permeable reactive barriers: a sustainable technology for cleaning contaminated groundwater in...

Permeable reactive barriers: A sustainable technology for

cleaning contaminated groundwater in developing

countries

D.H. Phillips

Environmental Engineering Research Centre, School of Planning, Architecture and Civil Engineering,

Queen’s University of Belfast, Belfast BT9 5AG, United Kingdom

Tel. þ44 (0)28 9097 55; Fax þ44(0)28 9097 42; email: [email protected]

Received 31 January 2008; revised accepted 15 May 2008

Abstract

Permeable reactive barriers (PRBs) are a proven technology for remediating contaminated groundwater, particularly on indus-

trial and mining sites. PRBs are a sustainable technology that can operate over a long time scale with low maintenance. Over the

past 10–15 years, there have been great strides in refining site characterisation techniques (i.e. geophysical techniques), develop-

ing/discovering reactive materials/sorbents (i.e. Fe0 filings), and the installation and design of PRBs (i.e. funnel-and-gate design)

which have increased the cost-effeciveness of this technology. Prior to installation, careful consideration of the ease of removal of

the PRB should be considered as part of the design. This is important as the PRB may eventually need to be decommissioned. PRBs

are a sustainable site specific remediation technology that has the great potential to work well as a part of a larger scale integrated

water resource management programme in developing countries.

Keywords: Permeable reactive barriers (PRBs); Reactive materials; Sorbents; Remediation; Site characterisation; PRB designs

1. Introduction

Permeable reactive barrier (PRB) technology has

been sucessful in remediating a variety of groundwater

contaminants including heavy metals [1], organics [2]

and radionuclides [1,3]. Most PRBs have been installed

on industrial, mining and agricultural sites around the

world [1–3]. PRBs use the natural hydraulic gradient

of the groundwater plume to move the contaminants

through the reactive zone giving it an advantage over

traditional pump-and-treat technologies by being more

cost effective and lower maintenance in the long-term

[3]. Over the past decade, much work has been done on

improving site characterisation techniques, developing

reactive materials/sorbents, and the installation and

design of PRBs. This work has increased the cost-

effeciveness of this technology making it a more viable

remediation option for developing countries.

Presented at the Water and Sanitation in International Development and Disaster Relief (WSIDDR) International

Workshop Edinburgh, Scotland, UK, 28–30 May 2008.

0011-9164/09/$– See front matter © 2009 Published by Elsevier B.V.doi:10.1016/j.desal.2008.05.075

Desalination 248 (2009) 352–359

Geophysical techniques such as magnetic and

ground probing radar are a form of non-invasive site

characterisation that yields valuable information about

site geology, in situ engineering properties, hidden cul-

tural features, and contamination in the shallow subsur-

face. Non-intrusive investigations are a quick and cost

effective means of obtaining data, especially when

combined with old site plans, and are useful in planning

the intrusive site investigations. These techniques are

reducing the need for more expensive trial pitting or

borehole drilling with lower risks and decreasing the

chances of missing buried targets. Some of the instru-

ments (EM units) are hand carried and generally do not

contact the ground. All accessible areas of a site can be

quickly surveyed (up to 2 ha/day) without disturbing

the surface [4]. This is important because there may

be considerable surface and near surface contamina-

tion on former industrial and mining sites. Comparing

site historical plans with their geophysical surveys is a

very beneficial ground truthing method and is gener-

ally part of the protocol of a study.

A variety of reactive materials and sorbents,

which can be used separately or in combination

depending on the groundwater contamination, have

been successful in remediating contaminated

groundwater in PRBs. These materials, such as Fe0

filings [4], peat [3], limestone [4,8], granular acti-

vated carbon (GAC) [5,6] and zeolite [1], are easily

available and some are fairly inexpensive. Bench-

scale treatability studies are carried-out in the initial

screening of the reactive or sorbent material to plan

the design of PRBs using site groundwater. Batch

studes using a number of likely reactive and sorbent

materials are conducted to determine the best per-

forming materials. Then column tests are carried-

out on the best performers. Column tests can give

information towards the design of the PRB and indi-

cations on how an in situ PRB will perform [7].

Installation of PRBs is a crucial stage, especially in

the excavation of geological material. Improved equip-

ment and techniques used to excavate geological mate-

rial without obstructing the flow of the contaminated

groundwater plume in and out of the PRB has helped

to increase the success rate of PRB performance. Dur-

ing installation, loose geological material and soil can

be packed, smeared and fill void space that the con-

taminated groundwater flows through adjacent to the

PRB [8,9]. Methods in depositing of the reactive mate-

rial in to the PRB have also been refined to reduce par-

ticle size grading which can alter groundwater flow

through the reactive zone [9]. Prior to installation, care-

ful consideration of the ease of removal of the PRB

should be considered as part of the design. This is

important as the PRB may eventually need to be

excavated due to the completion of remediation; there-

fore, decommissioning could become an issue [7].

Of the two basic designs, i.e. the continuous trench

and the funnel-and-gate, the funnel-and-gate design

with the reactive material placed in single or sequenced

containers is probably the most cost-effective design.

This is because the funnel-and-gate usually uses less

reactive material than the continuous trench. The reac-

tive material(s) is placed in the canister(s) (reactors)

and can be removed if it needs to be replaced. This is

an important consideration as there could eventually

be built-up of contaminant concentrations in the reac-

tive/sorbent material(s) from the remediation process.

The containers can be designed to be reused at other

sites once remediation is finished.

PRBs are a sustainable site specific remediation

technology that has the great potential to work well

as a part of a larger scale integrated water resource

management programe in developing countries. The

objectives of the paper are to illustrate how PRBs are

planned and installed, and highlight cost effectiveness

which may allow them to be installed in developing

countries.

2. Material and methods

If PRB technology is considered one of the

options for the remediation of contaminated ground-

water at a site, there are a series of steps that should

be taken to ensure that the PRB is viable, and that it is

designed and installed properly. One of the first steps

is to collect as much information about the site as

possible, such as blueprints, geological maps, and

records. This will help in ground truthing and deter-

mining contamination. What are the possible con-

taminants? Have there been any buildings on site?

Buried debris may imped PRB installation and

groundwater flow across the site. Knowing what

contaminants may be present at the site will also

allow for better health and safety plans.

D.H. Phillips / Desalination 248 (2009) 352–359 353

2.1. Site characterisation

Non-invasive characterisation of the site using geo-

physical techniques, especially when used along with

test pits and wells improves the detail of the site char-

acterisation and is cost effective and time saving. Gen-

erally, a site is first cleared of surface objects (i.e. scrap

metal, reinforced concrete rubble) that may interfere

with subsurface features. A grid (i.e. 5 m) is surveyed

out (i.e. using a Total Station Lecia 1010 Wild Heer-

brugg surveyor). Further points are measured at the site

boundaries and across the site to give a representation

of height A.O.D.

Electromagnetic (EM) and ground penetrating

radar (GPR) instruments are portable units that can

detect metal, debris, geological features, and contami-

nation in the shallow subsurface. A Geonics EM61

time domain metal detector is used to survey for fer-

rous and non-ferrous metal objects down to 5 m. The

instrument, operating at 75 Hz, is trawled across the

site by the operator. The Geonics EM31 can map geo-

logical variations and any other feature associated with

ground conductivity change down to 6 m. The EM31 is

in the form of a boom that is carried about 1 m above

ground. It can be used in a normal operating orientation

(vertical dipole mode) or turned 90� to its long axis

(horizontal dipole) and used to measure down bore-

holes. The Geonics EM38 also measures conductivity

and is carried 15–20 cm above the ground. It is particu-

larly sensitive to soil salinity which could be a charac-

teristic of contaminated land/groundwater. Similar to

the operation principle of the EM31, the EM38 surveys

to the depth of 1.5 m in the vertical dipole, and 0.75 m

in the horizontal dipole modes [10]. GPR is also used

for high resolution shallow subsurface investigations.

Ground truthing should be part of the protocol in a field

investigation by comparing the geophysical results to

historic plans and site visits.

2.2. Treatability studies

Batch tests are used to screen potential reactor mate-

rials. They can determine whether the contaminants are

amenable to sorption, degradation, or precipitation by

different types of media. They also can be used to com-

pare treatment efficiency of reactive and sorptive mate-

rials. Contaminant degradation rates are measured

quantitatively over time, and various parameters and

several sampling intervals can be tested. Batch tests are

carried-out in bottles or vials capped with inert septa. A

known weight of reactive material and a known concen-

tration and volume of contaminant solution (preferably

the contaminated groundwater) are added to the vials.

Samples are extracted at regular intervals and the con-

taminant concentration measured. Degradation curves

can be made from this data, and degradation products

can also be determined [7].

Column tests are used to collect detailed informa-

tion on the degradation, precipitates, removal and/or

sorption of contaminants by reactive/sorptive media.

Removal rate data under a range of flow conditions,

especially those that mimic groundwater flow velocity

in the field can be determined with column tests. Con-

taminated or simulated groundwater is passed through

the column at a known flow rate. The column design

allows changes in contaminant composition and other

parameters (e.g. major ions, pH) to be determined at

the influent, effluent and along the column length

(Fig. 1) [7]. Column dimensions are generally 10–

100 cm long, with a 2.5–3.8 cm inside diameter [8].

Glass columns are generally the least reactive or

adsorptive with chlorinated organic compounds. How-

ever, no significant loss of organics have been found

using Perspex columns [11]. Sampling ports along the

length of the column should be constructed of stainless

steel fittings or inert stoppers (i.e. Teflon coated or

Viton). The ports should allow the sampling needle

to be inserted into the centre axis of the column or a

needle to be fixed in place in each port. A three-way

port should also be positioned in the influent and efflu-

ent lines. All tubing and fitting for the influent and

effluent lines should be composed of an inert material.

Information from the column study can be used along

with the site characterisation and modeling to help to

design the field-scale PRB [7].

2.3. Excavation

In conventional excavation of continuous trenches,

the soil is removed and the trench is backfilled with

reactive/sobent materials. When the emplacement of

the reactive materials is completed, temporary retain-

ing structures which support the walls will be removed

from the ground. A backhoe comprised of a digging

bucket on the end of an articuated arm is frequently

used for rapid digging of shallow trenches [8] less that

354 D.H. Phillips / Desalination 248 (2009) 352–359

10 m deep. A continuous trenching machine can also

be used to create a narrow trench less than 7 m deep.

It is comprised of a chain saw cutting belt and a trench

box on the boom. The backfill of reactive/sorbent

materials is carred-out with a hopper on the top of the

trench box. The removal of soil and the backfilling of

reactive materials are simutanously carried-out. This

elminates the need for dewatering and temporary

retaining structures [11,12]. An excavation of up to

15 m depth can be carried-out with a caisson for

funnel-and-gate PRBs. Caissons with a circular

cross-section are frequently used. After it has been

installed, a large auger can be used to remove the soil

within the caisson. The reactive/sorbent materials can

be then backfilled into the caisson [11,12].

2.4. PRB design

The two common PRB designs are the continuous

trench PRB and the funnel-and-gate system (Fig. 2a

and b). The continuous trench PRB does not contain

any structures, so the contaminant plume flows through

the treatment zone using the natural hydraulic gradient.

This PRB, which is perpendicular to groundwater flow

direction, needs to be slightly larger than the cross-

sectional area of the contaminated groundwater inorder

to capture the contaminants in both vertical and hori-

zontal directions [9]. The top of the PRB should be at

least 0.60 m above the water table and the bottom of the

PRB should be extended at least 0.30 m into a low per-

meability zone (i.e. clay), if it is present. The PRB

thickness should be designed to provide sufficent resi-

dence time for the contaminants within the treatment

zone to be completely treated. The funnel-and-gate

system is composed of impermeable walls and at least

one reactive zone. The funnel structure could be sheet

piles or slurry walls. The function of the funnel is to

intercept the contaminated groundwater and lead it to

the treatment zone. The bottom of the funnel and reac-

tive zone needs to be extended at least 0.30 m into the

less permeable soil layer, while the top of the funnel-

and-gate needs to be set at least 0.60 m above the water

table [10]. The reactive material is directly implaced or

filled into the reaction vessel(s) [9]. Multi-sequenced

reactive barriers are also being installed, especially

on sites with multiple groundwater contaminants such

as gas works sites. Multi-sequenced PRBs use muliti-

ple reactive materials in more than one reactive zone

to treat the contaminated groundwater [13] (Fig. 2c).

3. Results and discussion

3.1. Cost-effective groundwater clean-up

with PRBs

Over the past 10–15 years since the first PRBs were

installed to remediate contaminated groundwater, there

Fig. 1. Diagram and photograph of a column test.


have been great strides in reducing their costs. Com-

pared to traditional pump-and-treat remediation, PRBs

can be more costly in the initial stages, especially dur-

ing installation. However, since PRBs are a passive

system, which relies on the natural hydraulic gradient

of the groundwater plume to move the contaminated

groundwater through the treatment zone, the long-

term costs are lower than traditional pump-and-treat

operations and maintenance [14]. Many PRB reac-

tive/sorbent materials can remediate a range of con-

taminates, while others are selective. PRBs can be

used as part of a treatment train with other technologies

to clean groundwater. PRBs are generally below

ground and ‘out of sight’ so the site can be used for

other uses. However, there are some disadvantages of

PRBs. Sometimes the PRB require more maintenance

than originally planned. PRBs can also become

clogged and the reactive material coated causing them

to become less effective.

Non-invasive site characterisation has also

decreased the cost of PRB installation. Many of the

magnetic and ground probing radar units are portable

and can easily be transported into sites that may be

remote. Data on the characteristics of a contaminated

site can be gathered rapidly and can help in planning

the invasive characterisation (trial pitting and bore-

holes). Data collected on buried features, geology and

hydrogeology can be used to model or better plan for

Fig. 2. Diagrams of PRBs. (A) Elevation view of a continuous trench or wall (after [14]), (B) plain view of a funnel-and-gate,

and (C) elevation view of a multi-barrier.


monitoring and remediation of contaminated land and

groundwater. The suite of chemicals contributing to

subsurface groundwater contamination may be electri-

cally conductive, either because the contamination is

acidic or contains salt. Therefore, the groundwater

plume is also electrically conductive [4]. Importantly,

the non-invasive site characterisation can also give

indications if the site is not geologically and hydrogeo-

logically fit for a PRB to be installed (i.e. absence of

low permeability material to secure the bottom of the

PRB), which can reduce money and time spent on a

remediation option that may fail.

Treatability studies may be carried-out at a number

of stages of PRB selection including remediation

options screening, PRB design and operation [7]. Treat-

ability studies are used to determine the best reactive/

sorbent media, to help design the PRB, and to predict

weaknesses in the PRB design. The cost of treatability

studies can vary greatly, and should be proportionate to

the total cost of remediation. Generally, treatability

studies should cost <10% of the total remediation costs.

To cut costs, reactive material from local sources can be

screened in a treatability study. For example, activated

carbon from coconut husk is used to treat arsenic

contaminated groundwater [6]. Other media such as

Fe0 [3], sawdust [1], plant material [1], and zeolite from

volcanic rock [1] are also used in PRBs as well as

microbes [1] in biobarriers. Using local sources may

save money on purchasing the media and also renewing

it. Additionally, the use of recycled materials makes

the PRB a sustainable remediation option.

The conventional excavation is a common PRB

construction technique and is more suitable for shallow

PRB systems such as a continuous trench <10 m deep.

Operational cost can increase with the depth of excava-

tion. The conventional excavation can be carried-out

with common excavation equipment such as a back-

hoe, a clamshell and a caisson [12]. Because the trench

is located in shallow strata, the whole installation of the

PRB can be easily monitored. Some potential problems

can occur with the conventional excavation. In addition

to a decrease in the permeability of the PRB [11] from

smearing, contaminated soil and water can potentially

be exposed during the excavation raising health and

safety issues. Nevertheless, the operational costs and

time are low for continuous trenching [12]. Although,

the funnel-and-gate PRB is physically easier to

decommission, the continuous trench PRBs can still

remain an option if the geology and hydrogeology is

favorable for installation, and if there is a lack of funds

and resources to install a funnel-and-gate PRB. In the

funnel-and-gate PRB, reactive material is stored in

canisters or reactors which can be removed after the

PRB has completed the remediation or if the PRB has

problems. This allows for removing of potentially

further contamination from the site as a result of con-

taminant build-up in the reactive material. Addition-

ally, funnel-and-gate PRBs generally require less

reactive material. This is a great cost saver, if the reac-

tive material is pricey. For these reasons, the funnel-

and-gate design should be considered if funds are

available. Therefore, it is important to take the design

of the PRB in to consideration and how this will affect

its long-term operation.

3.2. Case studies of PRBs

There are over 100 PRBs in operation around the

world at present; however, only a few are installed in

developing countries. Examples are pilot-scale PRBs

used to treat contaminated groundwater and leachate

from uranium mines in Hungary and Bulgaria, respec-

tively. The PRB in Pecs Hungary which was installed

in 2002, is a continuous trench containing shredded

Fe0 which removes uranium from the groundwater.

Sand layers have been added to the up-gradient and

down-gradient sides of the PRB to allow for better

groundwater flow through the reactive zone. The PRB

was emplaced in an underlying clay and geosynthetic

clay liner. Groundwater U concentrations were

reduced to <1% of influent concentrations after passing

through the reactive zone in year 2003. Uranium con-

centrations were reduced from *1000 mg/L to *100

mg/L in monitoring wells near the PRB and <10 mg/L

within the PRB. A negative performance issue is that

a high amount of precipitates has been estimated to

have formed in the PRB which may reduce ground-

water flow through the PRB and reduce the reactivity

of the Fe0. However, only a 1.6% loss in porosity was

calculated and the PRB is predicted to have a 62 year

lifespan [3]. The PRB in Western Bulgaria (installed

2004) treats acid drainage (pH 2.5–3.7) runoff from a

uranium mine contaminated with radionuclides

(mainly uranium and radium), heavy metals, arsenic


and sulphates. The PRB is part of a treatment train

(connected series) in the form of an alkalizing lime-

stone drain that removes Fe as hydroxides, the PRB for

biosorption and microbial reduced sulphates, and a nat-

ural wetland. The barrier is a ditch/continuous trench

with a reactive material consisting of a mixture of solid

biodegradable organic material (plant and mushroom

compost, wood chips, straw and cow manure), crushed

limestone and zeolite saturated with ammonium phos-

phate. A mixed microbial community consisting of

sulphate-reducing bacteria and other microorganisms

is also present. Uranium, arsenic and non-ferrous

metals were mainly removed in the PRB by the indi-

genous sulphate-reducing bacteria. Portions of the pol-

lutants, and most of the radium, were sorbed onto the

dead plant material in the PRB [1].

Another example of a PRB, but not in a developing

country, is a funnel-and-gate PRB in Monkstown,

Northern Ireland, United Kingdom. This PRB, estab-

lished in 1995, is a field-scale PRB developed to

remediate a small but highly concentrated accumula-

tion of a degreaser pollutant, trichlorethene (TCE), in

gravely glacial till on an industrial site. TCE is a dense

nonaqueous phase liquid (DNAPL) that is denser than

water and has a tendency to settle in the subsurface as

immiscible accumulations (slugs). The PRB consists of

a long tube-like structure that holds Fe0 filing reactive

material. This structure is emplaced in naturally occur-

ring clay at the site. The TCE contaminated ground-

water is funneled from the up-gradient position into

the gate of the PRB where the vessel that holds the

reactive material is present. The groundwater flows

down through the Fe0 reactive zone in the PRB where

it is treated and exits at the down-gradient. There is a

decrease in up-gradient TCE concentrations in this

Fe0 funnel-and-gate PRB over time which suggest that

Fig. 3. PRBs as part of a larger scale integrated water resource management programme in developing countries.


the PRB is remediating the TCE contaminated ground-

water at the site. However, there is also a possibility

that a great portion of the TCE slug was removed while

installing the PRB which would cause the concentra-

tions to be lower over time (Fig. 3). There is decrease

in TCE as it moves through the PRB indicating that

it is being remediated (dehalogenated) by the Fe0. A

pre-existing TCE contamination occurs adjacent to the

down-gradient portion of the PRB possibly due the

clipping of the end of the TCE contamination slug

when the PRB was being installed which hampers the

down-gradient monitoring of this PRB; however, data

shows that it is dramatically decreasing over time [2].

When PRBs were first being inplemented about 15

years ago, it was suggested that they become a reme-

diation option for developing countries. This is because

they are a sustainable remediation technology that

requires little maintenance and operational cost over

a long duration. Addtionally, reactive/sorbent materi-

als such as peat, sawdust, activated carbon, zeolites,

and limestone can be aquired easily and may be from

a local source. Generally, PRBs are used for site speci-

fic remediation. However, in areas where adequate

water supply is limited and there is contamination from

industrial, agricultural or mining sites, if properly

planned they could become part of a larger scale inte-

grated water resources management programme. For

example, an International Water and Sanitation Center

(IRC) project DREAM (Drainage and Reuse of Efflu-

ents for Agricultural Management) has planned PRBs

to be part of an integrated water resources management

programme as a low cost approach for wastewater

treatment for reuse in agricutural areas in developing

countries where water supply is low [15].

References

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