situ Treatment
By John Cherry and Robert C. Starr, Waterloo Centre for Groundwater Research
new type of containment
wall composed of scalable
steel sheet piling has been de-
veloped at the University of
Waterloo's Institute for
Groundwater Research. The
Waterloo Barrier serves the
same general functions as oth-
er types of containment wails.
However, it has a number of
unique advantages over con-
ventional sheet piling for con-
taining polluted ground water.
The materials and construc-
tion techniques make the Wa-
terloo Barrier less prone to
leaking than other types of
containment walls, thus pro-
viding a greater degree of con-
fidence in its performance.
The joints of conventional
sheet piling are designed for
mechanical strength but not
watertightness. Leakage of wa-
ter through the unsealed
joints is acceptable for most
civil engineering applications,
but generally not for environ-
mental applications. With the
Waterloo- Barrier, the inter-
locking joints between indi-
vidual sheet piles incorporate
a cavity that is filled with
sealant after driving to pre-
vent leakage through the
joints. The scalable cavities
can be formed in two ways.
An internal cavity can be
formed as the sheet pile itself
is manufactured. Or, an exter-
nal cavity can be produced
adjacent to each joint by at-
taching a steel 'L' section to
conventional sheet piling. At
sites where a very high degree
of watertightness is desired,
the Waterloo Barrier can be
constructed with both an in-
ternal and external cavity
at each joint. Cavities at the
joints provide access for
inspection after the sheets
have been driven to confirm
that the sheet piles were not
damaged during construction.
With the Waterloo Barrier,
excavation of subsurface ma-
terials is not required, thus
there is less damage to the site
and disruption of normal site
activities. Also, since workers
are not exposed to contami-
nated soil, health and safely
precautions can be reduced.
Disposal of large volumes of
contaminated soil is avoided.
Installation is relatively clean
and rapid; and, comers and ir-
regular wall geometries can be
easily constructed. Topogra-
phy and depth to water table
have little effect on installa-
non techniques; and, the Bar-
rier can even be installed
through surface water bodies.
The Waterloo Barrier offers
a number of design options
not available with other types
of containment walls. Various
options, such as single or
double scalable joints, and
single or double walls, can be
combined on a single project
where parts of the wall have
one design and other parts
have another design. The
Barrier can be used for
containment purposes only or
used in combination with var-
ious in situ remediation tech-
niques, such as Funnel-and-
Gate systems. The volume of
sealant required is relatively
small, so it is feasible to use
special sealants that are par-
ticularly resistant to chemical
degradation, but are too ex-
pensive to use in large quanti-
ties. The integrity of the
barrier can be confirmed by
inspection during construc-
tion. Potential leak paths
(SEE WATERLOO BARRIER, PAGE 4)
NEW FOR THE BOOKSHELF
DNAPL Site Evaluation
The EPA's Robert S. Kerr En-
vironmental Research Labo-
ratory has published a manual
that is designed to guide in-
vestigators involved in the
planning and implementation
of characterization studies at
sites suspected of having sub-
surface contamination by
dense nonaqueous-phase liq-
uids (DNAPLs). DNAPLs,
especially chlorinated sol-
vents, are among the most
prevalent subsurface contami-
nants identified in ground
water supplies and at waste
disposal sites. There are sev-
eral site characterization is-
sues specific to DNAPL sites
including (a) the risk of in-
ducing DNAPL migration by
drilling, pumping or other
field activities; (b) the use of
special sampling and mea-
surement methods to assess
DNAPL presence and migra-
tion potential; and (c) devel-
opment of a cost-effective
characterization strategy that
accounts for DNAPL chemical
transport processes, the risk of
inducing DNAPL movement
during field work and the data
required to select and imple-
ment a realistic remedy. This
manual provides informanon
to address these issues and de-
scribes and evaluates activities
chat can be used to determine
the presence, fate and trans-
port of subsurface DNAPL
contamination. The manual
discusses the scope of the
DNAPL problem, the proper-
ties of DNAPLs and subsurface
media affecting DNAPL trans-
port and fate, objectives and
strategies for DNAPL site
characterization, invasive and
non-invasive methods of site
characterization and laborato-
ry methods for characterizing
fluid and media properties.
The manual concludes with
several case histories illustrat-
ing problems specific to
DNAPL sites and priority re-
search needs for improving
DNAPL site characterization.
The manual entitled
DNAPL Site Evaluation (Order
No. PB93-150217) will be
available only from: National
Technical Information Ser-
vice, 5285 Port Royal Road,
Springfield, VA 22 16 1 (tele-
phone: 703-487-4650); the
cost, subject to change, is
$44.50. A free project summa-
ry (Document No. EPA/600/
SR-93/002) can be ordered
from EPA's Center for Envi-
ronmental Research Informa-
non, Cincinnati, OH 45268
(telephone: 513-569-7502).
Ground Water Currents
-------
Funnel and Gate
(from page 1)
through the gate. The resi-
dence time that is critical to
the selection and design of re-
action media for the gates can
be better calculated; and, the
least amount of cutoff wall
and number of gates can be
chosen in order to minimize
cost.
The modelling analysis does
not depend on the type of in
situ reactor. Also, the type of
cutoff wall used is not critical
as long as the gate area does
not become plugged with low
hydraulic conductivity materi-
al during wall construction.
The Waterloo Barrier, a seal-
able joint sheet piling, devel-
oped at the Waterloo Centre
for Groundwater Research, is
particularly well suited to
Funnel-and-Gate construction
because it can be easily con-
nected to screens that house
in situ reactors; and, the area
around the cutoff wall does
not become plugged with low
conductivity material. (The
Waterloo Barrier is discussed
in more detail in this issue of
Ground Water Currents, p. 2.)
In situ treatment reactors
and Funnel-and-Gate systems
are concepts developed very
recently; and, research on in
situ reaction media is in its in-
fancy. Rapid advances on in
situ reactors are expected in
the next few years. See the
previous issue of Ground Wa-
ter Currents, Document No.
EPA/542/N-93/003, pp. 1-2,
for a discussion of one such
development—the perme-
able reaction wall. Examples
of other in situ reactor re-
search in progress include: a
biotic treatment medium in a
removable basket or cassette;
an oxygenating medium in a
gate for treating hydrocar-
bons; and processes for the
treatment of nitrate, phos-
phate and chromium.
For more information on
the Funnel-and-Gate concept
and modelling analyses and
the reactor research, contact
Dr. John Cherry at 5 19-888-
45 16 or Dr. Robert C. Starr at
519-M-1211, ext. 6750, at
the Waterloo Centre for
Groundwater Research.
Waterloo Barrier
(from page 3)
through the Barrier are limit-
ed to the sealed joints; so, the
joints are the focus of quality
control procedures. Rigorous,
post-construction hydraulic
testing is possible with double-
walled configurations or small
enclosures. The use of a re-
movable sealant, such as a
bentonitic grout, allows the
sheet piles to be removed from
the ground and used elsewhere
once a site has been success-
fully remediated. The ability
to remove the sheet piles
makes it easy to: (1) isolate
portions of a site for pilot
scale tests; (2) progressively
remediate a site in sections; or
(3) temporarily install the pil-
ing for construction purposes.
The design specifications of
each Waterloo Barrier con-
tainment system must be cus-
tomized to meet the site
requirements. The design is
dependent on: surficial geolo-
gy; nature and depth of con-
tamination and plume
morphology; and flow rate.
The Waterloo Barrier offers
considerable versatility. It can
be installed to completely en-
close a site to prevent off-site
migration of contaminants un-
til a remedial plan can be im-
plemented, or, to isolate a site
while remedial actions are in
progress. In some situations an
open ended Barrier can be ef-
fectively used in conjunction
with extraction wells to pro-
vide hydraulic containment.
The Barrier can be used to di-
rect or funnel a contaminant
plume into a subsurface treat-
ment gate. At new industrial
sites the Waterloo Barrier can
be installed to enclose the site
as a preventive or security
measure to control chemical
releases that could occur in
the future. Enclosures around
new landfills can be coupled
with caps or infiltration sys-
tems to manage the race of
waste degradation and leach-
ate production.
For more information, call
John Cherry (519-888-4516)
or Robert C. Starr (519-
885-1211, ext. 6750) at the
Waterloo Centre for Ground-
water Research.
To order additional copies of Ground Water Currents, or to be included on the permanent mailing list send a fax request to the National
Center for Environmental Publications and Information (NCEPI) at 513-891-6685, or send a mail request to NCEPI, 11029 Kenwood Road.
Building 5, Cincinnati, OH 45242. Please refer to the document number on the cover of the issue if available.
Ground Water Currents welcomes readers' comments and contributions. Address correspondence to:
Managing Editor, Ground Water Currents (OS-110W), U.S. Environmental Protection Agency,
401 M Street S.W., Washington. DC 20460.
United States
Environmental Protection Agency
National Center for Environmental
Publications and Information
Cincinnati, OH 45242
Official Business
Penalty for Private Use $300
EPA/542/N-93/006
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