£EPA
United States
Environmental Protection
Agency
ENVIRONMENTAL
RESEARCH BRIEF
Long-term Performance of
Permeable Reactive Barriers
Using Zero-valent Iron:
An Evaluation at Two Sites
Richard T. Wilkin*, Robert W. Puls*, and Guy W. Sewell*
Background
The permeable reactive barrier (PRB) technology is an in-situ approach for remediating groundwater
contamination that combines subsurface fluid flow management with a passive chemical treatment
zone. Removal of contaminants from a groundwater plume is achieved by altering chemical
conditions in the plume as it moves through the reactive barrier. Because the reactive barrier
approach is a passive treatment, a large plume can be treated in a cost-effective manner relative to
traditional pump-and-treat systems. There have now been more than forty implementations of the
technology in the past six years, which have proven that passive reactive barriers can be cost-
effective and efficient approaches to remediate a variety of compounds of environmental concern.
However, in all of the installations to date comparatively few data have been collected and reported
on the long-term performance of these in-situ systems, especially with respect to the buildup of
surface precipitates or biofouling (O'Hannesin and Gillham, 1998; McMahon et al., 1999; Puls etal.,
1999; Vogan, 1999; Phillips et al., 2000; Liang et al., 2000).
A detailed analysis of the rate of surface precipitate buildup in these types of passive, in-situ systems
is critical to understanding how long these systems will remain effective and what methods may be
employed to extend their lifetime or to improve their performance. Different types of minerals and
surface coatings have been observed to form under different geochemical conditions that are
dictated by aquifer chemistry and the composition of the permeable reaction zone (Powell et al.,
1995; Mackenzie et al., 1999; Liang et al., 2000). Microbiological impacts are also important to
understand in order to better predict how long these systems will remain effective in the subsurface
(Scherer et al., 2000). The presence of a large reservoir of iron coupled with plentiful substrate
availability supports the metabolic activity of iron-reducing, sulfate-reducing, and/or methanogenic
bacteria. This enhanced microbial activity may beneficially influence zero-valent iron reductive
dehalogenation reactions through favorable impacts to the iron surface or through direct microbial
transformations of the target compounds. However, this enhancement may come at the expense of
faster corrosion leading to faster precipitate buildup and potential biofouling of the permeable
treatment zone.
This research brief presents findings overthe past fouryears at two sites where detailed investigations
by the U.S. Environmental Protection Agency (U.S. EPA) have focused on the long-term performance
of PRBs under a Tri-Agency Permeable Reactive Barrier Initiative (TRI). This initiative involves the
U.S. EPA, the Department of Defense, and the Department of Energy. The objectives of the TRI are
to leverage the technical and financial resources of the three agencies in order to examine the field
performance of multiple PRBs across the United States. A survey of existing PRBs indicated that
the two main challenges facing the technology were (1) uncertainties associated with the longevity
(geochemistry) of a PRB and (2) ensuring/verifying hydraulic performance. Therefore, this initiative
and our research have focused primarily on these challenges.
* U.S. EPA, Office of Research and Development, National Risk Management Research
Laboratory, Subsurface Protection and Remediation Division, Ada, OK.
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Introduction
Research described in this research brief explores the
geochemical and microbiological processes occurring within
zero-valent iron treatment zones in permeable reactive barriers
that may contribute to decreases in iron reactivity and decreases
in reaction zone permeability that, in turn, may eventually lead to
system plugging and failure. Using advanced surface analytical
techniques together with detailed coring and water sampling
programs at two geographically, hydrogeologically, and
geochemically distinct iron barrier installation sites, specific
objectives of this research project were to:
1) Characterize the type and nature of surface precipitates
forming over time at the upgradient aquifer/iron interface,
within the iron zone, and at the downgradient/iron interface.
2) Develop conceptual models that predict the type and rate of
precipitate formation based on iron characteristics and water
chemistry.
3) Identify type and extent of microbiological activity upgradient,
within and downgradient in at least one of the chosen sites
to evaluate microbiological response or effects from emplaced
iron into an aquifer system.
4) Develop practical and cost-effective protocols for long-term
performance assessments at permeable reactive barrier
installations.
In planning groundwater remediation systems using permeable
treatment walls, granulariron metal is the treatment material most
often proposed and used. The choice of zero-valent iron stems
from the reducing conditions created by reactions between
groundwater and iron metal. The "ideal" iron wall would support
fast and complete contaminant transformation, and have a
hydraulic conductivity that could be maintained over time.
However, physical and chemical properties of the iron treatment
zone will change with time as a result of the "aging" of the iron. The
lifetime of an iron wall will essentially be determined by how fast
and by what types of minerals precipitate within the reaction
zone. The corrosion of zero-valent iron in aqueous environments
has been widely studied (e.g., Reardon, 1995). In water, zero-
valent iron is oxidized by many substances to ferrous ion, leading
to dissolution and volume loss of the metal. Under anaerobic
conditions, reduction of water occurs:
Fe°
2H2O = Fe2+
H + 2Ohr
(1)
Under aerobic conditions, dissolved oxygen acts as the oxidant
and can lead to the production of ferrous and ferric iron:
2Fe° + O
2H2O
= 2Fe2+ + 4Ohr
4Fe2+ + O, + 2H+ = 4Fe3+ + 2Ohr
(2)
(3)
Both aerobic and anaerobic iron corrosion reactions lead to an
increase in pH. Aerobic corrosion is a more rapid process, as
evidenced by the rapid loss of dissolved oxygen in iron/water
systems. As the corrosion process proceeds, iron hydroxides
form, which increases the thickness of an iron oxide passivation
layer already present at the iron metal surface. Under anaerobic
conditions, hydrogen gas that is formed as a product of iron
corrosion may also temporarily passivate the iron surface. Many
species abundant in groundwater can affect the iron corrosion
process. For example, chloride, carbonate, and sulfate can all
accelerate the corrosion of iron by increasing the dissolution rate
of the protective oxide layer.
In groundwater, any of the common anions present may influence
the effectiveness of zero-valent iron barriers for contaminant
remediation. Carbonate is of particular interest. At the high pH
condition caused by the corrosion of iron, bicarbonate reacts with
OH'and Ca2+ ions to form insoluble CaCO3 that will precipitate as
calcite or the polymorph, aragonite. In addition, the bicarbonate
anion can precipitate with ferrous ion to form ferrous carbonate
(siderite):
Fe2+ + HCO,-+ OH- = FeCO.(s) + H.O
(4)
Under aerobic conditions, ferric oxides and oxyhydroxides
precipitate. Under anaerobic conditions and at high pH, ferrous
hydroxide or green-rust minerals are expected to form. Other
species in groundwater can also precipitate at the high pH,
reducing conditions created in the iron-water system, such as
iron sulfides. The formation of these various precipitates has
been observed in numerous studies investigating the use of zero-
valent iron for remediating contaminated groundwater (e.g.,
Powell et al., 1995; Mackenzie et al., 1999; Phillips et al., 2000).
These mineral precipitates may limit access to the iron surface
and thereby decrease reactivity and will decrease porosity and
therefore impact flow through the reactive media, resulting in
decreased residence time and incomplete treatment of
contaminants.
Many studies have investigated the interactions of bacteria with
metallic iron under both aerobic and anaerobic conditions. The
oxidation or corrosion of zero-valent iron may be stimulated or
inhibited by microorganisms (Morales et al., 1993; Hernandez et
al., 1994). However from an ecological perspective, metallic iron
in subsurface environments represents a significant energy
reservoir—an energy supply that microorganisms will utilize if
possible. Due to the limited solubility of oxygen in groundwater
and the rapid reduction of molecular oxygen by Fe(0) and Fe(ll),
permeable reactive barriers usually exist as anaerobic
environments. Under anaerobic conditions molecular oxygen-
driven chemical corrosion rates may be reduced, but biologically
mediated anaerobic corrosion may occur at rates exceeding
those seen under oxygenated conditions (Lee etal., 1995). In the
absence of oxygen, protons may serve as electron acceptors and
allow for the formation of oxidized iron species such as Fe(ll).
Under static and aseptic conditions this process is usually limited
by the formation of a stable hydrogen film produced from the
reduction of protons on the metal surface. However, processes
that consume hydrogen gas and thereby interfere with the
formation of this hydrogen film, such as mixing with water that is
undersatu rated with respectto hydrogen orthe activity of hydrogen-
consuming bacteria, will enhance anaerobic iron corrosion. Both
of these processes are likely to be important in PRBs installed in
subsurface environments.
The enhancement of the anaerobic corrosion process and the
formation of dissolved Fe(l I) is not necessarily detrimental to PRB
performance. If the target contaminant is reduced by Fe(ll) as
well as Fe(0), such as Cr(VI), a dispersion of aqueous Fe(ll) could
represent an increase in the size of the reaction/treatment zone
as compared to the surface contact area of Fe(0) alone. However,
the utilization of dissolved hydrogen may result in bacterial
growth and biofilm formation. The development of this biofilm in
a PRB may be detrimental to performance through several
mechanisms. A microbial biofilm containing iron-reducing and
sulfate-reducing bacteria as terminal oxidizers is a complex
structure containing both organic and inorganic materials (Cord-
Ruwish, 2000). These biofilms are typically only about 5% dry
weight, suggesting a disproportionate effect on hydraulics per
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unit of bacterial growth. Changes in PRB hydraulic conductivity,
the masking of active sites, the removal of active chemical
species, and the competition for reducing equivalents would all
seem to be processes that could negatively affect PRB
performance. Conversely some microbial processes could
enhance PRB performance. In some instances bacteria may be
more effective at contaminant transformation or may degrade
compounds unaffected by PRBs. It is therefore evident that a
clear understanding is needed of microbial/PRB interactions for
the design and efficient operation of PRBs and because inhibition
of biofilm formation under the conditions associated with
subsurface PRBs appears to be technically and economically
impractical.
Site Descriptions
Two field sites were evaluated in the U.S. EPA portion of the TRI:
the U.S. Coast Guard Support Center (USCG-SC) site near
Elizabeth City, North Carolina, and the Denver Federal Center
(DFC) in Lakewood, Colorado.
U.S. Coast Guard Support Center
The USCG-SC is located about 100 km south of Norfolk, Virginia
and 60 km inland from the Outer Banks region of North Carolina.
The base is situated on the southern bank of the Pasquotank
River, about 5 km southeast of Elizabeth City, North Carolina. A
hard-chrome plating shop was in operation for more than 30 years
in Hangar 79, which is only 60 m south of the river (Figure 1).
Following its closure in 1984, soils beneath the shop were found
to contain chromium concentrations up to 14,500 mg/kg.
Subsequent site investigations by U.S. EPA personnel identified
a chromate plume extending from beneath the shop to the river.
The plume has high (>10 mg/L) concentrations of chromate,
elevated sulfate(to 150 mg/L), and minor amounts of chlorinated
solvent compounds (trichloroethylene, cis-dichloroethylene, vinyl
chloride). The plating shop soils and related groundwater
contamination are referred to as solid waste management unit
(SWMU) number 9 by the state of North Carolina and the USCG.
Sampling results from a monitoring network consisting of more
than 40 monitoring wells and about 100 Hydropunch™ and
Geoprobe™ monitoring points indicate that the Cr(VI) plume is
about 35 m wide, extends to 6.5 m below ground surface and
extends laterally about 60 m from the hangar to the Pasquotank
River. Multilevel samplers installed near the barrier wall location
indicate that the bulk of the contamination resides from 4.5 to
6.5 m below ground surface.
The site geology has been described in detail elsewhere (Puls et
al., 1999), but essentially consists of typical Atlantic coastal plain
sediments, characterized by complex and variable sequences of
surficial sands, silts and clays. In general the upper 2 m of the
aquifer are sandy to silty clays that pinch out towards the north,
or near the Pasquotank River, where sandy fill predominates.
Fine sands, with varying amounts of silt and clay, and silty-clay
lenses form the rest of the shallow aquifer.
®MW46
MW35D
m
N
ML35
MW47
WC25
BUILDING 78
ML34
ML33
X
4k WC24X,^/IW50^MW49
ML14
ML31
MW38
PRB
MW Compliance well
© (5-10ft. screen)
ML Multilevel bundle
• (6 in. screens)
WC Well cluster
• (6 in. screens)
*
'
10m
Approx. Scale
HANGAR 79 I Cp'atin9
I ^hpp |
Groundwater
flow direction
Figure 1. Plan view map showing compliance well, bundle and well cluster locations relative to granular iron barrier and Cr plume
(June 1994 data).
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Groundwater flow velocity is extremely variable with depth, with
a highly conductive layer at roughly 4.5 to 6.5 m below ground
surface. This layer coincides with the highest aqueous
concentrations of chromate. The groundwater table ranges from
1.5 to 2.0 m below ground surface and the average horizontal
hydraulic gradient varies from 0.0011 to 0.0033. Slug tests
conducted on monitoring wells with 1.5 m screened intervals
between 3 and 6 m below ground surface indicate hydraulic
conductivity values of between 0.3 to 8.6 m/d. A multiple
borehole tracer test in wells screened between 3.9 to 5.9 m below
ground surface was conducted by Pulsetal. (1999). Groundwater
velocities of 0.13 and 0.18 m/day were measured in this test.
Assuming an average hydraulic gradient of 0.0023 and porosity
of 0.38, these velocities correspond to an average hydraulic
conductivity of about 26 m/day. The groundwater is contaminated
with chromate and volatile organic compounds including
trichloroethylene (TCE), cis-dichloroethylene (cis-DCE), and vinyl
chloride (VC).
In June of 1996, a 46 m long, 7.3 m deep, and 0.6 m wide
permeable reactive barrier (continuous wall configuration) of
zero-valent iron was installed approximately 30 m from the
Pasquotank River (Figure 1; Blowes et al., 1999a,b). The
reactive wall was designed to remediate hexavalent chromium-
contaminated groundwater, in addition to treating portions of a
larger overlapping plume of TCE. A detailed monitoring network
of over 130 subsurface sampling points was installed in November
of 1996 to provide detailed information on spatial and temporal
changes in porewater geochemistry.
Denver Federal Center
The Denver Federal Center (DFC) is located about 10 km west of
downtown Denver, Colorado. Aquifer materials at the site are
made up of alluvial sediments that overlie the Denver Formation.
The Denver Formation is Paleocene to Late Cretaceous in age
and consists of brown, yellowish-brown, gray, and blue-gray
intercalated sandstone, claystone, siltstone, shale and
conglomerate containing olive-brown andesitic sandstone beds.
It lies about 2 to 14m below ground surface at the DFC and can
attain a thickness of up to 260 m. The Denver Formation has
been divided into two zones, the upper weathered zone and a
lower unweathered zone. These two zones are lithologically
similar but differ in color and permeability, with the upper
weathered zone having greater fracture permeability. The upper
weathered zone is up to 7 m thick and exhibits a grayish brown
color with yellowish orange staining while the lower unweathered
zone has a diagnostic blue color, commonly called "Denver Blue."
There are two separate deposits of alluvial sediments in the
vicinity of the DFC. The Verdos Alluvium of Pleistocene age is a
poorly sorted, stratified gravel containing lenses of sand, silt, and
clay. In some drainages, the Denver Formation may be overlain
by the Piney Creek Alluvium, well-stratified, interbedded organic
sands, silts and clays with interbedded gravels.
Groundwater at the site generally moves from west to east and
the average hydraulic velocity is about 30 cm per day in the upper
alluvium and weathered Denver formation (McMahon et al.,
1999). It is contaminated with volatile organic compounds,
primarily TCE, cis-DCE, trichloroethane (TCA) and
1,1-dichloroethene(DCE). At the eastern boundary of the site,
maximum concentrations of TCE, cis-DCE, TCA, and DCEwere
about 700 ppb, 360 ppb, 200 ppb and 230 ppb, respectively
(FHWA, personal communication, 2002). At least one source of
these VOCs was a leaking underground storage tank located
near Building 52 that was used by the Federal Highway
Administration (FHWA) to store waste, primarily TCA.
Groundwater flow from the aquifer discharges into Mclntyre
Gulch (Figure 2). Mclntyre Gulch is a deep channel that penetrates
the aquifer along the southern edge of the contaminant plume.
Downing Reservoir is too shallow to be influenced by the aquifer,
but the reservoir stage does affect the ground-water level.
In the fall of 1996, FHWA and GSA installed a permeable reactive
barrier atthe eastern edge of the DFC property along north-south
trending Kipling Street (Figure 2). In contrast to the continuous
wall design used at Elizabeth City, the DFC PRB has a funnel-
and-gate design configuration. The funnel component of the
PRB employs metal sheet pile that was driven into unweathered
bedrock of the Denver Formation or into resistant, weathered
layers of the Denver Formation. The depth of penetration of the
funnel ranged from about 7.0 to 10 m. The PRB has 4 reactive
gates, each 12.2 m long, up to 9.5 m deep, and from 1.8 m to
(Gate 1) to 0.6 m (Gates 3 and 4) wide. The design thickness
varied because of anticipated differences of contaminant fluxes
to the PRB at different locations.
Table 1 provides a general comparison of the PRBs at Elizabeth
City and the Denver Federal Center.
Methods
Groundwater Sampling
Groundwaterwas sampled from monitoring wells using peristaltic
or submersible centrifugal pumps. Atthe USCG-SC, 10-2" PVC
compliance wells have been monitored on a quarterly basis since
November 1996 and up to 125 multi-level wells have been
sampled on an annual basis since November 1996. Atthe DFC
approximately 18 wells were sampled on an annual basis over
the course of this study (1999-2000). Pumping rates were always
between 150 and 250 mL/min to minimize chemical and
hydrological disturbances in and around the well (Puls and
Powell, 1992). Prior to sample collection, waters were pumped
through a flow-through cell containing calibrated electrodes for
pH, oxidation-reduction potential (ORP), specific conductance,
and dissolved oxygen. Stabilization of electrode readings was
assumed after 3 successive readings within ±0.10 units for pH,
±10 mV for ORP, ±3% for specific conductance, and ±10% for
dissolved oxygen. After stabilization of electrode read-outs,
turbidity was generally less than 5 NTUs. Filtered samples for
cation analyses were collected using 0.45 urn cartridge filters
(Gelman aquaprep). Analyses were made using a Perkin Elmer
Optima 3300 DV inductively coupled plasma spectrometer
(ICP-OES). Anion samples were unfiltered and unacidified and
analyses were carried out using capillary electrophoresis (Waters
Quanta 4000E).
Colorimetric techniques were used in the field for Fe(l I), dissolved
oxygen, and hydrogen sulfide. Ferrous iron and sulfide were
determined using the 1,10 phenanthroline and methylene blue
indicators, respectively, and a HACH DR2010 spectrometer.
Dissolved oxygen was measured using CHEMets test kits that
employ the rhodazine D (low range) and indigo carmine (high
range) colorimetric indicators. Alkalinitytitrations were conducted
in the field by titration with standardized sulfuric acid to the
bromcresol green-methyl red endpoint. Dissolved H2 analyses
were performed atthe Denver Federal Center using the bubble
strip method (Chapelle et al., 1997).
Core Collection and Analysis
To assess the extent of corrosion and mineral buildup on the iron
surfaces, 5 cm i.d. cores were collected using a Geoprobe™.
Core barrels were driven using a pneumatic hammer to the
desired sampling location and continuous, upto 110cm, sections
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1690
0 m 75
Figure 2. Plan view of Denver Federal Center PRB and groundwater elevations (after McMahon et al., 1999).
Table 1. Comparison of PRBs Investigated in this Study
Contaminants PRB Date Iron Iron Ground Groundwater, Groundwater,
Configuration Installed Dimensions Volume water, pH DO
SC (mg/L)
(US/cm)
U.S. Coast
Guard Cr(VI)
Support
Center TCE, cis-DCE
Denver TCE, TCA, cis-
Federal DCE
Center
Continuous 6/96 46 m length
wall 7.3 m deep
0.6 m wide
Funnel-and- 10/96 Gate 1
gate 12.2 length
8.5 m deep
1.8 m wide
Gate 2
12.2 length
9.5 m deep
1.2 m wide
GateS
12.2 length
7.3 deep
0.6 m wide
201 m3
Gatel
187m3
Gate 2
139m3
GateS
53m3
325±126
(n=15)
Gatel
1236±65
(n=3)
Gate 2
1358±10
(n=3)
GateS
1306±10
(n=2)
5.94±0.44
(n=15)
Gatel
7.14±0.15
Gate 2
7.19±0.08
GateS
7.06±0.07
0.5±0.4
(n=11)
Gatel
0.5±0.2
Gate 2
0.2±0.1
GateS
<0.05
Notes: Geochemical parameters from Elizabeth City are average values (± 1 s.d.) from upgradient monitoring well MW48. All
parameters monitored quarterly since 2/97 to 3/01. Geochemical parameters from DFC gates 1, 2, and 3 are average values
from wells GSA21, GSA26, and GSA31. Gate 1 and 2 parameters were measured on a yearly basis from 5/99 to 7/01, and
gate 3 from 7/00 to 7/01. Information for DFC gate 4 is not included here; only gates 1-3 were studied in this investigation.
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of iron, iron + soil, or iron + pea-gravel were retrieved. Angle
cores (30° relative to vertical) and vertical cores were collected
in order to assess the spatial distribution of mineral/biomass
buildup in the reactive media. Priorto pushing the core barrel, an
electrical conductivity profile was collected to verify the exact
position of the iron/aquifer interface (e.g., Beck et al., 2001). In
all cases core recovery was 60 to 85% of the expected value.
Core materials from Elizabeth City and the Denver Federal
Center (1999-2000) were broadly similar in appearance. In all
cases the iron particles were jet black in color without any obvious
signs of cementation or oxidation. Iron grains from the upgradient
interface of DFC gate 2 were noticeably enriched in a black-
colored, gel-like material. Immediately after collection the cores
were frozen and shipped back to the Subsurface Protection and
Remediation Division in Ada, Oklahoma, for sub-sampling and
analysis. The frozen cores were partially thawed and then placed
in an anaerobic chamber with a maintained H2-N2 atmosphere.
Each core was logged and partitioned into 5 to 10 cm segments.
Each segment was homogenized by stirring in the glove box and
then split into 4 sub-samples: (1) inorganic carbon analyses, (2)
sulfur analyses/X-ray diffraction (XRD), (3) Scanning electron
microscopy (SEM)/X-ray photoelectron spectroscopy (XPS)
analyses, and, (4) microbial assays (phospholipid fatty acids,
PLFA). All sub-samples were retained in airtight vials to prevent
any air oxidation of redox-sensitive constituents.
Inorganic carbon analyses were conducted using a UIC carbon
coulometer system. Weighed samples were placed in a glass
reaction vessel and purged with CO2- scrubbed air. The samples
were then acidified with hot, 5% perchloric acid and evolved CO2
gas was carried to the coulometer cell containing a CO2-sensitive
ethanolamine solution and quantitatively titrated. Hot, dilute
perchloric acid dissolves carbonate precipitates such as siderite,
calcite, aragonite, and carbonate forms of green-rust. The
perchloric acid extraction does not liberate carbide-carbon, which
is present at concentrations near 3 wt% in Peerless iron. Un-
reacted Peerless iron contains 15±5 ppm of acid extractable
carbon. Total sulfur measurements were made with a UIC sulfur
coulometer system. Iron samples were covered with V2O5 and
combusted in the presence of oxygen at 1050 °C. Evolved gases
are passed through a column of reduced Cu to quantitatively
convert all sulfur to SO2, which is then carried to the coulometer
cell where it is absorbed and coulometrically titrated. Un-reacted
Peerless iron contains 5±1 ppm of sulfur using this combustion
method. In addition, acid-volatile sulfide (AVS) and chromium-
reducible sulfide (CRS) extractions were performed using hot,
6 M HCI and 1 M CrCI2 in 0.5 M HCI, respectively (Zhabina and
Volkov, 1978). These acid extractions determine the quantities
of metal monosulfide precipitates (AVS) and iron disulfide
precipitates (i.e., pyrite; CRS).
XPS measurements were made using a PE Model 5500 X-ray
photoelectron spectrometer, operated with an Al Ka X-ray source
at a power of 400 W(T. Sivavec, General Electric, Research and
Development). Atomic concentrations were obtained from peak
areas using established sensitivity factors. Depth profiles were
measured by resting a 4 kV argon beam over an area of 2x2 mm.
X-ray diffraction patterns were collected of the fine-grained
materials removed by sonication from the iron grains. Samples
were mounted on a quartz plate and diffraction patterns were
obtained using a Rigaku Miniflex diffractometer using Cu Ka
radiation (0.01 degree step, 0.5 degrees two-theta per minute).
Scanning electron and optical microscopy were utilized to
determine the thickness of surface precipitates and evaluate
physical morphology of grains and extent of surface coverage.
Prior to microscopic characterization, samples were set in epoxy
resin, cured, and ground and polished using standard techniques.
Samples were analyzed forcontent and distribution of phospholipid
fatty acids (PLFA; Microbial Insights, Inc.). PLFA analyses are
based on the extraction and separation of lipid classes, followed
by quantitative analysis using gas chromatography/mass
spectrometry (GC/MS).
Results and Discussion
Groundwater Chemistry
Major anion and cation compositions of groundwater collected
upgradient from the iron treatment zones at Elizabeth City and the
Denver Federal Center are shown in Figures. Groundwaterfrom
Elizabeth City contains lower concentrations of total dissolved
solids (<400 mg/L) compared to groundwater from the DFC
(1000-1200 mg/L). At both sites, the concentration of dissolved
oxygen in upgradient groundwater is about 1 mg/L and pH is
near-neutral. There are, however, spatial variations in Eh values
HCO
Elizabeth City ML10
Denver FC
SOT*
Mg2
IDS, mg/L
O 900-1200
o 600-900
° 300-600
• 0-300
Figure 3. Upgradient groundwater compositions (molar ratios) and TDS values forthe Elizabeth City and Denver Federal Center
PRB sites.
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of groundwater entering the iron zones at both sites (Table 2).
Reactions between zero-valent iron and groundwater impact, to
some extent, the concentration of all major solute species
(Figure 4). Long-term trends indicate consistent removal of TCE
and Cr(VI) at Elizabeth City as well as, among other species,
sulfate, calcium, magnesium, and silica (Figure 4). Similartrends
are observed at the DFC and are consistent with the patterns
observed at other zero-valent iron PRB sites (Phillips etal., 2000;
Liang et al., 2000; Battelle, 2000; Morrison et al., 2001). In
downgradient wells, an analysis of data collected over five years
at Elizabeth City shows little variation for most species; however,
there are subtle time-dependent increases in pH and decreases
in Eh (Figure 5).
A comparison of groundwater chemistry between upgradientand
downgradient wells indicates that the iron zones at Elizabeth City
and the DFC are long-term sinks for C, S, Ca, Si, Mg, N, and Mn.
Concentrations of dissolved iron are elevated in downgradient
compliance wells at Elizabeth City (Figure 5). Dissolved iron is
not released from the zero-valent iron but probably from the
downgradientaquiferdueto decreased redox potentials in regions
immediately downgradient from the iron media. In the region
immediately upgradient from the iron wall at Elizabeth City,
increases in the concentration of ferrous iron have led to the
development of a reducing zone where a fraction of the hexavalent
chromium is removed from the groundwater plume (Figure 4).
Removal of chromium continues as the plume passes through
the iron media.
Reaction Path Modeling
Thermodynamic modeling can in principle be used as a predictive
tool for estimating the types and quantities of mineral precipitates
that form as groundwater reacts and approaches thermodynamic
equilibrium with Fe° (Gavaskar et al., 1998; Blowes and Mayer,
1999; Morrison et al., 2001). The Geochemist's Workbench
program was used to simulate the incremental dissolution of Fe°
into representative groundwater compositions from Elizabeth
City and the DFC. The primary thermodynamic database was
modified to include solubility data and reaction stoichiometry for
zero-valent iron, iron monosulfides, and green-rusts (Table 3).
Figure 6 shows the predicted changes in masses of minerals that
precipitate and dissolve as the systems approach equilibrium
with Fe°. In general, equilibrium modeling predicts that Fe° will
eventually react to form magnetite, mackinawite, magnesium
silicate, and/or magnesium hydroxide. Carbonates (calcite,
dolomite), pyrite, quartz, and green-rust are predicted to be
intermediate products. Although dolomite formation is predicted
based upon equilibrium modeling, the precipitation of dolomite is
unlikely in a PRB. Dolomite has a chemical formula CaMg(CO3)2
and a crystallographic structure similarto that of calcite. It is well
known that reactions to precipitate or dissolve dolomite are
extremely slow at low temperatures (Hsu, 1967) so that dolomite
precipitation is an unlikely cause of the comparatively rapid
accumulation of inorganic carbon and magnesium in PRBs.
Interestingly, the reaction path model predicts the formation of
carbonate-green-rust prior to magnetite, consistent with iron
corrosion studies (McGill etal., 1976; Bonin etal., 2000). Green-
rusts are unstable, mixed valence Fe(ll)-Fe(lll)hydroxy-salts that
are highly susceptible to oxidation in the presence of oxygen.
They are known to be transient products of iron corrosion that
precipitate as metallic iron oxidizes to form Fe(lll) oxyhydroxides
(McGill et al., 1976; Schwertmann and Fechter, 1994). The
structure of green-rusts consists of sheets of Fe(ll)(OH)6 in which
some of the Fe(ll) is substituted for by Fe(lll). This mixture of
ferrous and ferric iron results in a net positive layer charge, which
is balanced by interlayer incorporation of anions such as Cl",
CO32", and SO42". Sulfate and carbonate forms of green-rust
appear to be the most important in natural systems (e.g., Genin
etal., 2001).
Table 2. Groundwater Chemistry at Elizabeth City (June, 2000; Median Values Transect 2) and the Denver Federal Center
(July, 2000; Median Values)
Elizabeth City
aquifer Fe°
pH 5.84 9.51
Eh (mV) 491 -347
02(mg/l) 0.3 0.1
H2 (nM)
Na (mg/l) 50 29
K 3.0 3.1
Mg 9.1 3.2
Ca 16.4 5.3
Fe <0.04 0.05
Sulfate 49 <1.0
Chloride 51 25
Nitrate 0.9 <0.1
Silica 10.9 <0.2
DIG 15 8
DOC 1.2 0.9
IDS 290 143
DFC - gate 1
aquifer Fe°
7.24 10.30
233 -247
0.92 0.1
0.63 948
179 184
0.39 0.60
18.1 3.9
100 1.2
0.19 <0.04
234 <1.0
52 59.7
3.2 <0.10
13.2 0.3
86 60
2.3 1.8
1090 510
DFC - gate 2
aquifer Fe°
7.80 9.90
281 -35
0.3 0.1
2.38 304
167 248
0.27 0.95
31 61
114 4.1
<0.04 <0.04
286 359
66 83
2.9 1.1
14.8 0.3
95 46
1.6 3.3
1172 1011
-------
Eh (mV, SHE)
sulfate (ppm)
alkalinity (ppm)
0.0 1.0 2.0 3.0
calcium (ppm)
silica (ppm)
0.0 1.0 2.0 3.0
0.0 1.0 2.0 3.0 0.0 1.0 2.0 3.0 0.0 1.0 2.0 3.0
June, 2000
magnesium (ppm) iron (ppm) chromium (ppm) cis-DCE (ppb)
0.0 1.0 2.0 3.0
0.0 1.0 2.0 3.0
0.0 1.0 2.0 3.0 0.0 1.0 2.0 3.0
0.0 1.0 2.0 3.0
Distance, m
Figure 4. Cross-sectional profiles at the Elizabeth City PRB for selected species (June, 2000; Transect 2).
25
9.0
8.5 -
8.0 -
7.5 -
7.0-
6.5-
6.0 -
5.5 -
5.0
20-
Q.
Q.
CO
0
0.0
Oct-95 Mar-97 Jul-98 Dec-99 Apr-01 Oct-95 Mar-97 Jul-98 Dec-99 Apr-01
Oct-95 Mar-97 Jul-98 Dec-99 Apr-01
-o— MW13 ,
Upgradient wells
-•— MW48
-A-MW49
Downgradient wells
-A— MW47
Figure 5. Time-dependent pH values and concentrations of calcium and iron in compliance wells from Elizabeth City (MW13, 30 m
upgradient of PRB, screened interval 4.3-7.3 m; MW48, 0.6 m upgradient, screened interval 4.3-7.3 m; MW49, 1.8 m
downgradient, screened interval 4.3-7.3 m; MW47, 1.8 m downgradient, screened interval 4.3-7.3 m).
-------
Table 3.
Thermodynamic Constants Added to the Geochemist's Workbench Database (Bethke, 1998)
Phase
Fe°
Mackinawite
(ordered)
Mackinawite
(precipitated)
Green Rust SO4
Green Rust Cl
Green Rust CO3
Gibbs free energy of formation
(AG°fi kJ/mol)
0
-88.4
-83.7
-3785.0
-2145.0
-3588.0
Reference
Benning et al. (2000)
Benning et al. (2000)
Bourrie et al. (1999)
Bourrie et al. (1999)
Bourrie et al. (1999)
Reactions
FeS + H+ = Fe2+ + HS"
Fe6(OH)12SO4 + 12H+ = 4Fe2+ + 2Fe3+ + SO42" + 12H2O
Fe6(OH)12CO3 + 12H+ = 4Fe2+ + 2Fe3+ + CO32" + 12H2O
Fe4(OH)8CI + 8H+ = 3Fe2+ + Fe3+ + Cl" + 8H2O
Unfortunately, progress along the reaction path is arbitrary; the
x-axes in Figure 6 do not represent time or space. Reaction path
models may be used to qualitatively evaluate materials that might
form in a PRBfroman initial groundwater composition. However,
with respect to predicting the space- or time-dependent
accumulation of mineral mass or volume, reaction path modeling
is less useful because it does not account for reaction rates or
non-equilibrium processes (i.e., microbiological activity).
Numerical models that consider kinetic limitations are beginning
to provide reasonably accurate predictions of mineral accumulation
in PRBs(e.g., Mayer etal., 2001), although there is still a need to
collect field data to verify and refine these multicomponent
reactive transport simulations.
Mineral Precipitates
Solid-phase characterization studies implementing XRD, SEM-
EDX, reflected-light microscopy, transmitted-light microscopy,
XPS, and chemical extractions indicate the accumulation of
lepidocrocite, magnetite, carbonate minerals (aragonite, siderite,
or green-rust), and iron monosulfide as primary precipitates in the
Elizabeth City and DFC PRBs. Similar authigenic precipitates in
other Fe° PRBs were reported in previous field and laboratory
studies (e.g., Phillips et al., 2000). The formation of these
minerals is predictable based on the results of thermodynamic
models discussed above. However, as previously noted, these
equilibrium models are not able to successfully predict the rate of
mineral buildup or the position of precipitates within the reactive
media, both essential to understanding the impact of mineral
precipitation on long-term remedial performance.
XPS scans show that iron particle surfaces from Elizabeth City
and the DFC contain C,O, Fe, Si, S, Mg, Ca, Mn.andN. TheXPS
data indicate a surface layer dominated by iron oxyhydroxides,
an intermediate layer of iron oxide, and finally zero-valent iron at
the greatest sputtering depths. Surface carbon is present as
carbonate with some detected hydrocarbon (binding energy
284.6 eV). The oxidation state of sulfur is predominantly present
as sulfide (-2) but with minor amounts of sulfate (+6). Surface
enrichment in the elements Ca, Mg, S, and Si are consistent with
observed decreases in groundwater concentrations of these
elements. Chromium was sometimes detected by XPS in iron
samples from Elizabeth City, consistent with the reduction/
precipitation mechanism for chromium uptake (Cantrell et al.,
1995; Powell et al., 1995; Pratt et al., 1997). Table 4 lists the
elements that preferentially partition to the iron media at Elizabeth
City and the Denver Federal Center, the form of each element as
it is transported to the iron media, and the possible solid-phase
form of each element in the iron media. Research is continuing
to identify the structure and mineralogy of precipitates that form
near the surfaces of iron particles in the PRBs at Elizabeth City
and the Denver Federal Center.
Microscopic observations indicate that mineral accumulation is
mainly occurring on the surfaces of the iron particles where steep
gradients in pH and redox potential promote mineral precipitation
(Figure 7). After 3.5 years of mineral accumulation, a consistent
coverage of surface material ranging in thickness from 10 to 50
urn is observed on iron grains collected near the upgradient
interface at Elizabeth City (horizontal penetration <8 cm). At
greater penetration depths (>8 cm), surface coatings are
-------
I I I I I I I I I I I I I I I I I I I
.1 .2 .3 .4 .5 .6 .7 .8 .9 1
Reaction progress
I I I I I I I I I I I I I I I I I I I
Figure 6.
.1 .2 .3 .4 .5 .6 .7 .8 .9 1
Reaction progress
Reaction path model results showing mineral precipitation trends for representative groundwater compositions from
(a) Elizabeth City and (b) Denver Federal Center (gate 2).
Table 4. Elements Removed from Groundwater by Fe° PRBs, Input Forms, and Possible Solid Phase Associations
element
c
s
Ca
Mg
Si
input form
HC03-
SO42"
Ca2+
Mg2+
H4Si04°
solid phase association
CaCO3, FeCO3, Fe6(OH)12CO3
FeS
CaCO3, sorbed (?)
incorporation into CaCO3; Mg-OH-Si
sorbed onto Fe(OH)3 (?)
ppts (?)
10
-------
Figure 7. SEM photomicrographs of iron particles collected near the upgradient iron/aquifer interface (Elizabeth City, 2000).
11
-------
discontinuous and <5 urn thick. Where mineral accumulations
are the greatest, distinctive particle morphologies include radiating
needles, characteristic of carbonates formed at high degrees of
supersaturation; homogenous crusts without euhedral
morphologies, characteristic of iron oxyhydroxides; and thin
coatingsoraggregatesofveryfine-grained particles, characteristic
of iron monosulfides. Textural forms and coating thickness of
surface precipitates are broadly similar at the DFC and are
comparable to those reported from the Y-12 Fe° barrier (Phillips
etal.,2000).
Eighteen cores were analyzed to determine concentrations and
spatial distribution of inorganic carbon (1C) and sulfur accumulation
in the Elizabeth City and the DFC Fe° PRBs. At both sites,
accumulation of 1C and sulfur is greatest nearthe upgradient soil-
Fe° interface, with concentrations decreasing with increasing
penetration into the iron (Figure 8). Anomalous buildup of mineral
precipitates and microbial biomass is readily apparent in gate 2
of the DFC. Concentrations of 1C in gate 2 are as high as 8000
ppm and total sulfur values approach 4500 ppm, or a factor of
about 4x the amounts observed in DFC gate 1 or the Elizabeth
City PRB.
Total sulfur measurements compare well with the results of acid-
volatile sulfide (AVS) extractions (Figure 9). Sulfur in the reactive
barrier system is mainly present as labile, iron sulfide phases.
Iron sulfides could potentially enhance the degradation of
halogenated aliphatic compounds in PRBs (Butler and Hayes,
2000). The high pH, low Eh geochemical signature in iron zones
favors the chemical reduction of sulfate to sulfide. Rates of
chemical sulfate reduction at low temperatures are extremely
slow(Trudingeretal., 1985). It is expected, therefore, thatthe low
Eh (high hydrogen fugacity) environment supports metabolism
by sulfate-reducing bacteria in the iron (see Microbiology section
below). Bisulfide respired by these bacteria rapidly reacts at high
pH with ferrous iron produced via the dissolution of the zero-
valent iron to form acid-volatile sulfides. Less than about 5% of
the accumulated sulfur in the Fe° is extractable with hotchromous
chloride which suggests either that some transformation of the
iron monosulfides to pyrite (FeS2) has occurred orthat labile AVS
phases have transformed to more crystalline, acid-resistant iron
monosulfides (most likely greigite). The solubility of precipitated
mackinawite decreases with increasing pH and can be described
by:
FeS + H+ = Fe2+ + HS-
(5)
withlogK =-3.6(Benningetal.,2000). AtphMOandlog aFe2+ = -5
(typical of Elizabeth City water), the expected equilibrium activity
of bisulfide is 10~8-1, which is much lower than the nominal
detection limit of the methylene blue method (~10'58 mol/L) for
dissolved sulfide. Therefore, the detection of hydrogen sulfide in
groundwater samples collected from Fe° media is not necessarily
an indicator of whether iron sulfides are precipitating in the
reactive media. ConcentrationsofhydrogensulfideuptoO.5 mg/L
were observed in groundwater samples collected from the DFC.
These high dissolved sulfide values suggest that particulates
were entrained during well purging even at the low flow rates used
in this study.
Porosity loss in the iron zones due to the precipitation of inorganic
carbon and sulfur minerals was estimated by integrating the
concentrations of 1C and S as a function of distance in the iron and
estimating the volume loss by using the molar volumes of pure
calcite (CaCO3), siderite (FeCO3), and mackinawite (FeS). The
proportion of 1C present as calcite and siderite was calculated by
evaluating the excess drawdown of dissolved inorganic carbon
5 10 15 20 25 30 35
10 15 20 25 30 35
Distance, cm
Figures. Concentration distribution of (a) solid phase
inorganic carbon, (b) sulfur, and (c) calculated
porosity lost after ~4 years in the Fe° media at
Elizabeth City and the DFC. Concentrations of C
and S were obtained from cores EC060300-4
(Elizabeth City); C1-2-71000 (DFC, gate 1); and,
C2-17-71300 (DFC, gate 2). Porosity lost is
calculated with [(initial porosity-final porosity)/(initial
porosity)].
relative to dissolved calcium concentrations between upgradient
and downgradient monitoring wells. This method suggests that
inorganic carbon is evenly distributed between FeCO3and CaCO3
at Elizabeth City; the DFC gates are slightly enriched in FeCO3
(60%) relative to CaCO3(40%). Porosity gain due to the dissolution
of Fe° was estimated by assuming that all Fe2+ released was
precipitated as siderite and mackinawite. Because the cores
were collected at an angle, the measured concentration profile
12
-------
o>
0)
tJ
8 cm. In gate 1 of the
DFC, the precipitation front is spread out over a greater distance
in the iron and may be the result of higher flow rates in gate 1
(0.38 m/d) compared to Elizabeth City (0.15 m/d). In gate 2,
14.2% of the available porosity in first 2.5cm of iron was lost over
the first 3.75 years of operation.
Table 5.
Mass Accumulation of Inorganic Carbon and Sulfur and Estimated Annual Volume Loss
Elizabeth
City
date
6/98
6/98
6/99
6/00
6/00
mass 1C
(kg)
28.4
68.3
136.6
53.8
mass S
(kg)
11.7
7.6
13.1
28.9
29.8
flux 1C fluxS VFeCC-3 V CaCO3 V FeS V loss
(kgm'V1) (kgm'V1) (103xcm3y~1) (103xcm3y~1) (103xcm3y~1) (% total y'1)
0.058
0.093
0.140
0.055
0.024
0.016
0.018
0.030
0.031
17.3
27.7
41.6
16.4
21.9
35.1
52.7
20.7
3.9
2.5
2.9
4.8
5.0
0.05
0.08
0.12
0.05
DFC
gate 1
DFC
gate 2
7/00
7/00
34.2
481.9
41.2
179.4
0.204
2.158
0.246
0.803
13.3
188.0
11.3
158.7
7.4
32.0
0.03
0.43
Note: V loss is the net annual porosity lost assuming that all precipitates accumulate evenly throughout the PRB.
13
-------
Mass balance on C and S was estimated by evaluating solid
phase concentration data and the changes in the groundwater
concentrations of inorganic carbon and sulfate. Mass
accumulation of 1C and S in the Fe° media based on changes in
groundwater solute concentrations was estimated from Q-ACV
where Q is the volumetric flux of water through the iron media
(L/y) and AC. is the change in the concentration of dissolved 1C
and S (mg/L) between upgradient and downgradient sampling
points. Average groundwater flow velocities at Elizabeth City of
0.13 to 0.18 m/d were reported by Blowes et al. (1999). McMahon
et al. (1999) report flow median velocities in gate 1 and gate 2 at
the DFC of 0.38 m/d and 0.11 m/d, respectively, based on heat
pulse flowmetermeasurements. Results forcarbon mass balance
(Figure 10) agree to within a factor of 1.5x for the Elizabeth City
PRB and gate 2 of the DFC, but with poor agreement in gate 1.
Sulfur mass balance is in reasonable agreement for gates 1 and
2 of the DFC. Considerably more accumulation of sulfur is
expected in the Elizabeth City PRB than has been observed on
the core materials. In gate 2, more solid phase C and S is
observed on the iron grains than is predicted from the observed
changes in groundwater chemistry. These trends may indicate
that rates of mineral accumulation have decreased with time, i.e.,
the estimates in Table 5 are based on the assumption of steady-
state conditions throughout the lifetime of the PRBs. Several
factors lead to uncertainty in the mass balance calculations for
PRBs. Estimates of accumulation based on changes in
groundwater chemistry depend on chemical and hydrogeologic
measurements. Determination of dissolved constituents may be
analyzed at a high level of accuracy and precision. However,
estimates of the mass flux of ground water moving through PRBs
are prone to large uncertainties. Estimates of accumulation
based on characterization of core materials depend on analytical
measurements and also on estimates of emplaced iron density.
Spatial heterogeneity in groundwater flow velocity, concentration
of solutes, concentration of solid phase products, and emplaced
iron density all factor into the uncertainty analysis of mass
balance calculations.
o
Cell 1, DFC
Cell 2, DFC
E. City
Cell 1, DFC
Cell 2, DFC
E. City
Figure 10.
Mass balance estimate for inorganic carbon and
sulfur in the Fe° media at Elizabeth City and the
DFC. Solid bar based on solid-phase
characterization; open bar based on groundwater
composition and flow rate.
Several transformation pathways of iron minerals in Fe° PRBs are
important to evaluations of long-term performance by affecting
the surface properties and reactivity of the iron particles and
reducing available porosity. Oxidized, reduced, and mixed-
valence iron phases form and undergo transformation in Fe°
systems. Oxidized phases (lepidocrocite, goethite) apparently
are the result of reactions with dissolved oxygen; elevated DO
concentrations can lead to rapid corrosion and clogging of Fe°
due to ferric oxyhydroxide precipitate at the upgradient interface
(Liang et al., 2000). Akaganeite has been identified based on
X-ray diffraction analyses at the PRB from Y-12 plant site (Phillips
et al., 2000). Akaganeite is rare in nature; its formation is
restricted to Fe- and Cl-rich hydrothermal brines and as the result
of iron corrosion in Cl-rich fluids. Akaganeite also forms as an
alteration product of green-rust compounds (Schwertmann and
Taylor, 1989) and therefore may be related to sample preservation
and the oxidation of green-rust compounds rather than incipient
precipitation of this phase as an iron corrosion product in
groundwater systems.
Mixed valance state iron minerals include magnetite and green-
rust compounds. Green-rust compounds are corrosion products
that are expected to form under more reducing conditions than do
iron oxyhydroxides. Their precipitation is favored under alkaline
conditions and transformation of these compounds to magnetite
is expected based on the thermodynamic calculations presented
in a preceding section. Iron carbonate and monosulfides are the
primary reduced iron forms and are likely to persist as long as
reducing, alkaline conditions persist. Although iron monosulfides
are unstable relative to pyrite, the kinetics of this transformation
process decrease with increasing pH so that iron monosulfides
may be the long-term solid phase sulfide in Fe° PRBs. Similarly,
substantial iron carbonate dissolution is not expected so long as
alkaline conditions persist. Research at EPA is continuing on the
factors that govern iron mineral transformations in PRBs and
impacts to system longevity and performance.
Microbiology
Seventy-one core samples collected from the Elizabeth City and
DFC PRBs were analyzed for content and distribution of
phospholipid fatty acids (PLFA). These organic compounds can
be used as lipid biomarkers to provide a quantitative means to
evaluate viable microbial biomass, community composition, and
nutritional status. The cores contained sediments from upgradient
and downgradient locations adjacent to the PRBs, iron filings
from the PRB matrix, or a mixture of sediment and iron matrix
when the cores were collected at sediment/PRB interface. The
highest accumulations of microbial biomass were found at the
DFC in iron samples from gate 2. Concentrations as high as
4,100 pmoles/g dry wt were measured in iron matrix samples from
approximately 17 ft below ground surface in gate 2 which is
equivalent to 8.36 x 107 cells per gram of iron matrix. Sediment
samples from 0 to 5 cm upgradient of the gate 2 PRB had similar
biomass levels and composition. Microbial biomass correlates
well with total sulfur suggesting that the accumulated biomass
contains anaerobic, sulfate-reducing bacterial consortia. The
presence of this consortium is also supported by the analysis of
PLFA structural groups that suggest the presence of sulfate-
reducing bacteria (mid-chain branched saturate fatty acids;
Dowling et al., 1986; Parkes et al., 1992), of Gram negative
anaerobic bacteria (terminally branched saturate fatty acids;
Parkes etal., 1992; Guckertetal., 1985), and of anaerobic metal-
reducing strains (branched monenoics fatty acids; Parkes et al.,
1992; Edlund et al., 1985). These three PFLA structural groups
comprise approximately 30% of the total PFLA mass in gate 2
samples.
14
-------
It is perhaps of value to compare the biomass and community
structure information in gate 2 samples versus gate 1 samples
due to differences in performance observed at these two PRB
systems at the DFC (Table 6). The overall distribution of PLFA
structural groups varies between gates 1 and 2. The higher
biomass, % contribution of anaerobic bacteria, and bacteria to
eukaryote ratio in gate 2 are all consistent with an anaerobic PRB
environment with a higher biological energy sink, suggesting that
hydrogen levels would be lower and that less reducing power
would be available for chemical reduction (White et al., 1980).
This would seem to agree with the lower dissolved hydrogen
levels seen in gate 2 (Table 2), and with the breakthrough of
contaminants dueto less available reducing powerand/or biomass
impacting system hydraulics. The higher pH and lower Eh values
associated with gate 1 would also create an environment less
conducive to bacterial growth, although it is interesting to speculate
as to the conditions that govern microbial growth and the conditions
that are impacted by microbial growth.
The availability of 45 samples from the Elizabeth City PRB,
collected as vertical, upgradient and downgradient interfacial
cores, allows for some analysis of the spatial distribution of
bacterial biomass and biomass composition. The highest biomass
levels were observed in upgradient core subsections, which
contained the sediment/iron interface (Figure 11). These samples
contained from 600 to 900 pmoles of PLFA per gram of sediment
and iron. This is equivalent to 1 to 2 x 107 cells per gram of solid
matrix or less than half the maximum detected in DFC samples
from gate 2. While total biomass peaked at the sediment/iron
interface, the contribution of anaerobic biomarkers identified
above (TerBrSats, BrMonos, MidBrSats) as a percent of total
biomass peaked 1 to 3 inches into the iron matrix (Figure 12). The
localization of metal and sulfate reducing activity near the
upgradient sediment/PRB interface would seem to correlate
reasonably well with the iron, sulfate, and Eh contours shown in
Figure 4, and with the sulfur concentrations and porosity changes
illustrated in Figure 8. Evaluation of the microbial populations
through the PLFA data from downgradient PRB/sediment interface
is more difficult. As shown in Figure 13, no clear pattern of
biomass distribution or population composition is immediately
apparent, although there seems to be some enhancement of an
anaerobic population immediately down stream of the interface.
Microbial counts range from 1.85 x 106 to 2.51 x 103, thus
downgradient cores seem to contain a 10-fold reduction in the
range of values (at both the maximum and minimum) compared
to the upgradient cores. The lower counts associated with the
mid-barrier and downgradient samples suggest that the
environment at these locations is more challenging to bacterial
growth and survival. Examining the geochemical conditions
associated with these locations supports this hypothesis. Figure 4
indicates a decrease in biologically available electron acceptors
such as sulfate and cis-DCE in the mid-wall and downgradient
locations. The higher pH along with precipitated iron and sulfur
species would also tend to create a more severe environment for
bacterial growth. Table 2 also suggests that mid-barrier and
downgradient locations may be less conducive for growth due to
a higher pH and a lack of electron acceptors (oxygen, sulfate,
nitrate), which would seem to negate the positive influence of an
abundant electron donor, dissolved hydrogen.
The use of cathodic hydrogen as an energy source for biofilm
development has been a staple of the corrosion literature for
some time (Von Wolzogen Kuehr and van der Vlugt, 1934). The
high surface areas of PRBs and the supply of electrochemically
active species such as sulfur oxyanions insure the development
of dissolved hydrogen in the PRB and the local subsurface
environment. Using the USCG-SC PRB as an example, we can
conduct some theoretical exercises to evaluate the potential
impact of biomass development on PRB performance based on
direct geochemical measurements. Assuming that the PRB
represents a 300 m2 cross section, and has a flow of 0.15 m/day,
50 % porosity, and an average dissolved hydrogen level of 500
nm, the net hydrogen production would be 6.75 mmoles/day
(Cord-Ruwish, 2000). This could yield 0.675 crrWday (assuming
0.5 mol ATP/mol H2, 200 g Biomass/mol ATP, Biomass density
1 g/cm3) and the Fe(l I) concentrations in Figure 4 yield 24.2 cm3/day
(assuming 1 mol Fe(ll)/molH2). Whilethese estimates of biomass
formation seem insignificant, the localization of this activity in
discrete zones and interaction with inorganic precipitates may
magnify the effect. However, the bio-available energy and thus
potential biofilms generating capacity represented by the PRB is
enormous. A1 m2 surface area that corrodes at 0.1 mm per year
would yield, using a density of iron of 7800 kg/m3, 14 moles of
H2/day or approximately 40 mmoles/day. This is equivalent to
1.5 urn of biomass per day over the entire surface area of the
PRB. This assumes uniform growth and no loss to predation or
sloughing of biomass. Nevertheless, these numbers indicate a
significant potential for biofouling in PRBs.
Table 6.
PLFA Results for DFC Gates 1 and 2
site
DFC gate 1
DFC gate 2
ave. biomass
325
2283
cells/g
6.5 x106
4.6 x107
bact/euk ratio
156
>600
% anaerobic
-12%
-30%
Notes: Average biomass in pmoles PLFA/g dry wt. Cell/g is estimated bacterial cells per gram dry weight of sample based on 10s
pmoles PLFA per gram dry weight of cells and 2.0 x 1012 cells per gram dry weight cells. Bac/euk ratio is the average bacteria
to eukaryote ratio, ratio ofpolyenoic PLFA to total- polyenoic PLFA. %Anaerobic is the percent of total PLFA composed of
terminally branched fatty acids, branched monenoics fatty acids and mid-chain branched fatty acids. Values are average of
12 gate 1 samples or 14 gate 2 samples collected 7/2000.
15
-------
Sediment Fe°
i 1000n
-30 -20 -10 0 5 10 15 20 25 30 35 40
Subcore thickness relative to Fe°/Sediment interface, cm
Figure 11. Biomass concentration in Elizabeth City cores collected June, 2000 (upgradient aquifer/iron region). PLFA data collected
from core EC060200-1 (open circles) and core EC060300-4 (open squares).
Sediment Fe
100 1
75-
D SRBs
• Metal red.
D G+/AnG-
-O- Biomass
0
-600
-500
400
-300
-200
-100
T3
f
CO
JD
O
Q.
V)
CO
CO
g
in
-30 -20 -10 0 5 10 15 20 25 30 35 40
Subcore thickness relative to Fe°/Sediment interface, cm
Figure 12. Concentration and distribution of PLFA in Fe° from Elizabeth City core EC060300-4 (upgradient aquifer/iron region).
16
-------
a;
EL
0
Fe° Sediment
100
-25
0
f
CO
_(D
O
a.
co
CO
CD
E
g
CQ
40 35 30 25 20 15 10 5 0 10 20 30
Subcore thickness relative to Fe°/Sediment interface,
cm
Figure 13. Concentration and distribution of PLFA in Fe° from Elizabeth City (downgradient iron/aquifer region; core EC060300-6).
Summary: PRB Long-term Performance
The types of mineral precipitates that form in Fe° barriers are
consistent with those predicted with geochemical models. The
principal factors that determinethe amount of mineral precipitation
and the spatial distribution of precipitates in reactive iron media
are flow rate, groundwater chemistry, and microbial community
structure. After four years of operation, the Elizabeth City and
DFC reactive barriers have developed consistent patterns of
spatially heterogeneous mineral precipitation and microbial
activity. The development of precipitation and biomass fronts
result from the abrupt geochemical changes that occur at
upgradient interface regions coupled with groundwater mass
flux. Upgradient regions at both sites investigated in this study
have witnessed the greatest accumulation of mineral mass and
biomass. Neither of the sites of this study show complete filling
of available pore space after four years, suggesting that flow
characteristics should not be affected by the accumulation of
authigenic components. However, even relatively thin coatings
of mineral precipitates that do not affect flow patterns may affect
the reactivity of iron particles with respect to the degradation of
chlorinated organic compounds by diminishing electron flow
capacity. At Elizabeth City, chromium concentrations have been
reduced to less that 0.01 mg/L and detectable concentrations of
chromium have not been observed in any of the downgradient
compliance wells since November, 1996. Similarly, concentrations
of chlorinated organic compounds (TCE, cis-DCE, VC) at Elizabeth
City are below regulatory target levels in downgradient compliance
wells. At the DFC, treatment of chlorinated compounds with Fe°
has been equally successful in gates 1 and 3. In gate 2, detection
of 1,1-DCE at downgradient sampling points has been linked to
impacts of the funnel-and-gate system on groundwater flow and
bypass of contaminants underneath the reactive zone or residual
contamination in downgradient sediments (McMahonetal., 1999).
Contaminant breakthrough, particularly of 1,1 -DCE is likely related,
either directly or indirectly, to anomalous buildup of authigenic
precipitates and biomass on the iron surfaces which could cause
a loss of effective reactivity, and therefore decreased efficiency
of contaminant degradation processes in the reactive media.
Residual contamination downgradient of gate 2 is also a likely
source of contaminants (FHWA, 1999). In addition to mineral/
biomass accumulation in gate 2, potential indicators of decreased
performance are increased Eh values, decreased dissolved
hydrogen values, increases in relative specific conductance
values and concentrations of certain solutes, such as sulfate and
magnesium.
Acknowledgments
WethankF. Beck, P. Clark, M. McNeil, C. Paul, F. Kahn, K. Jones,
and J. Cloud for invaluable field and laboratory assistance. We
also gratefully acknowledge the support provided by ManTech
Environmental Research Services Corp. J. P. Messier (USCG)
is thanked for providing site assistance at the USCG-SC and
C. Eriksson (FHWA) and J. Jordan (FHWA) are thanked for site
assistance at the DFC.
Notice
The U. S. Environmental Protection Agency through its Office of
Research and Development funded the research described here.
This research brief has been subjected to the Agency's peer and
administrative review and approved for publication as an EPA
document. Mention of trade names or commercial products does
not constitute endorsement or recommendation for use.
17
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