influence of hydrogeochemical processes on zero-valent iron reactive barrier performance: a field...

Journal of Contaminant Hydrology 78 (2005) 291–312

www.elsevier.com/locate/jconhyd

Influence of hydrogeochemical processes

on zero-valent iron reactive barrier performance:

A field investigationB

Liyuan Liang*, Gerilynn R. Moline,

Wiwat Kamolpornwijit, Olivia R. West

Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6038, USA

Received 8 July 2003; received in revised form 25 May 2005; accepted 25 May 2005

Abstract

Geochemical and mineralogical changes were evaluated at a field Fe0-PRB at the Oak Ridge

Y-12 site concerning operation performance during the treatment of U in high NO3� groundwater.

In the 5-year study period, the Fe0 remained reactive as shown in pore-water monitoring data,

where increases in pH and the removal of certain ionic species persisted. However, coring

revealed varying degrees of cementation. After 3.8-year treatment, porosity reduction of up to

41.7% was obtained from mineralogical analysis on core samples collected at the upgradient

gravel–Fe0 interface. Elsewhere, Fe0 filings were loose with some cementation. Fe0 corrosion and

pore volume reduction at this site are more severe due to the presence of NO3� at a high level.

Tracer tests indicate that hydraulic performance deteriorated: the flow distribution was

heterogeneous and under the influence of interfacial cementation a large portion of water was

diverted around the Fe0 and transported outside the PRB. Based on the equilibrium reductions of

NO3� and SO4

2� by Fe0 and mineral precipitation, geochemical modeling predicted a maximum

0169-7722/$ -

doi:10.1016/j.

B bThe submAC05-00OR2

reproduce the

* Correspon

37831-6038, U

E-mail add

see front matter. Published by Elsevier B.V.

jconhyd.2005.05.006

itted manuscript has been authored by a contractor of the U.S. Government under contract No. DE-

2725. Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or

published form of this contribution, or allow others to do so, for U.S. Government purposes.Qding author. Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN

SA.

ress: [email protected] (L. Liang).

L. Liang et al. / Journal of Contaminant Hydrology 78 (2005) 291–312292

of 49% porosity loss for 5 years of operation. Additionally, modeling showed a spatial

distribution of mineral precipitate volumes, with the maximum advancing from the interface

toward downgradient with time. This study suggests that water quality monitoring, coupled with

hydraulic monitoring and geochemical modeling, can provide a low-cost method for assessing

PRB performance.

Published by Elsevier B.V.

Keywords: Heterogeneity development; Long-term performance; Mineral precipitation; Permeable reactive

barrier; Preferential flow; Zero-valent iron

1. Introduction

Long-term performance monitoring is essential for the successful application of Fe0

permeable reactive barriers (PRBs) for groundwater remediation. Although PRBs are

designed to remove or retain environmental contaminants, other biogeochemical

reactions occur simultaneously (Reardon, 1995; Liang et al., 2000; Scherer et al.,

2000), which lead to accelerated Fe0 corrosion (Gui and Devine, 1994; Sclicker et al.,

2000; Kamolpornwijit et al., 2004), mineral precipitation (Mackenzie et al., 1999;

Phillips et al., 2000; Furukawa et al., 2002), and gas production (Bokermann et al.,

2000). Changes in porosity and/or reduced reactivity of the Fe0 media are the common

results of these reactions (Mackenzie et al., 1999; Blowes et al., 2000; Farrell et al.,

2000). Media clogging due to mineral precipitation (Gu et al., 2002a; Kamolpornwijit et

al., 2003) and gas production (Bokermann et al., 2000) have been observed in both

laboratory and field studies.

The extent of PRB deterioration with time varies, depending on site hydrogeology

and geochemistry. A 2-year study using groundwater with low total dissolved solids

(TDS) reported a 10% loss in porosity (Vogan et al., 1999). In contrast, a recent

field column study, simulating ~17-year operation of a PRB in high nitrate (NO3�)

and high TDS groundwater, showed near 80% porosity reduction at the sand–Fe0

interface (Kamolpornwijit et al., 2003). Nitrate, SO42�, and HCO3

� are all corrosive

to Fe0 (Gui and Devine, 1994; Sclicker et al., 2000; Alowitz and Scherer, 2002;

Gandhi et al., 2002; Kamolpornwijit et al., 2003), their presence in groundwater,

thus plays a key role in both the biogeochemical reactions and PRB clogging

processes.

Simple yet effective detection of early changes in PRB performance is urgently

needed for sustained use of the technology. In this paper, we present a case study

from a Fe0-PRB site in Oak Ridge, Tennessee. Sampling results are reported for the

first 5 years of U treatment in high NO3� and high TDS groundwater. The

performance of this PRB with regard to the removal of U has been reported

previously (Gu et al., 2002a). The primary objectives of this paper are to 1) evaluate

PRB performance based on water quality and hydraulic monitoring data and identify

essential performance indicators, and 2) incorporate a geochemical model with the

known groundwater–Fe0 reactions and the mineral phases, thus predicting PRB

performance over time.

L. Liang et al. / Journal of Contaminant Hydrology 78 (2005) 291–312 293

2. Field site background and methodology

2.1. Site history and hydrology

The PRB site is located at the U.S. Department of Energy’s Y-12 National Security

Complex, Oak Ridge, Tennessee. Prior waste disposal activities created a mixed-waste

plume within the underlying fill, unconsolidated residuum (saprolite) and competent

highly-fractured bedrock to a depth of ~30 ft. Groundwater flow at the site generally

follows local topography toward Bear Creek (Fig. 1). However, a btransition zoneQ is

known to present between the highly weathered saprolite and the competent bedrock

(Solomon et al., 1992). Since this zone is more fractured than the underlying bedrock and

less weathered than the overlying saprolite it has higher permeability than either of these

(Moline et al., 1998). Additionally, a buried stream channel (Fig. 1) bisects the PRB site.

Permeability differences between filled and saprolite materials, together with the influence

of the buried stream channel, may have created two interlinked flow systems: a shallow

system with a primary flow direction toward Bear Creek, and a fracture-dominated system

with a primary flow direction parallel to the axis of the valley and geologic strike (Fig. 1).

In November 1997, a permeable gravel and iron trench-style barrier was constructed to

intercept the contaminant plume contained within the sediment and to remove the mixed

contaminants of U and NO3� with granular Fe0 (Gu et al., 2002a). The U level at this

location was on the order of 1 mg l�1 and NO3� was ~20–100 mg l�1. Upgradient

groundwater varied chemically; up to ~1000 mg l�1 NO3� was measured (Table 1). The

groundwater was high in TDS and the major constituents in upgradient-sampling wells

during 5-year monitoring are summarized in Table 1.

The trench was approximately 225-ft long, 2-ft wide, and 30-ft deep. Between two

~100-ft sections of granite/quartz pea-gravel, a 26-ft reactive section filled with Peerlesskiron filings was emplaced from the base of the trench to 10–12 ft below ground surface,

i.e., approximately to seasonal high water levels. The gravel sections of the trench were

designed to provide high permeability zones to intercept and channel groundwater along

the axis of the PRB through the Fe0 media. In year 2000, the trench was extended and a

sump installed at the distal end (angled PRB section in Fig. 1) to further direct flow along

the axis of the trench.

2.2. Field sampling and analyses

A series of piezometers and multilevel wells were installed in-and-around the PRB (Fig.

1) at the beginning of the operation. The multilevel wells (Insert, Fig. 1) were to sample

three zones; the shallow zone (12–20 ft) was sampled by two wells, one labeled bsQ, theother unlabeled. The middle zone (20–25 ft) and deep zone (25–30 ft) sampling lines were

labeled bmQ and bdQ (Fig. 2), respectively. Samples were obtained using peristaltic pumps

with in-line filtration (0.45 Am). In-line analysis was made for field parameters where

appropriate. A low-flow groundwater sampling technique was followed to minimize the

purged volumes (Puls and Barcelona, 1995). Filtered and acidified samples were analyzed

by inductively coupled plasma spectrometry for cations. Unfiltered and unacidified

samples were analyzed for major anions using a Dionex-120 ion chromatograph (IC) with

Fig. 1. Plan view of groundwater and surface water monitoring locations at theY-12 PRB site. Water table contours (on September 20, 2001), generalized flow directions

for shallow and fracture-dominated flow systems, and the approximate trace of a buried stream channel are indicated. The multilevel monitoring wells within the Fe0-PRB

are shown in the insert, together with locations of the 7 cores obtained in September 2001. Boundaries of the Fe0 are shown by the bold rectangle. Six of the cores were

vertical penetrations. The dashed line is the trace of an angled core that intercepts the upgradient gravel– Fe0 interface. Grid coordinates are in feet for scale.

L.Lianget

al./JournalofContaminantHydrology78(2005)291–312

294

Table 1

Groundwater physical chemical parameters in selected wells during the first 5 years of PRB operation through April 2002

Parameters Upgradient wells In Fe0-PRB wells

TMW05 TMW11a DP-13 All DP21m TMW09 DP18m DP21m TMW09 DP18m

Max Min Max Min Max Min average Early treatment, 3/27/1998 5-year treatment 4/26/2002

Al 0.4023 0.1 0.2317 0 23.0102 0.1 0.3854 0.1000 0.1523 0.1000 0.14 0.37 0.37

Ca 368.51 224.3 547.91 105.1 209.6 128.9 198.47 172.67 21.25 16.56 191 82.71 73.6

Fe 0.109 0 141.13 0 0.295 0 2.602 0.05 0.05 0.05 7.01 0.05 0.06

K 3.61 0.1 12.17 1.41 6.51 2.56 4.57 3.48 4.20 2.71 3.36 2.85 3.80

Mg 23.68 18.21 63.02 17.21 31.33 19.3 35.22 29.10 25.14 18.45 23.19 13.94 16.08

Mn 0.33 0.06 52.41 1.07 3.33 0.25 3.63 4.65 0.02 0.03 2.92 0.04 0.03

Na 10.73 7.9 57.96 5.43 41.8 5.36 20.26 40.44 42.47 35.30 11.32 14.38 18.48

Si 13.31 0.571 8.34 0.772 8.851 0.918 2.94 0.00 0.691 0.00 1.39 0.44 0.255

Fe2+ 0.27 0.01 23 0 0.05 0 1.133 0.003 0.090 0.080 4.560 0.00 0.030

S2� 0.006 0 5.225 0.001 0.01 0 0.069 0.003 0.514 0.006 b0.01 b0.01 b0.01

HCO3� 363.7 138.9 1100 91.1 571.4 184.1 392.7 408.06 60.56 8.96 108.5 121.19 20.12

NO3� 1138.02 276.17 71.94 0 427.97 0 238.25 12.20 0.00 0.00 100.34 0.0 0.0

Cl� 87.12 36.21 186.22 15.78 73.28 11.07 60.85 140.47 131.18 136.47 76.30 35.57 49.98

SO42� 98.07 39.41 147.2 2.5 157.33 23.85 91.37 53.51 17.97 0.00 52.55 0.0 0.0

D. O.b 4.72 1.47 3.1 0.2 5.44 0.64 2.36 1.40 2.13 0.77 0.8 2.44 0.92

ECc AS cm�1 2120 1427 1340 626 1334 705 1265 1387 583 491 1220 366 384

T, 8C 24.1 14.6 22.9 12.6 21.9 13.9 17.4 17.3 13.5 17.7 18.2 16.1 16.1

pH 6.8 6.2 7.5 5.8 6.8 5.7 6.6 6.3 8.3 8.4 6.7 8.2 9.3

Eh, mV 278.1 �105 234 �298.3 304 �69.7 90.2 150.1 �276.0 �299.2 �123 �12.6 �70

See Fig. 1 for well locations. Concentration units are in mg l�1, unless noted. Groundwater composition varied seasonally and with location. A range of these values and

the average are shown for the upgradient wells. Two sets of data are tabulated for examples of wells located inside Fe0.a Water chemistry influenced by PRB chemistry during storm-induced gradient changes.b Dissolved oxygen.c Electric conductance.

L.Lianget


295

89

9.5

s s s s s

m mm

m

ddddd

s

mm

Shallow zone

middle zone

deep zone

TMW11 DP22 DP21 DP20 DP19 DP18 DP23 TMW7TMW9

7

9

98

10

8

10upgr

adie

nt

dow

ngra

dien

t

Fig. 2. Profiles of maximum pH contour lines across the Fe0-PRB over the 5-year period of operation. TMW11 and TMW7 are in the upgradient and down-gradient

gravel respectively. Dashed lines are approximate gravel–Fe0 interface locations. Multilevel sampling wells located in shallow, middle and deep zones are labeled with

s, m, and d.

L.Lianget


296


a conductivity detector. Groundwater pH, Eh, dissolved oxygen, conductivity, and

temperature were measured in the field using an YSI XL600M probe. Sulfide was

analyzed by methylene blue method (ChemetricR) and Fe2+ was analyzed by a 1, 10-

phenanthroline method (HachR kits). Alkalinity of all samples, except those collected in

2002, was obtained in the field using the standard titration technique with 0.1N HCl. For

the 2002 samples, aliquots of the unacidified samples were preserved at 4 8C and analyzed

within a day using a Shimadzu TOC-5000A for total inorganic carbon, which was

converted to an equivalent HCO3� concentration.

2.3. Hydraulic monitoring

Hydraulic heads were obtained periodically and during storm events to obtain temporal

variability in hydraulic gradients. The multilevel sampling wells within the Fe0 media were

not designed for water level measurements. Tracer tests were planned to determine

changes in hydraulic conditions within the Fe0.

A bromide (Br�) tracer injection was performed in July 2001, 3.7 year after PRB

installation, to determine transport pathways and residence times in the Fe0 media. A

fully-screened well (TMW 11), located in the gravel immediately upgradient of the

Fe0, was used for injection (Fig. 1). NaBr solution was mixed in the injection well to

expose the tracer to the entire influent face of the Fe0-PRB. The tracer solution was

injected over a 6-h period, and Br� concentration was just under 3000 mg l�1. The

tracer density at this concentration should not differ significantly from the site water,

as up to ~1000 mg/L nitrate was present at the site (Table 1). Additionally earlier tests

using higher concentrations at this site reported the tracer breakthrough at the shallow

depth and density flow was not observed. However, to minimize density flow and to

ensure even distribution of the tracer in the well, pumping was applied to circulate

and mix Br� tracer by placing the inflow of the circulating line at the bottom of the

well and the outflow line slightly below the water level. Sampling with dedicated

tubing and minimal purged volumes began within 30 min of injection and continued

for 103 days in monitoring wells in-and-around the Fe0 PRB. Br� was analyzed using

a Dionex-120 IC.

2.4. Coring and mineralogical analysis

In September 2001, after 3.8 years of operation, coring was conducted using a drive

point method (Geoprobek rig) at locations shown in the insert of Fig. 1. Seven cores were

collected in 5-ft sections using an acetate-lined core barrel. Six cores were obtained

vertically at close proximity to multilevel wells in order to compare mineralogy with water

chemistry. The seventh core was angled at 608, beginning upgradient and extending

parallel to the trench through the Fe0–gravel interface. At most locations, continuous

coring succeeded for the entire thickness of the Fe0 where three 5-ft core sections were

obtained at depths of 15–20 ft (shallow), 20–25 ft (middle), and 25–30 ft (deep). A 15-ft

section was also cored within the angled penetration. Due to the softness of the material

where cementation was minimal, compaction and incomplete recovery occurred within the

cored sections. Thus, vertical depths within each section could not be assigned precisely.

Alkalinity (as HCO3)

0

200

400

600

800 4/24/1998

4/14/1999

4/17/2000

5/3/2001

4/29/2002

0

200

400

600

8004/24/1998

4/14/1999

4/17/2000

5/3/2001

4/29/2002

0

200

400

600

8004/24/1998

4/14/1999

4/17/2000

5/3/2001

4/29/2002

Calcium

0

100

200

300

400

500

600

Sh

allo

w P

ort

s

5/14/1998

4/14/1999

4/17/2000

5/4/2001

4/29/2002

pH

5

6

7

8

9

10

5/14/1998

4/14/1999

4/17/2000

5/4/2001

4/29/2002

0

100

200

300

400

500

600

Inte

rmed

iate

Po

rts

5/14/1998

4/14/1999

4/17/2000

5/4/2001

4/29/2002

5

6

7

8

9

10

5/14/1998

4/14/1999

4/17/2000

5/4/2001

4/29/2002

0

100

200

300

400

500

600

TM

W13

TM

W12

TM

W11

DP

22d

DP

21d

DP

20d

TM

W09

DP

19d

DP

18d

DP

23d

TM

W07

TM

W06

EW

01

sum

p

TM

W13

TM

W12

TM

W11

DP

22d

DP

21d

DP

20d

TM

W09

DP

19d

DP

18d

DP

23d

TM

W07

TM

W06

EW

01

sum

p

TM

W13

TM

W12

TM

W11

DP

22d

DP

21d

DP

20d

TM

W09

DP

19d

DP

18d

DP

23d

TM

W07

TM

W06

EW

01

sum

p

Dee

p P

ort

s

5/14/1998

4/14/1999

4/17/2000

5/4/2001

4/29/2002

5

6

7

8

9

105/14/1998

4/14/1999

4/17/2000

5/4/2001

4/29/2002

Fig. 3. Calcium, pH, and alkalinity values for 1998–2002, showing spatial and temporal variations in water chemistry resulting from geochemical reactions occurring

within the Fe0. Vertical lines on the graphs indicate the location of wells upgradient, within, and downgradient of the Fe0 from left to right of the figure. These cross-

sections are along the axis of the PRB and represent flow in the shallow, middle and deep zones.

L.Lianget


298


Subsamples were taken and sealed in airtight PVC tubes, which were purged in the

field with argon (Ar) at 5 psi pressure and re-purged twice weekly thereafter to prevent

chemical and mineralogical changes during storage. Core mineralogy was determined

using X-ray diffraction (XRD), scanning electron microscopy (SEM), and combined

thermogravimetric and mass spectrometry (TGA-MS) methods. A more detailed

description regarding sample preparation and analytical procedures is provided elsewhere

(Kamolpornwijit et al., 2004). Briefly, anoxically dried samples were impregnated in

epoxy resin, from which polished sections were made for SEM analysis, using an FEI

XL30FEG SEM. XRD samples were prepared by mixing with acetone, followed by

sonicating for 1 h and wet grinding. After drying under Ar flow, samples were sieved

through #125 sieves (120 Am). XRD analysis was made using a Scintag XDS2000

equipped with a cobalt tube X-ray source. TGA analyses were performed using a Netzsch

STA 409 TGA/DSC and a Pfeiffer QMS300 MS.

In the TGA-MS analysis, samples were heated in TGA chamber under a constant flow

of inert gas. Weight changes were monitored throughout the experiment and the off gas

was analyzed with a mass spectrometer. Based on the stability of mineral phases at a given

temperature the weight loss can be attributed to certain phases. For example, CaCO3

transforms to CaO by losing CO2 at 600–800 8C. Thus weight losses in this temperature

range can be used to quantify both CaCO3 and CaO masses.

3. Results and discussion

3.1. Evaluation of PRB performance

3.1.1. Pore water chemistry

Maximum pore water pH in the Fe0 is shown in Fig. 2 for the 5-year study period. The

highest pH (N10) was measured inside the Fe0-PRB, centered at well DP18 m. The highest

pH values fell in the range of the elevated pH observed in field PRBs (Morrison, 2003;

Yabusaki et al., 2001; Wilkin et al., 2003). Pore water monitoring data (Fig. 3) show the

changes in Ca and alkalinity corresponding to pH changes over 5 years. These

concentrations are plotted for wells located along the axis of the PRB in the shallow,

middle, and deep Fe0 zones. Data show that Ca and alkalinity were removed from the

influent solutions. Secondary mineral precipitation has been widely observed as a sink for

these dissolved species in Fe0-PRBs (Blowes et al., 2000; Phillips et al., 2000; Furukawa

et al., 2002; Morrison, 2003; Kamolpornwijit et al., 2004). In the shallow zone, the

removal of calcium and alkalinity began soon after PRB installation and was concentrated

near the upgradient interface (Fig. 3). Over time, removal occurred at more downgradient

locations, which has been observed in other field and column experiments (Kamolporn-

wijit et al., 2003; Mayer et al., 2001; Wilkin et al., 2003). In addition to Ca and alkalinity,

other species, such as Mg, Si, SO42�, and NO3

� also decreased from the influent values

after 5 month and 5-year treatment (1998 and 2002 data, Table 1).

In the deep zone of the PRB, near the influent interface (DP21d and DP20d) and in well

DP23d, only a small pH rise was observed (Fig. 2). The absence of pH increase has been

attributed to high velocity and short residence time in Fe0 media (Kamolpornwijit et al.,


2003; Liang et al., 1997; Morrison, 2003). However, Ca, alkalinity, and NO3� (not shown)

are higher in these wells than in the upgradient (TMW11-13), suggesting that these pore

waters originate from a different source. It is suggested that groundwater from deeper

bedrock sources has been upwelling into the fracture-dominated transition zone. This

upwelling also appears to affect chemical compositions in the middle zone wells: DP21m

and DP22m (Fig. 3). Over 5 years the chemical concentrations consistently show

sustained, though reduced, reactivity of Fe0 media (Fig. 3 and Table 1).

3.1.2. Coring observation

Fig. 4 summarizes the degree of pore filling based on examination of the cores. Cross-

sectional views show that the cores near the edge of the barrier penetrated the Fe0-saprolite

interfaces, which reveals that sampling wells, including DP20d, DP21d, DP19m, and

DP19d were not in the PRB.

3.1.2.1. Shallow zone (10–20 ft). The uppermost 2–4 ft of Fe0 is solidly cemented crust

consisting of rust-colored iron filings. Immediately beneath this zone the iron filings are

very dark and fine. Corrosion can reduced Fe0 size considerably (Liang et al., 2000).

A heavily cemented region was encountered in the angled core that extended from the

gravel–Fe0 interface to downgradient of DP22 (Fig. 4a). Cementation observed during

previous coring after 1.2- and 2.5-year operation (Phillips et al., 2000; Gu et al., 2002a)

had become more extensive over time. Cemented Fe0 pieces in otherwise loose filings

(i.e., patchy cementation) were encountered in the shallow zone adjacent to DP22. Patchy

cementation was also seen in the shallow zone adjacent to TMW09, DP19, and DP18,

which were further away from the influent zone.

3.1.2.2. Middle zone (20–25 ft) and deep zone (25–30 ft). The iron filings were patchily

cemented and secondary mineral precipitates were present throughout the middle zone.

Within the deep zone, patchy cementation was observed near DP21 and DP20,

corresponding to saprolite–Fe0 interfaces (Fig. 4).

3.1.3. Hydraulic investigation

Though pore water chemistry suggests the sustained reactivity of the Fe0, the coring

results indicate extensive Fe0 corrosion, especially at the interface. A more definitive

picture of transport within the Fe0 was obtained from tracer tests, last performed after 3.7-

year PRB operation. Concentration profiles along the axis of the Fe0-PRB (Fig. 5) show

distribution and progression of the tracer. These profiles to some extent resemble the pH

profile (Fig. 2), where high Br� levels correspond to high H+ levels (low pH values).

Tracer breakthrough of 690 mg l�1 was observed within the first 6 h in well DP22m,

located just inside the Fe0, but took 12 days to reach the upper two wells (DP22s, DP22)

and with low concentrations (~3–5 mg l�1) (Table 2). The absence of tracer strongly

suggests clogging in the shallow zone of the upgradient gravel–Fe0 interface as seen in the

coring results.

Despite the high concentrations in the deep upgradient sampling wells, Br� in the

middle wells remained 2 orders of magnitude lower (Fig. 5, day 5). Br� tracer continued

to increase in DP21d and DP20d, peaking on day 8 at ~1000 mg l�1 (Table 2), but the

Fig. 4. Core characteristics along the axis of the PRB. The distribution of mineral phases is in order of inferred predominance based on relative XRD intensity. An average

seasonal low water level for TMW09 is also indicated. Cross-sectional views (Section A-A and B-B) showing core penetrations relative to barrier walls and nearby

groundwater sampling wells.

L.Lianget


301

0 5 10 15 20 25 30 35

970

975

980

0

200

400

600

800

1000

1200

14006.0h

5 10 15 20 25 30 35

970

975

980 24h

TMW11 DP22 DP21 DP20 DP19 DP18 DP23 TMW7

0

200

400

600

800

1000

TMW9

0 5 10 15 20 25 30 35

970

975

980 5d

0

200

400

600

800

1000

0 5 10 15 20 25 30 35

970

975

98033d

0 5 10 15 20 25 30 35

970

975

980

0

200

400

600

800

1000

0

200

400

600

800

1000

104d

Fig. 5. Bromide tracer distribution (mg l�1) in a longitudinal cross-section through the Fe0-PRB. Time since

injection is noted in each plot. TMW11 and TMW07 are in the gravel immediately up-and-down gradient of the

Fe0, respectively. Flow is from left to right in the figures. Grid coordinates on the x-axis are in feet.


Table 2

Tracer test results

Well Distance

(ft)

Peak arrival

(days)

Peak concentration

(mg l�1)

Velocity

(ft d�1)

Maximum

pH

TMW11 0.00 Injection well 2849.00 7.50

Wells inside the Fe0

DP22s 6.72 12 3.76 0.55 7.50

DP22 6.72 12 1.40 0.55 9.30

DP22m 6.72 0.8 1160.00 8.95 6.90

DP21s 11.66 9 5.60 1.27 8.80

DP21 11.66 19 4.01 0.61 9.30

DP21m 11.66 13 229.00 0.90 7.20

DP21da 11.66 8 1037.00 1.46 6.70

DP20s 14.40 14 2.58 1.03 10.20

DP20 14.40 13 1.54 1.11 9.50

DP20m 14.40 40 209.00 0.36 9.00

DP20da 14.40 9 990.00 1.57 7.60

TMW09 21.86 40 0.26 0.55 9.60

DP19s 25.75 9 0.33 2.81 8.30

DP19a 25.75 12 0.38 2.11 9.20

DP19ma 25.75 21 0.22 1.23 10.00

DP19da 25.75 23 250.00 1.12 9.20

DP18s 28.43 33 0.35 0.86 10.20

DP18 28.43 21 1.00 1.35 10.20

DP18m 28.43 14 3.78 2.04 10.20

DP18d 28.43 58 112.00 0.49 9.80

DP23s 30.27 29 0.91 1.04 9.00

DP23 30.27 29 2.41 1.04 10.10

DP23m 30.27 19 25.00 1.59 9.90

DP23da 30.27 33 198.00 0.92 8.50

Downgradient wells

TMW07 37.83 16 0.86 2.36 9.50

TMW06 85.70 No breakthrough 0.00 0.00 7.20

EW01 105.99 64 0.13 1.65 8.00

DP15d 17.81 13 0.10 1.37 6.90

DP15s 16.05 6 0.28 2.71 7.70

DP14d 29.32 No breakthrough 0.00 0.00 6.30

DP14s 27.73 16 1.05 1.73 7.80

DP17d 26.64 103 0.98 0.26 6.60

DP17s 25.28 6 0.30 4.04 7.80

DP16d 36.81 16 1.86 2.30 6.80

DP16s 35.33 40 0.43 0.88 7.00

DP07 55.24 40 0.08 1.38 7.20

DP08 80.97 No breakthrough 0.00 0.00 7.30

DP09 105.91 103 0.20 1.03 7.20

DP10 69.52 6 0.26 11.12 6.90

DP11 46.41 58 0.68 0.80 7.00

GW836 33.37 40 0.70 0.83 7.40

TMW01 39.11 No breakthrough 0.00 0.00 N/A

TPB07 144.09 No breakthrough 0.00 0.00 7.20

TPB15 40.83 5 0.45 7.84 N/A

a Later determined to be located in the adjacent saprolite.



center of mass of the plume remained stationary for ~64 days. By day 33 Br� tracer of

~200 mg l�1 was observed at downgradient wells DP19d and DP23d, bypassing DP18d.

By day 104, low Br� remained upgradient in wells DP21d and DP20d but the center of

the plume had finally moved to the most downgradient well (DP23d).

The sharp contrast in tracer concentrations between the deep and middle wells (Fig.

5) indicates impaired connectivity between these zones, consistent with the coring

result, where patchy cementation occurred at the saprolite–Fe0 interfaces (Fig. 4).

Coring also showed that the deep wells (DP21d and DP20d) and the lower 3 wells at

DP19 were actually located in the saprolite, outside the Fe0-PRB. Although not cored,

well DP23d is assumed to be in the saprolite on the basis of its proximity to the

boundary of the PRB. The presence of tracer in saprolite zones and its absence in the

Fe0 media (TMW09 and DP18d) indicate that a large portion of water was diverted

around the Fe0 and transported within the highly-fractured transition zone outside the

PRB.

Peak tracer concentrations, breakthrough times, average velocities and maximum pH

values for wells in-and-around the PRB are summarized in Table 2. Pore water velocities

were calculated using the distance between injection and monitoring wells and the tracer

peak arrival times, ranging from 0.36 to 8.9 ft d�1. Small amounts of Br� were detected

in downgradient wells (b2 mg l�1), but it is difficult to determine the transport route

(i.e., whether through the Fe0 or a bypass). The high pH (9–10) in pore water was

expected based on reaction of Fe0 with water (Reardon, 1995; Liang et al., 2000). The

low pH (6–7) however, deserves some discussion. In DP22m, high Br� levels (1160

ppm), high flow velocity (8.9 ft d�1), and short residence time (0.8 d) correspond to little

pH rise, showing the same correlation established for preferential flow in column studies

(Kamolpornwijit et al., 2003). Short residence time can prevent chemical reactions from

complete, suppressing pH increase in DP22m. In DP22s, lack of pH rise is likely due to

the reduced reactivity as corrosion and cementation were observed in this and previous

studies (Gu et al., 2002a; Phillips et al., 2000). In DP21d and DP20d, although high Br�

and slow flow velocities were obtained, no reaction with Fe0 occurred to raise the pH, as

the wells were located in saprolite (Fig. 4). In well DP21m, absence of pH rise despite

low flow velocity (0.9 ft d�1) is due to upwelling of an acidic groundwater characterized

by higher levels of Ca, HCO3�, NO3

�, etc. than in the upgradient (Table 1).

Hydraulic investigation showed that flow at the site was inherently heterogeneous.

This could create an initial uneven distribution of groundwater velocities and solute

flux across the PRB (Eykholt et al., 1999; Gupta and Fox, 1999). The tracer transport

pattern reflected the loss of permeability due to mineral precipitation at the gravel–

Fe0 and saprolite–Fe0 interfaces, implying that the initial heterogeneity had been

enhanced by geochemical reactions. Groundwater bypassing and transporting through

preferential flow paths both impact on contaminant treatment. Along the preferential

flow path, pore-water velocity increases and residence time decreases, thus preventing

slow reaction to complete and allowing contaminant to breakthrough.

Even though hydraulic characterization is commonly regarded as necessary in

evaluation of PRB performance both for groundwater capture initially after the installation

and for sustained operation, it is often replaced by chemical monitoring solely to save the

cost. Pore water chemistry is a good indicator on media reactivity, and to a certain extent


flow distribution, but may not provide unambiguous results to the PRB performance

especially when there are changes in mass transport due to flow heterogeneity

development.

3.2. Prediction of PRB performance

Loss of porosity due to mineral precipitation affects both iron reactivity and hydraulic

capture of PRBs. With known information on the groundwater–Fe0 interaction, the

secondary mineral phases, and corrosion rate of Fe0, porosity loss can be predicted using

geochemical models.

3.2.1. Mineral phase identification and occurrence

Mineralogical analyses were conducted to identify the predominant phases and to

estimate degree of pore filling. Secondary mineral phases identified in the core samples

(Fig. 4) include aragonite, quartz, Fe2(OH)2CO3, siderite, goethite, amorphous iron oxide,

green rust, and mackinawite, with minor amounts of calcite, maghemite, magnetite,

hematite, and akaganeite. Some of these minerals have been identified previously at the

site (Gu et al., 2002a; Phillips et al., 2000) and in other PRBs (Furukawa et al., 2002; Roh

et al., 2000; Wilkin et al., 2003; Yabusaki et al., 2001). Aragonite, Fe2(OH)2CO3, and

quartz are widely spread in core samples. Calcite was less abundant than aragonite at the

site, which might be due to the inhibitory effect on calcite formation by ferrous and ferric

iron (Takasaki et al., 1994) whose levels were affected by nitrate enhanced corrosion. The

presence of quartz in abundance may mislead the previous identification of akaganeite

(Phillips et al., 2003). The quartz is believed to be transported as fines from the

surrounding quartz-rich saprolite and gravels, rather than forming by precipitation of

aqueous silica.

Precipitates were detected in core samples from all barrier locations. The majority

occurred as surface coatings, which did not completely fill the pore spaces. The occurrence

and predominance of the mineral phases vary with depth, flow-path length, and proximity

to the gravel–Fe0 or saprolite–Fe0 interfaces (Fig. 4), fundamentally reflecting the pore

water chemical conditions.

3.2.1.1. Shallow zone (10–20 ft). Goethite was the predominant pore-filler in the

solidly-cemented layer, which lies above the seasonal water table. Its formation is

favored by the presence of O2 (Cornell and Schwertmann, 1996). The suite of mineral

phases within the shallow zone samples below the oxidized crust was consistent,

including aragonite, Fe2(OH)2CO3, amorphous iron oxide, and green rust. Siderite and

mackinawite were also identified near DP22. A sulfurous odor was associated with most

samples, and occasional black silt layers were encountered. The mineral composition,

relative abundance, and degree of cementation varied in samples obtained within a given

5-ft section.

3.2.1.2. Middle zone (20–25 ft). These consisted primarily of aragonite, Fe2(OH)2CO3,

and amorphous iron oxide. Maghemite occurs near the upgradient locations (DP22 and

DP21) and green rust was predominant downgradient near DP20 and TMW09.


3.2.1.3. Deep zone (25–30 ft). Mineral phases included aragonite, Fe2(OH)2CO3, and

amorphous iron oxide, with an increasing occurrence of siderite downgradient of DP21.

3.2.2. Corrosion and cementation

Two cores, one from the cemented zone at the upgradient interface (near DP22s) and

the other from a downgradient uncemented zone (near DP18s), were used for TGA-MS

analysis. SO2, a thermodynamically stable form of S under the test condition, was present

in the off gas of both samples, confirming the presence of S-containing minerals.

However, XRD identified the crystalline mackinawite in the upgradient interface sample

only. The sulfide-containing phase must be in an amorphous form in the other samples.

Microbial analysis on core samples obtained after 2.5-year confirmed the presence of

microbial facilitated SO42� reduction (Gu et al., 2002b), and explains the sulfurous odor

associated with most of the core samples.

Porosity loss and corrosion rates were calculated based on TGA-MS analyses of the

core samples (Table 3). A fresh Fe0 sample was also analyzed to provide a baseline

correction for the pre-oxidized mass. The corrosion rate was calculated using the initial

and oxidized Fe masses from TGA results, (See details in Kamolpornwijit et al., 2004).

Mass of CaCO3 was calculated based on the weight loss during the transformation of

CaCO3 to CaO at 600–800 8C. The oxidized iron mass was determined with the loss of O-

atom from iron oxide during the final phase of TGA analysis when H2 gas was introduced.

Here H2 combines with O to form water. With the known masses of CaCO3, CaO, and

oxidized Fe, initial Fe was calculated. Porosity loss was estimated by assuming that all

iron precipitated as Fe2(OH)2CO3 and by neglecting the mass of quartz. The molar

volumes of the pure minerals of Fe0, CaCO3, and Fe2(OH)2CO3 are 7.11, 34.13, and 57.85

cm3mol�1, respectively. The porosity gain due to the loss of Fe0 was also accounted for

(Table 3). No attempt was made to quantify iron sulfide. Because the density of iron

Table 3

Mineral precipitate mass and corrosion rates based on mineralogy analysis of TGA data

Samples Precipitant (mmol) Initial Fea

(mmol)

Precipitate weight (%)b Porosity

lossc (%)

Corrosion

rated (mmol

kg�1 d�1)as CaCO3

(600–800 8C)as Fe3O4

e

(800–900 8C)CaCO3+Fe2(OH)2CO3

f

Fresh iron 0.024 1.75

#5 (DP22, shallow) 0.054 0.071 1.45 33.77 41.7 1.91

#38 (DP18, shallow) 0.020 0.033 1.65 12.26 15.9 0.78

a Calculated from the final mass subtracting the mass of CaO and the preexisting oxidized iron mass.b Total weight percentage of all precipitates corrected for the initially oxidized mass based on data from the

reduction of fresh iron.c Porosity loss=(precipitate volume�volume gain due to loss of Fe0)/initial pore volume; precipitate

volume=CaCO3 (mmol)�molar volume of CaCO3 (cc/mol)+Fe2(OH)2CO3 (mmol)�molar volume of

Fe2(OH)2CO3 (cc/mol), where Fe2(OH)2CO3 (mmol)=1.5�Fe3O4 (mmol); volume gain due to loss of

Fe0=3�Fe3O4 (mmol)�molar volume of Fe0 (cc/mol); initial pore volume=initial Fe0 (mmol)�molar volume

of Fe0 (cc/mol)�q / (1�q), where the initial porosity, q =60%.d Calculated from the weight ratio of total oxidized Fe / initial Fe, averaged over a period of 1380 days.e Calculated as magnetite based on the weight loss from 800 8C to the end of the TGA-test. The mole quantity

presented is then corrected for the oxidized mass that was present in fresh iron.f Calculated by assigning all oxidized iron as Fe2(OH)2CO3 at room temperature.


sulfide (4.3 g cm�3) is higher than that of Fe2(OH)2CO3 (3.59 g cm�3), the calculation

represents an upper limit of the porosity loss.

The calculated corrosion rates are 1.91 mmol kg�1 d�1 for the upgradient interface

sample and 0.78 mmol kg�1 d�1 for the downgradient sample (Table 3), both representing

averages over the 3.8-year operation. The latter is near to the value 0.6 mmol kg�1 d�1

reported for saline groundwater (Reardon, 1995). The lower corrosion rate in down-

gradient locations is to be expected since corrosive species, such as NO3�, has been

progressively removed along the flow path (Table 1). The porosity loss was 41.7% for the

cemented upgradient sample and 15.9% for the loose downgradient sample. These values

are very high compared to the results from PRBs operated in low nitrate groundwater

(Vogan et al., 1999; Wilkin et al., 2003) but are similar to studies using similar influent

groundwater (Kamolpornwijit et al., 2003).

SEM analysis provides a picture of the sequence of secondary mineral formation

(Fig. 6). In Fig. 6a, remnants of iron grains are highlighted in white. In Fig. 6b–d

respectively, bright zones indicate the presence and distribution of Ca, S, and Fe.

These images show that the Fe0 grains are completely surrounded by iron oxide, and

that Ca and S phases precipitate some distance away from the Fe0 surface. It is known

that even fresh Fe0 filings contain some oxide coating. As Fe0 corrodes pore water pH

increases, which leads to the super saturation and precipitation of CaCO3 minerals

(Reardon, 1995; Mackenzie et al., 1999; Liang et al., 2003). The clustered Ca signals

in Fig. 6b show substantial CaCO3 precipitation. Sulfur signals occur adjacent to

CaCO3 in the filled pores, indicating a delayed sequence of the sulfide mineral

formation. Understandably, microbial conversion of SO42� to S2�takes time (Gu et al.,

2002b). However, within 1.2 years of operation, this sequence of mineral precipitation

has occurred as reported previously (Phillips et al., 2000). The major contributors to

pore filling are iron oxide, CaCO3 and iron sulfide, in that order of abundance and

occurrence in the PRB. Each of these phases can act as a binding agent in natural

systems.

3.3. Geochemical modeling and analyses

Both pore water chemistry data and the coring results suggest that the removal of ionic

species by mineral precipitation rapidly occurred. Thus it would be reasonable to assume

that the pore water in a Fe0-PRB is at a pseudo, chemical equilibrium. Regardless of the

reaction pathways, each pore water composition along a flow path represents a new

equilibrium state. In any segment defined by adjacent well pairs, e.g., from DP22s to

DP21s (Fig. 3) a concentration change (such as mineral precipitation) may be evaluated by

mass balance of the equilibrium compositions of the well pair (Mayer et al., 2001;

Yabusaki et al., 2001). This was used previously in determining the average amount of Fe0

corrosion and mineral precipitation in Fe0-PRB (Liang et al., 2003).

Using pore water data, we modeled Fe0 corrosion and the precipitate volumes between

adjacent wells in the shallow zone along the axis of the Y-12 Fe0-PRB over a 5-year period

(Fig. 7). To obtain the dissolved Fe0 and assess the extent of Fe0 corrosion, an amount of

Fe0 is allowed to dissolve into the upgradient (DP13) pore water, according to:

Fe0=Fe+2+2e�, so that the equilibrium composition would match that in the down-

a

c) S d) Fe

Fig. 6. a) SEM and b–d) X-ray mapping for Fe, Ca, and S of a thin section prepared from a cemented core sample

near DP22. The image size is 1100�1200 pixels at 6 Am/pixel, 4 s dwell time, and 15 KV.


gradient pore water (DP22s). Both NO3� and SO4

2� enhance Fe0 oxidation (Gui and

Devine, 1995) and their effects due to high quantities at this site (Table 1) were modeled

using the following reactions: 4Fe0 +NO3�+ 7H2OY4Fe2+ +NH4

+ + 10OH� (log

K =39.533), and SO4�2+9H++8e�=HS�+4H2O with log K =33.66. Additionally, Fe0

was assumed to oxidize to Fe2+, which may further oxidize to Fe3+. When mineral

saturation states in the upgradient pore water exceed those in the downgradient, an amount

was allowed to precipitate. The mineral phases were selected based on coring results,

including CaCO3, goethite, mackinawite, magnetite, Fe(OH)2 and siderite. Fe2(OH)2CO3

was not used due to lack of equilibrium constants. The precipitate amount was then

converted to a volume with the known mineral molar volume (Fig. 7). PHREEQC code

was used for numerical calculations (version 2, Parkhurst and Appelo, 1999).

Model results (Fig. 7) show that precipitation concentrates at the upgradient gravel–Fe0

interface (between DP13 and DP21s) and decreases downgradient (e.g., between DP20s

Pre

cip

itat

e vo

lum

e cm

3 L-1

po

re w

ater

0.00

0.05

0.10

0.15

0.20

DP13 DP22s DP21s DP20s TMW09 DP18s DP23s

Apr-02 May-01 Apr-00Apr-99 Apr-98

TMW07

Fig. 7. Model estimates of precipitate volume between adjacent wells along the axis of Fe0-PRB at Y-12 site.

DP13 and TMW07 represent upgradient and downgradient wells of the Fe0, respectively. For April 2000

calculations, January 2000 data were used for DP13 as the April data were not available.


and DP23s), as observed in the cores (Fig. 4) and in other studies (Morrison, 2003; Phillips

et al., 2000; Wilkin et al., 2003; Yabusaki et al., 2001). Over time, the maximum of the

precipitate volume advanced: from the gravel–Fe0 interface in 1998, to down-gradient

locations (between DP21s and DP20s) by 2001 and 2002. This shows the general feature

of a migrating reaction front of these systems (Blowes et al., 2000; Mayer et al., 2001;

Sclicker et al., 2000). Using the maximum predicted Fe0 dissolution (12-mM), the initial

porosity (60%), and an average flow velocity (1 m d�1) in the Fe0-PRB an equivalent rate

of 1.53-mmol kg�1�Fe d�1 was calculated. This maximum dissolution rate, although

grossly simplified, is broadly similar to that of the cemented core sample (1.91 mmol kg�1

d�1, Table 3) and other data from the site (Kamolpornwijit et al., 2004; Liang et al., 2003).

Pore volume reduction (R%) due to the precipitate was calculated by R%=100CvolVT /

PVi, where Cvol is the modeled precipitate volume (Fig. 7), VT is the total treated volumes

in a given period, and PVi is the initial pore volume. Assuming an average flow of 1 m d�1

for the 5-year treatment, pore volume reduction of an average (24%) and a maximum

(49%) is predicted using the corresponding average (0.09 cm3 l�1) and maximum (0.18

cm3 l�1) precipitate volumes in pore water. The maximum would occur at the upgradient

interface as shown in the interface core (DP22s, Table 3) where 41.7% porosity loss

occurred in 3.8-year operation. The estimates have considered the effects of NO3� and

SO42� in the groundwater, which are relatively minor at other sites (Vogan et al., 1999;

Wilkin et al., 2003; Yabusaki et al., 2001).

We do not believe this high rate of porosity loss will persist. In field PRBs, groundwater

flux will greatly decrease due to bypassing when permeability is sufficiently reduced

(Fryar and Schwartz, 1998). Consequently, mineral precipitation will decrease over time

within zones of cementation. The moving precipitation front suggests that groundwater is

being treated downgradient, after bypassing or channeling through the cemented zones.

Using the average flow velocity of 0.5 m d�1 determined from the latest tracer test (Table


2), average pore volume reduction of ~ 2.5% per year is predicted for further use of the

Fe0-PRB after the initial 5-year operation.

4. Conclusions

In the first 5-year operation, significant degradation of performance has occurred

due to extensive mineral precipitation at the interface of the Y-12 PRB, resulting in

the loss of hydraulic capture. Fe0 corrosion and mineral precipitation occurred

throughout the media, but concentrated at the upgradient gravel–Fe0 and the

saprolite–Fe0 interfaces. Fe0 corrosion and pore volume reduction were more severe

compared to other sites (Vogan et al., 1999; Wilkin et al., 2003; Yabusaki et al.,

2001) due to very high NO3� in the groundwater. Permeability reduction has resulted

in the diversion of groundwater around cemented zones and in general, reduced flow

within Fe0 media.

Although mineral precipitation at the upgradient interface and the advancing

precipitation front generally agree with previous models and observations (Blowes et

al., 2000; Fryar and Schwartz, 1998; Sclicker et al., 2000; Yabusaki et al., 2001), the extent

of precipitation at this site has been more severe. Our modeling results show that

equilibrium prediction closely agrees with the corrosion rates determined from

mineralogical analyses of core samples taken from the PRB. The heavily-cemented

sample near the upgradient interface indicates that nearly a half of the porosity was lost in

3.8 years of operation (Table 3).

Coring and mineralogical analyses have confirmed observations and interpretations of

chemical and hydraulic monitoring of the PRB. When Fe0 is reactive, pH is a good

indicator of flow field, where low pH corresponds to fast flow path as shown in the pH

profile (Fig. 2), the tracer breakthrough curves (Fig. 5), similarly to the correlation

established for pH and flow residence time (Kamolpornwijit et al., 2003). However under

heterogeneous field conditions, interpretations of water chemistry data (such as pH)

require a good understanding of the hydrologic system. Good initial estimates of

groundwater flux into the PRB and changes in flux over time can then be coupled with the

results of geochemical modeling to quantify the degree of porosity loss and to predict PRB

life expectancy. More expensive techniques, such as coring and mineralogical analysis,

may be delayed or avoided altogether.

Acknowledgements

We thank T Hart at PNNL for helping with TGA-MS analysis for core samples. M Fox

and K Hyder provided technical assistance for field sampling and analysis. Y Roh helped

with XRD analysis. Field monitoring data prior to year 2000 were provided by B Gu and

D Watson. Funding for this research was supported by the Subsurface Contaminants Focus

Area of the Office of Science and Technology, U.S. Department of Energy, under contract

DE-AC05-00OR22725 with Oak Ridge National Laboratory, which is managed by UT-

Battelle, LLC.


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