2117
The Canadian Mineralogist
Vol. 43, pp. 2117-2140 (2005)

MINERALOGY OF PERMEABLE REACTIVE BARRIERSFOR THE ATTENUATION OF SUBSURFACE CONTAMINANTS
JOHN L. JAMBOR§
Leslie Research and Consulting, 316 Rosehill Wynd, Tsawwassen, British Columbia vV4M 3L9, Canada

MATI RAUDSEPP
Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada

KEITH MOUNTJOYHemmera Envirochem Inc., Vancouver, British Columbia V6Z 2K5, Canada

ABSTRACT
Permeable reactive barriers (PRBs) are a relatively recent development of a passive system to remediate subsurface waters
containing organic or inorganic contaminants. Groundwater flow under a natural gradient passes through a permeable curtain oftreatment medium that either precipitates the contaminants as relatively insoluble compounds or transforms the contaminants into
environmentally acceptable or benign species. The most widely adopted treatment medium is submillimetric zero-valent iron, asubstance that is highly reactive, environmentally acceptable, and is readily available as a manufactured product derived fromthe recycling of scrap iron and steel. Organic compost wastes have also been used to ameliorate inorganic contaminants, and twocase studies of the utilization of composts to reduce sulfate and precipitate metals are presented, primarily from a mineralogicalperspective. In cores of the reacted treatment media, the most abundant secondary product formed in situ is Fe oxyhydroxide,
but a variety of precipitates has been identified. For example, secondary pyrite, greigite, and native nickel are present at a site at
which replacement of organic material by sulfides is common. At an industrial site, secondary pyrite, covellite, chalcopyrite, andbornite have formed in the treatment medium, and whereas replacement of organic material by Fe oxyhydroxides is widespread,
replacement by sulfides is rare. The secondary sulfides and metals are volumetrically small and are unlikely to impede the permeability of the treatment medium, but the formation of Fe oxyhydroxides and secondary carbonates in the presence of zero-valentiron requires further monitoring to determine whether the secondary precipitates and the consumption of Fe0 will appreciably
lessen the effectiveness of such PRBs over the long term. Current indications are that PRBs are both an environmentally effective
and a cost-effective technique of remediation.
Keywords: permeable reactive barriers, metals contamination, organic treatment media, sulfate reduction, Fe oxyhydroxides,
secondary sulfide precipitates, zero-valent iron, Nickel Rim, Canada.

SOMMAIRE
L’installation de barrières réactives perméables est un développement assez récent pour dépolluer les eaux souterraines contaminées par des produits organiques ou inorganiques. L’écoulement souterrain suit un gradient naturel qui passe au travers d’unrideau perméable contenant un médium traitant qui soit précipite les contaminants sous forme relativement insoluble, ou bienles transforme en espèces acceptables ou bénignes pour l’environnement. Le traitement le plus répandu utilise le fer métalliquesubmillimétrique, substance très réactive, acceptable pour l’environnement, et facilement disponible comme produit du recyclage de ferraille et d’acier. On se sert aussi des déchets de composts organiques afin d’éliminer les contaminants inorganiques.
Nous présentons à titre d’exemples deux cas d’utilisation de composts pour réduire la teneur en sulfate et pour précipiter lesmétaux présents, surtout d’une perspective minéralogique. Dans les carottes du médium de traitement ayant servi, le produit
secondaire le plus abondant formé in situ serait un oxyhydroxyde de fer, mais divers précipités ont été identifiés. Par exemple,
les phases secondaires pyrite, greigite, et nickel natif sont présentes au site où il y a remplacement de matériaux organiques pardes sulfures. A un site industriel, la pyrite, covellite, chalcopyrite, et bornite secondaires se sont formés dans le médium de traitement. Tandis que le remplacement de matériaux organiques par des oxyhydroxydes de fer est répandu, un remplacement par dessulfures semble rare. Les sulfures secondaires n’occupent qu’un petit volume, et sont peu propices à réduire la perméabilité dumédium de traitement, mais la formation des oxyhydroxydes de fer et des carbonates secondaires en présence de fer métallique§

E-mail address: jljambor@aol.com

2118

THE CANADIAN MINERALOGIST

requiert une surveillance continue afin de déterminer si les précipités secondaires et la consommation du Fe0 diminuent de façon
importante l’efficacité de telles barrières à long terme. Il semble clair, d’après les indications actuelles, que ces barrières sontenvironnementalement efficaces et rentables.
(Traduit par la Rédaction)
Mots-clés: barrières reactives perméables, contamination par métaux, médium de traitement organique, réduction de sulfate,oxyhydroxydes de fer, précipités de sulfures secondaires, fer natif, Nickel Rim, Canada.

INTRODUCTION
A significant advance in remediation technology thathas been developed over the past decade to attenuate
high aqueous concentrations of environmentally
detrimental substances, both organic and inorganic,is the permeable reactive barrier or PRB (Blowes etal. 1995a, b, 2000, Naftz et al. 2002, Richardson &Nickelow 2002). PRBs, also commonly referred to astreatment walls or permeable reactive walls, are usable
to ameliorate contaminants in subsurface groundwater;
the demonstrated effectiveness of PRBs has resulted ina substantial recent increase in the number of full-scale
and pilot-scale units that have been installed. The total
number of full-scale installations at present is small,but most PRBs have been constructed only within thepast few years; for example, although the first full-scalecommercial application of a PRB, which was emplaced
to remediate chlorinated solvents, was in February
1995 at Sunnyvale, California (Warner et al. 1998),
a recent estimate is that there may be as many as 200
PRBs currently operating (ITRC 2005). Applicationsof PRB technology will likely continue to increase,especially when the technology is more widely adopted
in Europe.
Here we describe the case histories of two PRBs:
one treats acid mine-drainage (AMD), and the othertreats heavy metals at an industrial site. The barrier at
the AMD site relies on organic carbon as a medium forcontaminant attenuation, whereas both organic carbon
and Fe0 are used in various parts of the treatment wallat the industrial site.

BACKGROUND INFORMATION
Subsurface groundwater contaminants are commonly
remediated by a pump-and-treat system in which thecontaminant plume is intercepted by boreholes, andthe groundwater is brought to the surface for treatment.
PRBs have the advantage of being a passive systemthat, once installed, requires no maintenance or ongoing
expenditures beyond those of optional monitoring, andrequires no surface infrastructure. PRBs are also viewed
favorably because they are able to treat an extremely
wide variety of contaminants. About three-quarters of
the full-scale installations are designed to remediateorganic contaminants, predominantly chlorinatedsolvents such as trichloroethylene, tetrachloroethylene,

and carbon tetrachloride. In Figure 1, we provide anindication of the most common inorganic contaminants
that are treated, and in Table 1, list only a few of thetested elements and a few of the numerous pertinent
references that are available.
About three-quarters of the current full-scale PRBs
use iron metal as the reactant medium. Other metals,
such as Zn, Mg, and Sn, have proved effective in laboratory tests (Boronina et al. 1995, 1998, Warren et al.
1995), and applications of many of the host of other
tested reactive media, such as zeolites, Fe oxyhydroxides, and organic compounds, are given by Scherer etal. (2000) and Vidic & Pohland (2000). The advantage
of zero-valent iron is that, in addition to being effective,
it is inexpensive and readily available in bulk quantities, and unlike many other metals, it is non-toxic. The
iron, commonly referred to as “iron filings” or “granulated iron”, is commercially available and produced
from scrap iron and steel. It typically is impure. In the
manufacturing process, the filings are heated at 700

MINERALOGY OF PERMEABLE REACTIVE BARRIERS

2119

FIG. 1. Early distribution of full-scale PRBs and their applications (data from Smyth etal. 2002).

to 1200°C in rotary kilns to burn off the non-metallic
materials, especially the cutting oils (Ritter et al. 2002).
The heating stage promotes oxidation, and the final
product generally has a “rusty” appearance because of
the presence of surficial iron oxides. Particle sizes of theFe0 from various producers are typically submillimetric(Johnson et al. 1996).

GENERAL PRINCIPLESA PRB consists of a zone or curtain of permeable reactive material that is placed in the flowpathof contaminated groundwater so that, during passage
through the curtain under a natural gradient, reaction
between the contaminants and the emplaced material
improves the quality of the aqueous outflow. Because
the design is for flow under a natural gradient, thesystem is categorized as “passive” rather than “active”;
the latter, such as a pump-and-treat system, requires acontinued input of energy to maintain the remediation
process. The ideal physical setting for a PRB is one inwhich the contaminant plume is near surface, is laterally
of limited dimension, and is underlain by bedrock or asimilarly effective aquitard. These conditions serve todirect the contaminant plume to a focal point at which
the PRB can be conveniently installed to intercept
the plume. In less favorable circumstances, however,
impediments to lateral flow can be constructed to funnel
the contaminant plume toward an interception point.
Arrangements of this type are commonly referred to as
funnel-and-gate systems. Various funneling systems,
which may consist simply of interlocked, corrugatedsteel sheets driven vertically into the ground, have been
described by Gavaskar et al. (1998).
The within-barrier reactions depend on the geochemical nature of the inflow and the composition of thereactive medium. The emphasis here is on inorganic

subsurface contaminants, but PRB methodology isequally applicable to the treatment of surficial organicand inorganic seepages; the technology, for example,
has been investigated for remediation of drainage
from acid sulfate soils (Waite et al. 2002). Removalof contaminants by PRBs involves two main processes(Scherer et al. 2000): (1) transformation, whereby thecontaminant undergoes a change to a less harmful
compound, and (2) immobilization, whereby thecontaminant is retained within the barrier either bysorption to the reactive medium or by precipitation of
the dissolved phase. The transformation mechanism
applies mainly to organic materials, such as chlorinatedcompounds, that are transformed to less harmful or
to benign species. Immobilization within a barrier isthe principal mechanism that operates for dissolved
inorganic contaminants. Nevertheless, the exact natureof the immobilization, whether by precipitation of thedeleterious species or by sorption to either the barrier’s
original materials or to newly formed compounds, is
not yet understood well because few mineralogicalexaminations have been done on functioning field-scale
PRBs, most of which have been installed relatively
recently. Determination of the mineral–solution reactions that occur is important because the results not
only provide indications of the stability of the productsthat are formed, but they will also aid future design interms of the reactive mixtures employed. In the twocase studies presented here, the mineralogical datasuggest that precipitation is the principal attenuating
mechanism, and that sorption is associated with newly
formed products, chiefly Fe oxyhydroxides, rather thanwith the initially emplaced reactive medium.
Although sulfate-reducing bacteria have been shown
to be active where zero-valent iron is the treatment
medium (Rowland 2002), both of the case-study PRBs
use organic matter as a means of inducing reductive

2120

THE CANADIAN MINERALOGIST

biotransformation (Benner et al. 1998, 1999). The
organic matter provides both a nutrient source forbacteria and a high capacity to deplete dissolved
oxygen. Maintenance of anoxic conditions is most
readily attained in subaqueous domains because
dissolved oxygen has a relatively low solubility inwater. At sufficiently low Eh, dissolved metals suchas Fe2+ and Cu+ can be precipitated as native metals.
Under acidic conditions, the reaction for the reductionof dissolved sulfate can be written as
2 CH2O + SO → H2S + 2 HCO3–(1)

where CH2O is a generic term for simple organic carbon
compounds. In the presence of dissolved metals (Me):Me2+ + H2S → MeS(s) + 2 H+

(2)which for Fe2+ corresponds toFe2+ + H2S → FeS(s) + 2 H+

(3)Reaction (3) is considered to dominate at acid pH and S= <10–3 M, and becomes more important at temperatures<25°C (Rickard et al. 1995). The formation of Fe disulfide proceeds through loss of Fe from the monosulfide(Wilkin & Barnes 1996, Benning et al. 2000):
2 FeS(s) + 2 H+ → FeS2 + Fe2+ + H2(g)(4)

Although various other sequences have been observed(e.g., Wikjord et al. 1980, Shoesmith et al. 1980), it
is generally accepted that formation of FeS2 proceeds
via the following pathway (Rickard et al. 1995,
Lennie & Vaughan 1996), wherein amorphous FeS has
recently been determined to be disordered mackinawite(Wolthers et al. 2003, 2005):
cubic FeS or amorphous FeS →mackinawite → greigite →FeS (tetragonal) Fe2+Fe3+2S4
marcasite or pyrite
FeS2

(5)

Systems containing organic carbon are chiefly concernedwith treatment of heavy metals rather than sulfate, but
generation of hydrogen sulfide through the action of
sulfate-reducing bacteria has the beneficial effect ofimmobilizing metals as sulfides that have low solubilities and are likely to remain stable even undervariable conditions. Reaction (1) has the beneficial
effect of adding alkalinity to the system, and although
the large number of species referred to as sulfatereducing bacteria can operate effectively even at pH≤ 3 (Koschorreck et al. 2003), the bacteria generally
have optimal growth approximately in the pH range 6

to 7. Thus, for treatment of the effluents related to acidmine-drainage, addition of pH-raising material, such asa small amount of limestone, to the reactive medium
provides a means of optimizing conditions of growthduring the initial period of microbial growth.
Previous mineralogical studies
The solid-phase products formed during reactions
with PRB treatment media for the precipitation ofdissolved metals have been examined either in numerous
laboratory-scale experiments, especially those involving
zero-valent iron in the medium, or the mineral precipitates have been inferred on the basis of geochemical
modeling (Amrhein et al. 1998, Odziemkowski et al.
1998, Gu et al. 1999, McMahon et al. 1999, Herbert
2003; Table 1). Some of the laboratory-scale experiments have closely simulated the geochemical conditions occurring within PRBs, but only within the past
five years have significant numbers of mineralogicalstudies of cored material from field-scale operating
PRBs been undertaken. Vogan et al. (1998) cored pilotscale walls from two unspecified sites, one in New Yorkand the other in Colorado, that had been operating for
24 and 18 months, respectively. At both sites, zerovalent iron is the sole reactive medium and is used totreat organic contaminants. Vogan et al. (1998, 1999)
detected calcite, aragonite [CaCO3], siderite [FeCO3],
green rust [simplified formula Fe2+Fe3+(OH)5], magnetite [Fe2+Fe3+2O4], and unspecified Fe2O3 and FeOOH
as secondary products. No sulfides were detected, but
the probable presence of sulfate or sulfide precipitates
was suspected because of declines in aqueous sulfate
concentration at both sites. At Fry Canyon, Utah, afield-scale test-wall with Fe0 filings as the reactive
medium was installed to treat uranium-contaminatedgroundwater, and after operating from August 1997 to
May 1999, precipitation of calcite and “Fe sulfide” wasobserved in the cores (Naftz et al. 2000). Cores from apilot-scale PRB containing 100% granular iron to treat
organic contaminants at a former U.S. Naval station(Moffett Field, California) were obtained after approximately 20 months of field operation. The pH within thebarrier was 10 to 11, and Eh was –108 to –228 mV (Sasset al. 1998, Yabusaki et al. 2001). Corrosion coatings
were reported to be predominantly hematite [-Fe2O3]
and magnetite, both of which were also present in theunused iron, and minor aragonite and marcasite [FeS2],
neither of which was detected in the unused iron (Sasset al. 1998). Calcite with compositions ranging fromnear end-member to 50 at.% Fe, and ferrihydrite [nominally 5Fe2O3•9H2O] were identified as precipitates ina PRB at Durango, Colorado (Morrison et al. 2002).
Cores from Fe0 PRBs that had operated for aboutfour years at Elizabeth City, North Carolina (treating
chlorinated hydrocarbons), and at the Denver Federal
Center, Lakewood, Colorado (treating Cr6+ and chlorinated hydrocarbons), were reported by Furukawa et al.

MINERALOGY OF PERMEABLE REACTIVE BARRIERS(2002) and Wilkin et al. (2003) to contain ferrihydrite,
lepidocrocite [-FeOOH], goethite, magnetite, hematite, aragonite, calcite, Fe monosulfide, mackinawite,
greigite, pyrite, carbonate green rust, Fe2CO3(OH)2 andsiderite. Although the precipitates had reduced the porespace, neither site showed evidence of pervasive poreclogging (Wilkin et al. 2003).
A PRB installed in 1997 primarily for the treatmentof uranium-contaminated groundwater at the Y–12 Plant
site, Oak Ridge, Tennessee, has an Fe0 zone approximately 8 m long, 0.6 m thick, and 9 m deep. After 1.2
and 2.5 years of operation of the PRB, coring showedthat goethite [-FeOOH], akaganéite [-FeOOH],
amorphous FeS, aragonite, and siderite had formed
as secondary precipitates (Phillips et al. 2000, Gu etal. 2002). Significant changes observed in the cores
collected after 2.5 years of operation were an increasein goethite and siderite, the detection of Fe2CO3(OH)2,
lepidocrocite, calcite, and mackinawite, and confirmation of the presence of green rust (Phillips et al. 2003).
The rates of corrosion of the filings suggested that theFe0 in some portions of the PRB would be consumed
within less than five more years.
In summary, calcite, aragonite, siderite, Fe2CO3(OH)2,
goethite, lepidocrocite, hematite, magnetite, akaganéite,ferrihydrite, green rust, marcasite, amorphous FeS,
mackinawite, greigite, and pyrite have been reported
for the preceding PRBs, all of which use or used Fe0 as
the reactive medium. PRBs utilizing an organic-matterbased medium (rather than Fe0) typically treat relatively
sulfate-rich effluents, and whereas several of the abovementioned minerals are present, the precipitate assemblages formed from the sulfate-rich effluents differ
considerably, as is discussed in the two case studies
described below. A preliminary report on the case-study
sites was given by Jambor & Mountjoy (2003).
Current studies: methods
A core for mineralogical study was obtained fromthe Nickel Rim site near Sudbury, Ontario, by using
a wire-line rig in which the inner sleeve was 5 cm indiameter and penetrated the barrier vertically for 3.35
m. The core and sleeve were cut into easily handledlengths that were capped in the field and were stored
frozen until mineralogical examination. The industrialsite is near Vancouver, British Columbia, and has botha pilot-scale PRB and a subsequently installed full-scalewall. The samples from both of the Vancouver-site PRBs
were obtained with a split spoon and Becker Hammer
drill rig. All samples recovered from the pilot-scale PRB
were wrapped in plastic and frozen on-site; those fromthe full-scale wall were temporarily iced, and then werelaboratory-frozen on the day of collection.
The mineralogical studies for the three PRBs at thetwo sites were done in different years, but the sameprocedures were followed. All of the selected material
was placed on plastic receptacles and was allowed to2121

dry at room temperature. Some of the samples from the
British Columbia site contained drainable fluid, which
was collected and allowed to evaporate at room temperature. Fluid from one of the pilot-wall samples yielded
sparse crystals of gypsum, but otherwise no precipitates
were observed on the samples or from the evaporated
fluids; no color changes, such as would occur from theoxidation of green rust (“fougerite”; Génin et al. 1998,
2001), were observed in the as-received solids.
The bulk of the material in the retrieved samples
contains pea gravel, for which no evidence of alteration
is present. The gravel was removed by hand-picking toincrease the proportion of organic-matter-rich material,
but no attempt was made to remove the fines (<2 mm).
The hand-picked residue was prepared as standard (26 46 mm) polished thin-sections; to minimize dissolution of water-soluble precipitates, if present, no water
was used during section preparation.
The sections were studied by optical microscopyusing both transmitted and reflected light, and many of
the areas deemed to merit further study were examinedby scanning electron microscopy (SEM) using a Philips
XL–30 instrument with a coupled energy-dispersion
analyzer (EDS); supplementary chemical compositions
were obtained with a Cameca electron microprobe.
Several Debye–Scherrer X-ray powder-diffraction
patterns were obtained from mounts that were preparedwhile viewing the relevant particle under the petrographic microscope.
The iron filings used in a portion of the full-scalewall at the British Columbia site are produced from
scrap metal by Connelly GPM Inc., Chicago, Illinois.
According to the manufacturer’s 1998 data, the filings
contain 89.8 wt.% Fe0 and 2.85 wt.% carbon; a batchof the filings was examined by Gui & Gillham (2002),
who reported that particle sizes range from about 2 mm
to ~300 m, with an average surface-area of 1.80 m2g–1.
A split of the specific batch of filings that was emplacedin the Vancouver PRB is archived at the University ofWaterloo, and a portion of that sample was used for Xray diffractometry and optical microscopy.

CASE STUDY: NICKEL RIM MINESITEThe small underground Nickel Rim mine near
Sudbury, Ontario, ceased operations in 1958. Likeother deposits in the Sudbury camp, Nickel Rim ore
consisted largely of chalcopyrite [CuFeS2] and pentlandite [(Fe,Ni)9S8]; on-site milling yielded about 0.9
Mt of tailings that contain 3–5 wt.% Ssulfide, nearly allof it as pyrrhotite [Fe1–xS]. The tailings were depositedin an elevated impoundment, wherein oxidation of thesulfides has resulted in acidic drainage containing up to
9.8 g L–1 Fe, 24 g L–1 SO4, and 698 mg L–1 Ni (Johnson
et al. 2000). At the western part of the impoundment,
the poor-quality drainage forms a plume within abedrock-confined sandy gravel aquifer, 3 to 8 m thick,that pinches to about 15 m width and 3.6 m depth

2122

THE CANADIAN MINERALOGIST

before discharging to a pond and adjacent lake whose
near-surface pH is about 3 (Fig. 2).
Following laboratory testing of various mixtures ofreactive medium (Waybrant et al. 1995, 1997, 1998,
2002) and the 1993 installation and subsequent monitoring of field-site test cells (Blowes et al. 1995a), a
full-scale PRB was emplaced in 1995 (Benner et al.
1997, 1999). The PRB was the first of its kind in that
it was designed to treat dissolved metals by using asulfate-reduction process for remediation. The organiczone in the barrier is 15 m long, 4.3 m wide, and 3.7
m deep, within which the reactive medium does notcontain Fe0; rather, the medium consists of 20 vol.%each of leaf compost and a municipal compost of mixed
composition, 9% wood chips, and about 1% limestone,
all of which were blended with 50% pea gravel toenhance permeability and to ensure that the hydraulicconductivity of the mixture is greater than that of the
inflow aquifer (Blowes et al. 2003). The organic zone
is sandwiched between emplaced sand exteriors, each2 m in thickness.
After one year of operation, inflow concentrationsof 740–1000 mg L–1 Fe had decreased to <100 mg L–1
within the barrier, SO4 had decreased from 2400–3800
mg L–1 to 110–1900 mg L–1, Ni concentrations of up to
30 mg L–1 had declined to <0.2 mg L–1, and alkalinity
had increased from 60–220 mg L–1 to 850–2700 mg L–1(Blowes et al. 2000). The barrier ideally contains enoughorganic material to remove Fe and sulfate for >100 years(Benner et al. 1997), but declines in performance after
three years of operation were observed (Benner et
al. 2000, 2002). Although the rate of Fe removal had
decreased by about 50%, more than 250 mg L–1 Fe and>1000 mg L–1 SO4 were still being removed. Benner etal. (2001, 2002) suggested that the principal reasons forthe temporal decrease in performance are a diminution
in the availability and reactivity of the organic carbon

FIG. 2. General setting of the full-scale PRB at the Nickel
Rim site near Sudbury, Ontario.

over time, coupled with variations in flow rates throughdifferent portions of the barrier. The distance traversed
annually by groundwater flow through the barrier averages about 16 m, and average residence-time for the
fluid is about 60 days in the central part of the PRB
and up to 165 days in the surrounding zone (Smyth etal. 2001, Blowes et al. 2003).
The barrier was cored after two years of operation, at which time it was observed that mackinawite
had precipitated on the piezometer tubing and that
S accumulation in the organic material was chiefly
associated with acid-volatile sulfides (AVS), whichis the non-pyritic fraction as determined by chemical
extraction (Herbert et al. 2000). In a study of more
recently collected material, an increase in the AVS has
also been detected in the down-gradient portion of theaquifer receiving the effluent after its passage throughthe barrier (Doerr et al. 2003).
In July 2001, after almost six years of operation,
the PRB was cored for further mineralogical andgeochemical studies. The material examined mineralogically is from a borehole adjacent to piezometer nest
RW29, shown by Benner et al. (1999) and Herbert et al.
(2000) to be within the PRB and adjacent to its inflow
side, where the received contaminants should be attheir highest concentrations. The pH and Eh shown in
Figure 3 provide a general indication of the geochemical
conditions at the cored site.
The mineralogical study revealed that precipitationof Fe within the barrier is occurring in various forms,
the most common of which are sulfides (Figs. 4c, d,5a) and Fe oxyhydroxides in a ring-type form (Fig. 5b)
and in fragments with a complex-textured, commonly“swirled” form (Fig. 5a). The sulfides, most of which
are present as replacements of the cells within organic
matter, are less abundant than the oxyhydroxides assinks for Fe. Framboidal pyrite has been observed,
but is relatively rare. The framboids are intact, withno indications that some may have disintegrated frompolyframboidal aggregates during sample preparation.
None of the pyrite occurs as isolated single crystals,
such as cubes. Most of the Fe sulfide in the sections,in the few instances where optically resolvable, has
the appearance of greigite and pyrrhotite rather thanpyrite. Where greigite and pyrite coexist, the outward
succession is to peripheral pyrite (Figs. 6c, d). Where
greigite–pyrrhotite and Fe oxyhydroxides coexist, thesulfide mineral forms a narrow rind; the opposite relation, i.e., a rim of oxyhydroxide on the sulfide, has notbeen observed. Although there is undoubtedly overlap
in the precipitation of the various minerals, the generalprogression is Fe oxyhydroxide → greigite (or pyrrhotite) → pyrite. The presence of early-stage mackinawite,
as reported by Herbert et al. (2000), further suggeststhat greigite crystallization was preceded by that ofmackinawite. Thus, the typical sequence to formation of
pyrite in accordance with Reaction 5 seems to have been
followed. The general paucity of pyrite and the sparse-

MINERALOGY OF PERMEABLE REACTIVE BARRIERS

ness of framboids, together with the apparent relative
abundance of greigite, suggest that sulfide precipitationis still at an early stage and that transformations will
occur over time as H2S continues to be generated. Other
Fe minerals identified in the polished thin-sections aregoethite, hematite, maghemite, and isolated grains of
native iron, but all are quantitatively sparse relative tothe aforementioned sinks for Fe. The identification ofhematite was confirmed by a Debye–Scherrer X-ray
pattern of a large area (0.5 mm) of texturally complex
material resembling some of the fragmental-type
Fe oxyhydroxides (Figs. 6a, b). Fragmental-type Fe
oxyhydroxide is a generalized term insofar as the fragments consist predominantly of goethite, but some of
the intricately intergrown layers and swirls consist of
maghemite and hematite.
Rather than sorption to the Fe oxyhydroxides,
dissolved Ni in the PRB has precipitated as the native
metal (Figs. 4b, 6e). As well, some of the native nickel
has with it an Fe-bearing Ni sulfate (Fig. 6f) and apossible Ni sulfate–phosphate, both of which are later
than the native nickel.

2123

Distinctly secondary carbonate minerals areuncommon. Some of the material included within the
fragmental Fe oxyhydroxides is siderite, but other associations have not been observed. However, the opticalindications are that secondary carbon has been deposited; the carbon, which occurs as irregular particles
whose shapes are similar to those of the fragmental
Fe-oxyhydroxides, varies optically from completely
isotropic to particles whose strong anisotropy is similar
to that of graphite (Figs. 5c, d). Despite the apparent
crystallinity, however, both optical varieties proved
to be X-ray amorphous. Carbon, both isotropic andanisotropic, that apparently formed in situ has also
been observed at the second case-study site, described
below.
The principal purpose of the installation of the
Nickel Rim PRB was to decrease the content ofdissolved Fe because the relevant aquifer discharges
to adjacent Moose Lake (Fig. 2), whereupon oxidationof Fe2+ and the subsequent hydrolysis generate acidity.
The PRB has not only reduced the Fe content of theplume, but the emergent water now has an excess

FIG. 3. Geochemical characteristics of the influent, treatment zone, and effluent at the Nickel Rim site. SRB are sulfate-reducing bacteria, MPN (Most Probable Numbers); the slope of the line within the Exit Zone is toward data points that are farther
down-gradient in the sandy gravel aquifer. The Exit Zone includes 2 m of sand emplaced adjacent to the Organic Zone. Data
are from Benner et al. (1999).

FIG. 4. Nickel Rim PRB. (a) Overview of the treatment medium, in plane-polarized transmitted light and (b) in reflected light, bar scale: 0.5 mm. The brownish
material in (a) is the organic medium, ls is a fragment of limestone, and q is a fragment of pea gravel, here mainly quartz. The upper arrow in (b) points to aparticle of native nickel, and the lower arrow points to a particle of carbon that is shown at higher magnification in Figure 5. (c) Transmitted light and (d) thesame field in reflected light, bar scale: 100 m, showing replacement of cellular organic matter by sulfides, which are opaque in (c) and whitish in (d). The
main sulfide has the formula Fe0.97S and is strongly anisotropic.

2124
THE CANADIAN MINERALOGISTFIG. 5. Nickel Rim PRB. (a) Replacement of organic matter by Fe sulfides consisting mainly of greigite with subordinate pyrite; reflected light, bar scale: 100
m. The whitish particle on the far left is fragmental Fe oxyhydroxide. The greigite, whose identification was confirmed by XRD, has the composition Fe3.2S4.
(b) Ring-type Fe oxyhydroxide in reflected light, bar scale: 100 m. The cores in this type of texture typically consist only of the mounting medium, and the
Fe oxyhydroxide typically contains several percent S. (c) Reflected light and (d) the same field with polarizers crossed, bar scale: 50 m, showing the carbon
particle in Figure 4b and its strongly anisotropic, finely polycrystalline character.

MINERALOGY OF PERMEABLE REACTIVE BARRIERS

2125

2126

THE CANADIAN MINERALOGIST

FIG. 6. Nickel Rim PRB. (a) Complex replacement texture in reflected light, and (b) the same field with polarizers almost
crossed and internal-reflection mode. The reddish mineral is hematite, and the strongly anisotropic “threads” are carbon.
Bar scale: 100 m. (c) The large particle on the left is greigite rimmed by pyrite; the thin rinds on other particles are Fe
oxyhydroxide with an exterior film of Fe sulfide that is probably greigite (analyzed composition Fe3.2S4). The arrow points
to a group of white grains of native nickel. Reflected light, bar scale: 100 m. (d) The main mass is greigite, as confirmed
by X-ray powder diffraction, and the outer, yellower rim is pyrite. The adjacent grey grains on the left are primary ilmenite.
Reflected light, bar scale: 50 m. Figures (c) and (d) show the largest particles of secondary Fe sulfide observed in the Nickel
Rim samples. (e) The white grains, including the thread-like grain on the far right, are native nickel in mainly organic matter.
Reflected light, bar scale: 100 m. (f) Back-scattered-electron image of the native nickel shown in (c); bar scale: 10 m. The
nickel (white) is enclosed within a medium-grey, cracked Ni > Fe sulfate or similar compound. The slightly darker inclusions
are quartz and Al-bearing silicates.

MINERALOGY OF PERMEABLE REACTIVE BARRIERS

acid-neutralizing capacity, thereby helping to reduce
the existing acidity within the receiving lake (Benner
et al. 1997, Blowes et al. 2003).

CASE STUDY: INDUSTRIAL SITE NEAR VANCOUVERThe site is adjacent to a marine inlet, where oreconcentrate is mechanically transferred from railcars
and thence to ships that transport the material forfurther processing in other countries. The site consists
predominantly of unconsolidated deltaic sands andgravels, >45 m thick, to which fill from unknown
sources was periodically added to raise the groundlevel and extend the shoreline (McGregor et al. 2002).
The oxidation of sulfide minerals entrained within thesediments has resulted in groundwater contamination
by various dissolved heavy metals, including Cu, Zn,
Cd, and Ni, whose discharge to the marine inlet has
been deemed to be a potential threat to the shoreline
ecosystem (Ludwig et al. 2000). Metal contaminationis confined to the upper 15 m of the aquifer, with thehighest contamination in the uppermost 6 m. Dissolved
Cu at some sites on the property has exceeded 200 mg
L–1 (Ludwig et al. 2000, 2002).
A mineralogical study has shown that the sulfide
sources of Cu are mainly chalcopyrite and lesser bornite[Cu5FeS4] that are disseminated in the unconsolidated
arenaceous sediments and are also present locally as
thin sulfide-dominant layers. Sphalerite [(Zn,Fe)S] is
the main source of Zn and Cd; galena [PbS] and otherprimary sulfides, including pyrite, also are present.
Although the amount of pyrite is proportionally low
relative to both the total sulfide content at the site andthe high content that typically prevails at sites of acidmine-drainage, concentrations of groundwater sulfate
are nevertheless about 1000 mg L–1 (Blowes et al.
2003).
In March 1997, a pilot-scale PRB, 10 m long, 2–2½
m wide, and 6–6½ m deep, was installed to intercept thepath of groundwater flow. Descriptions and geochemical
data given below are from McGregor et al. (1999, 2000)
and Ludwig et al. (2000, 2002). Excavation of thetrench for the PRB was done with a hydraulic bucket,
and a slurry of biodegradable guar gum (Day et al.
1999) was pumped into the trench to prevent collapseof the unconsolidated-sediment walls. The reactive
mixture in the PRB consists of 15 vol.% leaf compost
and about 1% limestone, with 84% pea gravel (D50 = 8.5mm) in the blend to ensure an appropriate permeability.
Thus, like the unit at Nickel Rim, the pilot PRB at theindustrial site does not contain zero-valent iron in thetreatment medium.
Subsequent study of the hydraulic conductivity indicated that the influent residence-time within the barrier
is about three days. The pH of the influent is slightly lessthan 6, and that of the effluent is only marginally higher,
but influent Eh decreases from 416 mV to within-barrier
values of –18 to –27 mV; accordingly, concentrations

2127

of dissolved sulfide increase from <1.0 g L–1 towithin-barrier values of about 1700 g L–1. Figure 7
shows the effect on the concentration of dissolved Cu,
which decreases to <10 g L–1 in the outflow plume.
Concentrations of other dissolved metals show similar
patterns, with Zn decreasing from about 2400 g L–1
to <30 g L–1, and Cd and Ni decreasing from ~16 and118 g L–1 to <3 and <10 g L–1, respectively.
Mineralogy of the pilot-scale PRB
Covellite [CuS] and mackinawite were noted as
black precipitates on PVC tubing that was withdrawn
from the pilot-scale PRB after 21 months of operation(McGregor et al. 2002). Prior to construction of the fullscale wall, additional coring of the pilot PRB was done,
and filtrates on acetate filters from within the monitoring
wells were also collected to obtain the mineralogical
data reported herein. The filtrate and cores also containsmall amounts of primary sulfides, i.e., those that did
not form in situ, whose presence was readily distinguishable by textural features. In the samples examined,
the primary sulfides occur as typically isolated (single)
disseminated grains whose observed decreasing order ofabundance is chalcopyrite > pyrite > bornite > sphalerite>> galena >> molybdenite [MoS2].
Minerals designated here as secondary are thosethat have precipitated within the barrier. The sulfides
are dominated by framboidal pyrite (Fig. 8a) andblue-reflectance Cu sulfides that are mainly covellite.
As in the Nickel Rim PRB, secondary carbon varies
from optically isotropic to strongly anisotropic andoccurs in fragments, some of which have indicationsof layering that is made evident by the incorporationof masses of minute inclusions of lithic material orformer voids (Fig. 8a). Secondary chalcopyrite iscommon in small amounts, chiefly as rims on various
non-sulfide cores (Figs. 9c, 10); locally, the rims oscil-

FIG. 7. Distribution of Cu (g L–1) in the Vancouver pilotscale PRB, 21 months after installation (data from Ludwig
et al. 2002).

FIG. 8. Industrial site pilot-scale PRB; bar represents 100 m. (a) Reflected light, showing a fragment of optically isotropic carbon and numerous associatedcircular framboids of pyrite. The top and bottom of the carbon are riddled with inclusions, but the large grey “inclusion” at the arrow tip is a former void. (b)Reflected light, showing on the right a particle of optically isotropic carbon with attached peripheral grains of secondary chalcopyrite. (c) Reflected light and(d) the same field with internal reflection, showing Fe oxyhydroxide replacement of cellular organic matter, within which is also a micrometric bleb of native
copper (not visible at this scale). The cellular material yielded a very weak X-ray pattern of goethite.

2128
THE CANADIAN MINERALOGISTMINERALOGY OF PERMEABLE REACTIVE BARRIERS

2129

FIG. 9. Industrial site pilot-scale PRB; bar represents 100 m; all except (d) in reflected light. (a) Complex texture of fragmental
Fe oxyhydroxide. At the tip of the arrow is a minute, white bleb of MnS. The upper half of the fragment consists predominantly of goethite, and the lower, higher-reflectance half is mainly maghemite. (b) Fragmental Fe oxyhydroxide consistingof maghemite and goethite; the yellowish grain to its left is primary chalcopyrite, and on the far left is a large grain of grey,
primary magnetite. (c) Globules of malachite enclosed within a thin rind of secondary chalcopyrite. (d) Almost the same fieldas (c), but with internal reflection, showing the radial-fibrous, globular form of the malachite.

late between chalcopyrite and lesser amounts of bornite(Fig. 10e). One particle contains globular malachite[Cu2(CO3)(OH)2] rimmed by a phase that may be a
Fe–Cu (hydroxy)sulfate, and which in turn has an outerrind of chalcopyrite (Fig. 9c). The secondary-mineralcores upon which rinds of secondary chalcopyrite
have formed have variable compositions (Table 2). For
example, two cores, one of which is shown in Figure
10b, have compositions corresponding to that of spertiniite [Cu(OH)2], another corresponds to cuprite [Cu2O],
and yet others of similar form, including the presenceof a narrow rim of chalcopyrite, have more complex
compositions (Table 2). Illustrative of the mineralogicaldiversity of the cores is the fact that, in one particle,
secondary chalcopyrite has been deposited on carbon(Figs. 8b, 11b), and several cores of primary magnetite
have a rim of secondary pyrite rather than of chalcopyrite (Fig. 11a). However, most of the cores around which

secondary sulfides have formed are Fe oxyhydroxides
and probable Fe–Cu hydroxysulfates.
Other minerals detected in trace amounts are native
copper, a few blebs of native iron, and a bleb of MnS(alabandite) within Fe oxyhydroxide (Fig. 9a). Asidefrom framboidal pyrite, the principal sinks for Fe havebeen the fragmental-type Fe oxyhydroxides (Figs. 9a, b)
and sorption-replacement of organic matter that is now
distinctly Fe-rich (Fig. 8c). Some of the energy-dispersion spectra of the rudimentary Fe oxyhydroxides show
the additional presence of Cu, and electron-microprobe
analyses of the fragmental Fe oxyhydroxides indicate
the local presence of trace amounts (<0.05 wt.%) of Cu
and Ni. Debye–Scherrer X-ray patterns combined withthe optical properties of the fragmental Fe oxyhydroxides indicate that the principal components are goethiteand maghemite. The fragmental type of oxyhydroxides
cement detrital material, but such occurrences are rare.

FIG. 10. Industrial site pilot-scale PRB; reflected light, bar represents 100 m. (a) Partial rim of secondary chalcopyrite on a core of, possibly, Fe–Cu
hydroxysulfate (Table 2, anal. 4). (b) Rim of secondary chalcopyrite on a core of probable spertiniite (Table 2, anal. 1), below which is a grain of primarychalcopyrite and, at the bottom right, a grain of primary bornite. (c) Fibrous secondary chalcopyrite around a core of probable Fe–Cu sulfate. Yellowish grains on the right are primary chalcopyrite. (d) Possible Cu-rich S-bearing Fe-oxyhydroxide (anal. 3, Table 2) with associated whitish secondary
chalcopyrite. Near the rim of the particle is blue secondary covellite, and along the exterior is a thin rind of secondary chalcopyrite. (d) Secondary
covellite (blue, confirmed by XRD) with peripheral secondary chalcopyrite and secondary brownish bornite. The chalcopyrite and bornite locally show
oscillatory zoning.

2130
THE CANADIAN MINERALOGISTMINERALOGY OF PERMEABLE REACTIVE BARRIERSMcGregor et al. (1999) illustrated on an Eh–pH
diagram that influent versus effluent water at the pilot
PRB follows a trend primarily toward decreasing Eh,with little change in pH (6 ± 0.5). The within-barrier
mineralogical trends for Cu follow a similar patternof decreasing Eh, with early deposition of malachite
and Cu and Fe–Cu hydroxides and hydroxysulfates,
followed by later deposition of Cu sulfides.
Full-scale PRB
Following about four years of monitoring of thepilot-scale unit, a full-scale PRB was installed between
November 2000 and March 2001 (Mountjoy & Blowes
2002). Prior to the installation, two laboratory-column
tests were done using groundwater from a monitoring
well near the proposed site of the full-scale barrier. Oneof the tests used a reactive medium whose components
were like those emplaced in the pilot PRB, and the othertest used a similar mixture, but with iron filings added.
The iron-filings medium was observed to increase
both the extent and the rate of reaction (Mountjoy &Blowes 2002).
The full-scale PRB is approximately 400 m long,
2–2½ m wide, and up to 14 m deep. During construction, more than 16 million liters of guar-gum slurry
were pumped into the trench to prevent collapse of
the unconsolidated sediments into the excavation(Mountjoy & Blowes 2002). The “main” wall contains
a medium of compost, limestone, and pea gravel. As
well, about 130 m of the eastern portion of the wallis accompanied by an additional parallel wall thatwas installed along the inflow side of the main wall.
In this parallel section, the compost-based treatment
medium was supplemented by the addition of Connelly
iron filings, and this particular blend was emplaced in

2131

the lower portion of the wall; however, the shallower
portion contains only the compost-based mixture plus
sand. All of the wall was covered with a geosynthetic(PVC) layer, 30 mm thick and overlain by ~0.3 m ofsand, to minimize surficial infiltration from the adjacentcontaminated sediments.
After several months of functioning, the full-scale
PRB was cored for a mineralogical study. Like within
the pilot PRB, the principal sulfide in the full wall ispyrite that occurs predominantly as individual isolated
framboids rather than as polyframboidal clusters, andunlike the situation at Nickel Rim, little replacement oforganic matter by sulfides has occurred. The reactive
medium in the full wall also contains minute amountsof secondary marcasite, chalcopyrite, and covellite. As
was observed in samples of the pilot PRB, the deposition of secondary rinds of chalcopyrite was commonly
preceded by the formation of cores of Fe–Cu or Cu–Fe
oxides–hydroxides. Marcasite occurs both as isolated
polycrystalline particles and as aggregates of elongate to
fibrous grains that enclose framboids of pyrite.

FIG. 11. Industrial site pilot-scale PRB. Back-scatteredelectron images showing (a) secondary pyrite as a rim
on primary magnetite, and (b) chalcopyrite on carbon, as
illustrated in Figure 8b.

2132

THE CANADIAN MINERALOGIST

FIG. 12. Industrial site full-scale PRB. (a) Overview of the cored treatment-medium containing zero-valent iron, reflected light,
bar scale 0.5 mm. The whitish particles are variably oxidized Fe0, and the grey particles are pea-gravel lithic fragments. (b)Initial stage of oxidation of Fe0, reflected light, bar scale: 100 m. Grey Fe oxyhydroxide occurs at the rim and is also associated with veinlet-like brownish graphitic material in the interior. (c) The same field as (b), but with almost crossed polarizers.
The whitish, thread-like grains are graphite, whereas most of the carbonaceous material associated with alteration is finely
polycrystalline and therefore appears to have lower anisotropy. (d) Pseudomorph of compositionally and texturally complex
Fe oxyhydroxide after an iron filing. Reflected light. The brownish material within the pseudomorph and along the left edge isgraphite or carbon. The arrow points to a bleb of Ti metal. (e) Partly altered Fe0 with spheroidal “primary” graphite; reflected
light, bar scale: 100 m. Penetration of Fe-oxyhydroxide alteration along graphitic centers is evident at the far right. (f) Same
field as (e), but with polarizers almost crossed, showing the radial fibrous character of the graphite spheroids.

MINERALOGY OF PERMEABLE REACTIVE BARRIERSIn core samples containing the Fe 0 treatment
medium, the principal reaction has been partial replacement of the filings by Fe oxyhydroxides that form rims
and veinlets (Fig. 12). Mineralogical examination of
the archived, unreacted filings showed that, in addition
to metallic iron, the filings contain graphite, magnetite,maghemite, hematite, and traces of an unidentified optically anisotropic Fe oxide. The various oxides associated with the archived Fe0 are not uniformly distributed
insofar as many of the filings either lack or have little
oxide coating; only rarely do the particles consist chiefly
or wholly of oxides, and the oxides invariably show an
outward progression toward increased oxidation. Thus,
hematite in the archived material characteristicallyoccurs sporadically only as a thin coating and is the lastoxide formed in the outward progression Fe0 → magnetite → maghemite → hematite. Graphite commonlyoccurs as blebs that in detail have a radial fibrous
structure (Fig. 12f); even more commonly, however, thegraphite is present as elongate, thread-like particles that
impart a reticular texture to the iron (Fig. 12c).
In the PRB-reacted filings, the exteriors of the Fe0
particles and the sites containing graphite appear to havebeen the foci of initial replacement by oxyhydroxides(Fig. 12b). Where pseudomorphism of Fe0 has occurred,some of the pseudomorphs show vestiges of the original
distribution of graphite, but in other particles, thedistribution of graphite is unlike that observed in thearchived Fe0 (Figs. 13a, b). Moreover, no pseudomorph
containing blebs of graphite, as in Figure 12e, andnone of the magnetite–maghemite intergrowths thatare present in the archived Fe0, were observed in thereacted samples. Whether these differences are significant is an issue that requires further study to resolve.
For example, the apparent non-occurrence of Fe0-associated magnetite is at variance with the persistence andstability of primary (detrital) magnetite and ilmenite
within the PRB.
Limestone is a component of the reactive medium,but it is readily distinguishable optically from thesecondary carbonates that have formed. The latter occur
predominantly as a rim on lithic particles, as aggregatesof varied habit, and as finely disseminated grains that
may be the physically disintegrated fines of what wereoriginally rims and clusters (Figs. 13e, 14a). Electronmicroprobe analyses and Debye–Scherrer X-ray data
indicate that almost all of the secondary carbonate issiderite, but aragonite also has been detected within
pseudomorphs of the iron filings (Fig. 14c). Some ofthe peripheral Fe carbonate has a fibrous habit like thatof aragonite (Roh et al. 2000). Electron-microprobe
analysis of the siderite indicates highly variable compositions that range from almost the pure end-member to
Ca- and Mn-rich (up to 6.3 wt.% CaO and 16.5 wt.%
MnO). In samples containing abundant secondary
carbonates, secondary sulfides are extremely sparse.
Energy-dispersion analysis indicates that some of thesiderite rinds are heterogeneous and locally contain Cu

2133

oxides or hydroxides, as well as associated covellite,
and it seems that some of the dissolved Zn (and abundant Mn) has been incorporated in siderite precipitates;
nonetheless, at this early stage of operation of the PRB,
the principal repository for solubilized Cu and Zn seems
to be the Fe oxyhydroxides.

DISCUSSION
The formation of secondary Cu–Fe sulfides within
the PRBs is unusual but not unprecedented for this typeof geochemical setting. Copper-rich supergene environments are typically dominated by Cu–S minerals, but
Cu–Fe–S minerals also form, and the co-occurrenceof chalcopyrite, bornite, and native copper has been
observed in a few instances (e.g., Sinclair & Gasparrini
1980). In another type of setting, chalcopyrite as rims
on framboidal pyrite, with associated marcasite andcovellite, was described by Lett & Fletcher (1980) as
an authigenic mineral in reducing, organic-matter-richsoil layers. The precipitation of covellite (Martin et al.
2003) and of chalcopyrite and pyrite (Pirrie et al. 1999)
has been observed to occur under reductive conditions
in organic-matter-rich muds containing mine-waste
contaminants. The authigenic formation of chalcopyriteand covellite by replacement of anthropogenic Cu wirein estuarine sediments was reported by Moles et al.
(2003), and similarly related authigenic occurrencesof flocculent Zn–Fe sulfide of variable composition,and of Cu–Fe sulfide with Cu:Fe:S identical to thatof chalcopyrite, have been observed in anaerobic
freshwater sediments (Large et al. 2001). Althoughsupergene sphalerite has only rarely been reported
to occur in mineral deposits (Brown 1936, Sinclair& Gasparrini 1980), a large deposit of bacteriogenic
supergene framboidal sphalerite has been identified in
the Carlin Trend of Nevada (Bawden et al. 2003). Bothsphalerite and chalcopyrite have formed as nanoscalesecondary sulfides in sulfate-reducing environments
in mining-contaminated anoxic sediments associatedwith the Clark Fork River Superfund site in Montana(Hochella et al. 2005). Bornite has been synthesizedfrom aqueous solutions at room temperature (Cuthbert1962), and chalcopyrite has been reported by McNeil& Mohr (1993) to be a common product of corrosion
where natural conditions are appropriate.
Hardy & Gillham (1996) suggested that hydrophobichydrocarbons may form during the reaction of zerovalent iron with aqueous CO2, but Deng et al. (1997)
showed that carbon impurities associated with the Fe0
were the source of the hydrocarbons. However, insulfate-reducing PRBs, the organic medium of treatment
provides a pervasive and reactive source of carbon, andthere is evidence to indicate that the unprecedented
formation of secondary carbon is a characteristic featureof these PRBs. The efficient decomposition of CO2 tocarbon by reaction with “cation-excess” magnetite, as
contrasted with the cation deficiency of maghemite, has

2134

THE CANADIAN MINERALOGIST

FIG. 13. Industrial site full-scale PRB; bar represents 100 m except in (d). (a) Pseudomorph of Fe oxyhydroxide after aniron filing, showing three relatively large, lenticular particles of carbon at mid-height. (b) The same field as (a), but withpolarizers almost crossed and internal reflection. Arrows point to the two largest carbon particles, which are anisotropic but
finely polycrystalline. (c) Pseudomorph after an iron filing, showing the complex texture and multiple replacement-induced
phases. Reflected light. (d) Enlargement of the right half of (c), reflected light, bar scale: 50 m. The arrow points to ableb of secondary blue covellite; to the left of the covellite is thread-like Mo metal, possibly derived from the Fe0. (e) Feoxyhydroxide pseudomorph after an iron filing, reflected light. (f) The same field as (e), but with polarizers nearly crossed,internal reflection, showing a well-developed rim of near-end-member secondary siderite that is locally Zn-rich.

MINERALOGY OF PERMEABLE REACTIVE BARRIERS

2135

FIG. 14. Industrial site full-scale PRB; bar represents 100 m. (a) Reflected light, and (b) with polarizers nearly crossed,internal reflection. On the right in (a) is a pseudomorph of an iron filing containing abundant brownish carbon. On the left
is a large grain of albite with a rim of secondary siderite that contains ~13 wt.% MnO. (c) Part of a pseudomorph after aniron filing, in reflected light. At the bottom right are white remnants of Fe0 with abundant associated carbon. The interior of
the pseudomorph at the arrow consists of aragonite. (d) The same field as (c), but with polarizers almost crossed, internalreflection. White areas are aragonite.

been demonstrated by Tamaura & Tabata (1990), and
it is possible that analogous reactions occur at ambient
temperatures in PRBs. Detection of inorganic carbon
is commonly interpreted to represent the carbonatemineral fraction in PRBs, but the results may be
misleading if biotic reducing conditions are present.

CONCLUSIONSPRBs offer an effective method for the remediation of a variety of subsurface aqueous contaminants.
The question of long-term effectiveness remains tobe answered; although several studies have observed
declines in performance over time (Mackenzie et al.
1999, Yabusaki et al. 2001, Benner et al. 2002, Klausen
et al. 2003, Morrison 2003, Li et al. 2005), the declines
are not synonymous with a lack of effectiveness.
Aside from considerations about the design of PRBs

(Robertson et al. 2005), the principal concerns about
long-term treatment generally relate to (1) decrease inporosity and permeability through the accumulation of
secondary minerals (and gas; Zhang & Gillham 2005),
and (2) decrease in reactivity because of the armoring
effect of secondary coatings or because of the consumption of the more accessible portions of the reactive
medium, especially the organic carbon.
At the British Columbia site, consumption of theFe0 has been extensive over a short period, but the
rapid oxidation may be related to the initiation period,in which “fresh” Fe0 surfaces are at a maximum, rather
than reflecting steady-state, long-term conditions. The
PRB will be monitored to evaluate performance andtrends over time.
The cost of PRBs varies with site conditions, but
an indication of the order of magnitude is that the total
cost of the Nickel Rim barrier was about $35,000 US,

2136

THE CANADIAN MINERALOGIST

about half for the purchase and delivery of materials,
and the remainder for installation (Smyth et al. 2001,
Blowes et al. 2003). The industrial-site pilot PRB costslightly less, but unit costs were similar at about $190
US per cubic meter (McGregor et al. 2002). Thus, PRBs
potentially represent a highly cost-effective method for
remediation of a wide variety of contaminated sites.

ACKNOWLEDGEMENTS
D.W. Blowes of the University of Waterloo isthanked for significant contributions and for providing
the samples used in the Nickel Rim study. Assistancewith the X-ray powder-diffraction identifications was
kindly given by A.C. Roberts of the Geological Surveyof Canada, A. Lam of the Department of Chemistry,
University of British Columbia, and J.M.D. Wilson of
CANMET. We are grateful to F.C. Hawthorne and N.
Sidenko for their critical reviews of the manuscript.

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Received May 25, 2005, revised manuscript accepted
November 29, 2005.